# Voltage and current relation difference between Ohm's law and Power

Discussion in 'General Electronics Discussion' started by bunny, Nov 4, 2015.

1. ### bunny

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Nov 4, 2015
Hi I am new to this forum According to Ohms law, at constant temperature voltage is directly proportonal to current.
But when we come to power formula p=vi which means that V is inversly proportional to I. How can we justify this?

Last edited by a moderator: Nov 4, 2015

111
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Oct 14, 2014
Because resistance and power aren't the same thing.

3. ### bunny

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Nov 4, 2015
Yar i know that both are not same but they relate same things no? and that to a law must be satisfied for all the things. We know that ohms law is valid only for ohmic conductors but power is same formula no?

Last edited by a moderator: Nov 4, 2015

111
43
Oct 14, 2014
I'm not sure I understand the question. Here's an image that shows all of the different ways to relate power, voltage, current and resistance:

What do you believe isn't able to be justified?

hevans1944 and Martaine2005 like this.
5. ### AnalogKid

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Jun 10, 2015
No, it doesn't. There is nothing to justify because your statement is incorrect.

For a constant resistance, voltage is directly proportional to current.
For a constant power, voltage is inversely proportional to current.

ak

Last edited: Nov 4, 2015
6. ### duke37

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Jan 9, 2011
To go one step further, P= V*V/R and P=I*I*R.
These can be derived from Ohms law by substitution.

5,165
1,087
Dec 18, 2013
What do you mean justify?

8. ### GPG

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Sep 18, 2015
Marshall Givens?

9. ### Ratch

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Mar 10, 2013
First of all, Ohm's law is not V=I*R, and all its variations. Regardless of what predominates the electrical literature, a good physics book will tell you that Ohm's law is an electrical property of a material, specifically its electrical linearity. I can provide documentation for that statement if necessary.

So V=I*R is correct, but it is not Ohm's law. It is a definition. R is defined by the amount of current present at a specific voltage (R=V/I). Voltage is the electrical energy density of the charge (joules/coulomb). If V volts are lost to move Q coulombs of charge, then the energy expended is E=V*Q. Power is rate of energy and current is rate of charge. Dividing both sides of E=V*Q by t gives P=V*I.

Ratch

Last edited: Nov 18, 2015