elementary load resistance and load current relationship question.

[zoulzubazz]

hello folks,

this is a very elementary question which has been bothering me. According to ohms law the current through a load decreases as the load resistance increases (assuming constant voltage) and vice versa.

now this would suggest that a short circuit is a higher load than say a 1kW motor which does not make any sense to me.

also while testing current source circuits i have increased the load by increasing the load resistance so was it completely wrong to do this?

if someone could shed some light on this it would be great help.

Thanks very much

 

[albbg]

I don't understand what's your problem. Yes in case of constant voltage increasing the load resistance the current will decrease while decreasing the load resistance the current will increase.

I=V/R

from the delivered power point of view:

P=V²/R

then increasing R, the power will decreases, then the lower the resistance the heaviest the load.

If you test a constant current circuit (as far as I've understood well) the voltage will not be constant but it will vary in order to keep constant the current.

V=R*I

This from the delivered power point of view (considering ideal the source) will be:

P=R*I²

In this case the higher the load resistance the heaviest the load.

 

[Iron_Mike]

The misunderstanding is purely semantic. You are right about Ohm's Law: V = I * R. The word 'load' is ambiguous. Think of it as 'burden', which is more poetic than exact engineering terminology. A short circuit is a bigger burden to carry than any finite load, and no supply can sustain a short circuit, I = V / 0. And yes, you are working backwards in testing your current supply. Lower the resistance to find the max current that can be sourced. Try to make sure you have a fuse or circuit breaker or thermistor or something to prevent short circuiting.
Sounds like you are off to a good start. Keep questioning.

 

[zoulzubazz]

right i get the concept ok now, but this is another thing that's been confusing me, consider a iron box or any load for that matter that heats up, its common knowledge that heat increases electrical resistance. why does heating up of load pull more current than before the load was hot, if resistance is increasing due to heat (is this not contradicting ohms law).

 

[Iron_Mike]

I don't think I agree with you. Yes, in metals, resistivity increases with temperature. So, apply a voltage V across a length of metal with initial resistance R0. Current will = V / R0. As the metal heats, resistance increases to R1 > R0. So current *decreases* somewhat, but is still heating the material. Actually, what you're describing is an interesting differential equation, especially if you also account for heat loss (radiation, convection, etc) from the metal. The engineers who design electric heaters probably have to figure that out. My guess is that an equilibrium point is reached. The metal resistance does not increase infinitely.
This would make a good experiment. Set it up and measure it. Nature always has the last word.