Capacitors Flashcards
What’s a capacitor
Two parallel conducting plates separated by a dielectric
What’s capacitance
The charge stored per unit potential difference
What’s the unit for capacitance
Farads
How can you find capacitance using a charge, of graoh
It’s the gradient
What’s the uses of capacitors
- smoothing unwanted variations in the voltage in circuits
- back up power supplies
- timing circuits
What kind of energy is stored in a capacitor
Electrical potential energy
What happens to current, p.d and charge through a charging capacitor
- current decreases exponentially
- voltage and charge increase exponentially
What happens to current, p.d and charge through a discharging capacitor
They all decrease exponentially
What is the time constant
The time taken for charge, p.d and current of a discharging capacitor to decrease to 37% of their fully charged value
What affects the time taken to charge and discharge a capacitor
- capacitance (charge and p,d)
- resistance (affected by current)
How long roughly does it take for a capacitor to discharge
5RC
How long does it take for everything to half
0.69RC
What is permittivity
A measure of how difficult it is to create an electric field in a material
What is relative permittivity
The ratio of the permittivity material to free space
What happens during permittivity
When an electric field is applied the polar molecules line up creating their own electric field which opposes the capacitor
What happens if you have a larger permittivity
This reduces the resultant electric field and therefore reduces the potential difference needed to charge a capacitor so increasing the capacitance
How does a capacitor work
- close the switch and current flows
- electrons flow to the plate on the negative terminal making it negative
- electrons repeal other electrons on the other plate causing it to be positive
- an equal and opposite charge is built up
- as charge build within the plates electrostatic repulsion makes it harder for electrons to be deposited so current falls, finally to 0
