If I is the current flowing through a conductor of resistance R in time t, then the quantity of charge flowing is, q = It. If the charge q, flows between two points having a potential difference V, then the work done in moving the charge is = V. q = V It.
Then, electric power is defined as the rate of doing electric work.
Electric power is the product of potential difference and current strength. Since V = IR, Power = I2R
Electric energy is defined as the capacity to do work. Its unit is joule. In practice, the electrical energy is measured by watt hour (Wh) or kilowatt hour (kWh). 1 kWh is known as one unit of electric energy. (1 kWh = 1000 Wh = 1000 × 3600 J = 36 × 105 J)
Mention of expression for power loss :
In case charges moving freely through the conductor under the action of electric field, their kinetic energy would increase as they move with a steady drift velocity. This is because of the collisions with ions and atoms during transit. During collisions, the energy gained by the charges thus is shared with the atoms. The atoms vibrate more vigorously, i.e., the conductor heats up. Thus, in an actual conductor, an amount of energy dissipated as heat in the conductor during the time interval ∆t is,
∆W = I V ∆t
The energy dissipated per unit time is the power dissipated
P = ∆W/∆t and we have,
P = I V
Using Ohm’s law V = IR, we get P = I2 R = V2 /R
as the power loss (“ohmic loss”) in a conductor of resistance R carrying a current I. It is this power which heats up, for example, the coil of an electric bulb to incandescence, radiating out heat and light.
