Page 459 - Physics Coursebook 2015 (A level)

Chapter 28: Electromagnetic induction
separated by a non-conductor so eddy currents cannot flow from one sheet to the next. This reduces the eddy currents and the thermal energy that they create in the core.
There is much more about transformers in Chapter 29, where we will look at how they are used to change voltages in circuits which make use of alternating currents.
QUESTIONS
17 Figure 28.29 represents a coil of wire ABCD being rotated in a uniform horizontal magnetic field. Copy and complete the diagram to show the direction of the induced current in the coil, and the directions of the forces on sides AB and CD that oppose the rotation of the coil.
soft iron core
input voltage
primary (3 turns)
output voltage
secondary (7 turns)
Figure 28.30 The construction of a transformer.
A
B
C
D
18 Does a bicycle generator (Figure 28.7) generate alternating or direct current? Justify your answer.
19 The peak e.m.f. induced in a rotating coil in a magnetic field depends on four factors: magnetic flux density B, area of the coil A, number of turns
N, and frequency f of rotation. Use Faraday’s law to explain why the e.m.f. must be proportional to each of these quantities.
20 Explain why, if a transformer is connected to a steady (d.c.) supply, no e.m.f. is induced across the secondary coil.
Figure 28.29 A coil rotating in a magnetic field.
B
Summary
■■ In a magnetic field of magnetic flux density B, the magnetic flux passing through an area A is given by Φ = BA.
■■ The magnetic flux linking a coil of N turns is the magnetic flux linkage, NΦ.
■■ Flux and flux linkage are measured in webers (Wb). 1 Wb = 1 T m2.
■■ When a conductor moves so that it cuts across a magnetic field, an e.m.f. is induced across its ends. When the magnetic flux linking a coil changes, an e.m.f. is induced in the coil.
■■ Faraday’s law states that the magnitude of the induced e.m.f. is equal to the rate of change of magnetic flux linkage:
E = Δ(NΦ)
Δt
■■ Lenz’s law states that the induced current or e.m.f. is in a direction so as to produce effects which oppose the change that is producing it.
■■ In an a.c. generator, an e.m.f. is induced because the rotating coil changes the magnetic flux linking the coil.
447