Capacitors Flashcards
Capacitance definition
The charge stored per unit potential difference (between the plates)
What does a greater capacitor mean
the greater the energy stored in the capacitor
What does the capacitance equation mean
ratio of the charge stored by the capacitor to the potential difference between the plates
Describe how a dielectric works in an electric field (including when no charge is applied and when charge is applied) and its effect on capacitance
- A dielectric is made up of many polar molecules
- These are molecules that have a ‘positive’ and ‘negative’ end (poles)
- When no charge is applied to the capacitor:
There is no electric field between the parallel plates and the molecules are aligned in random directions
- When there is a charge applied:
One of the parallel plates becomes positively charged and the other negatively charged hence an electric field is generated between the plates (from positive to negative)
The negative ends of the polar molecules are attracted to the positive plate and vice versa
This means all the molecules rotate and align themselves parallel to the electric field
- The opposing electric field reduces the overall electric field, which decreases the potential difference between the plates
- Therefore, the capacitance of the plates increases
Permitivity def and what it means for a dielectric
Permittivity is the measure of how easy it is to generate an electric field in a certain material
the permittivity is how well the polar molecules in a dielectric align with an applied electric field
What is the definition for relative permitivity (also known as the dielectric constant)
The ratio of the permittivity of a material to the permittivity of free space
Describe why energy is stored on a capacitor
- When charging a capacitor, the power supply pushes electrons from the positive to the negative plate
- It therefore does work on the electrons and electrical energy becomes stored on the plates
What happens to the potential difference across a capacitor as the amount of charge increases
The potential difference across the capacitor increases
Why does potential difference on a capacitor increase with charge
- At first, a small amount of charge is pushed from the positive to the negative plate, then gradually, this builds up
- Adding more electrons to the negative plate at first is relatively easy since there is little repulsion
- As the charge of the negative plate increases ie. becomes more negatively charged, the force of repulsion between the electrons on the plate and the new electrons being pushed onto it increases
- This means a greater amount of work must be done to increase the charge on the negative plate and so the p.d across the capacitor increases
When does capacitor charging stop
When no more electrons can be pushed onto the negative plate
Draw a current-time graph for a capacitor charging
Explain the current-time graph for a capacitor charging
- At the start of charging, the current is large and gradually falls to zero as the electrons stop flowing through the circuit
- When no more electrons can be pushed onto the negative plate, the charging stops
Draw the voltage-time graph for a capacitor charging
Explain the voltage-time graph for a capacitor charging
Since an equal but opposite charge builds up on each plate, the potential difference between the plates slowly increases until it is the same as that of the power supply
Draw the charge-time graph for a capacitor charging