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Can you calculate load current using Ohm's Law?

If I had an open circuit with no load like the one pictured here on the left:

http://hyperphysics.phy-astr.gsu.edu/hbase/electri...

And I then calculated Voc. Then I added a load resistor and I had a circuit similar to that which is pictured on the right. Could I use my previously calculated Voc and Ohm's Law to determine the load current through the load resistor? why or why not?

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  • Anonymous
    8 years ago
    Favorite Answer

    Because Voc is affected by the load resistance RL. Consider an extreme case where RL is so low as to be zero Ohms; in other words, a straight piece of wire. In such a case, the the junction of R1 and R2 will be pulled down to zero volts because the straight piece of wire is connected to the zero volts of the battery. Increase the resistance of RL from a straight piece of wire and the voltage will appear. Taking the other extreme, the only time you can expect the output voltage to be Voc when a load is connected is when that load resistance, RL is very much grater than R2.

    Basically, the reason why you can't use Voc is because the calculated value of the LHS assumes R2, but the right hand side replaces R2 with the parallel combination of R2 in parallel with RL, and that will always be slightly less than RL and hence the voltage drop across that parallel combination will also be less.

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  • 8 years ago

    well no, not for the parallel circuit to the right. You could use ohms law in the first schematic, but you see it switches from a series circuit to a parallel in the second. So the rules change. In a series circuit, current is the same throughout and in parallel, the current is divided among resisters. So to get total resistance in a parallel circuit you have to use the the product over sum method. (RxR/R+R) and that will give you total resistance of the two. I hope this makes sence, it's really hard to explain over a computer. But good luck with your first period. It gets easier as you go along and the trade is awesome.

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  • 8 years ago

    You can't as R2 and RL are in parallel in the right hand example. The link you've posted explains it all perhaps the part that isn't clear is that R2 ║ RL means "R2 in parallel with RL"

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