Question 1

Which equation correctly describes the instantaneous emf of an AC voltage source, as shown in the first image?

Accepted Answer

$$\mathcal{E} = \mathcal{E}_0 \cos(2\pi f t)$$

Question 2

In a resistor connected to an AC source, which statement is true about the phase relationship between the voltage across the resistor and the current through it?

Accepted Answer

The voltage and current are in phase

Question 3

Given a peak voltage $$V_p in an AC $$circuit, what is the root-mean-square (rms) voltage $$V_{rms}$$?

Accepted Answer

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

Question 4

A transformer has 100 turns on the primary coil and 500 turns on the secondary coil. If the primary voltage is 120 V (rms), what is the secondary voltage (rms)?

Accepted Answer

$$600\ 	ext{V}$$

Question 5

Why is AC electricity preferred for power transmission over long distances?

Accepted Answer

AC can be easily transformed to higher or lower voltages using transformers

Question 6

If a household device uses 2 kW of power for 3 hours, how much energy does it consume in kilowatt-hours (kWh)?

Accepted Answer

$$6\ 	ext{kWh}$$

Question 7

A circuit breaker is rated for 15 A. If three devices, each drawing 6 A, are plugged into the same circuit, what will happen?

Accepted Answer

The circuit breaker will trip and shut off the circuit

Question 8

What is the main safety function of the third prong (round) on a household electrical plug?

Accepted Answer

It acts as a secondary ground connection to prevent electric shock

Question 9

A worker touches a live wire at 480 V with rubber boots whose resistance is $$R_{boots} = 10^6$\ \Omega. If $$the total resistance through the body and boots is $$1.1 	imes 10^6$\ \Omega$$, what is the current through the worker?

Accepted Answer

$$0.436\ 	ext{mA}$$

Question 10

Why can birds safely sit on high-voltage transmission lines without being electrocuted?

Accepted Answer

There is negligible potential difference between their feet

Question 11

Which equation correctly relates the voltages and number of turns in an ideal transformer?

Accepted Answer

$$\frac{V_2$}{V_1$} = \frac{N_2$}{N_1$}$$

Question 12

If the rms voltage of a US wall outlet is 120 V, what is the peak voltage?

Accepted Answer

$$170\ 	ext{V}$$

Question 13

Which device is designed to disconnect a circuit when the current exceeds a specified value and can be reset after tripping?

Accepted Answer

Circuit breaker

Question 14

A ground fault interrupter (GFI) will disconnect the circuit if the difference in current between the hot and neutral wires exceeds:

Accepted Answer

$$5\ 	ext{mA}$$

Question 15

Which formula gives the average power dissipated in a resistor connected to an AC source?

Accepted Answer

$$P_{avg} = I_{rms} V_{rms}$$

Question 16

If a device is plugged into a grounded outlet and a wire inside the device touches the metal case, what happens?

Accepted Answer

The current will flow to ground, preventing electric shock

Question 17

Which of the following is NOT a reason why transformers are essential for the power grid?

Accepted Answer

They can operate with DC electricity

Question 18

What is the main physiological danger of electric current passing through the human body?

Accepted Answer

It mimics nerve impulses and can cause involuntary muscle contractions

Question 19

If the resistance of dry skin is $$10^5$$\ \Omega$$ and the resistance of wet skin is 10^3$\ \Omega$$, which situation is more dangerous when touching a live wire?

Accepted Answer

Wet skin, because resistance is lower and more current flows

Question 20

A generator produces electricity at 25 kV, which is stepped up to 500 kV for transmission. If the transmission line has resistance $$R$$ and the current is $$I$$, what happens to power loss $$P_{loss} = I^2$ R$$ when voltage is increased?

Accepted Answer

Power loss decreases because current is lower at higher voltage

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

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