Current, Voltage and Power

  • Electricity is the flow of electric charge. We can describe the flow of electric charge in several ways. These include the quantities Current, Voltage and Power.


  • Current (I) is the rate of flow of Charge Carriers, such as electrons. Current is usually thought of as moving in the direction of positive charge, so from the positive power supply to the negative. However, since in metals it is electrons that carry electric charge, the actually flow is opposite to the way in which we think of it.
  • Current it the the amount of Charge, Q that passes a point in a set time, t. It is measured in Amps (A), and charge is measured in Coulombs (C). Since Amps are SI base units, Coulombs are defined as A×s, As.


  • Voltage (V) or Potential Difference (p.d.) is a measure of the Energy transferred per Charge Carrier between two points.
  • Voltage is the Energy, E per Charge, Q. Voltage is measured in Volts (V), which is defined as one Joule per Coulomb. Voltage can be defined in base units as Kgm2s-3A-1.


  • Power (P) is the rate of Energy transfer. It is measured in watts (W), where one watt is defined as one Joule per Second. Hence watts can be expressed in base units as Kgm2s-3
  • From this definition of Power, we can substitute the algebraic definitions above to produce a variety of other formulae, including ‘Power = Current × Voltage
  • Ohm’s Law states that ‘Voltage = Current × Resistance’. We can use this to produce two more definitions of Power.
Written by Sam Adam-Day.