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Electrical Fundementals > Capacitance And Capacitors > Flashcards

Capacitance And Capacitors Flashcards

1
Q

A capacitor is fully charged after 25 seconds to a battery voltage of 20 Volts. The battery is replaced with a short circuit. What will be the voltage across the capacitor after one time constant?

A

7.36 volts

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2
Q

The voltage rating of a capacitor is?

A

the max voltage that can be constantly applied.

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3
Q

The relative permittivity of a capacitor is?

A

the relative permittivity of the dielectric in relation to a vacuum.

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4
Q

When checking a capacitor with an ohmmeter, if the reading shows charging but the final reading is less than normal then the possible fault is?

A

leaking dielectric capacitor.

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5
Q

Three capacitors 10 microfarads, 10 nanofarads and 10 millifarads are connected in parallel. What is the total capacitance?

A

10.01001 millifarads.

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6
Q

If a 1 milliFarad capacitor has a potential difference across it of 5V, what is the energy stored?

A

12.5 milliJoules.

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7
Q

A capacitor with double the area and double the dielectric thickness will have?

A

the same capacitance.

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8
Q

An electrolytic capacitor is used where?

A

there is a polarized input.

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9
Q

What must you do to make a 3 F capacitor circuit into a 2 F circuit?

A

Put a 6 F capacitor in series.

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10
Q

The time required to fully charge a 1 µ(micro)F capacitor in series with a 100k ohm resistor is?

A

500 milliseconds.

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11
Q

A 50 μ(micro) Farad capacitor is fed with a current of 25 milliAmpere. How long will it take to charge it to 100Volts?

A

1 second.

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12
Q

In a circuit C = 25 microfarads and the current flow is 40 microamps for 4 seconds. What is the voltage?

A

6.4Volts.

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13
Q

Three 12 µ(micro)F capacitors are in series. The total capacitance is?

A

4 µ(micro)F.

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14
Q

The time constant of a capacitor is the time?

A

for the emf to reach 63.2% of maximum.

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15
Q

The amount of electricity a capacitor can store is directly proportional to the?

A

plate area and inversely proportional to the distance between the plates.

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16
Q

The capacitance of a capacitor is dependant upon?

A

the type of material separating the plates.

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17
Q

When different rated capacitors are connected in parallel in a circuit, the total capacitance is?

A

equal to the sum of all the capacitances.

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18
Q

One Picofarad is?

A

1 * 10-12 farad.

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19
Q

The switch on a DC circuit containing a fully charged capacitor is opened. The voltage across the capacitor?

A

remains equal to the original charging voltage supply.

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20
Q

The current in a DC circuit containing a fully charged capacitor is?

A

zero.

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21
Q

A capacitor is a barrier to?

A

D.C.

22
Q

What is the total capacitance of a parallel circuit containing three capacitors with capacitance of 0.25microfarad, 0.03microfarad, and 0.12microfarad, respectively?

A

0.4 µ(micro)F.

23
Q

When two capacitors are connected in series?

A

the charge stored on each is the same.

24
Q

The charge on a capacitor is expressed as?

A

the product C * V.

25
Q

When handling a high voltage capacitor in an electrical circuit, be sure it?

A

is fully discharged before removing it from the circuit.

26
Q

The dielectric constant of a capacitor is a measurement of?

A

the electrostatic energy storing capacity of the capacitor dielectric.

27
Q

Three 12 microfarad capacitors in parallel. The overall circuit capacitance is?

A

36 microfarads.

28
Q

A circuit has 1 megohm and 8 microfarads. What is the time constant?

A

8 seconds.

29
Q

A capacitor rating is?

A

the maximum continuous voltage it can take.

30
Q

The multiplier colour coding on a capacitor is in?

A

picofarads.

31
Q

The total capacitance of the circuit shown is?

A

4µ(micro)F.

32
Q

When a capacitor is charged from a DC supply, the voltage/time curve is?

A

exponential.

33
Q

An electrolytic capacitor is used because it has a?

A

small physical size for a large capacity.

34
Q

An electrolytic capacitor would be used in circuits supplying?

A

heavy / light loads.

35
Q

What is the correct description of Option C?

A

small physical size for a large capacity.

36
Q

In what type of circuits would an electrolytic capacitor be used?

A

heavy / light loads.

37
Q

How long does a 20 Picofarad capacitor with a 1 Megohm resistor take to charge?

A

100 microseconds.

Time constant (TC) = RC = 20/1,000,000,000,000 * 1,000,000 = 20/1,000,000 = 20 microseconds. But fully charged is 5 * TC = 100 microseconds.

38
Q

What is the discharge curve of a capacitor?

A

Exponential.

39
Q

What is the value of a monolithic capacitor with 103 on it?

A

10,000pf.

103 means 10 followed by 3 zeros. Capacitors are always measured in picofarads.

40
Q

What is the formula for working out the capacitance of a capacitor?

A

c = K*A/d.

41
Q

What is the total capacitance of 3 capacitors each 12milliFarad in a parallel circuit?

A

36milliFarad.

42
Q

What is the time constant of a combination of 1 megohm and 8 microfarad?

A

8 seconds.

1,000,000 * 8/1,000,000 = 8 seconds. Note: Time Constant is asked for, not ‘time to charge’.

43
Q

What is the voltage across a fully charged capacitor?

A

Same as circuit voltage.

Normal theory is that a fully charged capacitor has an equal (and opposite) voltage to the supply. However, a capacitor is never fully charged (something in the order of 99.999999% charged).

44
Q

When would you use an electrolytic capacitor?

A

Large output compared to size required.

45
Q

What is the value of a capacitor with Brown, Black and Orange bands?

A

10 nanofarads.

Brown = 1, Black = 0, Orange = x1000. Total is 10,000 picofarads (always picofarads with capacitors) = 10 nanofarads.

46
Q

What will the voltage measured between the plates of a capacitor generally be?

A

the same as applied voltage.

47
Q

What is the dielectric constant of a capacitor?

A

electrostatic storing capability of the dielectric.

48
Q

What is one microfarad in farads?

A

1 * 10-6 farads.

49
Q

What would be the charge of a capacitor with 6 volts and 2400 pico-coulombs if the voltage was 1.5 volts?

A

600 * 10-12 coulombs.

Q = CV, so direct relationship of charge to voltage. 1.5V is 1/4 of 6V, so charge is 1/4 of 2400 picocoulombs.

50
Q

How is the dielectric strength of a capacitor measured?

A

Farads per meter.

51
Q

What is the total capacitance of three capacitors connected in series, each having a value of 12 picofarads?

A

4 picofarads.

12/3 = 4.

Electrical Fundementals (19 decks)

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