Capacitance Flashcards
What is the capacitance of a capacitor?
C = Q/V, the charge stored per unit potential difference.
What is the symbol and units for capacitance (C)?
F, Farads, 1 CV^-1
What is the energy supplied by a power supply to charge a capacitor?
E = QV
What is the relative permittivity of a dielectric?
The ratio of the charge stored with the dielectric to the charge stored without it; C / C0 = Q / Q0 = e1 / e0
What do dielectrics do?
Increase the capacitance of a capacitor.
How do dielectrics work?
Contain polar molecules that align themselves with the electric field between the plates, creating a ‘counter’ electric fields that partially cancels electric field caused by the charge. Reduces the P.D across capacitor, so more charge can be stored per volt.
What is er if there is no dielectric?
1
What do graphs of charge, pd or current against time for discharging capacitors show?
Exponential decay
What is the time constant of a capacitor?
RC
What do the symbols represent in the capacitor discharge equation?
t is time, e is a constant, R is resistance, C is capacitance. V0/Q0/I0 is initial voltage/charge/current
What is the capacitor discharge equation?
V = V(0) x e^-t/RC
How do you find the RC?
RC is the time taken for the charge, current and P.D to fall to 1/e of their original value.
How long does it take to ‘fully discharge’ a capacitor?
5 time constants.
What is the half-life of a capacitor, and how does it relate to RC?
Half-life is time taken for charge, current and p.d to half, half-life = 0.69 RC
How can you find the time constant from a graph?
RC is equal to -1/gradient for graphs of lnV, lnQ etc. against time
