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# Charge and current

Electric current is the rate of flow of electric charge. No current can flow if the circuit is broken - for example, when a switch is open.

An electric current flows when electrons move through a conductor, such as a metal wire. Metals are good conductors of electricity.

Electricity passes through metallic conductors as a flow of negatively charged electrons. The electrons are free to move from one atom to another. We call them a sea of delocalised electrons.

Current was originally defined as the flow of charges from positive to negative. Scientists later discovered that current is actually the flow of negatively charged electrons, from negative to positive. They termed the original definition ‘conventional current’ so as not to confuse it with the newer definition of current.

# Calculating current

The size of an electric current shows the rate of flow of electric charge. You can calculate the size of a current using this equation:

or:

where:

I is the current in amperes (amps), A

Q is the charge in coulombs, C

t is the time in seconds, s

QQuestion

What is the current if 20 C of charge passes in 5 s?

Current = 20 ÷ 5 = 4 A

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Glossary
1. ampere Unit of current, eg the current in the bulb is 4 amps or amperes (A).
2. circuit A closed loop through which charge moves - from an energy source, through a series of components, and back into the energy source.
3. conductor A material which allows charge to move easily through it.
4. coulomb Unit of charge. Eg the charge on an electron is 1.6 × 10-19 coulombs.
5. current Moving electric charges, for example, electrons moving through a metal wire.
6. electric charge The electrical state of an object, which can be positively charged or negatively charged.
7. electrical component A device in an electric circuit, such as a battery, switch or lamp.
8. electron Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons.
9. in parallel In a parallel circuit, the current divides into two or more paths before recombining to complete the circuit. Lamps and other components in these different paths are said to be in parallel.
10. in series Connected to a circuit in such a way that the same current flows through each component in turn. Opposite of in parallel.
11. inversely proportional A relationship between two variables where as one variable increases, the other variable decreases, eg as the volume doubled, the pressure decreased by half.
12. ion Electrically charged particle, formed when an atom or molecule gains or loses electrons.
13. joules The unit of work or energy, written as J.
14. potential difference The potential difference (or voltage) of a supply is a measure of the energy given to the charge carriers in a circuit. Units = volts (V). This is the voltage between two points that makes an electric current flow between them.
15. proportional When two quantities have the same ratio or relative size. For example, current is proportional to voltage if the current doubles when the voltage is doubled.
16. resistance The opposition in an electrical component to the movement of electrical charge through it. Resistance is measured in ohms.
17. volt Unit of voltage. Eg the voltage across the lamp was 6 volts (V).