### 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.