### Concept Items

#### 20.1 Magnetic Fields, Field Lines, and Force

- Yes, the magnetic fields from the two north poles will point in the same directions.
- Yes, the magnetic fields from the two north poles will point in opposite directions.
- No, the magnetic fields from the two north poles will point in opposite directions.
- No, the magnetic fields from the two north poles will point in the same directions.

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.

- Outside the magnet the direction of magnetic field lines is towards the south pole of the magnet.
- Outside the magnet the direction of magnetic field lines is away from the south pole of the magnet.

#### 20.2 Motors, Generators, and Transformers

- 0^\circand 180^\circ
- 45^\circ and 135^\circ
- 90^\circ and 270^\circ
- 225^\circ and 315^\circ

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 noâ€”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 increases; the direction of the induced current is counterclockwise.
- Yes, because area of the coil increases; 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.

- If induced current flows, its direction is such that it adds to the changes which induced it.
- If induced current flows, its direction is such that it opposes the changes which induced it.
- If induced current flows, its direction is always clockwise to the changes which induced it.
- If induced current flows, its direction is always counterclockwise to the changes which induced it.

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

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