### Short Answer

## 20.1 Magnetic Fields, Field Lines, and Force

To produce a magnetic field of 0.0020 T, what current is required in a 500-turn solenoid that is 25 cm long?

- 0.80 A
- 1.60 A
- 80 A
- 160 A

Using four solenoids of the same size, describe how to orient them and in which direction the current should flow to make a magnet with two opposite-facing north poles and two opposite-facing south poles.

How far from a straight wire carrying 0.45 A is the magnetic field strength 0.040 T?

- 0.23 µm
- 0.72 µm
- 2.3 µm
- 7.2 µm

## 20.2 Motors, Generators, and Transformers

A laminated-coil transformer has a wire coiled 12 times around one of its sides. How many coils should you wrap around the opposite side to get a voltage output that is one half of the input voltage? Explain.

- six output coils because the ratio of output to input voltage is the same as the ratio of number of output coils to input coils
- 12 output coils because the ratio of output to input voltage is the same as the ratio of number of output coils to input coils
- 24 output coils because the ratio of output to input voltage is half the ratio of the number of output coils to input coils
- 36 output coils because the ratio of output to input voltage is three times the ratio of the number of output coils to input coils

Explain why long-distance electrical power lines are designed to carry very high voltages.

*P*_{transmitted}=*I*_{transmitted>}^{2}*R*_{wire}and*P*_{lost}=*I*_{transmitted}*V*_{transmitted}, so*V*must be low to make the current transmitted as high as possible.*P*_{transmitted}=*I*_{transmitted>}^{2}*R*_{wire}and*P*_{lost}=*I*_{lost}*V*_{lost}, so*V*must be low to make the current transmitted as high as possible.*P*_{transmitted}=*I*_{transmitted>}^{2}*R*_{wire}and*P*_{lost}=*I*_{transmitted}*V*_{transmitted}, so*V*must be high to make the current transmitted as low as possible*P*_{lost}=*I*_{transmitted}^{2}*R*_{wire}and*P*_{transmitted}=*I*_{transmitted}*V*_{transmitted}, so*V*must be high to make the current transmitted as low as possible.

## 20.3 Electromagnetic Induction

The plane of a square wire circuit with side 4.0 cm long is at an angle of 45° with respect to a uniform magnetic field of 0.25 T. The wires have a resistance per unit length of 0.2. If the field drops to zero in 2.5 s, what magnitude current is induced in the square circuit?

- 35 µA
- 87.5 µA
- 3.5 mA
- 35 A

Yes or no—If a bar magnet moves through a wire loop as shown in the figure, is a current induced in the loop? Explain why or why not.

- No, because the net magnetic field passing through the loop is zero.
- No, because the net magnetic field passing through the loop is nonzero.
- Yes, because the net magnetic field passing through the loop is zero.
- Yes, because the net magnetic field line passing through the loop is nonzero.

What is the magnetic flux through an equilateral triangle with side 60 cm long and whose plane makes a 60° angle with a uniform magnetic field of 0.33 T?

- 0.045 Wb
- 0.09 Wb
- 0.405 Wb
- 4.5 Wb