10.2 - capacitors

Question 1

What do capacitors do?

Answer 1

Store electric charge

Question 2

What is a capacitor made from?

Answer 2

A pair of electrical conductors close together

sheets of metal foil with an insulating layer between them

Question 3

How does a capacitor store charge?

Answer 3

Charges come in positive/negative pairs. Charging a capacitor involves using a potential difference to pull pairs apart and put a lot of positive charge on one plate and a lot of negative charge on the other. The charge then has nowhere to go if the circuit is not closed, so is stored on the plates

Question 4

What happens to the potential difference as the amount of charge stored increases?

Answer 4

Potential difference increases

Question 5

What is capacitance?

Answer 5

The charge stored per volt

Question 6

What is the symbol for capacitance?

Answer 6

C

Question 7

What is the equation used to work out capacitance?

Answer 7

C = Q/V

Question 8

What are the units of capacitance?

Answer 8

Farads, F

Question 9

What are the equivalent units to one Farad, F?

Answer 9

CV^-1

Question 10

What does a larger capacitance mean for the amount for charge that can be stored?

Answer 10

A greater charge can be stored for a given potential difference

Question 11

How does the charge on a capacitor decay when it is discharged?

Answer 11

Exponential decay

Question 12

What effect does moving the plates of a capacitor closer together have? Which equation shows this?

Answer 12

More energy can be stored

E = V/d

Question 13

What is the differential equation for rate of flow of charge when discharging a capacitor?

Answer 13

dQ/dt = -Q/RC

Question 14

What conditions do there have to be for charge to decay exponentially?

Answer 14

Current is proportional to potential difference, capacitance is constant

Question 15

What is the breakdown voltage of a capacitor?

Answer 15

The voltage at which a spark will cross between the plates

Question 16

What is the equation used to find the remaining charge on the plates of a capacitor at a given time?

Answer 16

Q = Q0 x e^-t/RC

Question 17

What is the symbol for the time constant?

Answer 17

τ (tau)

Question 18

What is the equation for the time constant?

Answer 18

τ = RC

Question 19

What does the time constant tell you?

Answer 19

Q is equal to Q0 x 1/e when t = τ

Q is equal to roughly 37% of its original value

Question 20

What is the equation for the energy stored on a capacitor?

Answer 20

E = 1/2QV, where Q and V are the initial values

NOT E=QV

Question 21

Why is E=QV NOT used for the energy stored on a capacitor?

Answer 21

E = 1/2QV is used because the potential difference drops as the capacitor discharges, the energy per unit charge falls during discharge. Energy is lost to resistance in the circuit.
The average is found be using 1/2

Question 22

What are the units of the time contant, RC

Answer 22

s^-1

Question 23

What are the units of -t/RC

Answer 23

No units
(you cannot raise a number to the power of something which has any units)

Question 24

What does the area under the curve represent on a charge/time graph?

Answer 24

The charge stored/moved

Question 25

What is the total capacitance for two capacitors in series?

Answer 25

1/CT = 1/C1 + 1/C2

Question 26

What is the total capacitance for two capacitors in parallel?

Answer 26

CT = C1 + C2

Question 27

For capacitors in series, is the charge stored on each capacitor the same?

Answer 27

yes

Question 28

For capacitors in series, does increasing the number of capacitors increase the capacitance?

Answer 28

Yes

Question 29

When can multiple capacitors behave as a single capacitor?

Answer 29

When they are connected in parallel