Question 1

Which of the following best describes the difference between alternating current (AC) and direct current (DC) as shown in the first two images?

Accepted Answer

AC periodically reverses direction and voltage, while DC flows in only one direction with constant voltage.

Question 2

The equation for the instantaneous emf in an AC circuit is given by $$\mathcal{E} = \mathcal{E}_0 \cos(2\pi f t). If $$the frequency $$f is $$doubled, what happens to the period $$T of $$the oscillation?

Accepted Answer

The period $$T is $$halved.

Question 3

In an AC circuit with a resistor, the instantaneous voltage and current are given by $$v_R = V_R \cos(2\pi f t)$$ and $$i_R = I_R \cos(2\pi f t). $$What is true about their phase relationship?

Accepted Answer

The voltage and current are in phase.

Question 4

A resistor is connected to an AC voltage source. If the peak current is $$I_0$$, what is the expression for the average power dissipated in the resistor?

Accepted Answer

$$P_{avg} = \frac{1}{2} I_0$^2 R$$

Question 5

The root-mean-square (rms) value of a sinusoidal voltage with peak value $$V_p is$$:

Accepted Answer

$$V_{rms} = \frac{V_p}{\sqrt{2}}$$

Question 6

A transformer has a primary coil with $$N_1$$ turns and a secondary coil with $$N_2$$ turns. If $$N_2$$ \u003e $$N_1$$, what type of transformer is it and what happens to the voltage?

Accepted Answer

Step-up transformer; voltage increases.

Question 7

In an ideal transformer, the relationship between the rms voltages and the number of turns is $$\frac{V_2$}{V_1$} = \frac{N_2$}{N_1$}. If$$ $$V_1 = 120$$\,	ext{V}$$, N_1$ = 400$$, and $$N_2 = 100$, what is V_2$?

Accepted Answer

$$V_2 = 30$$\,	ext{V}$$

Question 8

A power plant generates 40 MW of AC electricity at 25,000 V. The transmission line has a resistance of 7.0 $$\Omega. $$What is the current required to deliver 40 MW?

Accepted Answer

$$I = 1600\,	ext{A}$$

Question 9

Why is electricity transmitted at high voltages over long distances?

Accepted Answer

To decrease the current and reduce power loss.

Question 10

A household circuit is protected by a 15 A circuit breaker. If a computer (240 W), a lamp (120 W), and a heater (1200 W) are plugged in, will the breaker trip?

Accepted Answer

No, because the total current is less than 15 A.

Question 11

If a space heater draws an rms current of 12.5 A from a 120 V outlet for 5 hours, how much energy is used in kilowatt-hours?

Accepted Answer

7.5 kWh

Question 12

Which of the following is true about the physiological effects of AC and DC currents on the human body?

Accepted Answer

AC current is more dangerous than DC current at the same value.

Question 13

A worker touches a live wire at 480 V with dry skin (total resistance $$2\,	ext{M}\Omega). $$What is the current through his body?

Accepted Answer

$$I = 0.24\,	ext{mA}$$

Question 14

Why can birds safely sit on high-voltage transmission lines?

Accepted Answer

Because the potential difference across their bodies is extremely small.

Question 15

What is the function of a ground fault interrupter (GFI) in household circuits?

Accepted Answer

It disconnects the circuit if the current in the hot and neutral wires differs by more than about 5 mA.

Question 16

In a transformer, if the primary coil has 500 turns and the secondary coil has 20 turns, and the primary voltage is 120 V, what is the secondary voltage?

Accepted Answer

$$V_2 = 4.8$$\,	ext{V}$$

Question 17

Which device can be reset after it trips due to excessive current, and which must be replaced?

Accepted Answer

Circuit breaker can be reset, fuse must be replaced.

Question 18

If two devices are plugged into a standard household outlet, how are they connected?

Accepted Answer

In parallel.

Question 19

Which formula correctly relates the rms current in the secondary coil of an ideal transformer to the rms current in the primary coil and the number of turns?

Accepted Answer

$$I_{2,rms} = \frac{N_1$}{N_2$} I_{1,rms}$$

Chapter 26: AC Electricity – Principles, Applications, and Safety

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