Page 462 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
9a State Lenz’s law and explain how you would use a coil and a magnet to demonstrate the law.
Make clear any other apparatus that you use. [4]
b A vehicle brake consists of an aluminium disc attached to a car axle (Figure 28.34). Electromagnets cause an e.m.f. to be induced in the disc.
car wheel
aluminium disc car axle
electromagnets
Figure 28.34 For End-of-chapter Question 9.
i Explain how the induction of an e.m.f. causes the vehicle to slow down. [3]
ii Explain why the braking effect increases when the speed of the car increases. [2]
10 A bicycle wheel is mounted vertically on a metal axle in a horizontal magnetic field (Figure 28.35).
Sliding connections are made to the metal edge of the wheel and to the metal axle.
magnetic field
spoke
Figure 28.35 For End-of-chapter Question 10.
a i Explain why an e.m.f. is induced when the wheel rotates. [2] ii State and explain two ways in which this e.m.f. can be increased. [2]
b The wheel rotates five times per second and has a radius of 15 cm. The magnetic flux density may be assumed to be uniform and of value 5.0 × 10−3 T.
Calculate:
i the area swept out each second by one spoke [2]
ii the induced e.m.f. between the contacts. [2]
connections
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