Boys and Girls
I’ve noticed over time there appears to be some confusion regarding electricity and how it all comes together. I have no wish to be regarded as a clever dick … far from it, but I thought I’d throw these little thoughts together in the hope that they may clear up some common misconceptions. Let me state here and now I am no leccie
bright spark and that being the case would welcome correction where deemed necessary.
First off … consider a
water tank up on a stand, with a hose to ground level. At the bottom of the hose at ground level we have pressure, right??
Now one of the primary laws of physics is that energy cannot be created or destroyed, in other words it is merely changed into another form.
So, getting back to the
water tank, we have pressure at the bottom of the hose, this is referred to as potential energy, or in other words it has the potential to do work, such as in a hydroelectric scheme in a bigger picture, where the water pressure (energy) is converted to electricity (energy).
OK you say, what has that got to do with electricity?? Fair enough. Now think of a 10mm hose connected to the tank and also a 20mm hose. Which is going to carry the most water?? …. Correct. Now if you put a pressure gauge at the end of both hoses you would get the same reading .. same head of water, same pressure, get my drift?? Same as if you connected a long hose to your tyre, you’d get the same pressure at the end of the hose as at the valve.
However the 20mm hose will carry far more water, or do far more work (depending on your point of view). So, why is this? The 20mm hose offers less resistance than the 10mm, in this instance it’s less friction losses (an area separate to our discussion, but relevant in the big picture).
Now getting back to electricity, it’s the same story. A battery has potential difference between the two poles … the potential to do work. In this instance it’s called volts, but it means the same thing.
If you hook up 2 wires of greatly differing diameter to the +’ve and
check one against the other, back to the –‘ve with a voltmeter you will get the same result, let’s say 12 volts for the purpose of the discussion. The voltage, or potential difference remains constant, however the capacity to do work (carry current) varies greatly.
Let’s assume a wire has a resistance of 1 ohm/metre. V(voltage)=i(amps)*R(resistance)
So i=V/R … 1 metre of wire at 1 ohm/m means 12/1=12 amps capacity
2 metres of wire means 12/2=6 amps capacity.
The longer the wire the less the capacity to do work, due to losses .. in this instance resistance, rather than friction, as in a water hose.
Now P(power or watts)=V*i. Remembering V stays constant at 12, we get an increasing value of P as i increases. If i=6 amps then P=12*6 = 72 watts, but if 1=12amps then P=12*12 = 144 watts.
Resistance in conductors (wires) reduces significantly with increase in diameter ..
check the size of your battery cables. The bigger the wire, the less the resistance, the greater the capacity to do work.
This will hopefully explain why you need large diameter wires (say 6mm2) for your fridges, to ensure the losses are reduced to a minimum.
Now let’s consider 3 way fridges. The heating element is a large resistance, that’s why it gets hot, much the same as your toaster.
In this instance the value of R is significant. Let’s assume it’s rated at 160 watts, which is about average. P = V*i or i = P/V.
160/12 = 13.33 amps. But let’s say you battery has run down to 10 volts. 160/10 = 16 amps. Because the resistance is fixed and constant, the more your battery discharges, the greater the current draw. A vicious circle until the battery is stuffed.
I hope the above is of interest to some and apologise to those who find all a bit of a yawn.
Cheers