Capacitors Flashcards
What is capacitance?
It is defined as the charge stored per unit PD.
What is a capacitor?
It is an electrical component made up of two conducting plates separated by a gap ora dielectric
What a farad?
1F=1C/V
What are some of the uses of capacitors?
Smoothing circuits back up power supplies timing circuits pulse-producing circuits tuning circuits filter circuits
How is electrical energy stored in a capacitor?
Charge builds up on the plates of the capacitor.
The energy stored is equal to the work done to force charge onto the plate
How can the energy stored be calculated graphically?
Energy stored in the capacitor is equal to the area under a graph of charge stored against PD across the capacitor
How much energy is stored and wasted in a capacitor?
50% of the energy supplied is stored
50% is wasted due to resistance in the circuit
Describe what happens once a capacitor starts charging
Once the switch is closed, current flows and electrons flow to the plate connected to the negative terminal of the dc power supply and a negative charge builds.
Electrons are repelled to the positive terminal.
An equal but opposite charge builds up on each plate causing a PD between the plates.
As the charge builds up, it is harder for more electrons to be deposited.
When the PD across the capacitor is equal to the PD across the power supply, the current falls to zero
Describe what happens as a capacitor is discharging
The current flows in the opposite direction from the charging current
As the PD decreases, the current decreases as well
The current, charge and PD decreases exponentially with time
What is permittivity?
It is a measure of how difficult it is to generate an electric field in a certain material
What is relative permittivity?
It is the ratio of the permittivity of a material to the permittivity of free space
Why is a dielectric used in capacitors?
It increases the capacitance of the capacitor by increasing the amount of charge stored
Describe the motion of molecules in a dielectric when no charge is applied
A dielectric contains polar molecules
When there is no charge stored in the capacitor there is no electric field, therefore the molecules are orientated in random directions
Describe the motion of molecules in a dielectric when a charge is applied
When a charge is applied, an electric field is generated.
The negative ends o the molecules are attracted to the positively charged plate and vice versa
All the molecules rotate and align themselves with the electric field
How do dielectrics increase the charge stored?
The molecules in the dielectric have their own electric fields which oppose the applied electric field of the capacitor. The larger the permittivity, the larger the opposing field
This reduces the overall electric field which reduces the PD needed to charge the capacitor
Therefore the capacitance increases
