6.1 Capacitors

Question 1

What is a capacitor used for?

Answer 1

To store charge

Question 2

How does a capacitor store charge? (3)

Answer 2

Electrons from the negative terminal give the first plate a negative charge
Electrons on the other side are repelled by the negative charge, giving the other plate a positive charge
The difference in charge causes a potential difference between the plates

Question 3

How are charge and voltage related in capacitors?

Answer 3

Charge is directly proportional to voltage

Question 4

Define capacitance

Answer 4

Charge stored per unit voltage

Question 5

How is capacitance combined in series? (3)

Answer 5

1/C = 1/C1 + 1/C2
Charge stored across each capacitor is the same
The sum of the PDs across the capacitor adds up to the total PD

Question 6

How is capacitance combined in parallel? (3)

Answer 6

C = C1 + C2
PD across each capacitor is the same
Electrical charge is conserved

Question 7

Why can a capacitor store energy? (2)

Answer 7

It is able to do work on other components

As charge stored increases, there is more repulsion on more work must be done to charge the capacitor

Question 8

How is capacitance found experimentally? (6)

Answer 8

Connect a cell, ammeter, capacitor, variable resistor and voltmeter and switch in parallel with the capacitor
Use the variable resistor to choose a fixed value for the current
Open the switch to allow current to pass through the capacitor and start a timer simultaneously
Take voltage readings every 5 seconds, adjusting the variable resistor to keep the current constant
Use Q = It to find the charge at each time interval
Plot charge against voltage and find the gradient of the linear part of the graph to find capacitance

Question 9

When is the current in a capacitor circuit zero?

Answer 9

When the capacitor is fully charged

Question 10

What is an exponential function?

Answer 10

A function which, in equal intervals, the rate at which a quantity increases or decreases is proportional to its value

Question 11

What does the area under a voltage-charge graph represent?

Answer 11

Energy stored

Question 12

Why does work have to be done to store charge in a capacitor? (2)

Answer 12

The electrons moving towards the negatively charged plate experiences electrostatic repulsion so work is done to move it closer to the plate
Electrons being repelled on the other side are attracted to the positive charge on the second plate so work has to be done to overcome the attraction

Question 13

How is a capacitor discharged through a resistor? (4)

Answer 13

Set up a circuit with a cell, capacitor, resistor and voltmeter connected in parallel and a switch in series with the cell and capacitor
When the capacitor is fully charged, open the switch and allow the capacitor to discharge through the resistor
The charge and PD across the capacitor decreases with time, causing the current and PD across the resistor to decrease
When the charge and PD across the resistor are zero, the current in the resistor is also zero

Question 14

Define time constant

Answer 14

The time taken for the PD/current/charge across a discharging capacitor to decrease to e^-1 of its initial value

Question 15

What is the equation used for modelling a discharging capacitor?

Answer 15

ΔQ/ΔT = -Q/CR

Question 16

How is a capacitor charged?

Answer 16

Connect a cell, switch, resistor and capacitor in series
When the switch is closed, the capacitor begins to charge and the PD across it increases from zero
The sum of the PDs across the capacitor and resistor add up to the PD of the cell so as the PD across the capacitor increases. the PD across the resistor decreases
When the capacitor is fully charged, the PD across it is equal to the PD of the cell

Question 17

What are capacitors used for? (2)

Answer 17

Provide back-up power for computers and emergency lighting

Smoothing output voltage of domestic electricity supplies