Electricity Tutorial 5  Energy and Power in Circuits
Conventional current goes from positive to negative. Electrons carry energy around the circuit; they go from negative to positive. In the early days, physicists didn't know about the electron, which is why they got it all wrong. Correction would require a complex rewrite of the Laws of Physics, a task which noone is likely to be bothered to tackle. So all conventional currents are from positive to negative. All currents are treated as conventional.
We can measure the energy in a circuit by measuring the voltage and the current.
The voltage current graph looks like this:
Suppose a current I flows for t seconds in a component. The charge that flowed led to E joules being dissipated in the component.
We know that:
Q = It
E = QV
So if we substitute Q out of the second equation, we get:
E = ItV
Now
Power = energy
time
So we can write:
P = ItV
t
It doesn't take a genius to see that the term t cancels out to leave us with:
P = IV
Power is measured in watts (W). 1 watt = 1 joule per second
A 12 volt heater takes a current of 3.6 A. It is left to heat up an aluminium block for a period of 45 minutes. How much heat energy is transferred to the aluminium block?  
What current is consumed by a 60 W light bulb operating on the 230 V mains? 
The Heating Effect of a Current
We know that:
V = IR
P = IV
So we can write:
P = I × IR
So it doesn’t take a genius to see that by substituting the second equation into the first, we get:
P = I^{2}R
We know that:
I = V/R
P = IV
So we can write:
P = V × V/R
So it doesn’t take a genius to see that by substituting the second equation into the first, we get:
P = V^{2}/R
The graph looks like this:
The graph shows that if we double the current, we get four times the power, consistent with the idea that P µ I^{2}. If we were to plot P against I^{2} we would get a straight line graph.
Question 3 
A resistor of value 50 ohms is rated at 1 watt. This means that if it has to give out more power than 1 watt, it will start to get hot. What is the maximum current that it can handle? 

Question 4 
The same resistor as in Q 9 is then connected to a 20 volt supply. What power will it dissipate now? What do you think will happen to the resistor? 
The picture shows the heating effect of a current on a resistor!
This was a 33 W resistor connected to a 20 V supply. The current would be 20 V ÷ 33 W = 0.61 A
The power would be 0.61 A × 20 V = 12 watts. Plenty enough to fry a 1 watt resistor.
It is important that we ensure that any current limiting resistors can dissipate the power through them. The above situation could be highly dangerous.