If necessary, see the page Electrical Power and Energy
before this page. 



Consider the simple circuit below in which an alternating
current generator supplies energy to a light bulb rated at
6V and 6W.
All values of current, voltage and power in this section are assumed
to be r.m.s. values. 

These figures tell us that, to operate at normal brightness, the
bulb needs 1A of current to flow
though it. 



However, let us also consider that the bulb is connected to the
supply through rather long wires and that the resistor R represents
the (unavoidable) resistance of these long connecting cables. 





The 1A of current
must flow through the resistance R and there will therefore
be a voltage across it. 

This means that to light the 6V bulb at its normal bright ness
will require a supply of greater than 6V. 



The energy converted to thermal energy per second by the
resistance R (the wasted power) is given by 



which reminds us that, to keep this waste to a minimum we should
attempt to reduce I as much as
possible. 



Consider the following circuit, which might at first sight seem
a little strange because it suggests that we first step up the
voltage using a transformer and then, at the other end step the
voltage down again! 







However, on closer inspection it becomes
clear that to have the same power in the bulb (6W)
as in the previous situation we will need to have a smaller
current flowing in the primary coil of the step down
transformer. 

If we assume the transformers are 100% efficient then we can
write 



Thus, if V has been stepped up 8× then the
current through R is, in effect, stepped down by the same factor
(for a given power transfer to the bulb). 

So, in this case, to light the bulb to its normal brightness, we
will have 8× less current flowing through R than in the previous
situation and therefore 64 times less power wasted (power
being proportional to I_{2}). 



In the real case of electrical distribution to houses and
factories, the generated voltage is already quite high (thousands of
volts) but it is then stepped up to hundreds of thousands
of volts for transmission with minimum power wastage. 
