# Chapter Review

### 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.

- 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 Electromagnetic Induction

- 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 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.

#### 20.3 Motors, Generators, and Transformers

- ${0}^{\circ}$and ${180}^{\circ}$
- ${45}^{\circ}$ and ${135}^{\circ}$
- ${90}^{\circ}$ and ${270}^{\circ}$
- ${225}^{\circ}$ and ${315}^{\circ}$

- 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

### Critical Thinking Items

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

True or false—It is not recommended to place credit cards with magnetic strips near permanent magnets.

- false
- true

True or false—A square magnet can have sides that alternate between north and south poles.

- false
- true

You move a compass in a circular plane around a planar magnet. The compass makes four complete revolutions. How many poles does the magnet have?

- two poles
- four poles
- eight poles
- 12 poles

#### 20.2 Electromagnetic Induction

- A loop of wire should be placed nearest to the vacuum cleaner wire to maximize the magnetic flux through the loop.
- A loop of wire should be placed farthest to the vacuum cleaner wire to maximize the magnetic flux through the loop.
- A loop of wire should be placed perpendicular to the vacuum cleaner wire to maximize the magnetic flux through the loop.
- A loop of wire should be placed at an angle greater than 90° to the vacuum cleaner wire to maximize the magnetic flux through the loop.

- The electric field in the coil increases rapidly due to spinning of magnet which creates an emf in the coil that is proportional to the rate of change of the magnetic flux.
- The magnetic field in the coil changes rapidly due to spinning of magnet which creates an emf in the coil that is proportional to the rate of change of the magnetic flux.

- Yes, the induced current will be produced in the clockwise direction when viewed from above.
- No, the induced current will not be produced.

#### 20.3 Motors, Generators, and Transformers

- The peak emf from a generator can be maximized only by maximizing number of turns.
- The peak emf from a generator can be maximized only by maximizing area of the wired loop.
- The peak emf from a generator can be maximized only by maximizing frequency.
- The peak emf from a generator can be maximized by maximizing number of turns, maximizing area of the wired loop or maximizing frequency.

Explain why power is transmitted over long distances at high voltages.

*P*_{lost}=*I*_{transmitted}*V*_{transmitted}, so to maximize current, the voltage must be maximized.*P*_{transmitted}=*I*_{transmitted}*V*_{transmitted}, so to maximize current, the voltage must be maximized.*P*_{lost}=*I*_{transmitted}*V*_{transmitted}, so to minimize current, the voltage must be maximized.*P*_{transmitted}=*I*_{transmitted}*V*_{transmitted}, so to minimize current, the voltage must be maximized.

### Problems

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

A straight wire segment carries 0.25 A. What length would it need to be to exert a 4.0-mN force on a magnet that produces a uniform magnetic field of 0.015 T that is perpendicular to the wire?

- 0.55 m
- 1.10 m
- 2.20 m
- 4.40 m

#### 20.2 Electromagnetic Induction

What is the current in a wire loop of resistance 10 Ω through which the magnetic flux changes from zero to 10 Wb in 1.0 s?

- –100 A
- –2.0 A
- –1.0 A
- +1.0 A

An emf is induced by rotating a 1,000 turn, 20.0 cm diameter coil in Earth’s 5.00 × 10^{–5} T magnetic field. What average emf is induced, given the plane of the coil is originally perpendicular to Earth’s field and is rotated to be parallel to the field in 10.0 ms?

- –1.6 × 10
^{-4}V - +1.6 × 10
^{-4}V - +1.6 × 10
^{-1}V - –1.6 × 10
^{-1}V

### Performance Task

#### 20.3 Motors, Generators, and Transformers

Your family takes a trip to Cuba, and rents an old car to drive into the countryside to see the sights. Unfortunately, the next morning you find yourself deep in the countryside and the car won’t start because the battery is too weak. Wanting to jump-start the car, you open the hood and find that you can’t tell which battery terminal is positive and which is negative. However, you do have a bar magnet with the north and south poles labeled and you manage to find a short wire. How do you use these to determine which terminal is which? For starters, how do you determine the direction of a magnetic field around a current-carrying wire? And in which direction will the force be on another magnet placed in this field? Do you need to worry about the sign of the mobile charge carriers in the wire?