3.9: Capacitance

Question 1

235. The ability of an electronic component to store electric charge is called its:
     (a) Addmittance
     (b) Reluctance
     (c) Capacitance

Answer 1

(c) Capacitance

Question 2

236. The amount of electricity a capacitor can store is directly proportional to :
     (a) distance between the plates and inversely proportional to the plate area.
     (b) plate area and is not affected by the distance between the plates.
     (c) plate area and inversely proportional to the distance between the plates.

Answer 2

(c) plate area and inversely proportional to the distance between the plates.

Question 3

237. The subsatance that exists between the plates of a capacitor is more commonly
     known as the:
     (a) Electrode
     (b) Dialectric
     (c) Core

Answer 3

(b) Dialectric

Question 4

238. Placing two charges further from each other, will cause their resultant field force
     to:
     (a) Increase
     (b) Remain the same.
     (c) Decrease

Answer 4

(c) Decrease

Question 5

239. A 1 μF capacitor is equivalent to:
     (a) 1,000,000 Farads
     (b) 1/1,000,000 Farads
     (c) 1/1,000 Farads

Answer 5

(b) 1/1,000,000 Farads

Question 6

240. A capacitor that stores 6 coulombs of electrons when a potential of 2 volts is
     applied across its terminals has what total value of capacitance?
     (a) 12 farads
     (b) 3 farads
     (c) 6 farads

Answer 6

(b) 3 farads

**Note: **
Q = C * V
where Q = Charge in Coulombs
C = Capacitance in Farads
V = Voltage in Volts

Question 7

241. What is the charge on a capacitor of value 1μF when a voltage across it is 500
     volts?
     (a) 5 Coulombs
     (b) 500 μ Coulombs
     (c) 5,000 μ Coulombs

Answer 7

(b) 500 μ Coulombs

Note:
Q = C * V
where Q = Charge in Coulombs
C = Capacitance in Farads
V = Voltage in Volts

Question 8

242. A 28VDC supply is placed across a 100 microfarad capacitor. What will the charge
     be placed on it when fully charged?
     (a) 100 microcoulombs
     (b) 2.8 millicoulombs
     (c) 28 Coulombs

Answer 8

(b) 2.8 millicoulombs

Note:
Q = C * V
where Q = Charge in Coulombs
C = Capacitance in Farads
V = Voltage in Volts

Prefixes - Micro (u) (Micro-Pen** 6 inches) ( 10^-6)
- Nano (n) (Nanu steak house -90e for a steak) (10^-9)
- Pico (p) (10^-12)

Question 9

243. If the distance between the plates of a parallel plate capacitor is increased and the
     area of overlap of the plates is decreased the capacitance will be:
     (a) unaffected
     (b) increased
     (c) decreased

Answer 9

(c) decreased

Question 10

244. When a 2μF and 100 nF capacitor are connected in parallel, the total capacitance
     is equal to:
     (a) 3 μF
     (b) 102 μF
     (c) 2100 nF

Answer 10

(c) 2100 nF

Remember: Capacitors in parallel behave like resistors in series
2μF + 100 nF
(2x10-9)F + 100nF
(2000x10^-9)F + 100F = 2100nF

Question 11

245. The capacitance of two capacitors of 12 μF and 4 μF are connected in series. The
     total capacitance is?
     (a) 16 μF
     (b) 3 μF
     (c) 8μF

Answer 11

(b) 3 μF

Remember: Capacitors in series behave like resistors in parallel
1/Ctotal = 1/C1 + 1/C2 ….

Ctotal = (C1)(C2) / (C1 + C2)
Ctotal = 48 / 16 = 3

Question 12

246. Three capacitors are connected in series. They are 470 μF, 750μF and 0.00033
     μF respectively. The total capacitance is?
     (a) more than 330 pF
     (b) less than 330 pF
     (c) Approximately 1550 pF

Answer 12

(b) less than 330 pF

Note: Capacitors in series behave like resistors in parallel
Question too complex to work out mathematically, remember if resistors are in parallel, the maximum resistance is less than the smallest resistor value.

Question 13

247. Which statement is correct about polarity markings (+/-) on capacitors?
     (a) All capacitors have polarity markings.
     (b) Some capacitors have polarity markings.
     (c) Capacitors never have polarity markings

Answer 13

(b) Some capacitors have polarity markings.

Question 14

248. When the plates of a Capacitive Type Sensor are placed closer together the
     capacitance:
     (a) is increased
     (b) is decreased
     (c) will not change until the dialectric changes

Answer 14

(a) is increased

The capacitance of a capacitor depends on three factors:
1. The area of the plates
2. The distance between the plates
3. The material (dielectric) between the plates

Question 15

249. Which statement is correct about colour coding on capacitors?
     (a) All capacitors are colour coded..
     (b) Capacitors are never colour coded..
     (c) Some capacitors may be colour coded..

Answer 15

(c) Some capacitors may be colour coded..

Question 16

250. Which of the following will decrease the value of a capacitor.
     (a) Increase the area of the plates
     (b) Increase the thickness of the dielectric.
     (c) Decrease the spacing between the two plates.

Answer 16

(b) Increase the thickness of the dielectric.

The capacitance of a capacitor depends on three factors:
1. The area of the plates
2. The distance between the plates
3. The material (dielectric) between the plates

Question 17

251. Which of the following characteristics of a capacitor can be varied WITHOUT
     altering its capacitance?
     (a) Thickness of the plates
     (b) Area of the plates
     (c) Material of the dielectric

Answer 17

(a) Thickness of the plates

The capacitance of a capacitor depends on three factors:
1. The area of the plates
2. The distance between the plates
3. The material (dielectric) between the plates

Question 18

252. If a 3μF capacitor is placed in series with a 6μF capacitor, the total capacitance
     is:
     (a) 1.5 μF
     (b) 2.0 μF
     (c) 4.5 μF

Answer 18

(b) 2.0 μF

Remember: Capacitors in series behave like resistors in parallel
1/Ctotal = 1/C1 + 1/C2 + …..

Ctotal = (C1*C2) / (C1 + C2)
Ctotal = (3 * 6) / (3 + 6)
= 18/9 = 2

Question 19

253. A capacitor which stores a charge of one millionth of a coulomb when the emf
     of one volt is applied has a value of:
     (a) 1 farad.
     (b) 1 microfarad.
     (c) 1 picofarad.

Answer 19

(b) 1 microfarad.

Capacitance is measured in units called farads. A one-farad capacitor stores one coulomb (6.28 x10^18 electrons) of charge when a potential of 1 volt is applied across the terminals of the capacitor.

Question 20

254. A circuit has a 1μF and 2 μF capacitor in series what is the total capacitance?
     (a) .66 μF
     (b) 0.99 μF
     (c) 1.0 μF

Answer 20

(a) .66 μF

Remember: Capacitors in series behave like resistors in parallel
1/Ctotal = 1/C1 + 1/C2 + ….
Ctotal = (C1 * C2) / (C1 + C2)
Ctotal = (1 * 2) / (1 + 2)
=2/3 = 0.6666

Question 21

255. A circuit has a 1 μF and 2 μF connected in parallel . What is the total capacitance
     of the circuit?
     (a) 1 μF
     (b) 2 μF
     (c) 3 μF

Answer 21

(c) 3 μF

Remember: Capacitors in paralle behave like resistors in series
Ctotal = C1 + C2 + ….
Ctotal = 1 + 2
= 3

Question 22

256. A 500Ω resistor is connected in series with a 2 μF What is the time constant of the
     circuit.
     (a) .5 second
     (b) 1 msecond
     (c) 2 seconds

Answer 22

(b) 1 msecond

1 TC = C * R
Where TC is Time Constant in seconds
C = Capacitance in Farads
R = Resistance in Ohms

1 TC = time required to charge capacitor to 63% of full charge OR to discharge it to 37% of it’s initial voltage
5 TC = time required to be fully charged / discharged

Question 23

257. The capacitance of a capacitor is the ratio of :
     (a) voltage between plates to charge
     (b) voltage between plates to plate spacing
     (c) charge to voltage between plates

Answer 23

(c) charge to voltage between plates

Remember: Q = C * V
Where Q = Charge in Coulombs
C = Capacitance in Farads
V = Voltage in Volts

Question 24

258. Four 8 microfarad capacitors are connected in parallel. The equivalent
     capacitance is :
     (a) 2 microfarads
     (b) 32 microfarads
     (c) 8 microfarads

Answer 24

(b) 32 microfarads

Remember: Capacitors in parallel behave like resistors in series
Ctotal = C1 + C2 + ….
Ctotal = 8 + 8 + 8 + 8
Ctotal = 32 microfarads

Question 25

259. A 1 M ohm resistor is connected in series with a 1 μF capacitor across a 10V battery. What is the voltage across the capacitor 1 second after the capacitor starts charging. (a) 5.2 V (b) 6.32 V (c) 8.65 V

Answer 25

**(b) 6.32 V** 1 TC = C * R Where TC is Time Constant, C is capacitance in Farads and R is resistance in ohms 1 TC = (1x10^-6) * (1x10^6) Remember (10^a) *(10^b) = 10^(a+b) 1 TC = 1 second 1 TC = 63.2% of charge or 37% of discharge, therefore after 1 second, 1 TC = 63.2% of 10V = 6.32V

Question 26

260. The switch on a DC circuit containing a fully charged capacitor is opened. The voltage across the capacitor: (a) Starts to fall exponentially to zero. (b) Remains equal to the original charging voltage supplied (c) Drops immediately to zero.

Answer 26

(b) Remains equal to the original charging voltage supplied

Question 27

Answer 27

Question 28

Answer 28

Option B: 7.36 volts 1 TC = 63.2% of full charge OR 36.8% discharge of its initial voltage A capacitor is considered to be fully charged or discharged after 5 Time Constants 1 TC = C (Capacitance in Farads) * R (resistance in Ohms)

Question 29

Answer 29

Option A: the max voltage that can be constantly applied Note: Exceed the voltage at which the capacitor is rated, and you destroy the capacitor

Question 30

Answer 30

Option B: the relative permittivity of the dielectric in relation to a vacuum Note: 'Relative' means relative to 'free space' (a 'vacuum' in other words)

Question 31

Answer 31

Option A: leaking dielectric capacitor

Question 32

Answer 32

Option A: 10.01001 millifarads Remember: Capacitors in parallel behave like resistors in series Ctotal = C1 + C2 +....

Question 33

Answer 33

Option B: 12.5 milliJoules **Energy Stored in a Capacitor** E = 1/2 * C * (V^2) Where E is energy in Joules, C is capacitance in Farads and V is Voltage in Volts A capacitor can store electric energy when disconnected from its charging circuit, so it can be used like a temporary battery. Capacitors are commonly used in electronic devices to maintain power supply while batteries are being changed. This prevents loss of information in volatile memory.

Question 34

Answer 34

Option B: the same capacitance The capacitance of a capacitor depends on three factors: 1. The area of the plates 2. The distance between the plates 3. The material (dielectric) between the plates

Question 35

Answer 35

Option B: there is a polarized input Wiring an electrolytic capacitor wrong polarity will destroy it. AC will therefore destroy it. Current must be DC (i.e polarised) and the right way.

Question 36

Answer 36

Option B: Put a 6 F capacitor in series Remember: Capacitors in series behave like resistors in parallel. Ctotal = (C1 * C2) / (C1 + C2) = (3 * 6) / (3 + 6) = 18/9 = 2

Question 37

Answer 37

Option C: 500 milliseconds 1 TC = C * R 1 TC = (1x10^-6) * (100x103) 1 TC = 100 milliseconds **Note that question asks fully charged not 63.2% charged which is 1 Time Constant.** 5 TC = 500 milliseconds and fully charged

Question 38

Answer 38

Option A: 1 second 1 TC = C * R Need to work out R with Ohms law, R = V/I , R = 100V / 0.025 A, R = 4000 ohms 1 TC = (50x10^-6) F * (4 x10^3) ohms 1 TC = 200 x10^-3 seconds , 1 TC = 0.2 seconds However, 5 TC is required for full charge 0.2 seconds * 5 = 1 second

Question 39

Answer 39

Option A = 6.4 Volts Since Q = V*C and Q = I*t where I is current and t is time Then I*t = V*C V = I*t / C, V = (40x10^-6)(4) / (25x10^-6) = 6.4

Question 40

Answer 40

Option B: 4 u(micro)F Remember: Capacitors in series behave like resistors in parallel 1/Ctotal = 1/C1 + 1/C2 + 1/C3 + ...

Question 41

Answer 41

Option B: for the emf to reach 63.2% of maximum

Question 42

Answer 42

Option A: plate area and inversely proportional to the distance between the plates

Question 43

Answer 43

Option B: the type of material separating the plates Note: The capacitance of a capacitor is only dependant upon its physical properties (size and material) and not what you apply to it

Question 44

Answer 44

Option C: equal to the sum of all the capacitances

Question 45

Answer 45

Option C: 1x10^-12 farad

Question 46

Answer 46

Option C: remains equal to the original charging voltage supply Note: If there is no external circuit, the charge can go nowhere

Question 47

Answer 47

Option A: zero Note: If the capacitance is fully charged, it is equal and opposite to the source voltage, so no current flows

Question 48

Answer 48

Option C: D.C

Question 49

Answer 49

Option A: 0.4 u(micro)F Note: capacitors in parallel behave like resistors in series - Total capacitance is equal to the sum of all capacitors

Question 50

Answer 50

Option B: the charge stored on each is the same Note: If they were not the same, there would be a current flow between them, until they equalised out

Question 51

Answer 51

Option C: the product C * V Remember: Q = C * V

Question 52

Answer 52

Option B: is fully discharged before removing it from the circuit Note: Charged capacitors can be lethal, even when charged to say, 1 volt

Question 53

Answer 53

Option B: the electrostatic energy storing capacity of the capacitor dielectric

Question 54

Answer 54

Option C: 36 microfarads Remember: Capacitors in parallel behave like resistors in series. Just add them up

Question 55

Answer 55

Option A: 8 seconds 1 TC = C * R Where TC is the Time Constant, C is capacitance in Farads and R is resistance in ohms

Question 56

Answer 56

Option B: the maximum continuous voltage is can take

Question 57

Answer 57

Option A: picofarads

Question 58

Answer 58

Option C: expontential

Question 59

Answer 59

Option C: small physical size for a large capacity

Question 60

Answer 60

Option C: heavy / light loads Electrolytic capacitors are used in circuits of all sizes

Question 61

Answer 61

Option A: 100 microseconds 1 TC = C * R 1 TC = (20x10^-12) * (1x10^6) 1 TC = 20x10^-6 5 TC ( 5 time constants max charge) = 100 microseconds

Question 62

Answer 62

Option A: Exponential

Question 63

Answer 63

Option B: 10,000 pf 103 means 10 followed by 3 zeros. Capacitors are always measured in picofarads

Question 64

Answer 64

Option A: c = K*A / d

Question 65

Answer 65

Option A: 36 milliFarad

Question 66

Answer 66

Option C: 8 seconds

Question 67

Answer 67

Option B: Same as circuit voltage Note: The answer to this question depends upon how accurate you want to go. 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 99.99999% charged).

Question 68

Answer 68

Option C: Large output compared to size required An Electrolytic Capacitor is used where a large amount of capacitance is required. - Dielectric is very thin layer of oxide - so thin that is it possible to make capacitors with a large value of capacitance for a small physical size.

Question 69

Answer 69

Option B: 10 nanofarads

Question 70

Answer 70

Option B: 10 nanofarads ''Black boys R*** Our Young Girls But Violet Gives Willings''

Question 71

Answer 71

Option B: the same as applied voltage

Question 72

Answer 72

Option B: electrostatic storying capability of the dielectric

Question 73

Answer 73

Option C: 1 * 10^-6 Farads

Question 74

Answer 74

Option C: 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.

Question 75

Answer 75

Option A: Farads per meter

Question 76

Answer 76

Option B: 4 picofarads

Question 77

22. A capacitor that stores 16 coulombs of electrons when a potential of 2 volts is applied across its terminals has what total value of capacitance? (a) 32 farads (b) 8 farads (c) 6 farads

Answer 77

(b) 8 farads Q = C*V C = Q / V Q = 16 / 2, Q = 8 farads

Question 77

22. A capacitor that stores 16 coulombs of electrons when a potential of 2 volts is applied across its terminals has what total value of capacitance? (a) 32 farads (b) 8 farads (c) 6 farads

Answer 77

(b) 8 farads Q = C*V C = Q / V Q = 16 / 2, Q = 8 farads