Post on 29-Dec-2015
ConductorsMetals and graphite are electrical conductors.Electrons are free to move in conductors. Electrons are negatively charged. The electrons carry the energy from the power supply to the appliance.A movement of electrons is called an electric current.
Total charge = current x time Q = I x t
Coulombs , C Amps, A seconds, s( Basic electrostatic expts )
Calculation
• Calculate the current flowing if 500 C of charge flow around a circuit in 5 minutes.
• I = ? Q = 500 C, t = 5 mins = 300 s• Q = I x t
At
QI 67.1
300
500
Current
• Current is a measure of the number of charges that flow per second around a circuit.
• Units are Amps, A• Instrument : ammeter, connected is series.
Potential Difference (Voltage)
• Voltage is a measure of the energy given to each charge in the circuit . The bigger the voltage the more energy is transferred.
• Units : Volts, V• Instrument : Voltmeter • connected in • parallel
• Expts on series / parallel rules for • current/ potential difference• and useful circuits.
Series Circuits
• Components are connected in ‘line’
• Current is the same AT ALL points
Series Circuits
Sum of potential differences = Supply potential difference
Vs = V1 + V2 + V3
Remember that potential difference is a measure of energy. This equation tells us that the total energy available from the supply equals the energy used to drive current through resistor 1 plus the energy used to drive current through resistor 2 plus the energy used to drive current through resistor 3
Parallel Circuits
• Components are arranged ‘above each other’
• Total current drawn from supply equals sum of currents flowing through each component
• It = I1 + I2 + I3
Parallel Circuits
• Potential difference across each branch equals supply potential difference
• Investigation of ring circuit
Household wiring
• Appliances connected in parallel• 230 V across each• Independent switching• Power circuit( plugs ) is wired as a RING
circuit
Ring Circuit
• Current splits two ways• Thinner cable can be
used• Less heating effect in
each branch• Easy to add extra
sockets
Neutral
Live
Direct Current , D.C.
• Electrons flow in one direction only• A battery produces direct current as it is
always pushing electrons in the same direction.
Voltage
Time
Alternating Current, A.C.• Alternating current the current flows back / forwards around
the circuit.• The frequency of the supply is the number of cycles per
second. Mains frequency is 50 Hz• I cycle = 1/50 s• Time for 1 cycle ( period) = 1 / frequency ( T = 1 /f )
Time
Voltage
Alternating Current, 2
• The effective value of British mains is 230 V. This is called the quoted value and is less than the peak value.
Peak value, measured on an oscilloscopeQuoted value , measured using a meter.
To Measure Peak Voltage 1. Count the number of boxes from the trough to crest
2. Divide by 2 ( this is from the 0V line to the crest )
3. Multiply the number of boxes by the Y gain setting
Ex. 4 boxes from trough to crest
Therefore from 0V line to crest
= 2 boxes
2 x 5 = 10 V
Y gain = 5V per box
0V Line
Resistance
• This is a measure of the opposition to flow of current• Units are Ohms,Ω• Symbol for resistor• If resistance increases then current decreases
( provided supply pd is constant )
Ohm’s Law
• For a constant temperature
pd ( V ) current ( A ) resistance (Ω)
Calculate the resistance of a 12 V car bulb that draws a current of 5 A. V = 12 V I = 5 A R = ? V = I x R
RxIV
AI
VR 4.2
5
12
Ohm’s law 2
The gradient of the graph is equal to which equals = resistance
Heating conductors
A the temperature increases the gradient and hence resistance increases. As temperature increases the particles vibrate more making it more difficult for the power supply to push charges around the circuit.
Series ResistorsThe total resistance is the sum of the individual resistances.
Rt = R1 + R2 + R3
Parallel ResistorsThe effective resistance is always smaller than the smallest resistor in the network.
Ex : Calculate the effective resistance of the circuit shown.
R1 = 10 Ω R2 = 15 Ω Rp = ?
65
3030
5
30
2
30
3
15
1
10
1111
21
p
p
R
RRR
21
111
RRRp
Short circuits / open circuits
Potential DividersThese divide the voltage up . The resistors are in series therefore the same current flows through each.
The bigger the resistance the bigger the share of the voltage.
2
1
2
1
R
R
V
V
Potential divider circuitsExample:
VS = V1 + V2
VS = 18 + 6VS = 24 V
V1
V2
R1
R2
=
V1
612
4=
V1x 612
4=
V1 = 18 V
4 W
12 W
6 V
V1
VS
4 W
12 W
6 V
V1
VSVS
4 W
12 W V1
4 W
12 W
6 V
V1
VS
4 W
12 W V1
VS
Potential Dividers
Calculate the voltage across the
20 Ω resistor.
V2 = ? R2 = 20 Ω , R1 = 30 Ω, Vs = 10 V
V
RR
RVV s
450
2010
3020
2010
12
22
12
22 RR
RVV S
Electronics
Systems:
All electronic systems can be simplified to the
following block diagram,
input process output
e.g. for a radio receiver,
aerial tuner speaker
decoder
amplifier
Output Devices
examples are,- speaker (electrical energy to sound energy)- buzzer (electrical energy to sound energy)- lamp (electrical energy to light energy)- motor (electrical energy to kinetic energy)- relay (causes other circuits to be switched)- solenoid (causes a straight movement)- seven segment display
Output devices are energy changers.
Light emitting diode , LED
This device changes electrical energy into light energy. It allows current only one way through it.
Flow of electrons
LED 2Unlike a lamp it produces very little heat energy
as it does not contain a wire filament.
It lights when the current is small (e.g. 10 mA).
To prevent it being damaged by too large a
current, it always has a resistor connected in
series.
LED 3example:
An l.e.d. is designed to operate at 2.0 V and 10 mA.
What size of resistor is needed when it is powered by a 9.0 volt battery?
9.0 V
2.0 V
Pd across resistor = 9 -2 = 7V
R = V/I = 7 / 0.01 = 700 Ω
10 mA = 0.01 A
Input Devices
• Two main kinds : energy changers such as a microphone or thermocouple
• Or the component changes the size of the input voltage as a physical property changes e.g. LDR or thermistor
Microphone
energy change is,
sound energy to electrical energy
Thermocouple
energy change is,
heat energy to electrical energy
Used to measure high temperatures
+-
Solar Cell
energy change is,
light energy to electrical energy
Thermistor
The resistance of a thermistor usually decreases as the temperature increases .
Light Dependant Resistor
The resistance of an LDR decreases as the light level increases.
LDR in potential divider circuit
V4.7 kW
6.0 V
Explain what happens as it gets darker:
1. Resistance of LDR goes up
2. Voltage across LDR goes up
3. Hence voltage across fixed resistor goes down
Thermistor in potential divider circuit
V
2.4 kW
9.0 V Explain what happens as it gets colder;
1. Resistance of thermistor goes up
2. Voltage across thermistor goes up
3. Voltage across fixed resistor goes down
Transistors
• Two types• NPN and MOSFET• Both act as voltage operated switches
MOSFETMetal Oxide Semiconductor Field Effect Transistor
The MOSFET has a different construction from the
NPN transistor and switches on at a higher voltage
(about 2 volts).
Gate
Drain
Source
NPN transistor
This switches on when the voltage across the base emitter is 0.7 V
Collector
Emitter
Base
Example of transistor switching
5.0 V
As the temperature increases, the resistance of the thermistor decreases.
As a result, the voltage across the thermistor decreases.
This causes the voltage across the variable resistor to increase which switches on the MOSFET, causing the l.e.d. to light.
Electrical energy 1When a current flows through a component there is an energy change.
In a resistor Ee Eh
In a loudspeaker Ee Es
Electrical energy 2Power is the rate at which energy is transferred
I W is equivalent to 1 Js-1
Power can also be calculated from :
Power = voltage x current
P = V x I
t
EP Joules, J
Seconds, sWatts,W
Electrical energy 3There are 2 other equations that can be used to calculate power :
1: Combine P = V x I and V= I x R
Substitute for V = I x R into P = V x I
P = I x R x I = I2 x R
2 : Substitute for into P = V x I
R
V
R
VxVP
R
VI
2
R
VI
R
V
R
VxVP
2
Electrical energy 4
• Four equations to calculate power :When energy ( joules ) and time ( seconds ) is
known
When current ( amps ) and voltage ( volts ) are known
When current ( amps ) and resistance (ohms ) are known
When Voltage ( volts ) and resistance (ohms ) are known
t
EP
IxVP
RxIP 2
R
VP
2
Electrical energy 5
Calculate the current flowing through a 230 V mains, 2 kW kettle.
P = 2 KW = 2000 W V = 230 V I = ?
P = V x I
AV
PI 7.8
230
2000
Electrical energy 6Calculate the voltage across a 4 Ω loudspeaker when it produces 20 W of sound power.
P = 20 W R = 4 Ω V = ?
VV
xRxPV
RxPV
R
VP
9.880
420
2
2
Electrical energy 7
Calculate the current flowing through a 2000 W, 20Ω resistor.
P = 2000 W, R = 20 Ω, I = ?
P = I2 x R
R
PI 2
20
20002 I
1002 I
AI 10100
ElectromagnetismThe term ‘electromagnetism’ comes from the fact that there is a magnetic field around a wire when there is an electric current in the wire.
The magnetic field is stronger when,- the current is higher- the wire is longer
The direction of the magnetic field reverses if the current flow reverses direction.
Electromagnetism
When a wire is moved through a magnetic field avoltage is generated (induced) in the wire.This can be when the magnet is stationary and the wire is moved, or when the wire is stationary and the magnet is moved. The induced voltage is greater when,
- the magnetic field is stronger- the movement is faster- the wire is wound into a coil