A bar magnet is dropped vertically downwards through a long solenoid, which is connected to an oscilloscope . The oscilloscope trace shows how the e.m.f. induced in the coil varies with time as the magnet accelerates downwards.

$a$ A bar magnet falls through a long solenoid.$b$The oscilloscope trace shows how the induced e.m.f. varies with time.

Explain why no $e.m.f.$ is induced while the magnet is entirely inside the coil (section BC).

A bar magnet is dropped vertically downwards through a long solenoid, which is connected to an oscilloscope. The oscilloscope trace shows how the e.m.f. induced in the coil varies with time as the magnet accelerates downwards.

$a$ A bar magnet falls through a long solenoid. $b$ The oscilloscope trace shows how the induced e.m.f. varies with time.

Explain why section $CD$ shows a negative trace, why the peak $e,m,f,$ is greater over this section, and why $CD$ represents a shorter time interval than AB.

You can turn a bicycle dynamo by hand and cause the lamps to light up. Use the idea of Lenz's law to explain why it is easier to turn the dynamo when the lamps are switched off than when they are on.

Figure, 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 current caused by induced e.m.f. in the coil, and the directions of the forces on sides$AB$ and $CD$ that oppose the rotation of the coil.

A coil rotating in a magnetic field.

Does a bicycle generator, generate alternating or direct current? Justify your answer.

In a bicycle generator, a permanent magnet rotates inside a fixed coil of wire .

Which of the following units is not correct for magnetic flux?

$\mathrm{A})$ $\mathrm{weber}/{\mathrm{meter}}^2$

$\mathrm{B})$ $\mathrm{tesla}$

$\mathrm{C})$ $\mathrm{gauss}$

$\mathrm{D})$ $\mathrm{newton}/{\mathrm{amp}}^2$

A student thinks that electrical current passes through the core in a transformer to the secondary coil. Describe how you might demonstrate that this is not true and explain how an electrical current is actually induced in the secondary coil. Use Faraday's law in your explanation.