8/12/2019 A2 Unit 4 Topic 2 Electric and Magnetic Fields
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A2 Unit 4 Topic 2 Electric and Magnetic Fields
p41
Q1 What is the force on an electron in an
electric field of 300V.m-1?
19
17
17
300 1.602 10
4.806 10
4.8 10
F Eq
N
=
=
=
:
Q2 What is the strength of an electric field that
will !t a force of 1.28 " 10-1#$ on a roton?
1# 19
1#
19
3 1
1.28 10 1.602 10
1.28 10
1.602 10
7990
8.0 10 .
F Eq
E
E
V m
=
=
=
=:
Q3 %ow m!ch acceleration will an alha
article !ndergo whilst in an electric field of
10&V.m-1?
4 19
1#
1# 27
1#
27
11
11 2
10 2 1.602 10
3.204 10
3.204 10 4 1.67 10
3.204 10
4 1.67 10
4.7964 10
# 10 .
F Eq
N
F maa
a
m s
=
=
=
= =
=
=
:
Q4 'n the electron (eam of a cathode ra)
oscilloscoe* electrons are accelerated thro!gh a
otential difference of 3000V which is set !
(etween electrodes which are 3cm aart.
a+ ,alc!late the electric field strength (etween
these electrodes* ass!ming it is a !niform
field.
3
3
6 1
3000 3 10
3000
3 10
10 .
V Ed
E
E
V m
=
=
=
=
(+ %ow fast will the electrons (e moing when
the) emerge from this field?
31 2 19
19
31
7
7 1
19.109 10 3000 1.602 10
2
2 3000 1.602 10
9.109 10
3.2484 10
3 10 .
W Vq
v
v
m s
=
=
=
=
:c+ raw a ict!re to ill!strate the field rod!ced
() these lates.
d+ escri(e and e"lain how the field/s effects
wo!ld (e different if a roton were laced in
it.
The electron is accelerated to the right. A proton
would accelerate to the left, in the direction of the
electric field.
p43
Q1 "lain what wo!ld haen to an electron
which fo!nd itself at the e"act centre of the
electric field shown in fig 2.1.6(.
Assuming there are no other electric fields, the
electron would feel no force from either proton.
The net field is zero in the eact centre.
Q2 raw a air of electrons 8cm aart. n
)o!r diagram* add field lines to show the shae of
the electric field rod!ced () these electrons.raw in seeral e!iotential lines.
This diagram shows the actual equipotential lines
from this question. The !oundaries of the colours
give the equipotential lines.
8/12/2019 A2 Unit 4 Topic 2 Electric and Magnetic Fields
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The a!ove diagram shows the equipotential lines
and the field lines for two electrons.
Q3 air of flat* s!are metal lates is held
arallel to each other 10cm aart. ne is
connected to earth* and the other is connected to a
2000V "#$%s!l).
a+ What wo!ld the electrical otential (e at a
oint (etween the centres of the lates* which
are held 3cm from the earthed one?
4 1
4
2000
0.1
2 10 .
2 100.03
600
VE
d
V m
V
V V
=
=
=
==
(+ What difference in effect wo!ld there (e on
electrons within the field* one 1cm from the
earthed late* and the other in the e"act
midoint of the field?
A!solutel& nothing.
Q4 Wh) is the electric field in fig 2.1.6d
strongest near the oint of the steele?
$harge concentrates at points so the
steeple point would have the strongest electric
field around it.
Q5 'f fig 2.1.6a shows a h)drogen atom* then
the searation of the charges wo!ld (e #.3"10-11m.
'f the otential difference from the roton to the
electron5s osition is 27.2V* what is the strength
of the electric field along the line (etween the two
articles? ss!me the field is !niform* altho!gh
this is a simlification.+
11
11 1
27.2
#.3 10
#.1 10 .
VE
d
V m
=
=
=
p46
Q1 What is the force of attraction (etween a
!rani!m n!cle!s atomic n!m(er 92+ and an
electron at a distance of 0.1 nm?
( )
1 2
2
9 19 19
210
6
6
8.99 10 92 1.602 10 1.602 10
10
2.1226 10
2 10
'q qF
r
N
=
=
=
:
Q2
What is the strength of the electric field ca!sed ()
a gold n!cle!s atomic n!m(er 79+ at a distance of
1 " 10-12m from the centre of the n!cle!s?
( )
2
9 19
212
17
17
8.99 10 79 1.602 10
10
1.1378 10
1 10
'qE
r
N
=
=
=
:
What is the force of re!lsion (etween an alha
article and a gold n!cle!s when the alha article
asses () the n!cle!s at a distance of 1 m?
( )
1 2
2
9 19 19
212
2
2
8.99 10 79 1.602 10 2 1.602 10
10
3.64#4 10
4 10
'q qF
r
N
=
=
=
:
Q3 escri(e and e"lain a otential ro(lem
with the e"erimental set-! in fig 2.1.8 when
comaring res!lts with ,o!lom(/s law that
1 2
2
'( (F
r=
%int What wo!ld the metre r!ler (e meas!ring?+
8/12/2019 A2 Unit 4 Topic 2 Electric and Magnetic Fields
3/13
The charge is spread across the surface of the
spheres and the electric field is not linear !ut
radial. The distance value would !e ver& difficult
to find as it would not directl& relate to the gap
!etween the spheres.
The change in weight would !e ver& small
compared to the weight of the stand on the scale
pan. This could !e an issue in terms of accurac&.
Q4 stimate the distance that )o! float a(oe
)o!r (ed at night as a res!lt of the ,o!lom(
re!lsion (etween the electrons in the (edsheets
and those in )o!r )amas.
Assume the !ed sheets and panamas are pure
car!on. Assume the sheet is )*+g and &our
p&amas are also )*+g in mass.
1
23 1
26 19
7
2#0
12 .
2.0833
2.0833 6.022 10 . 1
1.#0#6 10 1.602 10
2.4118 10
gNo moles
g mol
mol
No electrons mol atoms mol
electrons $
$
=
=
=
=
= Assume the forces follow $oulom!-s law
Assume !od& mass is /+'g
( )
( )
1 2
2
29 7
2
29 7
10
11
8.99 10 2.4118 1070 9.81
8.99 10 2.4118 10
70 9.81
8.726 10
10
'q qF
r
r
r
m
=
=
=
=
:
This, of course, is utter ru!!ish0
p48
Q1 :ig!re 2.1.13 shows a ositiel) charged
oil dro held at rest (etween two arallelcond!cting latesAand1.
a+ ;he oil dro has a mass 9.79 " 10-1#&g. ;he
otential difference (etween the lates is
#000V and late1is at a otential of 0V. 's
lateAositie or negatie?
A is negative so the electric field is verticall&
upwards.
(+ raw a la(elled free-(od) force diagram
which shows the forces acting on the oil dro.
c+ ,alc!late the electric field strength (etween
the lates.
2
2
# 1
#000 2.# 10
#000
2.# 10
2 10 .
V Ed
E
E
V m
=
= =
=
d+ ,alc!late the magnit!de of the charge < on
the oil dro.
1# #
1#
#
19
9.79 10 9.81 2 10
9.79 10 9.81
2 10
4.802 10
3 electron e!ialent charges
F Eq
q
q
$
=
=
= = =
e+ %ow man) electrons wo!ld hae to (e
remoed from a ne!tral oil dro for it to
ac!ire this charge?
2 electrons would have to !e removed for this.
p53
Q1 What is the caacitance of a caacitor
which stores 2 co!lom(s of charge for eer) 100
olts alied to it?
8/12/2019 A2 Unit 4 Topic 2 Electric and Magnetic Fields
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2
2
100
2 10
20
q$
V
F
mF
=
=
= =
Q2 0.01 : caacitor is charged () and thenisolated from an 8V ower s!l).
a+ ,alc!late the charge stored.
2
0.018
8 10
q$
V
q
q $
=
=
=
(+ ;he caacitor is then connected across another
identical caacitor which is !ncharged.
escri(e and e"lain what will haen to the
charge and oltage on each caacitor.
The E3F from the charged capacitor will force a
current to flow until the E3F is !alanced across
the two capacitors. 1ecause the capacitances are
equal, the charge will also !e equal and the E3F
half of the original 4 5V.
Q3 %ow m!ch energ) is stored on a #0:
caacitor which is charged to 12V?
12
12
10
#0 1012
#0 10 12
6 10
q$
V
q
q
$
=
=
=
=
Q4 1200 : caacitor is connected to a
oltage s!l) !ntil f!ll) charged with 10.8m,. 'f
this caacitor is then disconnected andreconnected across a 100W light (!l(* for how
long co!ld it light the (!l(?
This is a ridiculous question !ecause the light
!ul! is a non6ohmic element. The cold resistance
is far lower than the operating resistance. The
!ul! will drain the capacitor at a much faster rate
than the time indicated.
7lus, the capacitor will discharge in an
eponential fashion with the pea' current at the
start of the discharge and then taper off over time.The power output of the !ul! will drop
accordingl&.
( )
2
26
12
2
2
2
4
10.8 10
1200 10
9.72 10
9.72 10100
9.72 10
100
9.72 10
E Vq
q$
V
qV
$
qE
$
8
E7
t
t
t
s
=
=
=
=
=
=
=
=
=
=
p59
Q1 What is the time constant for a car
co!rtes) light in which a 2 m: caacitor
discharges thro!gh a 1# &= resistor?
3 3
1# 10 2 1030
9$
s
=
= =
Q2 %ow m!ch charge remains on a 0.04:
caacitor after 8 seconds* if it is discharging
thro!gh a #00= resistance and initiall) held 2, of
charge?
0
8
#00 0.04
1
2
0.4038
4 10
t
9$q q e
e
$
=
=
=
:
Q3
8/12/2019 A2 Unit 4 Topic 2 Electric and Magnetic Fields
5/13
0
30
0.00669 0.03
0.00669 30ln
0.03
30
0.00669ln0.03
20.4127
> 20.41
t
9$
9$
: : e
e
9$
9$
s
=
=
=
=
=:
0.03 200
6
V :9
V
V
==
=
6200 100 10
0.02
9$
s
=
=
=
For the information given in the diagram, the
capacitance is too low !& a factor of ;+++. The
value of $ should !e ;++mF, not ;++>?A
(+ 'f this were a model of an a!tomatic hand
dr)er circ!it which re!ires 4.0V to oerate*
!se the grah to wor& o!t for how long it will
remain on.
4 200
0.02
V :9
:: A
=
=
=
t @ +.+)A Bs
c+ %ow wo!ld )o! alter the circ!it so that the
dr)er remains on for 30s?
4
4
0
30
10
30
10
4
4
#
4 6
4
6
4 30
ln 6 10
30
4ln 10
6
7.399 10
700
t
9$
9
9
V V e
e
e
9
9
'
=
=
=
=
=
= :
0
30
200
30
200
1
4 6
4
6
4 30ln
6 200
30
4ln 200
6
3.699 10
400
t
9$
$
$
V V e
e
e
$
$
mF
=
=
=
=
=
= :
Either increase the resistance to /++'C or the
capacitance to 5++mF. This will allow the voltageto !e at 5V after 2+s.
Again, this is a ridiculous question as a hand
dr&er uses )++W plus to operate, far more than is
availa!le in this scenario.
Q4 raw an acc!rate s&etch grah for the
c!rrent thro!gh a discharging #0@: caacitor
which is f!ll) charged () a 6V s!l) and then
discharged thro!gh a 10&= resistance* oer a
eriod of 2 seconds.
Q5 defi(rillator is a machine that is !sed to
correct irreg!lar heart(eats () assing a large
c!rrent thro!gh the heart for a short time. ;he
machine !ses a 6000V s!l) to charge a
caacitor of caacitance 20:. ;he caacitor is
then discharged thro!gh metal electrodes
defi(rillator addles+ laced on the chest of the
atient.
a+ ,alc!late the charge on the caacitor lates
when charged to 6000V.
8/12/2019 A2 Unit 4 Topic 2 Electric and Magnetic Fields
6/13
12
7
20 106000
1.2 10
q$
V
q
q $
=
=
=
(+ ,alc!late the energ) stored in the caacitor.
7
4
6000 1.2 10
7.2 10
E Vq
8
==
=
c+ When the caacitor is discharged* there is an
initial c!rrent of 40 thro!gh the atient.
,alc!late the electrical resistance of the (od)
tiss!e (etween the metal electrodes of the
addles.
6000 40
1#0
V :9
9
9
=
= =
d+ ss!ming a constant discharge rate of 40*
calc!late how long it wo!ld ta&e to discharge
the caacitor.
7
7
9
1.2 1040
1.2 10
40
3 10
3
q:
t
t
t
s
ns
=
=
=
= =
e+ 'n ractice the time for discharge is longer
than this calc!lated time. A!ggest a reason for
this.
$apacitors discharge through ohmic resistors
with an eponentiall& decreasing current. Also,there is no reason to thin' that a human !od& is
ohmic in its !ehaviour, i.e. V = :9.
p63
Q1 escri(e the !se of :leming/s left hand
r!le.
The thum!, first and second fingers are held at
mutuall& perpendicular directions. The thum!
represents the force eperienced !& a chargemoving in a magnetic field at a direction
perpendicular to the plane of motion and
magnetic field. The first or inde finger represents
the direction of the magnetic field. The middle
finger represents the direction of motion of a
positivel& charged particle or the conventional
current in a wire.
Q2 ,o) the diagrams in fig. 2.3.8 and draw
an arrow to show the direction of an) force acting
on the wire in each case.
Q3 Ban) real motors !se electromagnets to
create the magnetic field that ca!ses their rotor to
sin. Cie one adantage and one disadantage of
(!ilding a motor which !ses electromagnets rather
than ermanent magnets.
Advantages The strength of the magnetic field
can !e controlled to increase or decrease the
torque of the motor.
The motor can !e started in small stages rather
than with full power. A full power start uses ver&
high currents and creates ver& high torques at low
speeds.
Disadvantage The stator coils generate heat and
consume energ&.
7ermanent magnets tend to !e smaller in size than
compara!le electromagnets, so the motor size in
increased.
P67
8/12/2019 A2 Unit 4 Topic 2 Electric and Magnetic Fields
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# 19 2
21
21
# 10 1.602 10 # 10
4.00# 10
4 10
F 1qv
N
=
=
=
:
c+ %ow fast wo!ld a roton need to trael in
order for the electromagnetic force on it to (e
s!fficient to ma&e it or(it the arth at thes!rface? Dadi!s of arth E 6.4 " 106m.+
,omment on )o!r answer.
27
6
19 #
19 # 6
27
10
10 1
1.672 106.4 10
1.602 10 # 10
1.602 10 # 10 6.4 10
1.672 10
3.066 10
3 10 .
mvr
q1
v
v
m s
=
=
=
= :
This is 100 times the speed of light, which is
impossible.
Q2 Aeed E distanceFtime and c!rrent E
chargeFtime. "lain how : E G'l is act!all) the
same e!ation as : E G (!t considered for
man) charges in a gro!.
F 1:l
q:
t
qF 1 l
t
l1q
t
1qv
=
=
=
=
=
Q3 :or the inestigation on ta(le salt in the
wor&ed e"amle a(oe* calc!late the difference in
the radii of c!rat!re that wo!ld (e fo!nd if the
coman) inestigated the two isotoes of sodi!m23$aH and 22$aH. "lain wh) s!ch small
differences can (e easil) detected () a machine
li&e that shown in fig. 2.3.12.
2
22
12
23
12
22
27 12
19
6
23
27 12
19
1
2
2 2.
7.3737 10
7.2116 10
22 1.672 10 7.3737 10
1.602 10
1.69 10
23 1.672 10 7.2116 10
1.602 10
1
DE Vq mv
Vqv Vq
m m
For Na
v Vq
For Na
v Vq
mvr
q1
For Na
Vqr
1
r1
Vq
For Na
Vqr
1
r1
Vq
= =
= =
=
=
=
=
=
=
= 6
6
6
.73 10
1.73 101.024
1.69 109atio
= =
Therefore, there is a ).5 difference in theradius, assuming the ionisation, accelerating
voltage and magnetic field are the same for !oth.
This is easil& detected.
p71
8/12/2019 A2 Unit 4 Topic 2 Electric and Magnetic Fields
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Q1 What is the fl!" lin&age if a s!are coil
with 10 cm sides and haing #00 t!rns interacts
with a magnetic field* G E 0.33 m;?
( )2
4 1
3 2
3.3 10 10 #00
1.6# 10 .
1AN
T m
=
=
=
Q2 ;he grah shown in fig. 2.3.19 is a trace of
ind!ced emf against time as a magnet is droed
thro!gh a coil of wire.
a+ escri(e and e"lain the trace o(tained.
The positive voltage shows an increase of
magnetic flu as the magnet enters. The pea'
voltage is when the rate of change of flu is
greatest. The drop in voltage shows that the rate
of flu increase drops off as the magnetic field
aligns with the motion of the magnet in the plane
of the coil.
The negative voltage shows a decrease of
magnetic flu through the coil as the other pole of
the magnet passes through the coil.
ee diagrams !elow.
(+ 'f the coil were connected in series to a light
(!l(* descri(e and e"lain how the c!rrent
thro!gh the (!l( wo!ld ar) oer time.
The current in the !ul! would reflect the emf
created in the coil in terms of direction and
magnitude.
Q3 ;he coil from !estion 1 is t!rned thro!gh
90I within the 0.33m; magnetic field* moing
from a osition erendic!lar to the field* to a
osition arallel to the field. ;his action ta&es
12ms. What is the ind!ced emf in the coil? %ow
wo!ld JenK/s law ca!se the ind!ced c!rrent to
affect the moement?
3
3
1
1
1.6# 10
12 10
1.37# 10
1.4 10
t
1AN
t
V
=
=
=
=
:
Q4 transformer for a to) train set conerts
the mains s!l) from 230V down to 6V tooerate the model train. ;he transformer that
ma&es this conersion has 160 t!rns on its
secondar) coil. %ow man) rimar) t!rns does the
transformer hae?
6 160
230
2301606
6133
o s
i p
p
p
V n
V n
n
n
turns
=
=
=
=
p73
Q1 What was the diameter of Gr!sh/s wind
t!r(ine in metres?
*>ft in diameter.
Q2 'f the (r!sh s)stem were oerating at thehighest allowed oltage 90V+ and rod!cing
12&W* what c!rrent wo!ld (e deliered to each of
the 12 arallel (atteries of cells?
312 10 90
133.3
F 133.3 F12
11.1
11
7 V:
:
:
: cell
A
=
=
=
=
=
&
&
&
:
Q3 'f a st!dent were to (!ild a model wind
t!r(ine to light an J* list the main arts of the
model that wo!ld (e needed and what their
8/12/2019 A2 Unit 4 Topic 2 Electric and Magnetic Fields
9/13
f!nction is L a diagram ma) hel )o!r
e"lanation.
A wind tur!ine requires a device to garner energ&
from the wind. A !lade or tur!ine "as shown
a!ove% would do. This connects to a generator
which converts 'inetic energ& to electrical energ&.
The GE# then converts electrical energ& to heat
and visi!le light.
Q4 "lain the enironmental imortance of
deeloing increased !se of wind ower
electricit) generation s)stems. 'n )o!r answer*
disc!ss the ro(lems which will slow the ace of!ta&e of wind ower as an alternatie to non-
renewa(le energ) so!rces.
1est evidence and models indicate that the Earth
is warming as a result of 3an-s use of fossil fuels
that has increased the $H)concentration in the
atmosphere.
Ising renewa!le sources of energ&, such as wind
power, reduces the use of fossil fuels to generate
power and hence reduces the amount of $H)
!eing poured into the atmosphere.#isincentives include the relativel& high cost of
the infrastructure and the power produced, as
well as the intermittent nature of the wind,
resulting in unrelia!le power generation.
Q5 ,arr) o!t some research on the ro(lem of
aria(le wind ower s!l). Write a aragrah to
e"lain how this ro(lem is (eing oercome.
p74Q1 ne ractical arrangement for erif)ing
,o!lom(/s law is to !se a lightweight* negatiel)-
charged* freel)- s!sended (all. 't is reelled ()
the negatie charge on a larger shere that is held
near it* on an ins!lated s!ort. ;he small angle of
deflection Jis then meas!red.
;he weight of the
(all is W. Ahow that the force of re!lsion : on
the s!sended (all is gien ()
F = W.Tan"J%2+
st!dent ta&es seeral sets of readings () moing
the larger shere towards the s!sended (all in
order to increase the m!t!al force of re!lsion
(etween them. %e meas!res the angle of
deflection Jand the searation distance rin each
case. %e then calc!lates the magnit!de of the
force :.
%ere are some of his res!lts.
:orce :F10-3$ 142 #68
istance rF10-3m 36.0 27.0 18.0 9.0
,alc!late the al!es that )o! wo!ld e"ect the
st!dent to hae o(tained for the missing forces*
ass!ming that ,o!lom(/s law was o(e)ed.
Write )o!r answers in a co) of the ta(le. 4+
( )
( )
1 2
2
3
3
9
1 2
23
23 3
1 2 9
1# 2
142 10
8.99 10
18 10
142 10 18 10
8.99 10
#.1177 10
'q qF
rF N
q q
q q
$
=
=
=
=
=
8/12/2019 A2 Unit 4 Topic 2 Electric and Magnetic Fields
10/13
8/12/2019 A2 Unit 4 Topic 2 Electric and Magnetic Fields
11/13
3
# 1
# 19
14
14
3000
2# 10
1.2 10 .
1.2 10 1.602 10
1.922 10
2 10
VE
d
V m
F Eq
N
=
=
= =
= =
:
,o) and comlete the grah to show how the
force on the electron aries as the distance of the
electron aries from the (ottom late to the to
late.
2+
;his force ca!ses the electron to accelerate.
;he electron is initiall) at rest in contact with the
(ottom late when the otential difference is
alied. ,alc!late its seed as it reaches the !er
late. 3+
19
19
212
19
31
7
7 1
3000 1.602 10
4.806 10
2 4.806 10
9.109 10
3.248 10
3.2 10 .
W Vq
8
mv
v
m s
==
=
=
=
=
:
;otal 11 mar&s+
Q4 efine caacitance. 2+
$apacitance is the charge stored in a capacitor
divided !& the voltage across the terminal of a
capacitor.q
$V
=
n !ncharged caacitor of 200@: is connected in
series with a 470&= resistor* a 1.#0V cell and a
switch. raw a circ!it diagram of this
arrangement. 1+
,alc!late the ma"im!m c!rrent that flows. 2+
0
0
3
6
1.#
470 10
3.191# 10
3.2
t
9$: : e
V:
9
A
=
=
=
= :
A&etch a grah of oltage against charge for )o!r
caacitor as it charges. 'ndicate on the grah the
energ) stored when the caacitor is f!ll) charged.
4+
12
212
6 212
4
4
200 10 1.#
2.2# 10
2 10
E Vq
$V
8
=
=
=
=
:
8/12/2019 A2 Unit 4 Topic 2 Electric and Magnetic Fields
12/13
,alc!late the energ) stored in the f!ll)-charged
caacitor. 2+ ;otal 11 mar&s+
Q5 ;he diagram shows a grah of charge
against oltage for a caacitor.
What !antit) is reresented () the sloe of the
grah? 2+
lope is the capacitance.What !antit) is reresented () the shaded area?
2+
haded area is the energ& in the capacitor.
n electronic camera flash g!n contains a
caacitor of 100@: which is charged to a oltage
of 2#0V. Ahow that the energ) stored is 3.1M. 2+
12
212
6 212 100 10 2#0
3.12#
3.1
E Vq
$V
8
=
=
= =:
;he caacitor is charged () an electronic circ!it
that is owered () a 1.#V cell. ;he c!rrent drawn
from the cell is 0.20. ,alc!late the ower from
the cell and from this the minim!m time for the
cell to recharge the caacitor. 3+ ;otal 9 mar&s+
1.# 0.2
0.3
3.12#0.3
10.416
10
7 V:
W
E7
t
t
t
s
=
= =
=
=
=:
Q6 ;he magnit!de of the force on a c!rrent-
carr)ing cond!ctor in a magnetic field is directl)roortional to the magnit!de of the c!rrent in the
cond!ctor. With the aid of a diagram descri(e how
)o! co!ld demonstrate this in a school la(orator).
4+
The power suppl& provides the current in the
&ellow wire. This wire is held in place !& clamps
"not shown%. The la!orator& scales measure the
weight of the I6shaped magnet. When the current
flows, the weight of the magnet will change
amount of force eerted !& the current on the
magnetic field. F = !l
The second diagram showelectric current going into the
page. The direction of the
magnetic field and the
current gives a net
downwards force on the
conductor. This gives a net
upwards force on the magnets. "Newton-s 2rdlaw%
t a certain oint on the arth/s s!rface the
horiKontal comonent of the arth/s magnetic field
is 1.8 " 10
-#
;. straight iece of cond!cting wire2.0 m long* of mass 1.#g lies on a horiKontal
wooden (ench in an east-west direction. When a
er) large c!rrent flows momentaril) in the wire it
is !st s!fficient to ca!se the wire to lift ! off the
s!rface of the (ench.
Atate the direction of the c!rrent in the wire. 1+
The current must flow towards the West.
"Fleming-s left hand rule%
,alc!late the c!rrent. 2+
3 #
3
#
1.# 10 9.81 1.8 10 2
1.# 10 9.81
1.8 10 2
411.48
410
F 1:l mg
:
:
A
= =
=
=
=:
What other noticea(le effect will this c!rrent
rod!ce? 1+ ;otal 8 mar&s+
A current of this size would cause su!stantialheating of the wire.
Assume the wire is made from copper.
8/12/2019 A2 Unit 4 Topic 2 Electric and Magnetic Fields
13/13
3
2
3
4
1.# 108890
2
1.# 10
2 8890
#.1792 10
mass
Vol
r
r
m
=
=
=
=
( )
8
24
2
2
2 2
3
1.72 10 2
#.1792 10
4.0821 10
411.48 4.0821 10
6.9117 10
6.9
G9
A
7 V: : 9
'W
=
=
=
= =
=
= :
This situation produces approimatel& /'W of
power dissipated !& the wire. $opper has a
specific heat capacit& of 2B/8.'g6;.$K6;and a
melting temperature of ;2*>K$. Assuming a
starting temperature of )+K$
( )3387 1.# 10 13#6 20
776
776
6912
0.11
( $m t
8
W7
t
t
s
= =
=
=
=
=
:t would ta'e a!out a tenth of a second to melt the
copper wire.
Q7 ;he diagram shows a rectang!lar coil
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