Capacitors

Question 1

What is the function of a capacitor

Answer 1

To store electric charge

Question 2

What does a capacitor consist of

Answer 2

2 metal plates separated by a layer of electrical insulator - a dielectric

Question 3

Define Capacitance

Answer 3

The charge stored per unit potential difference across the plates

Question 4

Formula for capacitance

Answer 4

C=Q/V

Question 5

Describe the movement of electrons when a capacitor is charging

Answer 5

• The plates contain a large number of electrons moving freely within a lattice of positive metal ions. The number of free electrons is balanced by the positive charge of the metal ions
• Once attached electrons flow onto one metal plate giving it an excess of electrons making it negatively charged
• An equal number of electrons flow off the other plate to the positive cell making that plate positively charged
• As more electrons accumulate on the negative plate, they oppose further electrons flowing onto it
• Once the pd across the capacitor= EMF the flow of electrons stops and the capacitor is fully charged

Question 6

What is the unit of capacitance

Answer 6

Farad

Question 7

What is 1 Farad equal to

Answer 7

1CV^-1

Question 8

What does the gradient of a Charge vs Pd graph represent

Answer 8

Capacitance

Question 9

What does the area under a Charge vs Pd graph represent

Answer 9

Energy stored

Question 10

What does the capacitance of a parallel plate capacitor depend on

Answer 10

The area of the plates, the separation of the plates and the type of dielectric

Question 11

3 formulas for energy stored

Answer 11

1. E= 1/2QV
2. E= 1/2CV^2
3. E=1/2Q^2/C

Question 12

Describe the relationship between capacitance, area and plate separation

Answer 12

-Capacitance is directly proportional to the area of the overlap of plates,A
-Capacitance is inversely proportional to the plate separation,D
CαA/D

Question 13

Why is energy stored only 1/2QV, not just QV

Answer 13

because 1/2QV is always wasted in the wires

Question 14

What properties must dielectrics have and why

Answer 14

Good electrical insulator to prevent the flow of electrons between plates

Question 15

Define relative permittivity

Answer 15

The factor by which the electric field between 2 charges is decreased by the presence of the dielectric, relative to a vacuum. Also equal to the factor by which capacitance is increased when using that dielectric compared to a capacitor with a vacuum between its plates

Question 16

What is relative permittivity also known as

Answer 16

The dielectric Constant

Question 17

What is a polar molecule

Answer 17

A molecule that has a partial positive charge in one part of the molecule and a partial negative in the other, although overall the molecule is neutral

Question 18

Describe and Explain how the use of a dielectric increases capacitance

Answer 18

• The materials used as dielectrics contain polar molecules
• Before the capacitor is charged the molecules have random orientations, but on experiencing the electric field they rotate and align themselves with the field
• The polarisation of the dielectric creates an internal electric field, which partially cancels the electric field created by the charge on the plates, so reducing pd across capacitor
• To reurn pd to its original value requires the addition of more charge onto the plates
• Therefore the overll effect is to increase the amount of cahrge a capacitor can storefor a given pd, hence increasing its capacitance

Question 19

What is present between the charged plates

Answer 19

A uniform electric field

Question 20

When are capacitors particularly useful

Answer 20

When a high power output is required for a short time

Question 21

How does the resistance effect the charging of a capacitor

Answer 21

The greater the resistance of the resistor, the more slowly the charging/discharging takes place

Question 22

Why is the current always greatest just as the switch is closed

Answer 22

Because no charge has built up on the capacitor, therefore the pd across the resistor is equal to the EMF. This means the rate of flow of charge is at its greatest, as charge builds up on the capacitor electrostatic repulsion means the rate of flow of charge decrease

Question 23

The graph of charging current verus time is a ………… curve

Answer 23

Exponential Decay (meaning the charging current reduces by the same factor in equal time intervals)

Question 24

What is T1/2

Answer 24

0.69RC, This is the time taken for the charge/potential difference/current of a discharging capacitor to reach half of the value it was when it was fully charged

Question 25

Define Time Constant

Answer 25

The time for the charging current to fall to 1/e of its original value, which is approximately 37%

Question 26

What is the time constant equal to

Answer 26

It is the time taken for the charge/potential difference/current of a discharging capacitor to fall to 37% of its value when fully charged
Also, the time taken for the charge/potential difference of a charging capacitor to rise to 63% of its value when fully charged

Question 27

How do you calculate time constant

Answer 27

Resistance x Capacitance

Question 28

What is an estimate for the time it takes to fully charge/discahrge a capacitor

Answer 28

5RC

Question 29

When using log graphs what does the gradient of the straight line represent

Answer 29

gradient=-1/Time Constant

Question 30

If the charging voltage is doubled, how does this affect the total charge stored, the capacitance and the time taken to discharge

Answer 30

• The total charge stored doubles, as V is proportional to Q
• The capacitance wouldn’t change
• The time taken wouldn’t change