Energy in general is defined as the capacity for doing work.
Power is the rate of doing work or the rate of using energy:
Power=Work/t=Energy/t,
where t is time.
Although casually the terms energy and power are often used interchangeably, technically they have different
meanings. Energy comes in many forms, such as heat, motion, gravitational, and electrical. The SI unit of
energy and work (which are numerically the same) is the
joule. By
definition, work done by a constant force F when it moves an object by distance L in the direction of force is
Work=F×L.
A joule (J) is the work done by a force of one newton for a distance of one meter. This unit is usually used in physics. For different types of energy other physical units are also used. For example, the British Thermal Unit (Btu) is often used to
measure the heat energy or compare fuels. One Btu is the energy needed to heat one pound of water one degree F. The SI
derived unit of power is
watt (W).
Watt is power required to produce or use of one joule of energy per second.
In an uniform electric field with voltage
V over distance
L, the force acting on a change
Q is equal
to
F=V/L×Q (particularly, in the field of 1 volt/meter, force of 1 newton is
acting on one coulomb charge). Substituting this into the above general
expression of power gives the equation for power required to move a charge
Q
in an electric field:
P = F×L/t = V×Q/t.
The rate of charge flow
Q/t
is called electric current
I.
Replacing
Q/t with
I in
the above expression yields a familiar formula for instantaneous value
of electric power:
P=V×I.
In AC circuits voltage and current are often shifted in phase and are not sinusoidal. In such cases the quantity volt-amps is used to describe the apparent power. For basic relationships between various AC power characteristics see
Understanding watts, VA and power factor. For the mathematics of these electric quantities also see
Electric power primer.
