CHAPTER 14: CAPACITIVE REACTANCE

In a circuit containing pure capacitive reactance and no resistance, the phase angle is always

(a) + 45º

(b) 0º

(c) - 45º

(d) −90º

D

In a circuit in which the resistance and the capacitive reactance are equal and opposite (the

resistance positive, the reactance negative), the phase angle is always

(a) + 45º

(b) 0º

(c) - 45º

(d) −90º

C

In a circuit containing pure resistance and no reactance, the phase angle is always

(a) + 45º

(b) 0º

(c) - 45º

(d) −90º

B

A capacitor has a value of C = 200 pF. We apply a signal at f = 4.00 MHz. What's XC ?

(a) - 498 Ω

(b) -995 Ω

(c) -199 Ω

(d) -3.98 k

C

In an RC circuit containing a finite nonzero resistance, as the ratio XC /R approaches zero

(from the negative side), the phase angle approaches

(a) -90º

(b) - 45º

(c) 0º

(d) negative infinity.

C

A capacitor has a value of 0.0330 μF and a reactance of -123 Ω at a certain frequency.

What frequency?

(a) 39.2 kHz

(b) 19.6 kHz

(c) 78.4 kHz

(d) We need more information to calculate it.

A

What happens to the value of a capacitor (in microfarads) as we decrease the spacing between

the plates without changing anything else?

(a) It does not change.

(b) It increases.

(c) It decreases.

(d) We need more information to say.

B

What's the reactance of a 470-pF capacitor at 12.5 MHz?

(a) -2.71 k

(b) -271 Ω

(c) -27.1 Ω

(d) -2.71 Ω

C

What happens to the reactance of the capacitor described in Question 8 if we reduce the

frequency by a factor of 10?

(a) It becomes 100 times what it was (negatively).

(b) It becomes 10 times what it was (negatively).

(c) It becomes 1/10 of what it was (negatively).

(d) It becomes 1/100 of what it was (negatively).

B

A capacitor has a reactance of -100 Ω at 200 kHz. What's its capacitance?

(a) 7.96 nF

(b) 79.6 nF

(c) 796 nF

(d) 7.96 μF

A

A series RC circuit comprises a capacitor whose reactance is -75 Ω at the frequency of

operation, connected to a 50-Ω resistor. What's the phase angle?

(a) -34º

(b) -56º

(c) -85º

(d) -90º

B

A series RC circuit comprises a capacitor whose reactance is -50 Ω at the frequency of

operation, connected to a 75-Ω resistor. What's the phase angle?

(a) -34º

(b) -56º

(c) -85º

(d) -90º

A

A series RC circuit comprises a capacitance of 0.01 μF along with a 4.7-Ω resistor. What's

the phase angle for a signal with a constant frequency?

(a) - 60º

(b) - 45º

(c) -30º

(d) We need more information to answer this question.

D

What will happen to the phase angle in the circuit of Question 13 (whether or not we know

its actual value) if we short out the resistor but leave the capacitor alone?

(a) It will become -90º

(b) It will become - 45º

(c) It will become 0º

(d) Nothing

A

What will happen to the phase angle in the circuit of Question 14 (not 13!) if we short out

the resistor and double the capacitance?

(a) It will become - 60º

(b) It will become - 45º

(c) It will become -30º

(d) Nothing

D

What will happen to the phase angle in the circuit of Question 15 (not 13 or 14!) if we triple

the frequency while leaving all other factors constant?

(a) We need more information to answer this question.

(b) It will increase negatively (get closer to -90º).

(c) It will decrease negatively (get closer to 0º).

(d) Nothing.

D

A 470-pF capacitor has a reactance of -800 Ω at a certain frequency. What frequency?

(a) 423 kHz

(b) 846 kHz

(c) 212 kHz

(d) We need more information to answer this question.

A

In the situation of Question 17, what happens to XC if we cut the frequency in half?

(a) It increases negatively by a factor of the square root of 2.

(b) It increases negatively by a factor of 2.

(c) It increases negatively by a factor of 4.

(d) Nothing.

B

Figure 14-1.

In the scenario portrayed by Fig. 14-1, the XC /R ratio is roughly

(a) - 0.66.

(b) - 0.75.

(c) -1.5.

(d) -3.0.

C

In Fig. 14-13, the R and XC scale divisions differ in size. We can nevertheless calculate the

phase angle as roughly

(a) -19º

(b) -56º

(c) -37º

(d) -33º

B