Electric Current: Calculations (CIE AS 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

Charge equation

Answer: B

Step 1: Write out the equation relating current, charge and time

Q = It

Step 2: Rule out any obviously incorrect options

• • Option A does not contain charge or time, so can be ruled out

Step 3: Try the rest of the options to determine the correct answer

• • Consider option B:

I = 4 / 500 = 8 × 10^–3 = 8 mA

• • Consider option C:

I = 8 / 100 = 80 × 10^–3 = 80 mA

• • Consider option D:

I = 1 / 8 = 125 × 10^–3 = 125 mA

• • Therefore, the correct answer is B

• 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

Conduction in a current-carrying conductor

• 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

Current in a conductor equation

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