Phy Notes Manhatten 3
Transcript of Phy Notes Manhatten 3
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Chapter 18Chapter 18
ElectrostaticsElectrostatics
Chapter 18Chapter 18
ElectrostaticsElectrostatics 18.118.1 Electric ChargesElectric Charges
18.218.2 Different Charging MethodsDifferent Charging Methods
18.318.3 Relation between ElectricRelation between Electric
Current and Electric ChargesCurrent and Electric Charges
18.418.4 Electric FieldsElectric Fields
18.518.5 Electrostatic Hazards andElectrostatic Hazards and
ApplicationsApplications
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Section 18.1Section 18.1
Electric ChargesElectric Charges
Section 18.1Section 18.1
Electric ChargesElectric Charges
Charges in atomCharges in atom
Conservation of chargesConservation of charges
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??
18.1 Electric charges (SB p. 2)
Electrostatics Whycanthecombattract
thehairsaftercombing?
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Charges charges in atom
Atoms consist of nucleus and electrons
-
-
-
-
-
+ proton
neutron
electron
++
++
electrons
nucleus
Charges in atom18.1 Electric charges (SB p. 3)
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Nucleus consists of protons and
neutrons
neutron
proton++
++
-
-
-
-
-
+ proton
neutron
electron
nucleus
Charges in atom18.1 Electric charges (SB p. 3)
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proton carriespositive charge
neutron has nocharge
electron carries
negative charge
and circulatesaround nucleus--
-
-
-
+ proton
neutronelectron
++
++
nucleus
Protons, neutrons and electrons
Charges in atom18.1 Electric charges (SB p. 3)
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Electrical neutrality
Apositivechargeand
anegativecharge
canceleachother
whentheymeet.The
atomissaidtobeelectrically
neutralNormally,anatom
hasanequal
numberofelectronsandprotons(i.e.
carriesnonetcharge)
Charges in atom18.1 Electric charges (SB p. 3)
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Neutralization
-
-
-
-++
++
proton 4 1.6 1019 Celectron 4 1.6 1019Cnet charge 0
charges
neutral
Charges in atom18.1 Electric charges (SB p. 3)
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-
-
-++
++
Positive ions
-
lost electron
-
proton 4 1.6 1019 Celectron 3 1.6 1019C
net charge 1.6 1019 C
charges
carriespositive charge
Charges in atom18.1 Electric charges (SB p. 3)
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Negative ions
-
-
-
-++
++
-
-
gain electron
proton 4 1.6 1019 Celectron 5 1.6 1019C
net charge 1.6 1019 C
charges
carries
negative charge
Charges in atom18.1 Electric charges (SB p. 3)
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When a body loses a certain amount ofcharge, another body will gain the same
amount of charge at the same time
--
-
-++ ++
-
--
-
-++ ++
-
transfer
of electron
Conservation of charges
Conservation of charges18.1 Electric charges (SB p. 4)
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Section 18.2Section 18.2
Different Charging MethodsDifferent Charging Methods
Section 18.2Section 18.2
Different Charging MethodsDifferent Charging Methods
Charging insulatorsCharging insulators
Charging conductorsCharging conductors
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Conductors and insulators
Conductors Insulators
silver
copper
aluminium
iron
earthhuman
bodies
water
wood
18.2 Different charging methods (SB p. 4)
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Charging
electrically neutral
charging
positively charged negatively charged
or
+++
++
++
+++
--
--
--
--
-
The process of converting an electricallyneutral object to a charged object
Charging insulators18.2 Different charging methods (SB p. 4)
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Discharging
electrically neutral
discharging
positively charged negatively charged
or
++
++
++
+
+++
--
--
--
-
-
-
The process of converting a charged object toan electrically neutral object
Charging insulators18.2 Different charging methods (SB p. 4)
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Charging by friction - negatively
charged
polythene rod
Rubbing a polythene rod
with a neutral dry cloth
Electrons are transferred
from the cloth to the rod
Charging insulators18.2 Different charging methods (SB p. 5)
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Charging by friction - positively charged
acetate rod
Rubbing an acetate rod
with a neutral dry cloth
Electrons are transferred from
the acetate rod to the cloth
Charging insulators18.2 Different charging methods (SB p. 5)
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Experiment 18AExperiment 18A
Charging by friction
Intro. VCD Expt. VCD
Charging insulators18.2 Different charging methods (SB p. 5)
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Experiment 18A
Like charges repel
rubbed by
fingers
two strips carry like charges
and repel each other
two strips carry like charges
and repel each other
Charging insulators18.2 Different charging methods (SB p. 6)
Ch i i l18 2 Diff h i h d (SB 6)
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rubbed with a
piece of
woollen cloth
two balloons carry like
charges and repel each other
two balloons carry like
charges and repel each other
Charging insulators18.2 Different charging methods (SB p. 6)
Experiment 18A
Like charges repel
Ch i i l t18 2 Diff t h i th d (SB 6)
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Experiment 18A
Unlike charges attract
two balloons
are rubbed
against each
other
two balloons carry unlike
charges and attract each other
two balloons carry unlike
charges and attract each other
Charging insulators18.2 Different charging methods (SB p. 6)
Ch i i l t18 2 Diff t h i th d (SB 7)
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Class Practice 1Class Practice 1
If a polythene strip and an acetate strip are rubbed
with a dry cloth, explain what happens when the
strips are brought close together.The strips ____________ each other because the
polythene strip and the acetate strip are charged
____________ and _______________ respectively.Ans
wer
positivelynegatively
attract
Charging insulators18.2 Different charging methods (SB p. 7)
Ch i i l t18 2 Diff t h i th d (SB 7)
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- - - - - -
+ + + + + +
induced
charges
neutralpaper scrap
Why can a charged object
attract a neutral object ?
- - - - - -
+ + + + + +
+ + + + + + + + + +
a positively charged ruler
paper scrap is
attracted upwards
Charging insulators18.2 Different charging methods (SB p. 7)
Ch i i l t18 2 Diff t h i th d (SB 8)
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Class Practice 2Class Practice 2
What will happen to the water flowing from a tap
if a charged rod is brought close to it?
The water ___________________ (will beattracted / will be repelled / will not be affected)
by the charged rod. It is because
________________________________________
______________________
will be attracted
water molecules become polarized. The
attractive force between the rod and water
molecules is stronger than the repulsive force
between them. Ans
wer
Charging insulators18.2 Different charging methods (SB p. 8)
Ch i d t18 2 Diff t h i th d (SB 8)
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Experiment 18BExperiment 18B Charging by EHT power supply
Expt. VCD
Charging conductors18.2 Different charging methods (SB p. 8)
E.H.T. power
supply
metal strips
insulating rods
Charging conductors18 2 Different charging methods (SB p 9)
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_ +Unlike charges
+
+
+
+
+
+
+
+-
-
--
-
-
-
-
two strips attracttwo strips attract
Charging conductors18.2 Different charging methods (SB p. 9)
Charging conductors18 2 Different charging methods (SB p 9)
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Like charges_
-
-
--
-
-
--
-
-
-
-
-
-
--
two strips repeltwo strips repel
_
Charging conductors18.2 Different charging methods (SB p. 9)
Charging conductors18 2 Different charging methods (SB p 10)
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Experiment 18CExperiment 18C Charging by sharing
Expt. VCD
Charging conductors18.2 Different charging methods (SB p. 10)
foam board
Van de Graaff
generator
Charging conductors18 2 Different charging methods (SB p 11)
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Experiment 18C
Van de Graaff
generator
The hairs stand
on their ends!
-
-
--
-- - -
-
-
-
-------
--
- ---
---
--
---
-
--
Charging conductors18.2 Different charging methods (SB p. 11)
Charging conductors18 2 Different charging methods (SB p 11)
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A metal sphere is charged by sharing of
charges
Charging conductors18.2 Different charging methods (SB p. 11)
Charging conductors18 2 Different charging methods (SB p 12)
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Sharing of charges between two spheres of
different sizes
conductor less charges more charges
Charging conductors18.2 Different charging methods (SB p. 12)
Charging conductors18 2 Different charging methods (SB p 13)
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__
_
_
_
__
_
_
_
the sphere
acquires anegative net
charge
Charging by induction
Charging conductors18.2 Different charging methods (SB p. 13)
electrons flowelectrons flow
positivelycharged
metal rod
metal sphere
insulated stand
Charging conductors18 2 Different charging methods (SB p 13)
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EarthingAll charges ofthe conductor
willmovetotheearth
Charging conductors18.2 Different charging methods (SB p. 13)
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Section 18.3Section 18.3Section 18.3Section 18.3
Relation between ElectricRelation between ElectricCurrent and Electric ChargesCurrent and Electric Charges
Relation between ElectricRelation between ElectricCurrent and Electric ChargesCurrent and Electric Charges
18 3 Relation between electric current and electric charges (SB p 14)
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Experiment 18DExperiment 18DElectric current and electrostatic charges
Expt. VCD
18.3 Relation between electric current and electric charges (SB p. 14)
light beam
galvanometer
earthsocket
18 3 Relation between electric current and electric charges (SB p 14)
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Electric current
Electronsremain
stationarywhenthe
re
isnopathforthemt
o
flow.Theyarecalled
electrostaticscharg
es
Theflowof
electronsiscalledelectriccurrent
18.3 Relation between electric current and electric charges (SB p. 14)
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Section 18.4Section 18.4Section 18.4Section 18.4
Electric FieldsElectric FieldsElectric FieldsElectric Fields
18.4 Electric fields (SB p. 15)
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ExperimentExperiment18E18EDifferent electric field patterns
Expt. VCD
18.4 Electric fields (SB p. 15)
EHT power supply
+ electrodeelectrode
point
electrodecastor oil
semolina
18.4 Electric fields (SB p. 15)
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Electric field lines
Anelectricfieldisa
regioninwhichan
electriccharge
experiencesa
force
Thepatternsformedbysemolina
representtheelectricfieldlines
Electricfieldlinesindicatesthedirectionofelectricfield
18.4 Electric fields (SB p. 15)
18.4 Electric fields (SB p. 16)
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Different electric field patterns
18.4 Electric fields (SB p. 16)
18.4 Electric fields (SB p. 16)
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Positive charge
+
Electricfieldlinesaredirectedfrompositivecharge
( p )
E18.4 Electric fields (SB p. 16)
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Negative charge
Electricfieldlinesaredirectedtowardsthenegativecharge
( p )
18.4 Electric fields (SB p. 16)
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Direction of electric field lines
+
( p )
18.4 Electric fields (SB p. 16)
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Electric field lines
Do not
cross one
another
Do not have
branches
Each point of the
electric field has onedirection only
( p )
18.4 Electric fields (SB p. 17)
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Sketch the electric field lines between theelectrodes in the electric field apparatus shown
below:
Class Practice 3Class Practice 3
Ans
wer
( p )
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Section 18.5Section 18.5
Electrostatic Hazards andElectrostatic Hazards and
ApplicationsApplications
Section 18.5Section 18.5
Electrostatic Hazards andElectrostatic Hazards and
ApplicationsApplications Electrostatic hazardsElectrostatic hazards
Electrostatic applicationsElectrostatic applications
18.5 Electrostatic hazards and applications (SB p. 17)
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Lightning
- - - --- - - - ---
++
+
+
+ +
Electrostatic hazards
18.5 Electrostatic hazards and applications (SB p. 18)
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Lightning conductor
- - - --- - - - ---
++
+
+
+ +-
-
-
install a
lightning
conductor
Electrostatic hazards
18.5 Electrostatic hazards and applications (SB p. 20)
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Oil tanker
metal chain
connected
to theground
Electrostatic hazards
18.5 Electrostatic hazards and applications (SB p. 20)
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Aircraft landing
aircraft tyre madefrom conducting
rubber
Electrostatic hazards
18.5 Electrostatic hazards and applications (SB p. 21)
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Electrostatic nuisance
The screens of
TV sets,
monitors and
plastics attractdust particles
When taking off
woollen clothes,
crackling sound
will be heard
CD, VCD
attract dust
particles easily
On dry days,
touching metallic
doors may get
electric shock
Electrostatic hazards
18.5 Electrostatic hazards and applications (SB p. 21)
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Electrostatic precipitatorElectrostatic applications
18.5 Electrostatic hazards and applications (SB p. 21)
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Electrostatic precipitator
exhaust gas
clean gas
negatively
charged side wall
positively chargedcentral part
high voltage
source
in
out
positively chargedsmoke particles
attract to side wall
Electrostatic applications
18.5 Electrostatic hazards and applications (SB p. 22)
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positively
chargedmetal
surface
nozzle of spray gun
Electrostatic sprayingElectrostatic applications
nozzle of spray gun
negatively
charged droplet
nozzle of spray gun
negatively
charged droplet
nozzle of spray gun
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Chapter 19Chapter 19
CircuitsCircuits
Chapter 19Chapter 19
CircuitsCircuits19.1 Electric Circuit
19.2 Electromotive Force and
Potential Difference
19.3 Ohms law and Resistance
19.4 Simple Circuits
19.5 Electrical Power and Energy
19.6 Domestic Wiring and Electrical
Safety
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Section 19.1Section 19.1
Electric CircuitElectric Circuit
Section 19.1Section 19.1
Electric CircuitElectric Circuit
Electric currentElectric current
Circuit diagramCircuit diagram
19.1 Electric circuit (SB p. 30)
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Electric circuit
Combination of
different
electrical
components
No electric19.1 Electric circuit (SB p. 30)
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Open circuit
the switch
is opened
No electric
current flows
through the
circuit. The lightbulb doesnt light
19.1 Electric circuit (SB p. 30)
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A closed circuit
the switch
is closed
Electric current
flows through
the circuit. Thelight bulb lights
Electric current19.1 Electric circuit (SB p. 31)
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Direction of electric current
conventional current
flow of electrons
electrons
Conventional current
Electric current19.1 Electric circuit (SB p. 31)
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Electric current
t
QI=
=
i.e.
takenTimeflowChargecurrentElectricElectric
current (I)
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
quantity of
charge (Q)
time (t)
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Electric current19.1 Electric circuit (SB p. 32)
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Unit of current: ampere (A)
-1
sC1A1 = -1-3 sC10re)(milliampemA1 =-1-6 sC10re)(microampeA1 =
ammeter milliammeter microammeter
Circuit diagram19.1 Electric circuit (SB p. 33)
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Circuit symbols of electric components
battery light bulb switch
Circuit diagram19.1 Electric circuit (SB p. 33)
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ammeter voltmeter
Circuit symbols of electric components
Circuit diagram19.1 Electric circuit (SB p. 33)
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resistor rheostat potential divider
Circuit symbols of electric components
Circuit diagram19.1 Electric circuit (SB p. 33)
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Class Practice 1Class Practice 1 The following figure shows an electric circuit.
Draw the circuit diagram for it.Ans
wer
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Section 19.2Section 19.2
Electromotive Force andElectromotive Force and
Potential DifferencePotential Difference
Section 19.2Section 19.2
Electromotive Force andElectromotive Force and
Potential DifferencePotential Difference
Electromotive force
Potential difference
Relation between e.m.f. and p.d.
Electromotive force
19.2 Electromotive force and potential difference (SB p. 34)
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Electromotive force
Q
E=
=
e.m.f.Or
cellthethroughCharge
cellthebysuppliedEnergy
forceiveElectromot
e.m.f.
Electromotive force
Potential difference
19.2 Electromotive force and potential difference (SB p. 35)
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Potential difference
Q
EV =
=
Or
pointsthethroughCharge
pointstwobetweenformsotherintoconvertedEnergy
differencePotential
potential difference (V)
Potential difference
Potential difference
19.2 Electromotive force and potential difference (SB p. 36)
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Connection of ammeter and voltmeter
A
V
V
I
ammeter
in series
voltmeterin parallel
Potential difference
Relation between e m f and p d
19.2 Electromotive force and potential difference (SB p. 36)
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electromotive force = 6 V
V V
I
2 V 4 V
2 V 4 V
potential
difference = 6 V
Relation between e.m.f. and p.d.
6 V
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Section 19.3Section 19.3
Ohms law and ResistanceOhms law and ResistanceSection 19.3Section 19.3
Ohms law and ResistanceOhms law and Resistance
Ohms law
Change of resistance with
temperature
Change of resistance with
dimensions of a wire
Resistor and rheostat
Intro VCD Expt VCD
Ohms law19.3 Ohms law and resistance (SB p. 37)
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Experiment 19AExperiment 19A
Ohms law
eureka wires
voltmeter
ammeter
switch
batteryrheostat
Intro. VCD Expt. VCD
E i t 19A R lt
Ohms law19.3 Ohms law and resistance (SB p. 37)
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Potential
difference (V) / V
1 2 3 4 5 6
Current (I) / A
potential
difference
current
Experiment 19A Results
0.1 0.2 0.3 0.4 0.5 0.6
IV
Oh l
Ohms law19.3 Ohms law and resistance (SB p. 38)
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77
Ohms law
IV
The potential difference across a conductoris directly proportional to the current passing
through it, provided that the temperature
and other physical conditions remain
unchanged
R i t (R)
Ohms law19.3 Ohms law and resistance (SB p. 38)
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78
Resistance (R)
):symbolohm,:(Uniti.e.
conductorthroughcurrentconductoracrossp.d.Resistance
I
VR=
=
V
I
R
Change of resistancewith temperature
19.3 Ohms law and resistance (SB p. 40)
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79
Experiment 19BExperiment 19BChange of resistance with temperature
12 V d.c. supplyswitch
ammeter
voltmeter
light bulb
Expt. VCD
E i t 19B R lt
Change of resistancewith temperature
19.3 Ohms law and resistance (SB p. 41)
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80
Experiment 19B Results
Potential
difference (V) / V
1 2 3 4.5 6 8
Current (I) / A 0.1 0.2 0.3 0.4 0.5 0.6
Potential difference
Current
Do not
obey
Ohms law
T t i t
Change of resistancewith temperature
19.3 Ohms law and resistance (SB p. 42)
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81
-
-
-
-
-
-
-
-
-
-
-
-
Temperature , resistance
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
When the temperature increases, the atoms
of the conductor vibrate more violently andhinder the motion of the electrons. Hence,
the resistance increases
Change of resistance with
dimensions of a wire
19.3 Ohms law and resistance (SB p. 43)
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82
l
ammetervoltmeter
battery
Experiment 19CExperiment 19C
Change of resistance with dimensions of wire
eureka wiresof different
thickness
Expt. VCD
Cl P ti 2Cl P ti 2
Change of resistance with
dimensions of a wire
19.3 Ohms law and resistance (SB p. 44)
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83
Class Practice 2Class Practice 2
A uniform metal wire has a length of 2 m and aresistance of 10 .If the wire is cut into two halves, what would be the
resistance of 1-m wire?
ByRl, the resistance = ______________________
If the two 1-m wires are twisted to form a thicker wire,
what would be its resistance?The resistance of the wire would be in the range of
_________ (0-5 / 5-10 / 10-20). Ans
wer0-5
52
10
=
Resistor and rheostat
Resistor and rheostat19.3 Ohms law and resistance (SB p. 44)
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84
Resistor and rheostat
resistors rheostat
Rheostat
Resistor and rheostat19.3 Ohms law and resistance (SB p. 46)
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85
Rheostat
AB
C
By varying the position
of the movable contact
of the rheostat, the
length of the uniform
resistance wire in whichthe current flows is
changed
Resistor and rheostat19.3 Ohms law and resistance (SB p. 47)
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86
Class Practice 3Class Practice 3 A student tries to find theresistance of a semiconductor. The data is given below.
(a)(a) Plot a graph of voltage
against current.
(b)(b) Does the semiconductor
obey Ohms law? Explain
briefly.
Voltage / V 0.1 0.2 0.3 0.4 0.5 0.6
Current / mA 0.01 0.02 0.03 0.04 0.08 0.12
No. Because the voltage
is not proportional to
the current
Ans
wer
voltage
current
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Section 19.4Section 19.4
Simple CircuitsSimple Circuits
Section 19.4Section 19.4
Simple CircuitsSimple Circuits
Combination of resistors
R i t i i
Combination of resistors19.4 Simple circuits (SB p. 48)
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88
Resistors in series
Combination of resistors19.4 Simple circuits (SB p. 48)
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89
VVVIII
=+==
21
21
( )IIIRRR
RIRIIR
VVV
==+=
+=
+=
2121
2211
21
As
Resistors in series
Combination of resistors19.4 Simple circuits (SB p. 48)
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90
Resistors in series
R= R1 + R2 + R3 + R4 + ...
Resistors in parallel
Combination of resistors19.4 Simple circuits (SB p. 50)
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91
Resistors in parallel
Combination of resistors19.4 Simple circuits (SB p. 51)
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92
21 VVV ==
21
21
21
21
21
or
111
As
RR
RRR
RRR
R
V
R
V
R
V
III
+
=
+=
+=
+=
Resistors in parallel
Combination of resistors19.4 Simple circuits (SB p. 51)
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93
Resistors in parallel
.........
11111
4321 ++++= RRRRR
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Section 19.5Section 19.5
Electrical Power and EnergyElectrical Power and Energy
Section 19.5Section 19.5
Electrical Power and EnergyElectrical Power and Energy
Electrical power
Electrical energy
Electric bill
Electrical power
Electrical power19.5 Electrical power and energy (SB p. 53)
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95
( )WWatt,:Unit
takenTime
dtransferreenergyElectricalpowerElectrical
t
EP=
=
Electrical power
Electrical power
Electrical power19.5 Electrical power and energy (SB p. 53)
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Electrical power
220 V 220 V
each second
consumes 100 J
each second
consumes 60 J
Electrical power
Electrical power19.5 Electrical power and energy (SB p. 53)
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Electrical power
( )
IVP
V
t
Q
QVEt
QV
tEP
=
=
==
=
Electrical power
Electrical power19.5 Electrical power and energy (SB p. 54)
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Electrical power
( ) ( )
RIP
IRVIRI
IVP
2=
===
Electrical power
Electrical power19.5 Electrical power and energy (SB p. 54)
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Electrical power
R
VP
R
VIR
R
V
RIP
2
2
2
=
=
=
=
Electrical power
Electrical power19.5 Electrical power and energy (SB p. 54)
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Electrical power
VIP=
RIP2
=
RVP
2
=
or
or
Electrical energy
Electrical energy19.5 Electrical power and energy (SB p. 54)
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Electrical energy
( )
( )
==
==
===
R
VPt
R
V
RIPRtI
VIPVIt
PtE
22
22
Electrical energy
Electrical energy19.5 Electrical power and energy (SB p. 54)
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Electrical energy
J103.6kWh1
s6003sJ0001
h1W0001
kWh1kWh1
6
1
=
=
=
=
Experiment 19DExperiment 19D Electrical energy
Electrical energy19.5 Electrical power and energy (SB p. 55)
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Experiment 19DExperiment 19D Electrical energy
kilowatt-hour meter
hair dryer
rotating
disc
to mains
Expt. VCD
Calculate electrical power
Electrical energy19.5 Electrical power and energy (SB p. 55)
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Calculate electrical power
t
EE
t
EP
12
(PowerElectrical
==)
Initial kWh meter reading : E1
Final kWh meter reading : E2
Electrical energy consumed by
the appliance : E2 E1
Time : t
Class Practice 4Class Practice 4 C l h f ll i bl
Electric bill19.5 Electrical power and energy (SB p. 56)
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AppliancePower
/ WPower/ kW
Time/ h
Energyconsumed
/ kWh
Kettle 1500 3Light bulb 60 6
Vacuumcleaner
0.8 5
Iron 3 3
Complete the following table:
Ans
wer
1.5 4.50.06 100
800
1000 1
4
Class Practice 5Class Practice 5 From the electric bill shown, findthe cost of electricity per kWh.
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y p
Cost for two months =__________________
Energy consumed = ___________________________ kWh
Cost per kWh =________________________Ans
wer
$206.88
25974 25734 = 240
862.0$240
88.206=
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Section 19.6Section 19.6
Domestic Wiring andDomestic Wiring and
Electrical safetyElectrical safety
Section 19.6Section 19.6
Domestic Wiring andDomestic Wiring and
Electrical safetyElectrical safety
Electricity supply
Wiring of electric appliance
Domestic wiring Electrical safety
DC and AC
Electricity supply19.6 Domestic wiring and electrical safety (SB p. 57)
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DC and ACvoltage / V
time / s
direct current (d.c.)direct current (d.c.)
voltage / V
1 cycle
time / s
alternating current (a.c.)alternating current (a.c.)
Live wire (L) and neutral wire (N)
Electricity supply19.6 Domestic wiring and electrical safety (SB p. 58)
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Live wire (L) and neutral wire (N)
zero
voltage
LN
positive and
negativevoltages appear
alternately
Alternate change of positive and
Electricity supply19.6 Domestic wiring and electrical safety (SB p. 58)
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Alternate change of positive and
negative voltages in a live wire
L: positive voltage
N: zero voltage
L: negative voltage
N: zero voltage
0.01 s
0.01 s
Wiring of electric applianceWiring of electric appliance
19.6 Domestic wiring and electrical safety (SB p. 59)
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Wiring of electric appliance
plug socket
E
N LN
L
E
plastic coated wire
brown wire is
connected to
the live (L) pin
cartridge
fuse
blue wire is
connected to the
neutral (N) pin
yellow-green
wire is
connected to the
earth (E) pin
SwitchWiring of electric appliance
19.6 Domestic wiring and electrical safety (SB p. 60)
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S c
FuseWiring of electric appliance
19.6 Domestic wiring and electrical safety (SB p. 60)
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Installing earth wireWiring of electric appliance
19.6 Domestic wiring and electrical safety (SB p. 61)
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Installing earth wire
fault occurs
electric current
flows to the ground
No earth wire Wiring of electric appliance
19.6 Domestic wiring and electrical safety (SB p. 61)
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No earth wirefault occurs
electric current flows to the
ground via the human body
in parallel
Domestic wiringDomestic wiring19.6 Domestic wiring and electrical safety (SB p. 62)
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lightning circuitconsumer unit (fuse box)
kWh
meter
cableN L
main
fuse at
electric
company
to water
heater
to air-
conditioner
earth
high power appliances ring mains
Class Practice 6Class Practice 6 On a Christmas tree, the bulbs are
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connected in series. Each bulb has rated value of 5 V, 5 W.
(a)(a) Calculate the resistance of one bulb.
(b)(b) If each bulb operates at rated value from themains supply (220 V), how many bulbs can be
connected?
(c)(c) What is the total power output at that time?
Answer
=== 55
522
P
VR
44
5
220
bulboneofVoltage
voltageMainsbulbsofNumber ===
W220544outputpowerTotal ==
Do not overload a socket
Electrical safety19.6 Domestic wiring and electrical safety (SB p. 67)
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Replace the worn leads and do not join wires
Electrical safety19.6 Domestic wiring and electrical safety (SB p. 67)
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p j
Pull out the plug before filling an electric kettle
Electrical safety19.6 Domestic wiring and electrical safety (SB p. 67)
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p g g
Do not run extension leads into the bathroom
Electrical safety19.6 Domestic wiring and electrical safety (SB p. 68)
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Do not poke anything into sockets or appliances
Electrical safety19.6 Domestic wiring and electrical safety (SB p. 68)
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Chapter 20Chapter 20
Magnetic Effect of a CurrentMagnetic Effect of a Current
Chapter 20Chapter 20
Magnetic Effect of a CurrentMagnetic Effect of a Current
20.120.1 Magnetic EffectMagnetic Effect
20.220.2 ElectromagnetElectromagnet
20.320.3 Force on Current-CarryingForce on Current-Carrying
Conductor in Magnetic FieldConductor in Magnetic Field
20.420.4 Moving-Coil GalvanometerMoving-Coil Galvanometer
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Section 20.1Section 20.1Section 20.1Section 20.1
Magnetic EffectMagnetic Effect
Permanent magnetPermanent magnet Magnetic fieldMagnetic field Current-carrying conductorsCurrent-carrying conductors
Permanent magnetPermanent magnet20.1 Magnetic effect (SB p. 87)
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SN
North
pointing to
Arctic
South
pointing to
Antarctic
Everymagnet
hasnorthpole
andsouth
pole
Like poles
Permanent magnet20.1 Magnetic effect (SB p. 87)
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S N N SS N N SS N N S
repelrepel
Unlike poles
Permanent magnet20.1 Magnetic effect (SB p. 87)
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S N NSS N NSS N NS
attracattractt
The earth is like a magnetPermanent magnet20.1 Magnetic effect (SB p. 88)
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Arctic
Antarctic
The earth is like a large magnet
Arctic
Antarctic
N-poles of magnets point to Arctic
Magnetic effectPermanent magnet20.1 Magnetic effect (SB p. 89)
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iron rod
S N
Iron objectsare
magnetized
Which objects can be attracted by magnet?
Permanent magnet20.1 Magnetic effect (SB p. 89)
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N
iron
gold
silver
copper
aluminium
Class Practice 1Class Practice 1
Permanent magnet20.1 Magnetic effect (SB p. 89)
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There are three metalsX
,Y
andZ
.X
is attracted bya magnet no matter which pole of the magnet is
facing it. Ymay be attracted or repelled depending
on the pole of the magnet. The magnet cannot
attractZat all. From these results, decide which ofthem is a piece of iron, a piece of aluminium and a
magnet. Ans
werXis a piece of iron.
Yis magnet.Zis a piece of
aluminium.
Magnetic fieldMagnetic field20.1 Magnetic effect (SB p. 89)
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Amagnetwill
produceamagneticfieldinthe
space
aroundit,justlikea
chargewilles
tablish
anelectricfield
Asmagneticfieldscanexertforcesoneachother,twomagnetscanattractorrepeloveradistance
Experiment 20AExperiment 20A
Magnetic field20.1 Magnetic effect (SB p. 90)
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pMagnetic field of magnet Intro. VCD Expt. VCD
Magnetic field patternsMagnetic field20.1 Magnetic effect (SB p. 91)
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bar magnet
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Magnetic field patternsMagnetic field20.1 Magnetic effect (SB p. 91)
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Two bar magnets with like
poles facing each other
Magnetic field patternsMagnetic field20.1 Magnetic effect (SB p. 91)
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A slab-shaped magnet withunlike poles facing each other
Class Practice 2Class Practice 2
Magnetic field20.1 Magnetic effect (SB p. 91)
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For a bar magnet, the magnetic field is the
strongest at its two _____________. The
magnetic field lines are directed form one pole to
_____________ pole. Ans
wer
poles
another
Plotting a magnetic field lineMagnetic field20.1 Magnetic effect (SB p. 92)
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N Scompass
Amagneticfieldlineshowsthe
directionofthemagneticfield
Magnetic field linesMagnetic field20.1 Magnetic effect (SB p. 92)
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Direction:
from north to south
Strength:
higher density of
magnetic field lines,
greater magnetic
field strength
Experiment 20BExperiment 20B
Current-carrying conductors20.1 Magnetic effect (SB p. 92)
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Magnetic effect of current
Expt. VCD
Straight current-carrying wireCurrent-carrying conductors20.1 Magnetic effect (SB p. 94)
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currentpoints
upwards
Straight current-carrying wireCurrent-carrying conductors20.1 Magnetic effect (SB p. 94)
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currentpoints
downwards
Right-hand grip rule for current-carrying
t i ht i
Current-carrying conductors20.1 Magnetic effect (SB p. 94)
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right hand
field lines
straight wire
current
Current flows upwardsCurrent-carrying conductors20.1 Magnetic effect (SB p. 94)
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right hand
field lines
current
Current-carrying conductors20.1 Magnetic effect (SB p. 94)
Current flows downwards
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current
Current-carrying flat coilCurrent-carrying conductors20.1 Magnetic effect (SB p. 95)
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Right-hand grip rule for current-carrying
fl il
Current-carrying conductors20.1 Magnetic effect (SB p. 96)
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flat coil
Magnetic field
points upwards
Magnetic field points
into the paper
Current-carrying solenoidCurrent-carrying conductors20.1 Magnetic effect (SB p. 96)
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Right-hand rule for current-carrying
l id
Current-carrying conductors20.1 Magnetic effect (SB p. 97)
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solenoid
S i 20 2S ti 20 2S ti 20 2S ti 20 2
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Section 20.2Section 20.2Section 20.2Section 20.2
ElectromagnetElectromagnet
Applications of electromagnetsApplications of electromagnets
Electromagnet consists of soft-iron core
d l id
20.2 Electromagnet (SB p. 98)
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and solenoid
soft iron
U-cores
low voltage d.c.power supply
Experiment 20CExperiment 20C Electromagnet Expt. VCD20.2 Electromagnet (SB p. 98)
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soft iron
cores
low voltage
d.c. power supply
Showing direction of magnetic field20.2 Electromagnet (SB p. 99)
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When the power supply is turned on,the needle of the compass shows
the direction of the magnetic field
20.2 Electromagnet (SB p. 99)
Strength of magnetic field
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number of
turns of
wire
Number of turns of wire Magnetic field strength
Direction of magnetic field20.2 Electromagnet (SB p. 99)
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S N
Direction of magnetic fielddepends on direction of current
Applications of electromagnetsApplications of electromagnets20.2 Electromagnet (SB p. 99)
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Electricb
ell Telephonereceiver
Ticker-tapetim
er
Cranein
scrapya
rd
Experiment 20DExperiment 20D Model electric bellExpt. VCD
Applications of electromagnets20.2 Electromagnet (SB p. 100)
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Working principle of a
model electric bell
A, B are in contact
th i it i l d
Applications of electromagnets20.2 Electromagnet (SB p. 100)
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model electric bell
to low voltage
d.c. power supplywire
tape
screwbladesupporting block
B A
the circuit is closed
the electromagnet
attracts the blade
the blade bends
downwards
A, B are not in contact
the electromagnet loses its magnetism
the blade rebounds upwards
Telephone receiverApplications of electromagnets20.2 Electromagnet (SB p. 101)
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Manhattan Press (H.K.) Ltd. 2001160
mouthpiece
earpiece
plate
electromagnet
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Ticker-tape timer
Applications of electromagnets20.2 Electromagnet (SB p. 102)
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Manhattan Press (H.K.) Ltd. 2001162
iron strippaper
tape
dipperdiode
electromagnetcoil
spring
Current
Time
Crane in scrapyardApplications of electromagnets20.2 Electromagnet (SB p. 102)
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Section 20 3Section 20 3Section 20 3Section 20 3
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Section 20.3Section 20.3Section 20.3Section 20.3
Force on Current-CarryingForce on Current-Carrying
Conductor in Magnetic FieldConductor in Magnetic Field
Flemings left hand ruleFlemings left hand rule Moving-coil loudspeakerMoving-coil loudspeaker Electric motorsElectric motors
Experiment 20EExperiment 20E
Magnetic force on conductor
Expt. VCD20.3 Force on current-carrying conductor in magnetic field (SB p. 103)
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Magnetic force on conductor
12 V d.c. power supply
Flemings apparatus
When a current flows through the rider,
the rider moves
20.3 Force on current-carrying conductor in magnetic field (SB p. 103)
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Manhattan Press (H.K.) Ltd. 2001166
the rider moves
12 V d.c. power supply
Flemings apparatus
rider
the rider moves
Flemings left-hand rule
f
Flemings left hand rule20.3 Force on current-carrying conductor in magnetic field (SB p. 104)
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field
force
current
forceForce acting current-
carrying conductor in B-field
Flemings left hand rule20.3 Force on current-carrying conductor in magnetic field (SB p. 104)
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Manhattan Press (H.K.) Ltd. 2001168
current
field
current
carrying conductor in B-field
Turning effect of a coilFlemings left hand rule
20.3 Force on current-carrying conductor in magnetic field (SB p. 104)
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field
current
force
Class Practice 3Class Practice 3 A copper rod is placed on an opencircuit. When the switch is closed and the resistance of the
rheostat is increased gradually, state and explain the motion
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Manhattan Press (H.K.) Ltd. 2001170
of the rod when it is placed at positionsAB and CD.
AtAB:
At CD:
Ans
wer
magnetic
field regionThe rod moves to the right
according to Flemings left hand
rule. It moves at a decreasing
acceleration because the current
is decreasing.
The rod does not move
because there is no
magnetic field around it.
Moving-coil loudspeakerMoving-coil loudspeaker
20.3 Force on current-carrying conductor in magnetic field (SB p. 106)
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Structure of moving-coil loudspeakerMoving-coil loudspeaker
20.3 Force on current-carrying conductor in magnetic field (SB p. 106)
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permanent magnetpaper cone
voice coil
back panel of
receiver
speaker
terminals
Working principle of moving-coil
loudspeaker
Moving-coil loudspeaker20.3 Force on current-carrying conductor in magnetic field (SB p. 107)
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loudspeaker
magnetic field
paper
cone
voice
coil
force
force
Electric motorsElectric motors
20.3 Force on current-carrying conductor in magnetic field (SB p. 107)
d ill
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d.c. electric fans
a.c. washing
machine
d.c. drill
a.c. drill
Experiment 20FExperiment 20F
Model electric motor
20.3 Force on current-carrying conductor in magnetic field (SB p. 108)Electric motors
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Model electric motor
Expt. VCD
Working principle of electric motor20.3 Force on current-carrying conductor in magnetic field (SB p. 109)
Electric motors
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rotation
carbon brush
commutator
Rotation of a coil - at the beginning20.3 Force on current-carrying conductor in magnetic field (SB p. 109)
Electric motors
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Rotation of a coil - rotate 9020.3 Force on current-carrying conductor in magnetic field (SB p. 109)
Electric motors
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Rotation of a coil - rotate 18020.3 Force on current-carrying conductor in magnetic field (SB p. 109)
Electric motors
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Rotation of a coil - rotate 27020.3 Force on current-carrying conductor in magnetic field (SB p. 109)
Electric motors
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Ways to increase the turning speed
of the coil
20.3 Force on current-carrying conductor in magnetic field (SB p. 110)Electric motors
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Increase the current
Use a stronger magnet Increase the number of turns of the coil
Use a coil with larger surface area
Section 20 4Section 20 4Section 20 4Section 20 4
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Section 20.4Section 20.4Section 20.4Section 20.4
Moving-Coil GalvanometerMoving-Coil Galvanometer
Conversion of galvanometer toConversion of galvanometer toammeter and voltmeterammeter and voltmeter
Moving-coil galvanometerElectric motors20.4 Moving-coil galvanometer (SB p. 111)
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A galvanometer Circuit symbol
Experiment 20GExperiment 20G
Model moving-coil galvanometer
20.4 Moving-coil galvanometer (SB p. 111)
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g g
Expt. VCD
Structure of a moving-coil galvanometer20.4 Moving-coil galvanometer (SB p. 111)
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2
1
iron yoke
split pin
straw pointerabout 10 turns of insulated wire
wire wound in a loose
spiral to form a spring
thin rod
magnets
Conversion of milliammeter to galvanometer20.4 Moving-coil galvanometer (SB p. 112)
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to terminals
pointer
fixed soft-iron
cylinder
permanent
magnet
insulating support
lower hairspring
cylinder
support
pointer counter
balance
scale
upper hairspring
zero adjuster
coil
Ways to increase the sensitivity of
moving-coil galvanometer:
20.4 Moving-coil galvanometer (SB p. 113)
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g g
Use a stronger magnet
Use weaker hairsprings Increase the number of turns of the coil
Increase the surface area of the coil
Full-scale deflection (f.s.d.)Conversion of galvanometer to
ammeter and voltmeter
20.4 Moving-coil galvanometer (SB p. 114)
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to terminals
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Structure of ammeter
ammeter
Conversion of galvanometer to
ammeter and voltmeter
20.4 Moving-coil galvanometer (SB p. 114)
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Rs
IIg
r
I Ig
ammeter
shunt
Resistance of shunt (Rs)
V lt th l t
Conversion of galvanometer to
ammeter and voltmeter
20.4 Moving-coil galvanometer (SB p. 114)
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Voltage across the galvanometer
= Voltage across the shunt
( )gIII
rgIsR
gII
rgI
sR
sR)gI(IrgI
>>=
=
=
Conversion to voltmeterConversion of galvanometer to
ammeter and voltmeter
20.4 Moving-coil galvanometer (SB p. 115)
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multipliers
Structure of a voltmeterConversion of galvanometer to
ammeter and voltmeter
20.4 Moving-coil galvanometer (SB p. 115)
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Rmr
Ig
Ig
voltmeter
V
multiplier
Resistance of a multiplier
Voltage across the voltmeter
Conversion of galvanometer to
ammeter and voltmeter
20.4 Moving-coil galvanometer (SB p. 115)
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Voltage across the voltmeter
= Voltage across the galvanometer +
Voltage across the multiplier
rgI
VmR
)mR(rgImRgIrgIV
=
+=+=
Class Practice 4Class Practice 4 A milliammeter of resistance100 and full-scale deflection current 10 mA isconverted to an ammeter by using a resistor of
resistance 11.
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(a)(a) Draw a circuit to show how the milliammeter canbe converted to the ammeter.
(b)(b) Find the maximum current that can be measured by
the ammeter.
Answer
A101.0
11.0111
)11)(1010()100)(1010(
)(33
=
=
=
=
I
I
I
RIIrI sgg
Class Practice 5Class Practice 5 A milliammeter of resistance1 000 and full-scale deflection current 10-3 A isconverted to a voltmeter of full-scale deflection voltage
10 V.
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(a)(a) Draw a circuit to show the connection.
(b)(b) Find the resistance of the multiplier and the
voltmeter.
Ans
wer
==
+=
+=
=
+=
+=+=
k1000010
00090001
multiplierofResistanceermilliammetofResistancevoltmetertheofResistance
0009
)0001)(10(10
)(
m
m
3
mgmgg
R
R
RrIRIrIV
Chapter 21Chapter 21Chapter 21Chapter 21
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Electromagnetic InductionElectromagnetic Induction Electromagnetic InductionElectromagnetic Induction 21.1 Induced EMF and Induced Current21.1 Induced EMF and Induced Current
21.2 Generators21.2 Generators
21.3 Transformer21.3 Transformer
21.4 Transmission of Electrical Energy21.4 Transmission of Electrical Energy
Section 21.1Section 21.1Section 21.1Section 21.1
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Induced EMF and InducedInduced EMF and Induced
CurrentCurrent
Lenzs lawLenzs law Induced e.m.f. and induced current in aInduced e.m.f. and induced current in a
conducting wireconducting wire
Applications of induced e.m.f. in coilsApplications of induced e.m.f. in coils
Experiment 21AExperiment 21A
Electromagnetic induction
Intro. VCD Expt. VCD21.1 Induced EMF and induced current (SB p. 133)
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light-beam
galvanometer
magnet coil
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Ways to increase the induced e.m.f.:
21.1 Induced EMF and induced current (SB p. 133)
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Manhattan Press (H.K.) Ltd. 2001201
Move the magnet faster
Use a stronger magnet
Increase the number of turns in the coil
Faradays law of electromagnetic induction
21.1 Induced EMF and induced current (SB p. 133)
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Manhattan Press (H.K.) Ltd. 2001202
conductor
+ change of
magnetic field
e.m.f.
strength of e.m.f.
rate of change of magnetic field
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Electromagnetic induction - induced current
Lenzs law21.1 Induced EMF and induced current (SB p. 134)
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NS
induced current
Lenzs law
Lenzs law21.1 Induced EMF and induced current (SB p. 134)
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Lenzs law states that the induced
current always flows in a direction such
that it opposes the change producing it.
repulsive force
repulsive force
To prove Lenzs law
Lenzs law21.1 Induced EMF and induced current (SB p. 135)
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N
repulsive forcep
attractive force
attractive force
To prove Lenzs law
Lenzs law21.1 Induced EMF and induced current (SB p. 135)
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S
Flemings right hand rule
motion
Induced e.m.f. and induced
current in a conducting wire
21.1 Induced EMF and induced current (SB p. 137)
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field
current
Flemings right hand ruleInduced e.m.f. and induced
current in a conducting wire
21.1 Induced EMF and induced current (SB p. 137)
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Manhattan Press (H.K.) Ltd. 2001209
motion
current
field
Flemings right hand ruleInduced e.m.f. and induced
current in a conducting wire
21.1 Induced EMF and induced current (SB p. 137)
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motion
field
current
Ways to increase the induced
e.m.f.
Induced e.m.f. and induced
current in a conducting wire
21.1 Induced EMF and induced current (SB p. 138)
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Move
thewire
faster
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Ways to increase the induced
e.m.f.
Induced e.m.f. and induced
current in a conducting wire
21.1 Induced EMF and induced current (SB p. 138)
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Manhattan Press (H.K.) Ltd. 2001213
Useastron
ger
magnet
Class Practice 1Class Practice 1 In which direction will theinduced current flow (if any) when the conductor or
magnet is moved in the ways shown below?
21.1 Induced EMF and induced current (SB p. 139)
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Manhattan Press (H.K.) Ltd. 2001214
Answer
no current
Class Practice 2Class Practice 2 When a copper rod is moving along a metal frame, an
induced current flows as shown.
(a)(a) In which direction is the copper
21.1 Induced EMF and induced current (SB p. 140)
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( )( ) pp
rod moving?
(b)(b) Due to the induced current, the
rod is experiencing a force inside the
magnetic field. In which direction
does this force act on the rod?
Ans
wer
induced
current
metal frame
By using Flemings right
hand rule, the copper rod
is moving to the left.
By using Flemings left hand rule or
Lenzs law, the force on the rod acts to
the right. It opposes the motion of the rod.
Applications of induced e.m.f. in coils
Moving-coil
Applications of induced e.m.f. in coils21.1 Induced EMF and induced current (SB p. 140)
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induced e.m.f.induced e.m.f.
microphone
toamplifier
magnet
diaphragm
moving coil
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Applications of induced e.m.f. in coils
Magnetic tape recording and
Applications of induced e.m.f. in coils21.1 Induced EMF and induced current (SB p. 141)
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playback
current coil
magnet
tape
motionmagnetic tape
Section 21.2Section 21.2Section 21.2Section 21.2
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GeneratorsGenerators
A coil moving in a magnetic fieldA coil moving in a magnetic field
AC generator (alternator)AC generator (alternator)
DC generator (d.c. dynamo)DC generator (d.c. dynamo)
Bicycle alternatorBicycle alternator
Alternators in power stations and carsAlternators in power stations and cars
A coil moving in a magnetic field
A coil moving in a magnetic field21.2 Generators (SB p. 142)
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When the coil starts to turn
When the plane of
A coil moving in a magnetic field21.2 Generators (SB p. 142)
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Manhattan Press (H.K.) Ltd. 2001221
the current flows from
the right to the left
the coil ishorizontal, the rate of
cutting the
magnetic fieldlines is the
highest
the inducedcurrent is the
maximum
The coil is turned 90When the plane of
the coil is vertical,
A coil moving in a magnetic field21.2 Generators (SB p. 142)
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no current
the coil is vertical,
no field linesare cut
no current is
inducedAfter passing the
vertical position,
the induced
current recurs,
but the direction
is reversed
When the plane of
A coil moving in a magnetic field21.2 Generators (SB p. 143)
The coil is turned 180
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the direction of
induced current isreversed
the coil ishorizontal,
the rate of
cutting themagnetic field
lines is the
highest
the inducedcurrent is the
maximum
The coil is turned 270When the plane of
the coil is vertical,
A coil moving in a magnetic field21.2 Generators (SB p. 143)
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no current
no field linesare cut
no current is
induced
After passing the
vertical position,the inducedcurrent recurs,
but the direction
is reversed
A coil moving in a magnetic field21.2 Generators (SB p. 143)
The coil is turned 360
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The coil isturned to the
starting position
The coil isturned
continuously
An alternating
current isproduced
AC generator (alternator)
AC generator (alternator)21.2 Generators (SB p. 144)
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external
circuit
carbon
brush
slip rings
rotation a current
is induced
Induced e.m.f. and the number of revolutions
AC generator (alternator)21.2 Generators (SB p. 144)
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Ways to increase the e.m.f.
AC generator (alternator)21.2 Generators (SB p. 144)
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Rotate the coil at a higher speed
Increase the number of turns of the
coil
Wind the coil on a soft-iron core
(armature)
Use a stronger magnet
Experiment 21BExperiment 21B
Model dynamo
DC generator (d.c. dynamo)21.2 Generators (SB p. 145)
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Expt. VCD
DC generator (d.c. dynamo)
DC generator (d.c. dynamo)21.2 Generators (SB p. 146)
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commutator
carbon
brush
rotation
a current
is induced
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Bicycle alternator
cylindricaldriving
wheel
Bicycle alternator21.2 Generators (SB p. 147)
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output terminals
magnet (rotor)
coil
soft iron
axle
Experiment 21CExperiment 21C
Model bicycle alternator
Bicycle alternator21.2 Generators (SB p. 148)
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Expt. VCD
Display of output of a bicycle alternator
Bicycle alternator21.2 Generators (SB p. 149)
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Class Practice 3Class Practice 3
The graph of output voltage against time for an a.c.
generator is shown below:
21.2 Generators (SB p. 150)
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Manhattan Press (H.K.) Ltd. 2001235
(a)(a) Find the peak voltage and
the frequency of the output
voltage.
Peak voltage = ___________
Period = ________________
Frequency = _____________Ans
wer
0.02 s
5 V
Hz5002.0
1=
Class Practice 3 (Contd):Class Practice 3 (
Contd):
(b)(b) Draw the new graph when the number of
turns of the coil is trebled.
21.2 Generators (SB p. 150)
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Answer
Alternator in power station
Alternators in power stations and cars21.2 Generators (SB p. 150)
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Section 21.3Section 21.3Section 21.3Section 21.3
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TransformerTransformer
Efficiency of a transformerEfficiency of a transformer
Transformer
21.3 Transformer (SB p. 151)
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Mutual induction of a transformer21.3 Transformer (SB p. 151)
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at the instant when
the switch is closed
Mutual induction of a transformer
at the instant when
the switch is closed
21.3 Transformer (SB p. 151)
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at the instant when
the switch is opened
Experiment 21DExperiment 21D
Simple transformer
Expt. VCD
21.3 Transformer (SB p. 152)
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Transformer
21.3 Transformer (SB p. 153)
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p
s
p
s
N
N
V
V
=
Vp NpVs
Ns
A step-up transformer
soft-iron core
21.3 Transformer (SB p. 154)
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input
voltage
output
voltage
secondary
coilprimary
coil
circuit symbol
(Ns > Np)
A step-down transformer
soft-iron core
21.3 Transformer (SB p. 154)
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input
voltageoutput
voltagesecondary
coil
primary
coil
circuit symbol
(Ns < Np)
2 5001 000 :
Find the number of turns in the
secondary coil
21.3 Transformer (SB p. 154)
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220 V
p
s
p
s
N
N
V
V=
10002500
220s =V
550s =V
Experiment 21EExperiment 21E
Winding a transformer
21.3 Transformer (SB p. 155)
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Expt. VCD
Efficiency of a transformer
Efficiency of a transformer21.3 Transformer (SB p. 155)
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100%
powerelectricalInput
powerelectricalOutputrtransformeaofEfficiency =
100%=pp
ss
IV
IVe
Ideal transformer
Efficiency of a transformer21.3 Transformer (SB p. 156)
IVIV
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ppss IVIV =
s
p
p
s
I
I
V
V=
p
s
s
p
p
s
N
N
I
I
V
V==
Ideal transformer VS Practicaltransformer
Efficiency of a transformer21.3 Transformer (SB p. 156)
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transformer
input
power
output
power
input
poweroutput
power
power
loss
transformer
ideal practical
Reason for energy loss (1)
iron core
Efficiency of a transformer21.3 Transformer (SB p. 156)
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input
voltage
output
voltagesecondary
coil
primary
coil
the coils have
resistance
current in the coil produce
heating effect
electrical energy
converts to heatenergy is lost
Waystomin
imizeenergyloss:
Waystomin
imizeenergyloss:
useconduc
tingwiresof
smallerresis
tance
useathickerwire
iron core
eddy
currentReason for energy loss (2)
Efficiency of a transformer21.3 Transformer (SB p. 156)
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input
voltage
output
voltagesecondary
coil
primary
coil
current in the primary coil
produce magnetic field
induce eddy
current in the
iron coreheating effect of
the iron coreenergy is lost
Waystomin
imizeenergy
loss:
Waystom
inimizeenerg
yloss:
useanironcoremad
eformastac
kof
thinmetalsli
cesthatareinsulatedf
rom
oneano
ther,toincrea
setheresist
ance
andreducethe
eddycurren
t
Reason for energy loss (3)
Alternating current through the transformer
Efficiency of a transformer21.3 Transformer (SB p. 157)
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continuous magnetization and
demagnetization of iron core
the iron core is heated up
energy is lost
Waystominim
izeenergylos
s:
Waystominim
izeenergylos
s:
useasoftironcore,
whichcan
bemagnetize
danddemagnetized
easily,soen
ergylossisre
duced
Class Practice 4Class Practice 4 A transformer is used to stepdown the 220 V mains supply to run an appliance of
110 V, 550 W.
( )( ) If th i il h 1 000 t h t i th
21.3 Transformer (SB p. 158)
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(a)(a) If the primary coil has 1 000 turns, what is thenumber of turns in the secondary coil?
(b)(b) What is the current drawn by the appliance?
Ans
wer
turns500
220
100
0001
By
s
s
p
s
p
s
=
=
=
N
N
V
V
N
N
A5110
550===
V
PI
(c)(c) It is found that the current drawn from the
i i 2 8 A Wh t i th ffi i f th
Class Practice 4 (Contd):Class Practice 4 (
Contd):
21.3 Transformer (SB p. 158)
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mains is 2.8 A. What is the efficiency of thetransformer?
Ans
wer
%89
%100616
550
Efficiency
W616
8.2220powerInput
W550powerOutput
==
===
=
VI
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Experiment 21FExperiment 21F
Transmission of electrical power
Expt. VCD
21.4 Transmission of electrical energy (SB p. 159)
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transmission line
step-downtransformer
step -uptransformer
Transmission of electricity
21.4 Transmission of electrical energy (SB p. 160)
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Power loss of cables
RIP
2=
21.4 Transmission of electrical energy (SB p. 160)
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RIP
cable
12 V12 V
I
Power loss of cablesIsIp
21.4 Transmission of electrical energy (SB p. 160)
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Np : Ns
Vp Vs