Capacitance

Question 1

What is Capacitance

Answer 1

The charge per unit voltage stored by a capacitor

Question 2

What is a Capacitor and what is it made up of

Answer 2

• A device that stores charge
• It is made up of two conducting parallel plates separated by a thick layer of insulating material called a dielectric
• When the capacitor is connected to a source of power, opposite charges build up causing a uniform electric field to be formed

Question 3

What is the equation for Capacitance

Answer 3

C = Q / V

Capaciatance (F) = Charge (C) / Voltage (V)

Question 4

What is the Unit for Capacitance

Answer 4

Farad

Question 5

What are the Uses of Capacitors

Answer 5

• A power supply (Short period of Time)
• Power circuits to smooth signals

Question 6

What is Permittivity

Answer 6

A measure of the ability to store an electric field in the material (The bigger the permittivty the more charge is needed to generate an electric field)

Question 7

What is Relative Permittivty

Answer 7

The permittivity of the dielectric

Question 8

What is Relative Permittivty also known as

Answer 8

The Dielectric Constant

Question 9

What is the Equation for Capacitance in terms of Relative Permittivty

Answer 9

C = A ε_o x ε_r / d

Capacitance = Area between plates x Permittivty of Free Space x Permittivty of Dielectric / Distance

Question 10

How do Dielectric Materials work

Answer 10

• A dielectric are made of polar molecules
• Usually, when there is no electric field, these molecules are arranged in random directions, however when an electric field is present these polar molecules will move and align themselves with the field
• The negative ends will be rotated towards the positive plate of the capacitor and the positive ends to the negative plate; each molecule has its own electric field, the strength of which depends on the dielectric’s permittivity, which will now oppose the field formed by the capacitor, reducing this field.
• Due to this, the potentail difference required to charge the capacitor decreases (as electric field strength is decreased),

Question 11

What is a Polar Molecule

Answer 11

Molecules with one end which is positive and one which is negative

Question 12

What are the Factors Effecting Capacitance

Answer 12

• Area between plates
• Distance between parallel plates
• Permittivty of Dielectric

Question 13

How do you Calculate Capacitance in Series and Parallel Circuits

Answer 13

In Parallel -

C = C1 + C2 + C3

In Series -

1/C = 1/C1 + 1/C2 + 1/C3

(Opposite of Resistance Equations)

Question 14

Describe the graph of Charge against Volatage of a Capacitor

Answer 14

Gradient = Capacitance

Area = Energy stored in Capacitor

Question 15

What are the Equations for Energy Stored in a Capacitor

Answer 15

• E = 1/2 x Q x V

Energy = 1/2 x Charge x Voltage

• E = 1/2 x C x V²

Energy = 1/2 x Capacitance x Voltage²

• E= 1/2 x Q² / C

E = 1/2 x Charge² / Capacitance

Question 16

Describe the graphs of Charging a Capacitor

Answer 16

Question 17

Describe the graph of Discharging a Capacitor

Answer 17

Question 18

Explain what happens when a capacitor is charged

Answer 18

• Once the capacitor is connected to a power supply, current starts to flow and negative charge builds up on the plate connected to the negative terminal
• The electrons are repelled by the negative charge building up on the initial plate, therefore these electrons move to the positive terminal and an equal but opposite charge is formed on each plate, creating a potential difference
• As the charge across the plates increases, the potential difference increases but the electron flow decreases due to the force of electrostatic repulsion also increasing, therefore current decreases and eventually reaches zero

Question 19

Explain what happens when a capacitor is discharged

Answer 19

• The current flows in the opposite direction, and the current, charge and potential difference across the capacitor will all fall exponentially
• Meaning it will take the same amount of time for the values to halve

Question 20

What are the Equations for Charing and Discharging Current in a Capacitor

Answer 20

Charging -

• I = I₀ x e ^-t/RC
• Current = Initial Current x e ^{-Time / Time Constant}

Discharging -

• I = I₀ x e ^-t/RC
• Current = Initial Current x e ^{-Time / Time Constant}

Question 21

What are the Equations for Charging and Discharging Voltage in a Capacitor

Answer 21

Charging -

• V = V₀ x (1 - e ^-t/RC)
• Voltage = Initial Voltage x ( 1 - e ^{-Time / Time Constant})

Discharging -

• V = V₀ x e ^-t/RC
• Voltage = Initial Voltage x e ^{-Time / Time Constant})

Question 22

What are the Equations for Charging and Discharging Charge in a Capacitor

Answer 22

Charging -

• Q = Q₀ x (1 - e ^-t/RC)
• Charge = Initial Charge x ( 1 - e ^{-Time / Time Constant})

Discharging -

• Charge = Charge₀ x e ^-t/RC
• Charge = Initial Charge x e ^{-Time / Time Constant})

Question 23

What is the Time Constant

Answer 23

The product of Resistance and Capacitance