Capacitance and Capacitive Reactance Flashcards
Capacitance is the property of an electric circuit that opposes a change in:
A) voltage
B) current
C) power
D) electrons
A) voltage
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
B) distance between the plates and the dielectric constant
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
False
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
A) Q = C x V
What is one time constant when a 100uF capacitor is connected in series with a 150 ohm resistor?
0.015s
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
A) increase
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
C) t = R x C
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
A) flow of alternating current
If the frequency of the voltage across a capacitor is decreased, then the capacitive reactance will:
A) increase
B) decrease
C) stay the same
A) increase
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
D) fixed and variable
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
A) a dielectric
What type of capacitors can be used in ac circuits?
Non-polarized
What can happen if a polarized electrolytic capacitor is connected improperly in a circuit?
It may explode
Name four applications where capacitors are used
- Timers
- Rectifiers smoothing and filtering
- Power factor correction
- Motor starting
What three factors affect the capacitance of a capacitor?
- Area of the plates
- Distance between plates
- Type of dielectric that separates the plates
What are the two ratings that describe the effectiveness of a dielectric?
- dielectric constant
- dielectric strength
On a capacitor rated at 300 WVDC, what does WVDC stand for?
working volts dc
Calculate the charge required to keep a current of 5A flowing for 4 seconds.
20 C
Q = I x T
= 5A x 4s
= 20 C
Calculate the charge of a 100 uF capacitor when its measured voltage is 200 Vdc
0.02 C
Q = C x V
= 100uF x 200 V
= 0.02 C
Calculate how long it will take a 80uF capacitor to charge when it is connected in series with a 30 KOhm resistor
12 seconds
5 time constants = 5 x R x C
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
63.2 V
The voltage after one time constant is 63.2% of the supply voltage
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
B) decreases
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
A) increases
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
C) stays the same
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
A) leads
When the supply voltage is at its maximum in a purely capacitive circuit, what is the value of the current?
A) maximum
B) minimum
B) minimum
