What is a capacitor
2 plates with a insulator in between them, used to store charge. When a power supply is connected to them electrons are taken from one plate and place on the other (think of a power supply as an electron pump). So one plate becomes negative and the other positive. The plates now have a pd across them and are storing charge, they can discharge and act like a power supply
What is capacitance
The amount of charge something is able to store per unit of potential difference across it.
units of Farads or CV^-1
Voltage of the capacitor goes up to the voltage of the power supply.
A capacitor is like a bucket with base capacitance and hight of voltage, the volume of water it stores is the charge
Uses of capacitors
Can’t store much charge but can discharge quickly
Ultra capacitors used as back up power supplies
Smoothing capacitors used to smooth out DC voltage
Energy stored in capacitor derivation
As you charge the capacitor one plate becomes negatively charged and you are trying to add more electrons which takes energy. The energy put in is stored as electric potential until you allow the charges to repel.
Energy taken to move the charge against potential difference is E = QV
But as Q changes so does V so use the average V which is 1/2 V.
E=1/2QV
Which is also the area under the Q V graph of a capacitor.
Energy given by the power supply is E=QV meaning that half the energy is wasted to charge the capacitor
Explain permittivity and effect on capacitance
The insulating material between the 2 plates is called a dielectric. It has a high permittivity.
A high permittivity means a small field is created for the same amount of charge, meaning it will be easier to add more charges i.e. a higher capacitance
Explain why higher permittivity decrease the field size
Polar molecules in the dielectric, randomly arranged. When the plates are charged an electric field is produced that causes them to align anti parallel with the field. The positive sides of the polar molecule getting attracted to the negative plate.
The polar molecules have a small field them self this causes their field to cancel out with the field between the plates. Making it easier to add charge
