Power calculations 

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Ohm's Law - revision

Power calculations require a good understanding of Ohm's Law. For revision click here to go to:    OHM'S LAW

You will recall Ohm's law, where E = Volts; I = current in amperes, and R = resistance in ohms. By transposing we get the following: 

Remember: Cover up the value you seek and the formula to get it using the two remaining values is given  

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We can develop a diagram to include "power calculations" too, see the circular diagram below. It looks very complicated but it is just a lot of variations of the same thing! You know many of these already - they appear in our triangle. Here, for convenience, the Power is shown as P. 

There are four sectors, for P, I, E and R. Look at the three relationships from the triangle and locate them on this diagram. 

The  unit of electrical power is the watt (W). 

Power is the rate of doing work. (Remember: With higher power you can cook something faster!) 

Power is dissipated in a resistance - the resistor warms up! 

The relationship between the watt W, volts E, and amperes I, is:  

W = E times I   or   E . I = W.  Find this on the diagram. 

E . I = W  is perhaps the most fundamental of all the "power" relationships. 

To involve resistance (R) in a circuit, we can find a substitute for E or for I. 

For example, W = E . I. But perhaps we don't know E but we know both I and R. 

So we can substitute E = I . R. for E in this W = E . I .expression. 

This now means that W = I . R . I. 

This can also be written as W = I²R. (Say it out loud! Watt = I "squared" R) 

This diagram looks complicated, but what it really tells us is that there are many ways of saying the same thing! All it shows is that the "basic" Ohm's Law formula (I = E/R) can be used to make substitution in the basic formula W = E x I in many different ways! 

Check another case too - for when you don't know I, but you know both E and R and wish to find W! 

Power is dissipated in a resistor 

Remember: When current flows in a resistance, heat is produced. 

(This is how a fuse works! Heat resulting from excessive current melts the metal link in the fuse!) 

  Go here for some practice exercises:   RESISTOR CIRCUITS

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