EXPERIMENT :-1
Measurement of magnetic force using current balance
Souvik Sarkar & Uddhipan Thakur
Chennai Mathematical Institute
B.Sc Physics 1st year
30th August, 2009
1
Aim of experiment :-
In this experiment ,we will measure the magnetic force exerted on current carrying wire due to magnetic
field and will find a relation between force and current,length,magnetic field.
2
Introductory theory :-
If a charge particle q is moving with velocity v in a magnetic field B ,then force exerted on the particle is
given by
~
F~ = q(~v × B)
Using this equation, we can derive that if a wire of length l carrying current i is placed in a magnetic field
B,then total force on the wire is given by
~
F~ = i(~l × B)
where ~l has direction same as current through the wire and direction of force is determined by right hand
screw rule
Magnitude of force = i · l · Bsinθ
In our experiment θ is fixed at 90◦
So our equation is reduced to
~
|F~ | = i · |~l| · |B|
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Apparatus :1. Current Balance
2. Wires of different length
3. Magnets
4. Mass Balance
5. Power Supply with Ammeter
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4
4.1
Experiment :Expt 1
We first pick the plate with longest wire of known length in horizontal and fit it in front of the main unit.
Now the wire is placed in between the magnet assembly in such a way so that it doesn’t touch the magnet
in any way.We measure the weight of magnet assembly with mass balance. Now we complete the circuit
with power supply and set the current at 1 amp.Now weight of magnet assembly is changed and new weight
is recorded.This is repeated for several times with different values of current and all corresponding values
are recorded.And then we substract the new weights from original weight and put them in the column
named “Change”.
4.2
Expt 2
In the similar way as above we set the wire in between magnetic assembly and measure the weight.Now
aconstant current of 4 amp is allowed to pass through the wire.New weight and change are recorded in the
corresponding columns of table.This experiment is done several times using different numbers of magnets
to find relation between force and magnetic field.( All other parameters kept constant)
4.3
Expt 3
Following as above we do the experiment varying only length of wire ,all other keeping constant and record
them in tables.
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Results :-
1)Weight of 6 magnets when I=0 is 163.96 gm
2)Length of wire taken is 4.2 cm
Table 1 :: Current v.s Force trends
Current(amp) New weight(gm)
1
163.84
2
163.72
2.5
163.66
3
163.61
3.5
163.55
4
163.50
5
163.38
change(gm)
.12
.24
.30
.35
.41
.46
.58
3) Length of the wire taken is 8.4 cm
Table 2 :: Magnetic field v.s Force trends
No. of magnets
6
5
4
3
1
Weight(I=0) Weight(I=4 amp) Change(gm)
(gm)
(gm)
163.95
166.21
2.26
150.86
152.76
1.90
137.89
139.40
1.51
124.82
125.95
1.13
98.7
99.08
.38
2
4) Current though wire is 4 amp
5) Number of magnets is 6
Table 3 :: Length v.s Force trends
Length of wire(cm) New weight(gm)
1.2
164.09
2.2
164.21
3.2
164.28
4.2
164.42
6.4
164.58
8.4
164.87
6
Change(gm)
.13
.25
.32
.46
.62
.91
Analysis of results :-
After plotting the results on the graph paper we see that current,length of wire,magnetic field each one
maintains approximately linear relationship with force when the other parameters are kept unchanged.
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Sources of error :1. If the wire is not properly inserted in the magnetic assembly or it touches the magnets then possible
error may occur.
2. Systematic error may occur as least count of mass balance is .01 gm.
3. Due to high sensitivity of mass balance random error may occur.
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Precautions :1. Wire shouidn’t touch the magnets.
2. Similar poles of magnets should be kept in one side.
3. Readings should be taken without parallax.
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