Experiment 9 Electromagnet

Experiment 9

Topic: Electromagnetic Induction

Title: Induced e.m.f.

Objective: To investigate the law of electromagnetic induction and estimate the value of permeability of free space

Theory:

The theory of this experiment is being discussed in STPM PHYSIC

Apparatus:

                     i.            A 300 turn[2]-solenoid,S

                      ii.            A set of coils 10 turns C1 (10 turns), C2 (20 turns), C3 (30 turns), C4 (40 turns) and C5 (50 turns)

                        iii.            A transformer (0 - 12 V a. c.)[3]

                        iv.            A rheostat (15 Ω, 5 A)

                         v.            A signal generator

                           vi.            Two digital multimeters

                       vii.            Some connecting wires

                        viii.            A vernier calipers

                           ix.            A metre rule

Procedure:

Part I: To investigate how the induced e.m.f. depends on the number of turns in an induction coil.

Figure 17

a)   The circuit is set up as shown in Figure 17. The solenoid S[4] is connected in series with the rheostat R and the transformer. Multimeter  is used to measure the potential difference across the solenoid S. 

    (Caution: The rheostat itself is also a solenoid and should be placed away from the other apparatus).

b)       The output voltage of the transformer is set to 2.0 V. The rheostat is adjusted to fix the potential difference across the solenoid to 1.0 V.

c)       The multimeter is connected  to the coil, . The number of turns N of  is recorded. The coil  is inserted into the centre of the solenoid S.

d)     The induced e.m.f.  is measured and recorded.

e)       Repeat step (c) and (a) with coils. Plot a graph of e.m.f  against N.

Part 11: To determine the permeability of free space.

Figure 18

a)       The circuit is set up as shown in Figure 18. The output gain control of the signal generator or the rheostat or both is adjusted so that the reading of multimeter  is about 10 mA. The reading of  is recorded. This current is made sure that it is kept constant throughout the experiment.

                (Note: The multimeter measures r.m.s values).

b)     The length  of the solenoid is measured and the number of turns per unit length n is calculated.

c)       The diameter D of the coil  is measured, and its cross-sectional area A is calculated.

d)       The coil C is inserted into the centre of the solenoid.

e)       The frequency of the a.c. signal is adjusted to 100 Hz. The induced e.m.f. in  is measured and recorded.

f)      The frequency is increased in steps of 100 Hz. The respective values of  are measured and recorded.

g)     A graph of  against f is plotted.

h)       From the gradient of the graph, the value of  is determined,.

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[1] A solenoid is a coil with length that is bigger than its coil diameter.

[2] 20 cm long, swg 24 copper wire about 5 cm diameter wind on PVC pipe

[3] 1V&2V power supply

[4] The solenoid,S must be 300 turn, if less than 300, the experiment shows that the induced e.m.f  is not directly proportional to its turn,N in the secondary coil.