Capacitance and Capacitive Reactance

Question 1

Capacitance is the property of an electric circuit that opposes a change in:

A) voltage
B) current
C) power
D) electrons

Answer 1

A) voltage

Question 2

As well as the area of the plates, the other two factors that affect the capacitance of a capacitor are the:

A) distance between the plates and the dielectric strength
B) distance between the plates and the dielectric constant
C) material between the plates and the dielectric constant
D) dielectric constant and the voltage

Answer 2

B) distance between the plates and the dielectric constant

Question 3

The ratio of an insulating materials ability to affect the capacitance of a capacitor as compared to air is known as the dielectric strength.

True or False

Answer 3

False

Question 4

The equation for the stored charge of a capacitor is:

A) Q = C x V
B) C = Q x V
C) Q = C/V
D) V = Q x C

Answer 4

A) Q = C x V

Question 5

What is one time constant when a 100uF capacitor is connected in series with a 150 ohm resistor?

Answer 5

0.015s

Question 6

A variable resistor is connected in series with a capacitor across a dc supply. If the resistance value is increased, then the time taken for the voltage across the capacitor to reach its steady value would:

A) increase
B) decrease
C) stay the same

Answer 6

A) increase

Question 7

The formula for the time constant of a resistive-capacitive circuit is:

A) t = R/C
B) t = C/R
C) t = R x C
D) t = C x E

Answer 7

C) t = R x C

Question 8

Capacitive reactance is the opposition of a capacitor to a:

A) flow of alternating current
B) flow of direct current
C) change in current
D) leakage through the capacitor dielectric

Answer 8

A) flow of alternating current

Question 9

If the frequency of the voltage across a capacitor is decreased, then the capacitive reactance will:

A) increase
B) decrease
C) stay the same

Answer 9

A) increase

Question 10

Capacitors can be divided into two major classifications. These two are:

A) paper and ceramic
B) mica and electrolytic
C) ceramic and tubular
D) fixed and variable

Answer 10

D) fixed and variable

Question 11

What is the material between the two plates of a capacitor called?

A) a dielectric
B) a diamagnetic
C) a semi-conductor
D) a metallic foil

Answer 11

A) a dielectric

Question 12

What type of capacitors can be used in ac circuits?

Answer 12

Non-polarized

Question 13

What can happen if a polarized electrolytic capacitor is connected improperly in a circuit?

Answer 13

It may explode

Question 14

Name four applications where capacitors are used

Answer 14

• Timers
• Rectifiers smoothing and filtering
• Power factor correction
• Motor starting

Question 15

What three factors affect the capacitance of a capacitor?

Answer 15

• Area of the plates
• Distance between plates
• Type of dielectric that separates the plates

Question 16

What are the two ratings that describe the effectiveness of a dielectric?

Answer 16

• dielectric constant

- dielectric strength

Question 17

On a capacitor rated at 300 WVDC, what does WVDC stand for?

Answer 17

working volts dc

Question 18

Calculate the charge required to keep a current of 5A flowing for 4 seconds.

Answer 18

20 C

Q = I x T
= 5A x 4s
= 20 C

Question 19

Calculate the charge of a 100 uF capacitor when its measured voltage is 200 Vdc

Answer 19

0.02 C

Q = C x V
= 100uF x 200 V
= 0.02 C

Question 20

Calculate how long it will take a 80uF capacitor to charge when it is connected in series with a 30 KOhm resistor

Answer 20

12 seconds

5 time constants = 5 x R x C

Question 21

Calculate the voltage across a 50uF capacitor after one time constant if the capacitor is connected in series with a 1200 ohm resistor on a 100 Vdc supply

Answer 21

63.2 V

The voltage after one time constant is 63.2% of the supply voltage

Question 22

When the frequency of a circuit is increased, what happens to the capacitive reactance of the circuit?

A) increases
B) decreases
C) stays the same

Answer 22

B) decreases

Question 23

If a capacitor in a circuit is replaced with one of smaller capacitance, what happens to the capacitive reactance of the circuit?

A) increases
B) decreases
C) stays the same

Answer 23

A) increases

Question 24

If a 400 V, 60 Hz supply to a 75 uF capacitor is reduced to 240V, 60Hz, what happens to the capacitive reactance of that capacitor?

A) increases
B) decreases
C) stays the same

Answer 24

C) stays the same

Question 25

In a capacitive ac circuit, how does the current rise and fall compared in time to the rise and fall of the voltage?

A) leads
B) lags
C) same time

Answer 25

A) leads

Question 26

When the supply voltage is at its maximum in a purely capacitive circuit, what is the value of the current?

A) maximum
B) minimum

Answer 26

B) minimum