Charge & Current (Edexcel IGCSE Physics: Double Science)

• Electric current is defined as

The rate of flow of electric charge

• Current is measured in units of amperes or amps (A)
  
  • 1 amp is equivalent to a charge of 1 coulomb flowing in 1 second, or 1 A = 1 C/s

• This means the size of an electric current is the amount of charge passing through a component each second

• Current flows
  • when a circuit is formed e.g. when a wire connects the two oppositely charged terminals of a cell
  • from the positive terminal to the negative terminal of a cell

Charge flows from the positive terminal to the negative terminal

Measuring current

• Current can be measured using an ammeter
• Ammeters must be connected in series with the component being measured

An ammeter can be used to measure the current around a circuit

• The wires in an electric circuit are made of metal because it is a good conductor of electric current
• In the wires, the current is a flow of negatively charged electrons

In metal wires, the current is a flow of negatively charged electrons. When a voltage is applied, electrons flow through the lattice of metal ions

• Current, charge and time are related by the equation:

charge = current × time

• Where:
  • Q = charge, measured in coulombs (C)
  • I = current, measured in amps (A)
  • t = time, measured in seconds (s)

• The current, charge and time equation can be rearranged with the help of the following formula triangle:

Current charge time formula triangle

Formula triangle for the charge, current and time equation