### Concept Items

## 20.1 Magnetic Fields, Field Lines, and Force

If you place a compass at the three points in the figure, at which point will the needle experience the greatest torque? Why?

- The density of the magnetic field is minimized at B, so the magnetic compass needle will experience the greatest torque at B.
- The density of the magnetic field is minimized at C, so the magnetic compass needle will experience the greatest torque at C.
- The density of the magnetic field is maximized at B, so the magnetic compass needle will experience the greatest torque at B.
- The density of the magnetic field is maximized at A, so the magnetic compass needle will experience the greatest torque at A.

## 20.2 Motors, Generators, and Transformers

What is a voltage transformer?

- A transformer is a device that transforms current to voltage.
- A transformer is a device that transforms voltages from one value to another.
- A transformer is a device that transforms resistance of wire to voltage.

Why is electric power transmitted at high voltage?

- To increase the current for the transmission
- To reduce energy loss during transmission
- To increase resistance during transmission
- To reduce resistance during transmission

## 20.3 Electromagnetic Induction

Yes or no—Is an emf induced in the coil shown when it is stretched? If so, state why and give the direction of the induced current.

- No, because induced current does not depend upon the area of the coil.
- Yes, because area of the coil decreases; the direction of the induced current is counterclockwise.
- Yes, because area of the coil decreases; the direction of the induced current is clockwise.
- Yes, because the area of the coil does not change; the direction of the induced current is clockwise.

Explain how magnetic flux can be zero when the magnetic field is not zero.

- If angle between magnetic field and area vector is 0°, then its sine is also zero, which means that there is zero flux.
- If angle between magnetic field and area vector is 45°, then its sine is also zero, which means that there is zero flux.
- If angle between magnetic field and area vector is 60°, then its cosine is also zero, which means that there is zero flux.
- If the angle between magnetic field and area vector is 90°, then its cosine is also zero, which means that there is zero flux.