Capacitors

Question 1

Capacitance definition

Answer 1

The charge stored per unit potential difference (between the plates)

Question 2

What does a greater capacitor mean

Answer 2

the greater the energy stored in the capacitor

Question 3

What does the capacitance equation mean

Answer 3

ratio of the charge stored by the capacitor to the potential difference between the plates

Question 4

Describe how a dielectric works in an electric field (including when no charge is applied and when charge is applied) and its effect on capacitance

Answer 4

• A dielectric is made up of many polar molecules
• These are molecules that have a ‘positive’ and ‘negative’ end (poles)
• When no charge is applied to the capacitor:

There is no electric field between the parallel plates and the molecules are aligned in random directions
- When there is a charge applied:

One of the parallel plates becomes positively charged and the other negatively charged hence an electric field is generated between the plates (from positive to negative)
The negative ends of the polar molecules are attracted to the positive plate and vice versa
This means all the molecules rotate and align themselves parallel to the electric field

• The opposing electric field reduces the overall electric field, which decreases the potential difference between the plates
• Therefore, the capacitance of the plates increases

Question 5

Permitivity def and what it means for a dielectric

Answer 5

Permittivity is the measure of how easy it is to generate an electric field in a certain material

the permittivity is how well the polar molecules in a dielectric align with an applied electric field

Question 6

What is the definition for relative permitivity (also known as the dielectric constant)

Answer 6

The ratio of the permittivity of a material to the permittivity of free space

Question 7

Describe why energy is stored on a capacitor

Answer 7

• When charging a capacitor, the power supply pushes electrons from the positive to the negative plate
• It therefore does work on the electrons and electrical energy becomes stored on the plates

Question 8

What happens to the potential difference across a capacitor as the amount of charge increases

Answer 8

The potential difference across the capacitor increases

Question 9

Why does potential difference on a capacitor increase with charge

Answer 9

• At first, a small amount of charge is pushed from the positive to the negative plate, then gradually, this builds up
• Adding more electrons to the negative plate at first is relatively easy since there is little repulsion
• As the charge of the negative plate increases ie. becomes more negatively charged, the force of repulsion between the electrons on the plate and the new electrons being pushed onto it increases
• This means a greater amount of work must be done to increase the charge on the negative plate and so the p.d across the capacitor increases

Question 10

When does capacitor charging stop

Answer 10

When no more electrons can be pushed onto the negative plate

Question 11

Draw a current-time graph for a capacitor charging

Answer 11

Question 12

Explain the current-time graph for a capacitor charging

Answer 12

• At the start of charging, the current is large and gradually falls to zero as the electrons stop flowing through the circuit
• When no more electrons can be pushed onto the negative plate, the charging stops

Question 13

Draw the voltage-time graph for a capacitor charging

Answer 13

Question 14

Explain the voltage-time graph for a capacitor charging

Answer 14

Since an equal but opposite charge builds up on each plate, the potential difference between the plates slowly increases until it is the same as that of the power supply

Question 15

Draw the charge-time graph for a capacitor charging

Answer 15

Question 16

Explain the charge-time graph for a capacitor charging

Answer 16

the charge of the plates slowly increases until it is at its maximum charge defined by the capacitance of the capacitor

Question 17

Describe how a capacitor is discharged

Answer 17

• Capacitors are discharged through a resistor with no power supply present
• The electrons now flow back from the negative plate to the positive plate until there are equal numbers on each plate and no potential difference between them
• Charging and discharging is commonly achieved by moving a switch that connects the capacitor between a power supply and a resistor

Question 18

Draw a current-time graph for capacitor discharge

Answer 18

Question 19

Draw a voltage-time graph for capacitor discharge

Answer 19

Question 20

Draw a charge-time graph for capacitor discharge

Answer 20

Question 21

Explain the graphs for Capacitor discharge

Answer 21

• At the start of discharge, the current is large (but in the opposite direction to when it was charging) and gradually falls to zero
• As a capacitor discharges, the current, p.d and charge all decrease exponentially
• This means the rate at which the current, p.d or charge decreases is proportional to the amount of current, p.d or charge it has left

Question 22

What does the rate of capacitor discharge depend on?

Answer 22

• Resistance
• If the resistance is high, the current will decrease and charge will flow from the capacitor plates more slowly, meaning the capacitor will take longer to discharge
• If the resistance is low, the current will increase and charge will flow from the capacitor plates quickly, meaning the capacitor will discharge faster

Question 23

What does the area under a current-time graph for a capacitor mean?

Answer 23

the charge stored for a certain time interval

Question 24

What does the gradient of a charge-time graph for a capacitor mean?

Answer 24

the current at that time

Question 25

Capacitor time constant def (for a discharging capacitor)

Answer 25

The time taken for the charge, current or voltage of a discharging capacitor to decrease to 37% of its original value

Question 26

Capacitor time constant def (for a charging capacitor)

Answer 26

The time taken for the charge or voltage of a charging capacitor to rise to 63% of its maximum value

Question 27

def of half life for a discharging capacitor

Answer 27

The time taken for the charge, current or voltage of a discharging capacitor to reach half of its initial value

Question 28

Draw a graph and diagram for the values of the capacitor discharge equation

Answer 28