S-Cool Revision Summary

S-Cool Revision Summary


Charge is a property of certain particles. A particle with charge will experience a force in an electric field (or in a magnetic field if the charge is moving).

Charge is either positive or negative. Objects with a similar charge will repel. Objects with opposite charges will attract.

Charge is measured in coulombs, C. The amount of charge on an object can be found using a coulomb meter.

An electron always has a negative charge of -1.6 x 10-19 coulombs. Protons have an equal amount of positive charge. One coulomb is equal to the charge on 6.25 x 1018 electrons, which is a serious number of electrons.

The Conservation of Charge

It is not possible to destroy or create charge.

You can cancel out the effect of a charge on a body by adding an equal and opposite charge to it, but you can't destroy the charge itself. That's the Principle of the Conservation of Charge.

Static Electricity

Static electricity is caused by the transfer of electrons from one object to another. Normally neutral atoms can lose or gain electrons to become either positively or negatively charged. These charged atoms are called ions.

Static electricity is never caused by the movement of protons.

The easiest way to charge an object with static electricity is by using friction. The Van de Graaff generator uses friction to charge up a metal dome.

Current Electricity

Current electricity is about moving charged particles. If you allow the charge that builds up in static electricity to flow, you get a current.

Current is the rate of flow of charge; it is the amount of charge flowing per second through a conductor.

The equation for calculating current is:

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I = current (amperes, A)

Q = charge flowing past a point in the circuit (coulombs, C)

t = time taken for the charge to flow (seconds, s)

How can you get the Charge to Flow?

Well, first you need to have a conductor for it to flow through and then you need to attract or repel the charged particles to make them move. The amount of attracting or repelling you do is measured in volts and is called the voltage or the potential difference (p.d. for short).

Work is being done on these charged particles to make them move, so the voltage is a measure of the amount of energy that is provided per coulomb of charge.

1 volt = 1 joule per coulomb.

The equation for calculating voltage is:

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V = voltage (volt, V)

W = amount of energy (joule, J)

Q = charge (coulomb, C)

Circuit Rules

As the charged particles flow around a circuit they don't get used up; it is the energy that the charged particles carry that decreases as they move around the circuit.

Voltage changes as the charge moves around the circuit.

There are two main types of circuits you need to know about and each has two rules that make calculations simpler:

Series circuits:

Current Electricity diagrams

In a series circuit...

  • the current is the same all the way around the circuit.
  • the voltage is divided between the components in the circuit.

Parallel circuits:

Current Electricity diagrams

In a parallel circuit...

  • the current divides to travel along each loop.
  • the voltage is the same across each loop.

Conventional Current

Originally scientists believed that it was positively charged particles that flowed in circuits and so circuits are always labelled with the current flowing from the positive to the negative terminal of a cell in a circuit. We call this current the conventional current. The electrons are actually flowing in the opposite direction!

Conventional current is the flow of positive particles. All references to current in diagrams and questions at A-level refer to conventional current, unless it's specifically stated otherwise in the question.

Measuring Current and Voltage:

To measure current we use an ammeter. It is placed in series in a circuit to measure the amount of charge flowing through it per second. (You can compare it to a turnstile counting people into a stadium.)

To measure voltage we use a voltmeter. It is placed in parallel to compare the potential at two different points, either side of a component. It can then measure the potential difference, or voltage across the component.

Drift Velocity

You will now already know that current is a measure of the amount of charge moving per second. This means that current is dependent on:

  • the speed at which charged particles are moving.
  • the charge they are carrying.
  • the number of charged particles that are moving.

Charged particles do not travel in a straight line through a conductor, because they collide with other particles in the material. We therefore use the average speed the particle travels at along the conductor. This is called the drift velocity.

Current can be calculated using the equation:

I = vAnq


I = current (amperes, A)

v = drift velocity (m/s)

A = cross-sectional area of the conductor (m2)

n = charge density (m-3) This is the number of charge carriers that can move per m3

q = charge on each charge carrier (coulombs, C)

Comparing Materials

Different materials will have different values of n, the number of charge carriers per m3.

Good conductors such as metals have the most charge carriers. Semiconductors have about 1 x1010 times fewer charge carriers than metals. At low voltages insulators have no free electrons so that a current is unable to flow.

Conductors and Insulators

Metals are good conductors (poor insulators). Electrons in the outer layers of metal atoms are free to move from atom to atom. So if one end of a piece of metal is made positive, the electrons will be attracted towards it and because they are free, they can move towards it.

Static charge only builds up on insulators. These are materials that will not allow the flow of charged particles (nearly always electrons) through them. Insulators are materials made from atoms that hold onto their electrons very strongly. The voltage across an insulator has to be extremely high before an electron is given enough energy to free itself and move through the material.


Semi-conductors have far fewer free electrons than metals so do not conduct as well. However, if they are given energy electrons are able to free themselves from their atom and flow, which greatly reduces the resistance of the material. Some semi-conductors are light sensitive, as the light energy is able to free the electrons. There are about 5 naturally occurring semi-conductors.

Solids, Liquids and Gases

Although in circuits we deal with electrons carrying charge, in liquids and gases other particles are also able to carry charge, such as ions in the process of electrolysis.


Current, charge and voltage

Q = It

W = QV


Current, charge and voltage

Q = charge, C

I = current, A

t = time, s

W = work, J

V = potential difference, V