Capacitance Flashcards
What is capacitance?
Charge stored by a capacitor per unit potential difference.
What is a capacitor?
A capacitor is an electrical component which stores charge. It is made up of two conducting parallel plates with a gap in between them - they are separated by an insulating material called a dielectric.
What happens when a capacitor is connected to a source of power?
Opposite charges build up on the two parallel plates causing an electric field to be formed.
What is permittivity?
A property that dielectrics have, it is the ability to store an electric field in the material.
What is the relative permittivity?
Ratio of permittivity of the dielectric to the permittivity of free space.
What is a dielectric formed up of?
Polar molecules - one end is positive and one end is negative.
Explain what happens to the polar molecules when an electric field is present.
They will move and align themselves with the field. Each molecule has its own electric field, the strength of this field depends on the dielectric’s permittivity. This will oppose the field formed by the capacitor, therefore reducing it. Potential difference required to charge the capacitor therefore decreases and capacitance increases.
How do we calculate energy stored by a capacitor?
Area under charge-potential difference graph.
Explain capacitor charge.
Once the capacitor is connected to a power supply, current begins to flow and negative charge builds up on the plate connected to the negative terminal. On the opposite plate, the electrons are repelled by the negative charge and therefore the electrons move to the positive terminal. An equal but opposite charge is formed on each plate creating a potential difference. As charge increases, potential difference increases, but electron flow decreases due to the forces of electrostatic repulsion. Current decreases and eventually becomes zero.
How do we charge a capacitor?
Connect it to a circuit with a power supply and resistor.
Draw out the charge graphs.
I-t, V-t, Q-t
How do we discharge a capacitor?
Connect it to a closed circuit with just a resistor.
Draw out the discharge graphs.
I-t, V-t, Q-t
What happens when the capacitor is discharging?
Current flows in the opposite direction. Current, charge and potential difference will all fall exponentially
