current & voltage

• Electric current is caused by the movement of negative charge (electrons) through a conductor.

  • Electron flow: negative → positive
  • Conventional current: positive → negative (this is the defined direction used in circuits)

• Current is the rate of flow of charge:

  

  • Where:
    • I = current (A)
    • Q = charge (C)
    • t = time (s)
  • Charge is measured in Coulombs (C).
  • Current is measured in Amperes (A).

Electric circuits may use direct current (DC) or alternating current (AC).

• DC flows in one direction only (e.g. batteries).
• AC changes direction periodically.

The number of cycles per second is the frequency, measured in hertz (Hz).

Mains electricity typically has a voltage of about 230 V and a frequency of 50 Hz.

• Voltage is the energy transferred per coulomb of charge.
• Unit = Volt (V)
• The supply voltage is called the e.m.f (electromotive force).

Potential difference means the difference in electrical energy between two points.

Voltage allows electrons to transfer energy into:

• Heat
• Light
• Movement
• Sound

Work done = energy transferred.

In a circuit: