Calculating Electric Current & Charge (CIE A Level Physics)

• Current can also be defined as the charge passing through a circuit per unit time
• Electric charge is measured in units of coulombs (C)
• Charge, current and time are related by the following equation:

Q = It 

• • Q = charge (C)
  • I = current (A)
  • t = time (s)

• In a conductor, current is due to the movement of charge carriers
• These charge carriers can be negative or positive, however the current is always taken to be in the same direction
• In conductors, the charge carrier is usually free electrons
• In the image below, the current in each conductor is from right to left but the charge carriers move in opposite directions shown by the direction of the drift speed v
  • In diagram A (positive charge carriers), the drift speed is in the same direction as the current
  • In diagram B (negative charge carriers), the drift speed is in the opposite direction to the current

Current in a Current Carrying Conductor

The charge carriers move in opposite directions shown by the direction of the drift speed v.

• The drift speed is the average speed the charge carriers are travelling through the conductor. You will find this value is quite slow. However, since the number density of charge carriers is so large, we still see current flow happen instantaneously
• The current can be expressed in terms of the number density (number of charge carriers per unit volume) n, the cross-sectional area A, the drift speed v and the charge of the charge carriers q

I = Anvq

• • I = current (A)
  • A = cross-sectional area (m²)
  • n = number density of charge carriers (m^-3)
  • v = average drift speed of charge carriers (ms^-1)
  • q = charge of each charge carrier (C)

• The same equation is used whether the charge carriers are positive or negative