Chapter 11 - Capacitors

Question 1

Explain the composition of a capacitor?

Answer 1

Insulating material (dialetric) in between two metal plates, with wires going from each metal plate

Question 2

Explain how a capacitor is charged? (3)

Answer 2

1) connect up so positive side of capacitor is connected to the positive terminal
2) when switched on, negative terminal pushes electrons onto negative plate
3) tf electrons are repelled from other plate so it becomes positively charged

Question 3

Note about plates?

Answer 3

Equal and opposite charge

Question 4

Area under a current-time graph?

Answer 4

Charge

Question 5

Capacitance equation?

Answer 5

C=Q/V

Question 6

Capacitance unit and si unit?

Answer 6

F - farad

CV^-1

Question 7

Define capacitance?

Answer 7

The charge stored per unit potential difference across a capacitor

Question 8

What happens if you exceed the safe working voltage?

Answer 8

Charge may leak across plates

Question 9

Energy stored in a capacitor equation?

Answer 9

W = 0.5CV^2

Question 10

Investigating E stored by capacitor?

Answer 10

Learn diagram from notes

1) capacitor charges
2) capacitor discharges through joulemeter

Question 11

How to derive W=0.5QV?

Answer 11

1) area under p.d. against charge graph = work done W=QV
2) since it requires more energy for every additional electron, V-Q graph for capacitor is a straight line so W=0.5QV
3) Q=CV tf W=0.5CV^2

Question 12

Capacitance, charge and voltage totals in parallel?

Answer 12

Ctotal = C1 + C2
Qtotal = Q1 + Q2
Vtotal = V1 = V2

Question 13

Capacitance, charge and voltage totals in series?

Answer 13

1/Ctotal = 1/C1 + 1/C2

Qtotal = Q1 = Q2

Vtotal = V1 + V2

Question 14

Comparing capacitors and resistors table?

Answer 14

Notes

Question 15

Capacitor discharge diagram and steps?

Answer 15

Check notes

1) no resistance tf capacitor charges fast
2) flip switch, discharges through resistor

Question 16

3 things that change as charge leaves capacitor and note?

Answer 16

Charge stored by capacitor
Current in circuit
P.d. across capacitor and resistor

ALL DECAY EXPONENTIALLY WITH TIME

Question 17

Define exponential decay graph?

Answer 17

A decaying graph that has a constant-ratio property for a given interval of time

Question 18

5 equations for Q, I and V for a discharging capacitor?

Answer 18

Initial charge: Q0 = V0C
Initial current: I0 = V0/R
Q=Q0e^(-t/CR)
I=I0e^(-t/CR)
V=V0e^(-t/CR)

Question 19

Time constant τ = ?

Answer 19

CR

Question 20

What is the gradient and area under a Q-t graph?

Answer 20

Gradient = current
Area = total charge that has flowed from capacitor

Question 21

Define time constant?

Answer 21

The time taken for the current, charge stored or p.d. to fall to 1/e (approx. 37%) of its initial value

Question 22

3 uses of capacitors?

Answer 22

Camera flashes
Lasers in nuclear fusion experiments
Computers to give them time to save data

Question 23

What do capacitors do?

Answer 23

Charge up in normal use, then gradually discharge when power fails