Capacitors Flashcards
What is a capacitor?
A device that can temporarily store energy in the form of electrical charge accumulated on their plates
What are the two parallel plates speedster by in a capacitor ?
A insulating layer ( dielectric ) typically air
Define capacitance :
The charge stored per unit potential difference measured in Farads (F)
What does the gradient of a Q against v graph give ?
The capacitance of the capacitor
How do capacitors work ?
When a capacitor is connected to a source of potential difference , electrons flow from the negative terminal of the cell onto one metal plate of the capacitor making it negatively charged. This causes electrons from the other plate to flow into the positive terminal of the cell, making that plate of the capacitor positively charged.
What does the dielectric do ?
It prevents any electron/ charge flow between the plates as the dielectric is an insulating material
How is charge conserved throughout the circuit ?
If a certain amount is charge flows into any point the same amount will flow out , so current is the same at all points
When does the flow of electron/ charge stop in a capacitor ?
When the potential difference across the capacitor is equal to the emf. The current in the circuit falls to zero and the capacitor is now fully charged
How do capacitors work ( discharging ) ?
When the power supply is disconnected the capacitor retains its charge when it’s disconnected from the cell. When the capacitor is connected to a component such as a lightbulb electrons flow from the the negative plate to the positive plate briefly lighting the lamp. When no more current flows the capacitor is fully discharged
What is meant by a capacitance of 370 microfarads ?
The capacitor stores charge of 370 micro C per volt of of across its plates
What is permittivity?
The ability of a material to hold electric charge. It is a measure of how easy it is to generate an electric field in a certain material
What is the relative permittivity ?
The ratio of permittivity of a material to the permittivity of free space
What are polar molecules?
Molecules with a positive and a negative pole
What is a dielectric made up of ?
Polar molecules
What is the alignment of molecules when there is no charge applied to the capacitor?
As their is no electric field between the parallel plates so the molecules are aligned in random directions
What is the alignment of the molecules when a charge is applied to the capacitor?
An electric field is produced between the plates . As a result the negative end of the polar molecules are attracted to the positive plate .. aligning themselves parallel to the field
What produces a greater permittivity ?
The larger the opposing electric field from the polar molecules in the dielectric the greater the permittivity
What happens to your the capacitance as the opposing electric field increases ?
The overall electric field is reduced therefore decreasing the pd between the plates resulting in an increase in the capacitance
What are the factors affecting the capacitance of a parallel plate capacitor?
- area of the plate ( greater the area the higher )
- the greater the distance the lower the capacitance
- the permittivity of the dielectric between the plates
What is work done / energy stored in the capacitor ?
When charging capacitor the electrons are pushed from the positive plate to the negative plate. As more electrons are pushed into the negative plate the plate gets more negatively charged increasing the electrostatic force of repulsion . More work is required to overcome come the repulsive force and move electrons to the negative plate
On a c against v graph of V what does the are under the graph ?
The energy stored
How can you discharge a capacitor ?
Disconnecting the power supply and discharge through a resistor ( rc ) circuit
What affects the rate at which a capacitor discharges ?
The resistance the higher - the longer to discharge
