Capacitance revision Flashcards
What is a capacitor? What does it consist of?
A device designed to store charge. Consists of two parallel oppositely charged plates.
What is setup used to test for p.d against charge?
Capacitor connected in series with an ammeter, a variable resistor to keep constant current and a switch. Voltmeter is connected in parallel to the capcitor.
What is capacitance defined as and what is the formula?
C = Q/V. Defined as the charge stored per unit pd. Unit is a farad (F).
What type of energy is stored in a capacitor as it charges?
Electric potential energy, due to electrons being forced onto the plates.
What are the formula for energy stored by capacitor?
E = 1/2QV, 1/2CV^2 and 1/2Q^2/C
What happens to other half of energy which isn’t stored on the capacitor?
Transferred as heat to surroundings due to the resistance.
What is formula for time constant?
T = RC. t in seconds, r is resistance and c is capcitance. Its the time taken for voltage/current/charge to fall to 0.37 of its initial value.
What increases exponentially as capacitor charges and what decreases exponentially?
Current decreases while charge and voltage increases.
What is point of discharging and charging through a resistor in the experiment?
So that it doesn’t occur too quickly
How do we add capacitors together?
When they are in series, use the parallel formula. When they are in parallel, use the series formula.
What is a dielectric used for?
Used to increase the charge stored on the plates of a capacitor. They reduce the electric field in a capacitor as they are made of insulating materials.
What happens to the dielectric as it is placed between the two charged plates?
Dielectric becomes polarised. Electrons are pulled slightly towards the positive plate. So, surface of dielectric near the positive plate becomes negatively charged, and surface of dielectric near negative plate gets a positive charge. Now, positive side of dielectric attratcs more electrons from negative plate and increases the capacitance of the capacitor.
What is definition of relative permittivity?
The ratio of the charge stores with dielectric to the charge stored without a dielectric. Or ratio of charge stored with capacitor to without capacitor.
What do most dielectrics consist of?
Twp strips of aluminium foil separated by a layer of dielectric, all rolled up to increase the surface area. Also spacing between them is very small too to increase capacitance.
