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When to use I²R, V²/R or VI while calculating Electric Power?

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I came across a question that was asked to me quite a couple of time: When to use I²R, V²/R or VI while calculating power? At first, the question looks simple and as if it needs no formal explanation but that's not the case. Use of any of them will give the correct result provided that you apply it correctly. Correct application is possible only when you understand the type of circuit you are dealing with. Another important point is to use the one which is going to take less time and help you calculate easily. All of the three equations tell the same thing but their usage is subject to the type of problem you're trying to solve. Let us understand when, how an what to use while calculating power in an electrical circuit.
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How can a battery of 12V, 65Amp-hr throw 300Amps to a starter motor in car?

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Ampere-Hour and Ampere difference

A battery in cars or vehicles is used to start a starter motor which a DC motor which helps in cranking the engine. One such car has a battery rated 12V, 65Ampere-hour. But it throws 300Amps. to the motor. How's that possible? Wondering the same, here's the answer.


[image source: supercheapauto.com]

It is because Ampere-hour and Ampere are not the same thing. This is a concept many of us don't have idea about. Even I hadn't before.

Let me make this clear using the same data provided in the question. The battery in question has a capacity of 65Amp-hr. That's abstarct of the fact which says the battery can deliver 65A for one hour. The Ampere-hour rating is the time-based capacity of the battery. So, we can say,

65A can be delivered in 1 hour.
(65×2)=130A can be delivered for 1/2 an hour.
(65×3)=195A can be delivered for 1/3 an hour.

And so on..

So, the battery can easily deliver 300A for (65/300) hour i.e, 13 minutes. So there you go. I leave the calculation upto you. 

I hope you liked the topic. Please do share and support the blog.

 Thanks :)


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