AC Circuit

Multiple Choice

In a capacitor in an AC circuit, the voltage:

• leads the current by 90°.

• lags the current by 90°.

• may lead or lag the current depending on the frequency.

• is in phase with 70.7% of the current.

Multiple Choice

The rms current is equal to the direct current that:

• produces the same average voltage across a resistor as in an AC circuit.

• dissipates an equal amount of energy in a resistor at the same rate as in an AC circuit.

• provides the same average current in a resistor as in an AC circuit.

• results in the same peak power in a resistor as in an AC circuit.

Multiple Choice

Consider the AC circuit in Figure 33.8. The frequency of the AC source is adjusted while its voltage amplitude is held constant. When does the lightbulb glow the brightest?

• It glows brightest at high frequencies.

• It glows brightest at low frequencies.

• The brightness is the same at all frequencies.

Fill in the Blanks

CYU Figure 23.1 shows three ac circuits: one contains a resistor, one a capacitor, and one an inductor. The frequency of each ac generator is reduced to one-half its initial value. Circuit _(A/B/C)experiences the greatest increase in current. Circuit _(A/B/C) the least change in current?

Multiple Choice

Consider the AC circuit in Figure 33.12. The frequency of the AC source is adjusted while its voltage amplitude is held constant. When does the lightbulb glow the brightest?

• It glows brightest at high frequen­cies.

• It glows brightest at low frequencies

• The brightness is the same at all frequencies.

Multiple Choice

Which of the graph representing $$X_L<X_C$$ ?

• a

• b

• c

Multiple Choice

A long wire of finite resistance is connected to an ac generator. The wire is then wound into a coil of many loops and reconnected to the generator. Is the current in the circuit with the coil greater than, less than, or the same as the current in the circuit with the uncoiled wire?

• greater than

• less than

• the same

Multiple Choice

A light bulb and a parallel plate capacitor (containing a dielectric material between the plates) are connected in series to the 60-Hz ac voltage present at a wall outlet. When the dielectric material is removed from the space between the plates, does the brightness of the bulb increase, decrease, or remain the same?

• increase

• decrease

• remain the same

Multiple Choice

An air-core inductor is connected in series with a light bulb, and this circuit is plugged into an ac electrical outlet. When a piece of iron is inserted inside the inductor, does the brightness of the bulb increase, decrease, or remain the same?

• increase

• decrease

• remain the same

Multiple Choice

Consider the circuit in Figure. With the capacitor and the inductor present, a certain amount of current is in the circuit. Then the capacitor and the inductor are removed, so that only the resistor remains connected to the generator. Is it possible that, under a certain condition, the current in the simplified circuit has the same rms value as in the original circuit?

• No

• Yes when $$X_L=R$$

• Yes when $$X_C=R$$

• Yes when $$X_C=X_L$$

Fill in the Blanks

An inductor and a capacitor are connected in parallel across the terminals of an ac generator. Does the current from the generator decrease, remain the same, or increase as the frequency becomes (a) very large and (b) very small?

(a)______(i/d/s)

(b)______(i/d/s)

i:increase d: decrease s: same