Note: You may or may not (but don't worry!), have come across all of these terms or formulae, it depends on how far your studies have got.

In your course, you might not need to know every formula or have to be able to do all the calculations explained and follow-up questions - that's up to you to find out.

If a UK GCSE student you will find your science course help links via

Find your GCSE/IGCSE science course for more help links to all science revision notes

Most of the practice questions and worked out answers based on the equations explained on this page are found in Part 1.6

V the potential difference

(p.d., commonly called 'voltage') is the driving potential that moves the electrical charge, hence the electrical energy transfer, around a circuit - electrical charge usually carried by electrons.

Potential difference is the work done in moving a unit of charge.

It indicates how much energy is transferred per unit charge when a charge moves between two points in a circuit e.g. between the terminals of a battery.

(See energy transferred further down the page)

The p.d. across any part of a circuit is measured in volts, (unit symbol V).

I the electrical current

The rate of flow of electrical charge in coulombs/second (C/s), measured in amperes (amps, unit symbol A).

The quantity of electric charge transferred in a give time = current flow in amps x time elapsed in seconds

Formula connection: Q = It    I = Q/t    t = Q/I

I = current (A),  Q = electrical charge moved in coulombs (C),  time t (s)

It is better to be able to rearrange a formula rather than using a triangle

R the electrical resistance

Electrical resistance in a circuit, measured in ohms (unit symbol Ω).

A resistance slows down the flow of electrical charge - it opposes the flow of electrical charge.

Formula connection: V = IR     I = V/R     R = V/I 

(This is the formula often quoted as Ohm's Law)

I = current (A),  V = potential difference in volts (V),  R = resistance in ohms (Ω)

It is better to be able to rearrange a formula rather than using a triangle

P is the electrical power of a device

The power delivered by a circuit = the rate of energy transfer (symbol units J/s) and is measured in watts (symbol unit W).

Formula connection: P = IV    I = P/V    V = P/I  

also  P = I²R    R = P/I²   I = √(P/R)   (see also P = E/t below)

P = power in watts (W),  I = current (A),

V = potential difference in volts (V),  R = resistance in ohms (Ω)

It is better to be able to rearrange a formula rather than using triangle like those illustrated below

E is the energy transferred

Energy transferred by the quantity of electric charge by a potential difference of V volts.

energy transferred (joules) = quantity of electric charge (coulombs) x potential difference (volts)

Formula connections E = QV    Q = E/V    V = E/Q

E = energy transfer in joules (unit symbol J)

Q = electrical charge moved (unit symbol C)

V = p.d. in volts (unit symbol V)

Formula connections E = Pt    P = E/t    t = E/P

where P = power (W), E = energy transferred (J), t = time taken (s)

Energy transferred in joules = power in watts x time in seconds

Formula connection: Since E = Pt and P = IV, energy transferred E = IVt

It is better to be able to rearrange a formula rather than using a triangle