Capacitors Flashcards
What is a capacitor?
A device designed to store charge.
How does a capacitor charge?
- Two parallel metal plates placed near each other.
- When connected to a battery, electrons move through the battery and are forced onto one of the plates.
- This makes the first plate become negative.
- On the other plate, an equal number of electrons are repelled making that plate positive.
What is the symbol for a capacitor in a circuit?
Two equal length parallel lines.
What would a charge against pd graph for a capacitor look like?
What can be found from this graph?
A straight proportional line passing through the origin.
Area = work done = energy stored
gradient = capacitance
Define capacitance
The charge stored per unit pd.
C = Q/V
Define a farad
One coulomb of charge stored per volt
C = Q/V
1 F = 1CV-1
What factors affect the capacitance of a capacitor?
- surface area of plates
- Separation of plates
- Dielectric
Define permittivity
Permittivity is a measure of how difficult it is to generate an electric field in that medium.
The higher the permittivity of the material the more charge needed to generate an electric field.
Define relative permittivity
Relative permittivity is the ratio of the permittivity of a material to the permittivity of free space, it is sometimes known as the dielectric constant.
Explain permittivity
- When no charge is stored by a capacitor, no electric field is generated, the polar molecules are randomly aligned.
- When a charge is applied to the plates of a capacitor an electric field is generated between them.
- The negative ends of the molecule are attracted to the positive plate and vice versa.
- This causes the molecules to rotate and align them selves anti-parallel to the field.
- The molecules each have their own electric field, which now opposes the applied electric field of the capacitor.
- The larger the permittivity, the larger the opposing field.
- This reduces the overall electric field between the plates and therefore reduced the p.d. needed to transfer a given charge to the capacitor.
- This increases the capacitance (Q = CV).
When capacitors are placed in series how can the total capacitance be found?
1/CT = 1/C1 + 1/C2 + 1/C3 + ……
When capacitors are placed in parallel how can the total capacitance be found?
CT = C1 + C2 + …..
For a capacitor discharging sketch a graph of voltage against time.
Exponential decay - see diagram
For a capacitor discharging sketch a graph of charge against time.
What can be found from this graph?
Exponential decay - see diagram
Gradient = current
For a capacitor discharging sketch a graph of current against time.
What can be found from this graph?
Exponential decay - see diagram
Area = charge