Capacitors Flashcards
What is the definition of capacitance?
amount of charge it is able to store per unit potential difference
Describe the structure of a capacitor and purpose of dielectric in this structure
Consist of two conductive metal plates connected to voltage supply
Dielectric is between parallel plates to ensure charge does not flow freely across plates
Therefore charge builds on plates
Equation for capacitance (relating to definition)
Charge / Potential difference
(Note: Charge stored on plates not charge of capacitor)
Describe the graph for charge against potential difference
Straight line through origin
Shows proportionality
Gradient is capacitance
How do you find the energy stored by a capacitor
Area under graph of charge against potential difference
How much of total energy does capacitor store
Only stores half (Due to equation 1/2 x base x height)
Half of energy is always lost to heat due to resistance
One use of a capacitor
Used to smooth out variations in DC voltage
Absorbs peaks and troughs
Since it discharges slowly
Equation for energy stored on a capacitor
E = 1/2 QV
Half x base x height
What molecules is a dielectric made up of?
Polar molecules
Positive end and negative end
Describe the arrangement of polar molecules when no charge is applied to the capacitor
No charge applied = no electric field between plates
Therefore molecules aligned in random directions
Due to lack of electromagnetic force influencing their direction
Describe the arrangement of polar molecules when charge IS applied to capacitor
When charge is applied it generates an electric field between the plates
Therefore due to the positive and negative charges on the plates, the polar molecule is attracted to its respected charge and thus directs itself to that way
Molecules aligned themselves parallel to electric field due to electromagnetic force of attraction
What is meant by relative permittivity/ dielectric constant
How easy it is to generate and electric field in a certain material
Explain how a larger permittivity will increase capacitance (long)
When electric field present, polar molecules also have their own electric field which oppose the applied electric field == known as opposing electric field
Therefore the overall electric field reduces
Larger permittivity == larger opposing electric field from polar molecules
Therefore decrease in overall electric field = decrease in potential difference between plates
Therefore increase in capacitance
Since Q = CV and Q is constant
Explain how a capacitor charges
Electrons are pulled from the plate closest to the positive terminal of power supply
Therefore plate closest to positive terminal == Becomes positively charged
Electrons then travel around the circuit are pushed onto plate connected to negative terminal
Therefore plate closest to negative terminal == Becomes negatively charged
Explain why when capacitor becomes fully charged current stops
As the negative charge on negative plate builds up == Less electrons can be pushed onto said plate == Due to electrostatic repulsion
Therefore eventually none can be pushed on so flow of electron stops
At this point capacitor is fully charged
