11: Capacitors Flashcards
What is Capacitance?
The charge stored by a capacitor per unit potential difference
What is the equation for capacitance?
C = Q/V
What is a capacitor?
An electrical component which stores charge
What is a capacitor made up of?
Two conducting parallel plates with a gap between them which may be separated by an insulating material known as a dielectric
What happens when you connect a capacitor to a source of power?
- Opposite charges build up on the two parallel plates causing a uniform electric field to be formed
What is permittivity (ε)?
A measure of the ability to store an electric field in the material
How can you work out relative permittivity (εr)
- aka dielectric constant
- εr = ε/ε0 (permittivity of free space)
How can you calculate the capacitance of a capacitor?
C = A ε0 εr/d
What is a dielectric formed of?
Polar molecules, which are molecules with one positive end and one negative end
What happens to molecules in the dielectric when there is no electric field?
The polar molecules are arranged in random directions.
What happens to molecules in the dielectric when there is an electric field present?
- The polar molecules will move and align themselves with the field
- Positive ends rotate to the negative plate of the capacitor
- Negative ends rotate to the positive plate
- Each molecule has its own electric field strength, which now oppose the field formed by the capacitor, reducing its field
- The potential difference required to charge the capacitor decreases, causing capacitance to increase
What is the electrical energy stored by a capacitor (E)
The area under a graph of charge against potential difference
what is the equation to work out electrical energy stored by a capacitor?
E = 1/2 QV
or
E = Q^2/2C
how do you charge a capacitor?
Connect it in a circuit with a power supply and a resistor
What happens when a capacitor is charging?
- current starts to flow and negative charge build up on the plate connected to the negative terminal
- on the opposite plate, electrons are repelled by the negative charge building up on the initial plate, therefore electrons move to the positive terminal and an equal but opposite charge is formed on each plate, creating a potential difference
- As charge across the plates increases, potential difference increases but electron flow decreases due to the force of electrostatic repulsion also increasing, therefore current decreases and reaches zero.
How do you discharge a capacitor?
Connect it to a closed circuit with just a resistor
What happens when a capacitor is discharging?
The current flows in the opposite direction, and the current, charge and potential difference will fall exponentially
What is the time constant?
-The product of resistance and capacitance
- The value of time to:
– discharge a capacitor to 1/e (0.37 of its initial value)
– charge a capacitor to (1 - 1/e) (0.67 of its initial value)
What is the equation to work out the current when charging or discharging a capacitor?
I = I0 x e^-t/RC
What is the equation to work out the potential difference when charging a capacitor?
V = V0 x (1 - e^-t/RC)
What is the equation to work out potential difference when discharging a capacitor?
V = V0 x e^-t/RC
What is the equation to work out charge when charging a capacitor?
Q = Q0 x (1 - e^-t/RC)
What is the equation to work out charge when discharging a capacitor?
Q = Q0 x e^-t/RC
What is the time to halve?
The time taken for I, Q or V to discharge to half the initial value
What is the equation to work out time to halve?
T1/2 = 0.69RC
