chapter 21 - capacitance

Question 1

Describe the construction of a capacitor.

Answer 1

Two conducting plates seperated by an insulator.

Question 2

What is the relationship between charge stored and pd across a capacitor?

Answer 2

Charge is proportional to voltage

Question 3

Describe what happens to the two plates of an uncharged capacitor when a p.d. is applied to it?

Answer 3

Electrons/charge flows off of one plate and flows on to the other causing one to become positive and the other negative.

Question 4

What units are equivalent to a Farad?

Answer 4

CV-1 or coulombs per volt

Question 5

How do you calculate the total capacitance for capacitors in parallel?

Answer 5

Ct = C1 + C2 + C3 + …. Cn

Question 6

How do you calculate the total capacitance for capacitors in series?

Answer 6

1/CT = 1/C1 + 1/C2 + 1/C3….

Question 7

If capacitors are connected in series what quantity is the same for all capacitors?

Answer 7

Charge stored

Question 8

If capacitors are connected in parallel what quantity is the same for all capacitors?

Answer 8

p.d.

Question 9

What is represented by the area under a p.d. - charge graphs?

Answer 9

Electrical energy transferred OR work done.

Question 10

What is represented by the gradient of a charge-p.d. graph for a capacitor?

Answer 10

Capacitance

Question 11

What is the relationship between energy stored and p.d. on a capcitor?

Answer 11

Energy stored is proportional to p.d. squared

Question 12

What is the relationship between energy stored and charge on a capacitor?

Answer 12

Energy stored is proportional to charge squared

Question 13

Out of Q, V and I which variable(s) follow the relationship x = x0 e-(t/RC) when discharging a capacitor?

Answer 13

Q, V and I

Question 14

Out of Q, V and I which variable(s) follow the relationship x = x0 e-(t/RC) when charging a capacitor?

Answer 14

I

Question 15

Out of Q, V and I which variable(s) follow the relationship x = x0 (1 - e-(t/RC) ) when charging a capacitor?

Answer 15

Q and V

Question 16

How do you calculate time constant in a capcitor circuit?

Answer 16

Capacitance x resistance

Question 17

What is meant by the constant-ratio nature of exponential decay?

Answer 17

In equal time intervals the quantity decreases by equal factors.

Question 18

How much charge remains after one time constant when discharging a capacitor?

Answer 18

e-1 OR 0.37 OR 37%

Question 19

How much charge remains after n time constants when discharging a capacitor?

Answer 19

Xo x e^-n

Question 20

What is represented by the gradient of charge-time graph when charging a capacitor?

Answer 20

Charging current.

Question 21

State a two uses of capacitors.

Answer 21

Energy storage e.g. for camera flashes, spot welding…, timing circuits, voltage smoothing, audio filters.