Capacitance Flashcards
What is Capacitance
The charge per unit voltage stored by a capacitor
What is a Capacitor and what is it made up of
- 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
What is the equation for Capacitance
C = Q / V
Capaciatance (F) = Charge (C) / Voltage (V)
What is the Unit for Capacitance
Farad
What are the Uses of Capacitors
- A power supply (Short period of Time)
- Power circuits to smooth signals
What is Permittivity
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)
What is Relative Permittivty
The permittivity of the dielectric
What is Relative Permittivty also known as
The Dielectric Constant
What is the Equation for Capacitance in terms of Relative Permittivty
C = A εo x εr / d
Capacitance = Area between plates x Permittivty of Free Space x Permittivty of Dielectric / Distance
How do Dielectric Materials work
- 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),
What is a Polar Molecule
Molecules with one end which is positive and one which is negative
What are the Factors Effecting Capacitance
- Area between plates
- Distance between parallel plates
- Permittivty of Dielectric
How do you Calculate Capacitance in Series and Parallel Circuits
In Parallel -
C = C1 + C2 + C3
In Series -
1/C = 1/C1 + 1/C2 + 1/C3
(Opposite of Resistance Equations)
Describe the graph of Charge against Volatage of a Capacitor
Gradient = Capacitance
Area = Energy stored in Capacitor

What are the Equations for Energy Stored in a Capacitor
- 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
Describe the graphs of Charging a Capacitor

Describe the graph of Discharging a Capacitor

Explain what happens when a capacitor is charged
- 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
Explain what happens when a capacitor is discharged
- 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
What are the Equations for Charing and Discharging Current in a Capacitor
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
What are the Equations for Charging and Discharging Voltage in a Capacitor
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)
What are the Equations for Charging and Discharging Charge in a Capacitor
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)
What is the Time Constant
The product of Resistance and Capacitance