Capacitance

Question 1

What are capacitors?

Answer 1

Electrical devices used to store energy in electronic circuits, commonly for a backup release of energy if the power fails.
They store electric charge which creates a build up of electric potential energy.
They are made in the form of two conductive metal plates connected to a voltage supply (parallel plate capacitor)

Question 2

Capacitance equation

Answer 2

Capacitance (F) = Charge stored (C) / potential difference (V)

Question 3

Energy stored in a capacitor

Answer 3

The power supply pushes electrons to one of the metal plates. It therefore does work on the electrons and electrical energy becomes stored on the plates

Question 4

Energy stored in a capacitor equation

Answer 4

Energy stored (J) = 1/2 QV

Question 5

Charging graphs:

Answer 5

• The shapes of the p.d. and charge against time graphs are
  identical
• The current against time graph is an exponential decay curve
• The initial value of the current starts on the y axis and
  decreases exponentially
• The initial value of the p.d and charge starts at 0 up to a
  maximum value

Question 6

Discharge graphs:

Answer 6

• The shape of the current, p.d. and charge against time graphs
  are identical
• Each graph shows exponential decay curves with decreasing
  gradient
• The initial values (typically called I0, V0 and Q0 respectively)
  start on the y axis and decrease exponentially

Question 7

Time constant definition

Answer 7

The time taken for the charge, current or potential difference of a discharging capacitor to decrease to 37% of its original value

Question 8

Charging capacitor definition

Answer 8

The time taken for the charge or potential difference of a charging capacitor to rise to 63% of its maximum value

Question 9

Time constant equation

Answer 9

τ = RC