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Transmission of Electrical Energy
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 I2).  
   
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.  
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