Download EM6 Experiment: Magnetic fields around electric currents

SKH Li Fook Hing Secondary School
Physics
EM6
Name:_________________________ Class No.:_____
Group No.:_______
Date of Experiment: ___________
EM6 Experiment: Magnetic fields around electric currents
Objective: To investigate the magnetic fields due to an a.c. current carrying straight wire by a
search coil and CRO
Apparatus: lateral search coil, CRO, signal generator, ammeter (0-1A and 0-5A a.c.), signal
generator, PVC-covered copper wire (26 s.w.g.), rheostat, slotted base (2),
crocodile clips (2) ruler and connecting leads.
(The search coil consists of 5000 turns of very fine insulated copper wire
mounted on a small babbin. When the coil is placed near a conductor
carrying an a.c. current, an induced e.m.f. is produced in the coil. The induced
e.m.f. is directly proportional to the field strength and the strength of the
magnetic field can be seen and measured on a CRO)
Procedure and results:
1. Set up the apparatus as shown in Fig.1
Ensure that the search coil is at the same
level as the straight wire.
PQ1.
2
Fig.1
Set the signal generator to 5kHz. Adjust
the CRO such that a trace is displayed.
Change the frequency and find out how the trace on the CRO is affected.
PQ2.
3.
Why is the search coil placed at
the same level as the wire?
How can you increase the sensitivity of the search coil?
Adjust the output of the signal generator to produce an a.c. current of 0.1A.
Switch off the time base and measure the length of the vertical trace on the CRO.
Repeat the above step for different value of current. Tabulate the results:
Length of trace
l/cm
Current
I/A
Plot a graph of current I against length of trace l.
p.1
SKH Li Fook Hing Secondary School
Physics
EM6
Name:_________________________ Class No.:_____
Group No.:_______
Date of Experiment: ___________
PQ2.
What is the shape of the graph?
PQ3.
What do you deduce from the shape of the graph?
(Hint: The length of the vertical trace is directly proportional to the magnetic field.)
4.
Place the centre of the search coil 1cm away from the wire. Adjust the CRO such that the
vertical trace occupies the full screen.
5.
Increase in steps the distance of the search coil from the wire. Measure the length of vertical
trace on the CRO. Tabulate the results and complete the following table:
Length of trace
l/cm
distance
r/cm
ln l
ln r
Plot a graph of ln l against ln r.
PQ4.
What is the shape of the graph?
PQ5.
What do you deduce from the shape of the graph?
PQ6.
From PQ3 and PQ5, What is the relationship between magnetic field, current and
distance from the wire?
Questions:
1. What is the exact formula of magnetic field for an infinitely long wire?
2. Show that the induced e.m.f in the search coil is directly proportional to the frequency of the
a.c. current.
3. Why is the measured magnetic field intensity is exactly equal to the magnetic field intensity at
the specific point?
Reference: Further Physics Book 2 p.128 - p.132, New Way Physics Book 3 p.220
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