Energy in general is
defined as the capacity for doing work.
Power (P) is the
rate of doing work or the rate of using energy:
P=Work/t=Energy/t,
where t is time. Although casually the terms energy and power are often
used interchangeably we see 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.
For the mathematics of electric energy, power and
power factor, see
Electric
power primer.
