ZIMSEC O Level Combined Science Notes: Electrical Power

  • Electric power is the rate of doing work or the rate of of using energy
  • Electric power is the rate which an electrical device is converting electrical energy into heat or light or any other form of energy e.g kinetic energy in an electric motor
  • When current flows through a wire some of it lost as heat energy
  • If the wire’s resistance is high the wire becomes very hot
  • For example a water heating element or an electric bulb
  • A lot of electrical energy is lost as heat energy
  • The amount of heat energy produced by a wire can be calculated from the electrical energy used
  • The electric energy used can be calculated by the following formula:
  • Electrical energy used=volts x amps x seconds
  • E=VIt
  • \dfrac{\text{Work done}}{\text{time taken}}=\dfrac{\text{Volts x Amps x Seconds}}{Seconds}
  • Electrical power=voltage x  current
  • \text{P=VI}
  • It is measured in Watts
  • The formula can thus be expressed as:
  • watts=volts x amps
  • But from Ohm’s law we know that:
  • \text{V=I x R}
  • Note that we have changed the subject of the formula in the equation:
  • R=\dfrac{V}{I}
  • If we substitute the value for V in the formula above we get
  • \text{P=I x (RxI)}
  • This can be further simplified into:
  • P=I^{2}R
  • This formula is used to calculate how much power is lost due to resistance in Transmission lines used by Power companies such as ZESA.
  • The resistance of the transmission wires lead to energy being lost during the process in the form of heat energy
  • See the calculations section to see examples of electrical equation calculations in action

To access more topics go to the Combined Science Notes page.


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