Electrical charge and current

• An electric current is a rate of the flow of charge. Whenever a charge is flowing, there is a current. So an electric current is a measure of the rate of flow of electrical charge over time.

  charge = current X time
• Q = charge (unit: coulomb, C)
• I =current (unit: ampere, or amp, A)
• t = time (unit: second, s)
• For electrical charge to flow through a circuit, the circuit must be closed and at least one of its components must be a source of potential difference.
• In a single closed loop of a circuit, the current is the same at any point
• The free electrons in a circuit fill all wires and components of the circuit all the time, even when the circuit is broken (or open). So, when a potential difference is applied to a circuit, the charges (electrons) will all start flowing along circuit at the same time, wherever they are in the circuit. That is why a light bulb lights up almost straight away after the switch is turned on
• The free electrons move inside the circuit at a speed of about 1cm per minute, so if you had to wait for them to go the battery to the light bulb every time you turned the switch on you would be in darkness for quite a long time
• A charge of 0.24 C flows through a resistor in a circuit over a time of 14s. What is the current in the circuit? Write your answer in milliamps

  I = Q/t

  I = 0.24/14

  = 0.017 A

  0.017 x 1000 = 17 mA