Capacitors

Question 1

Charge / capacitance / voltage equation

Answer 1

Q = CV

Question 2

Capacitance / permittivity of dielectric / area / distance between plates equation

Answer 2

C = epsilon * area / distance

Question 3

Capacitance unit

Answer 3

Farads

Question 4

Total capacitance (parallel circuit)

Answer 4

C(total) = C1 + C2 + C3 + … + Cn

Question 5

Total capacitance (series circuit)

Answer 5

1/C(total) = 1/C1 + 1/C2 + 1/C3 + … + 1/Cn

Question 6

Charging capacitor voltage equation

Answer 6

V(capacitor) = V0 * (1 - e^(-t/RC))

V(capacitor) = V(t) = p.d. across capacitor at a point in time (volts)
V0 = initial p.d. across a capacitor (volts)
RC = constant tau (seconds)

Question 7

Discharging capacitor voltage equation

Answer 7

V(capacitor) = V0 * e^(-t/RC)

V(capacitor) = V(t) = p.d. across capacitor at a point in time (volts)
V0 = initial p.d. across a capacitor (volts)
RC = constant tau (seconds)

Question 8

Charging + discharging current equation

Answer 8

I = I0 * e^(-t/RC)

Question 9

Charging charge equation

Answer 9

Q(t) = Q0 * (1 - e^(-t/RC))

Question 10

Discharging charge equation

Answer 10

Q(t) = Q0 * e^(-t/RC)

Question 11

Time for capacitor to decrease its charge by 2 (half-life)

Answer 11

T(1/2) = RC * ln2

Question 12

Dielectric definition

Answer 12

An electrical insulator that can be polarised by an applied electric field

Question 13

Capacitance definition

Answer 13

The charge stored per unit p.d.

Question 14

Energy stored by capacitor equations

Answer 14

E = 1/2 QV
E = 1/2 CV^2
E = 1/2 (Q^2)/C

Question 15

Relative permittivity (epsilon r)

Answer 15

Dielectric constant
epsilon r = Q/Q0
epsilon r = C/C0

Question 16

How to increase capacitance

Answer 16

Increase area
Decrease space between plates
Filling space between plates with dielectric which has a relative permittivity as large as possible