CHAPTER 28 ELECTROMAGNETIC INDUCTION QUESTIONS AND ANSWERS
1. Calculate the magnetic flux through a rectangular loop of wire 0.75 m2 in area, when it is placed perpendicular to a magnetic field of flux density 4 T.
Solution Ф = BA ⇒ Ф = (4)(0.75) ⇒ Ф = 3 Wb
H
2. If a rectangular coil of sides 50 cm by 0.2 m is placed at 60° to a uniform magnetic field of flux density 5 T, calculate the magnetic
flux passing through it. Solution
Remember to convert area to m2 and resolve magnetic flux density into perpendicular and parallel components. Area = (50 × 10−2)(0.2) = 0.1 m2
Parallel component of B: B cos q = (5)(cos 60°) = 2.5 T Perpendicular component of B: B sin q = (5)(sin 60°) = 4.33 T Magnetic flux Ф = BA ⇒ Ф = (4.33)(0.1) ⇒ Ф = 0.433 Wb
You may now complete Exercise 28A (page 310). Faraday’s law of electromagnetic induction
Faraday’s law of electromagnetic induction The magnitude of induced emf is directly proportional to the rate of change of flux.
We will carry out three experiments to demonstrate electromagnetic induction. The conclusion in all three experiments is that a changing magnetic field induces emf in a coil.
To demonstrate Faraday’s law of electromagnetic induction 1
This demonstrates Faraday’s law on the relative movement of coil/magnet. 1. Set up the experiment as shown in Fig. 28.2. 2. As the north pole approaches the coil, the galvanometer needle twitches one way.
3. As the north pole moves away, the galvanometer needle twitches the other way.
4. If the magnet and coil are kept stationary, no movement takes place on the galvanometer.
5. It will be seen that the speed of approach of the magnet is proportional to the induced current. It will also be observed that the orientation of the magnetic pole affects the induced current direction.
galvanometer FIG. 28.2 main coil
Electricity is often called wonderful, beautiful; but it is so only in common with the other forces of nature. The beauty of electricity or of any other force is not that the power is mysterious, and unexpected, touching every sense at unawares in turn, but that it is under law. − Michael Faraday
