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 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.

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.