• Power is defined as the rate of energy transfer or rate of doing work. Power is expressed by the product of voltage and current. From the equation (1) and {2), we represent power measured in watts as 

P=v\;\times\;i\;=\;\frac{dw}{dq}\times\frac{dq}{dt}=\frac{dw}{dt}                      (3)

  • The energy measured in joules can be expressed by 

w=\int_0^tp\operatorname dt                      (4)

  • Example: Find the power in watts if the applied constant voltage is 100 V and the current flowing in the circuit is 0.5 A.

p= vi = 100\times0.5 = 50W