Chapter 21 Capacitance

Question 1

What is a capacitor? What is it made from?

Answer 1

It is an electrical component that stores charge. Made from two plates separated with an insulator material.

Question 2

What is capacitance? What is the equation?

Answer 2

It is the coulombs stored per unit of potential difference

Question 3

What is the unit of capacitance? And how big is one unit?

Answer 3

The Farad

1 farad is very large

Question 4

What is the total capacitance for capacitors in parallel?

Answer 4

The sum of the capacitance of individual capacitors

Question 5

What is the equation for capacitors in series?

Answer 5

1/C1 + 1/C2 + 1/C3 = 1/CT

Question 6

Why do capacitors in series combine like they do?

Answer 6

Because the PD over each capacitor is equal to the total PD. Capacitance is proportional to Potential difference.

Question 7

How do capacitors work?

Answer 7

Electrons are pushed onto a metal plate. The more join on, the harder they repel each other so adding more electrons requires more energy. This causes energy to be stored

Question 8

How can the energy stored be calculated from a graph of potential difference and charge??

Answer 8

It is the area under the graph

Question 9

What is the equation of work for a capacitor?

Answer 9

ΔW = V x ΔQ

Work = voltage x change in charge

Question 10

What are the 3 equations for work done on a capacitor??

Answer 10

W = 1/2QV
W = 1/2V^2C
W = 1/2xQ^2/C

Question 11

How does the potential difference of a capacitor vary whilst it discharges??

Answer 11

Exponentially

Question 12

What is the equation for the discharge of a capacitor involving voltage?

Answer 12

V = V0e^-(t/CR)
V = Current voltage
V0 = Starting voltage at time=0
t = time since V0
C = Capacitance
R = Resistance

Question 13

What is the equation for the discharge of a capacitor involving current and charge?

Answer 13

I = I0e^-(t/CR)
Q = Q0e^-(t/CR)

Question 14

What is CR in the equation: V = V0e^(t/CR) mean?

Answer 14

CR = time constant for the rate of 'decay' for a capacitor
C = Capacitance
R = Resisitance

Question 15

What happens if the time of discharge is the same as the charge constant? what does this show?

Answer 15

V = V0e^-(t/CR)
V = V0e^-(CR/CR)
V = V0e^-1
V = 0.37V0
So the time constant is equal time that 63% of the charge has dissipated. Instead of a half life, it is like a two thirds life.

Question 16

What is the equation for discharging a capacitor through a resistor?

Answer 16

ΔQ / Δt = -Q / CR

Question 17

What does the graph of potential difference against time for a charging capacitor look like?

Answer 17

The mirror image of exponential. The rate of charge at the beginning is large and then the rate gets smaller. Starts steep and then flattens out

Question 18

At any time, V0 = …..

Answer 18

...V0 = Vr + Vc
where:
V0 = Voltage at start
Vr = Voltage across the resistor
Vc = Voltage across the capacitor

Question 19

What is the equation for charging a capacitor?

Answer 19

Vc = V0(1-e^-(t/CR))

Question 20

What are some uses for capacitors?

Answer 20

Motherboards
Backup power
Rectifier circuites

Question 21

How are capacitors used in rectifier circuits?

Answer 21

They are used to smooth out the ripple in the DC current produced