Ch. 20

Question 1

In circuit A the battery that supplies energy has twice as much voltage as the battery in circuit B. However, the current in circuit A is only one-half the current in circuit B. Circuit A presents ____ the resistance to the current that circuit B does.

Answer 1

Four times

Question 2

Two materials have different resistivities. Two wires of the same length are made, one from each of the materials. Is it possible for each wire to have the same resistance?

Answer 2

Yes, if the material with the greater resistivity is used for thicker wire.

Question 3

How does the resistance of a copper wire change when both the length and diameter of the wire are doubled?

Answer 3

It decreases by a factor of two.

Question 4

A resistor is connected between the terminals of a battery. This resistor is a wire, and the following five choices give possibilities for its length and radius in multiples of L0 and r0, respectively. For which one of more of the possibilities is the current in the resistor a minimum?

Answer 4

(1/2)L0 and (1/2)r0

2L0 and r0

8L0 and 2r0

Question 5

A toaster is designed to operate with a voltage of 120V, and a clothes dryer is designed to operate with a voltage of 240V. Based solely on this information, which appliance uses more power?

Answer 5

Insufficient info is given for an answer.

Question 6

When an incandescent lightbulb is turned on, a constant voltage is applied across the tungsten filament, which then becomes white hot. The temperature coefficient of resistivity for tungsten is a positive number. What happens to the power delivered to the bulb as the filament heats up?

Answer 6

It decreases.

Question 7

A circuit that includes a bimetallic strip (made from brass and steel) with a resistance heater wire wrapped around it. When the switch is initially closed, a current appears in the circuit because charges flow through the heater wire (which becomes hot), the strip itself, the contact point, and the light bulb. The bulb glows in response. As long as the switch remains closed, the bulb..

Answer 7

Flashes on and off

Question 8

The drawing shows a circuit in which a light bulb is connected to the household ac voltage via two switches, S1 and S2. This is the kind of wiring, for example, that allows you to turn on a carport light from either inside the house or out in the carport. Which one or more of the combinations of the switch positions will turn on the light?

Answer 8

S1 set down and S2 set down

S1 set up and S2 set up

Question 9

Two light bulbs are designed for use with an ac voltage of 120V and are rated at 75W and 150W. Which bulb, if either, has the greater filament resistance?

Answer 9

The 75W bulb

Question 10

An ac circuit contains only a generator and a resistor. Which one of the following changes leads to the greatest average power being delivered to the circuit?

Answer 10

Double the peak voltage of the generator and reduce the resistance by a factor of two.

Question 11

A car has two headlights, and their power is derived from the cars battery. The filament in one burns out, but the other headlight stays on. Are the headlights connected in series or in parallel?

Answer 11

In parallel

Question 12

Two identical light bulbs are connected to identical batteries in two different ways. In method A the bulbs are connected in parallel, and the parallel combination is connected between the one battery’s terminals. In method B the bulbs are connected in series, and the series combination is connected between the other battery’s terminals. What is the ratio of the power supplied by the battery in method A to the power supplied in method B?

Answer 12

4

Question 13

You have three resistors, each of which has a resistance R. By connecting all three together in various ways, which one or more of the following resistance values can you obtain?

Answer 13

3R
(3/2)R
(2/3)R
(1/3)R

Question 14

You have four resistors, each of which has a resistance R. It is possible to connect these four together so that the equivalent resistance of the combination is also R. How many ways can you do it yet? There is more than one way.

Answer 14

There are two ways. One is to form two groups of two parallel resistors and then connect the groups in series. The other is to form two groups of two series resistors and then connect the groups in parallel.

Question 15

An ideal ammeter has ___ resistance, whereas an ideal voltmeter has ___ resistance.

Answer 15

zero, infinite

Question 16

What would happen to the current in a circuit if a voltmeter, inadvertently mistaken for an ammeter, were inserted into the circuit?

Answer 16

The current would decrease markedly

Question 17

The time constant T of a series RC circuit is measured in seconds (s) and is given by T=RC.

R measured in ohms
C measured in farads

Verify that an ohm times a farad is equivalent to a second.

Answer 17

(Ohm)(farad)
= (volt/ampere)(coulomb/volt)
= coulomb/ampere
= coulomb/(coulomb/second)
= second

Question 18

A single resistor is connected across the terminals of a battery. Which one or more of the following changes in voltage and current leaves unchanged the electric power dissipated in the resistor?

Answer 18

• doubling the voltage and reducing the current by a factor of two
• doubling the voltage and increasing the resistance by a factor of four
• doubling the current and reducing the resistance by a factor of four

Question 19

A battery has an emf of V and an internal resistance of r. What resistance R, when connected across the terminals of this battery, will cause the terminal voltage of the battery to be (1/2)V?

Answer 19

R = r

Question 20

Three capacitors are identical, each having a capacitance C. Two of them are connected in series. Then, this series combination is connected in parallel with the third capacitor. What is the equivalent capacitance of the entire connection?

Answer 20

(3/2)C