Class:
Name:
(
Experiment 5d
) Date:
5d Measuring magnetic field using a search
coil
Objective
To measure the magnetic fields set up by a straight wire and
solenoids using a search coil.
Background information
1
A search coil is a device that makes use of electromagnetic induction
for measuring the strength of a varying magnetic field. It consists of a
circular coil with about 5000 turns and the induced e.m.f. in the coil
is measured by a cathode-ray oscilloscope (CRO).
2
When a search coil is placed inside a changing magnetic field
perpendicular to the coil, a varying e.m.f. will be induced across the
ends of the coil. For magnetic field produced by a.c. with constant
freqnency, the maximum induced e.m.f. is proportional to the
maximum field strength.
3
By connecting a search coil to a CRO and measuring the amplitude
of the induced e.m.f., the magnetic field strength can be obtained.
Apparatus
❏ 1 lateral search coil
❏ 1 small solenoid
❏ 1 axial search coil
❏ 1 signal generator
❏ 1 cathode-ray oscilloscope (CRO)
❏ 1 ammeter
❏ 1 long PVC-coated copper wire
❏ 1 rheostat
❏ 1 slinky solenoid
❏ 2 retort stands and clamps
❏ 2 rectangular solenoids (one with
❏ 2 slotted bases
a smaller number of turns per unit
❏ several connecting leads
length)
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Experiment 5d
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) Date:
Procedure
Straight wire
1
Set up a lateral search coil and connect it to a CRO (Fig 5d-1).
cathode-ray oscilloscope (CRO)
✐ A signal generator
of frequency of 5 kHz
gives an induced e.m.f.
of much larger value
than a low voltage power
supply.
lateral search coil
✐ A large current is
used in order to produce
a large e.m.f. However,
the rheostat commonly
found in school can
withstand a maximum
current of 5 A only.
Note
This ensures that the
search coil is perpendicular
to the magnetic field.
✐ The resultant trace
in CRO is not sharp due
to the noise signal from
surroundings. Switch
off projector or other
sources which may
produce noise signal.
Fig 5d-1
2
(a) Connect a long PVC-coated copper wire in series with a
rheostat, an ammeter and a signal generator (Fig 5d-2).
(b) Switch on the signal generator. Set the output voltage to 0–6 V
of frequency 5 kHz in sine wave.
(c) Place the search coil near one side of the wire. Adjust the
time base of the CRO until a waveform is shown on the CRO
screen clearly. Rotate the orientation of the search coil until the
amplitude of the induced e.m.f. shown on the CRO screen is
maximum.
signal generator
long PVC-coated
copper wire
✐ The following methods
can increase the
sensitivity of the search
coil:
rheostat
ammeter
1 Increase the number
of turns of the search
coil.
lateral
search coil
2 Increase the area of
the search coil.
3 Use alternating current
of higher frequency.
(By increasing the
frequency, the
magnetic field changes
with a faster rate.
Therefore, the induced
e.m.f. increases.)
102
cathode-ray oscilloscope (CRO)
Fig 5d-2
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3
Place the search coil at different distances from the wire and take the
amplitude of the induced e.m.f. shown on the CRO screen. Record
the results in Table 5d-1.
✎
Results:
Distance from wire r / cm
Experiment 5d
) Date:
2
3
4
5
6
7
1
1
( ) / cm–1
Distance r
0.50
0.33
0.25
0.20
0.17
0.14
Amplitude of induced e.m.f. / V
(∝ magnetic field strength B)
0.235
0.165
0.130
0.110
0.095
0.085
Table 5d-1
4
1
. Record the results in Table 5d-1.
r
(b) Plot a graph of the amplitude of induced e.m.f. (directly
1
proportional to the magnetic field strength B) against
in
r
Figure 5d-3.
(a) Calculate the value of
amplitude of induced e.m.f. / V
(u magnetic field strength B)
0.30
0.25
0.20
0.15
0.10
0.05
0
0.1
0.2
0.3
0.4
0.5
0.6
1
( 1 ) / cm–1
distance
r
Fig 5d-3
✎
How is the magnetic field strength B related to the distance r from
the wire?
The magnetic field strength is inversely proportional to the distance from the wire, i.e.
1
B∝ .
r
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Experiment 5d
Class:
✐ The reading of
a current smaller
than 0.5 A should
not be taken since
the amplitude of
noise signal may be
comparable to the
induced e.m.f.
Name:
(
5
(a) Place the search coil at a fixed distance from the wire.
(b) Vary the current flowing through the wire by changing the
resistance of the rheostat.
(c) Take the ammeter reading and the amplitude of the induced
e.m.f. shown on the CRO screen. Record the results in
Table 5d-2.
6
Repeat several times with other current values through the wire.
Record the results in Table 5d-2.
✎
Results:
Current through wire I / A
0.5
Amplitude of induced e.m.f. / V
(∝ magnetic field strength B)
) Date:
0.6
0.120 0.145
0.7
0.8
0.9
0.170
0.193
0.215
Table 5d-2
7
Plot a graph of the amplitude of induced e.m.f. (directly proportional
to the magnetic field strength B) against the current I through wire in
Figure 5d-4.
amplitude of induced e.m.f. / V
(u magnetic field strength B)
0.30
0.25
0.20
0.15
0.10
0.05
0
0.2
0.4
0.6
0.8
1.0
current through wire I / A
Fig 5d-4
✎
How is the magnetic field strength B related to the current I through
the wire?
The magnetic field strength is directly proportional to the current through the wire, i.e.
B ∝ I.
104
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Experiment 5d
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Slinky solenoid
8
(a) Set up an axial search coil and connect it to a CRO.
(b) Connect a slinky solenoid in series with a rheostat, an ammeter
and a signal generator (Fig 5d-5).
signal generator
ammeter
rheostat
cathode-ray oscilloscope (CRO)
slinky solenoid
axial search coil
Fig 5d-5
9
Place the search coil at different positions inside the slinky solenoid
and note any change in the amplitude of the induced e.m.f. shown
on the CRO screen.
✎
How does the magnetic field strength (directly proportional to the
amplitude of induced e.m.f.) change inside the slinky solenoid?
The magnetic field strength does not vary inside the slinky solenoid.
✐ However, the
magnetic field strength
decreases significantly
at the two ends of the
slinky solenoid. Ts may
tell Ss to check this out.
10 (a) Place the search coil at a fixed position inside the slinky
solenoid.
(b) Vary the current flowing through the slinky solenoid by
changing the resistance of the rheostat.
(c) Take the ammeter reading and the amplitude of the induced
e.m.f. shown on the CRO screen. Record the results in
Table 5d-3 on p.106.
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Experiment 5d
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11 Repeat several times with other current values through the slinky
solenoid. Record the results in Table 5d-3.
Results:
✎
Current through slinky solenoid I / A
Amplitude of induced e.m.f. / V
(∝ magnetic field strength B)
0.2
0.4
0.012 0.022
0.6
0.8
1.0
0.032
0.042
0.052
Table 5d-3
12 Plot a graph of the amplitude of induced e.m.f. (directly proportional
to the magnetic field strength B) against the current I through slinky
solenoid in Figure 5d-6.
amplitude of induced e.m.f. / V
(u magnetic field strength B)
0.06
0.05
0.04
0.03
0.02
0.01
0
0.2
0.4
0.6
0.8
1.0
current through slinky solenoid I / A
Fig 5d-6
✎
How is the magnetic field strength B related to the current I through
the slinky solenoid?
The magnetic field strength is directly proportional to the current through the slinky
solenoid, i.e. B ∝ I.
13 (a) Place the search coil at a fixed position inside the slinky
solenoid.
106
(b) Vary the length of the slinky solenoid.
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(c) Measure the length of the slinky solenoid and take the amplitude
of the induced e.m.f. shown on the CRO screen. Record the
results in Table 5d-4.
14 Repeat several times with other lengths of the slinky solenoid. Record
the results in Table 5d-4.
✎
Results:
Length of slinky solenoid l / m
1.0
0.9
0.8
0.7
0.6
1
1
( ) / m–1
Length l
1.00
1.11
1.25
1.43
1.67
Amplitude of induced e.m.f. / V
(∝ magnetic field strength B)
0.043
0.047
0.052
0.059
0.068
Table 5d-4
1
. Record the results in Table 5d-4.
l
(b) Plot a graph of the amplitude of induced e.m.f. (directly
1
proportional to the magnetic field strength B) against
in
l
Figure 5d-7.
15 (a) Calculate the value of
amplitude of induced e.m.f. / V
(u magnetic field strength B)
0.06
0.05
0.04
0.03
0.02
0.01
0
0.5
1.0
1.5
1
( 1 ) / m–1
length l
Fig 5d-7
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Experiment 5d
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✎
(
) Date:
How is the magnetic field strength B related to the length l of the
slinky solenoid?
The magnetic field strength is inversely proportional to the length of the slinky
1
solenoid, i.e. B ∝ .
l
Rectangular solenoid
16 Connect a rectangular solenoid in series with a rheostat, an ammeter
and a signal generator (Fig 5d-8).
signal generator
solenoid
rheostat
ammeter
axial search coil
cathode-ray oscilloscope (CRO)
Fig 5d-8
17 Place the search coil at different positions inside the solenoid and
note any change in the amplitude of the induced e.m.f. shown on the
CRO screen.
✎
The magnetic field strength does not vary inside the solenoid.
✐ However, the
magnetic field strength
decreases significantly
at the two ends of the
solenoid. Ts may tell Ss
to check this out.
108
How does the magnetic field strength (directly proportional to the
amplitude of induced e.m.f.) change inside the solenoid?
18 (a) Place the search coil at a fixed position inside the solenoid.
(b) Vary the current flowing through the solenoid by changing the
resistance of the rheostat.
(c) Take the ammeter reading and the amplitude of the induced
e.m.f. shown on the CRO screen. Record the results in
Table 5d-5 on p.109.
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Experiment 5d
19 Repeat several times with other current values through the solenoid.
Record the results in Table 5d-5.
✎
Results:
Current through solenoid I / A
Amplitude of induced e.m.f. / V
(∝ magnetic field strength B)
Table 5d-5
20 Plot a graph of the amplitude of induced e.m.f. (directly proportional
to the magnetic field strength B) against the current I through
solenoid in Figure 5d-9.
amplitude of induced e.m.f. / V
(u magnetic field strength B)
current through solenoid l / A
Fig 5d-9
✎
How is the magnetic field strength B related to the current I through
the solenoid?
The magnetic field strength is directly proportional to the current through the solenoid,
i.e. B ∝ I.
21 (a) Place the search coil at a fixed position inside the solenoid and
note the amplitude of the induced e.m.f. shown on the CRO
screen.
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Experiment 5d
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(b) Repeat by using a rectangular solenoid with a smaller number of
turns per unit length (Fig 5d-10) and of a smaller size (Fig 5d-11)
in turn.
solenoid of
smaller size
solenoid with smaller
number of turns per unit length
Fig 5d-10
✎
Fig 5d-11
How does the magnetic field strength change with the number of
turns per unit length and the size of the solenoid?
The magnetic field strength increases with the number of turns per unit length of the
solenoid. It does not change with the size of the solenoid.
Discussion
✎
Where can a uniform magnetic field be detected for the two
solenoids?
A uniform magnetic field can be detected inside the two solenoids.
1 The strength of a varying magnetic field can be measured
search coil
using a ____________________________
.
2 The strength of the magnetic field set up by a current-carrying
directly proportional
straight wire is ________________________
to the current flowing
inversely proportional
through the wire and __________________________
to the distance
from the wire.
3 The magnetic field set up by a current-carrying solenoid is
uniform
______________________
inside the solenoid. It is
directly proportional
_______________________________
to the current flowing through
increases
the solenoid and _______________________________ with the
number of turns per unit length of the solenoid. It is
independent
_______________________________
of the area of the solenoid.
110
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