OCR A-Level Physics/Fields, Particles and Frontiers of Physics/Capacitors

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The circuit symbol for a capacitor

Capacitors are simply devices capable of storing electrical charge, and so capable of acting as a cell for a short period of time while discharging. They are made up of two metal plates, separated by an insulator, as illustrated by their circuit symbol (shown on the right).

Capacitance[edit | edit source]

Capacitance - Amount of charge stored per unit potential difference

Capacitance has the symbol C and the unit for capacitance is Farad, F and it is usually expressed with prefixes because capacitance is usually very small. from this definition, the equation can be defined as

However you will see it in the formula booklet as

Total Capacitance[edit | edit source]

The equations for finding the total capacitance are similar to the equations for total resistance.

For parallel we use

For series we use

Energy in Capacitors[edit | edit source]

=== P.d stored in a capacitor is proportional to the charge stored in it. If this was plotted, it would be a straight line through the origin. On a V against Q graph, the area under the graph is the energy stored by the capacitor. This is an area of a triangle where the base is charge and the height is p.d From this we can derive another equation. ===

Charging and Discharging[edit | edit source]

When a capacitor charges or discharges, it does so exponentially. This means that when charging, it takes the same amount of time to double and when discharging, it takes the same amount of time to halve.

Charging Capacitors[edit | edit source]

When a voltage is applied across a capacitor, electrons from one metal plate travel around the wire in the circuit to the other plate. By definition, the electrons can not travel across the insulator. For this reason, electrons are lost from one plate, which develops a positive charge, and added to another plate, which accumulates a negative charge.

Discharging Capacitors[edit | edit source]

To discharge a capacitor, connect the charged capacitor to a resistor or an electrical component without a power supply. The current flows through the wire and the electrons reach equilibrium when the capacitor is fully discharged. The potential difference across the plates is zero. The time taken for the capacitor to discharge depends on:

  • Capacitance of the capacitor.
  • Resistance of the circuit.

Capacitance affects the amount of charge stored and the resistance affects the current flowing in the circuit. Due to capacitors discharging exponentially, we have the equation for voltage.

However the can be applied to charge also.

For this reason, the formulae booklet gives the formula as

Modelling decay[edit | edit source]

For a discharging capacitor, the p.d. across the capacitor and the p.d. across the resistor sum up to zero, according to Kirchhoff's Second Law. Therefore;

Using Ohm's law and the capacitance equation, we derive the following;

Current is defined as the rate flow of charge therefore;

Rearrangement and calculus;

Arriving at the discharge equation;

Time Constant[edit | edit source]

Time constant, , is the time it takes for a capacitor to discharge by 37% of Q0. The symbol , is the Greek letter 'tau'.
The time constant is also the time it takes a capacitor to charge by 63% of Q0 The equation we are given for the time constant is
If we substituted




Parallel Plate Capacitor[edit | edit source]

The capacitance of a capacitor depends on both, the separation between the parallel plates and the area of overlap between the plates. Experimental results have shown that . The constant of proportionality in this case is the permittivity of free space which gives the equation:

This is only true when the dielectric is a vacuum. Realistically, a vacuum will not be used so we use a relative permittivity for different dielectrics.

depends on the dielectric. (You do not need to learn these values for the exam, they will be provided)

vacuum 1 (by definition)
air 1.0006
perspex 3.3
mica 4.0
barium titanate 1200

The general equation for a parallel plate capacitor is: