# Ohms Law

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#### mechie

##### New Member
Ohms Law

The relationship between current(I), voltage(V or E), and resistance(R) was discovered by a German scientist named Georg Ohm. This relationship is named Ohms Law in his honor. Ohm found that the current in a circuit varies directly with the voltage when the resistance is constant. Ohm varied the voltage across the resistance and measured the current through it. In each case, when he divided voltage by the current, the result was the same. In short, Ohms Law, which can be stated as, "The current is directly proportional to the voltage and inversly proportional to the resistance".

Written as a mathematical expression, Ohms Law is

Resistance(R)=voltage(V)/current(I) or R=V/I

The above equation allows you to determine the value of the resistance when the voltage and the current are known.

Of course, Ohms Law can be rearranged to solve for either current or voltage. The rearranged relationships are

current(I)=voltage(V)/resistance(R) or I=V/R

and

voltage(V)=current(I)*resistance(R) or V=IR

Here is a list of equations for power, voltage, current, and resistance

Power(W)= EI, I(squared)*R, E(squared)/R

Voltage(V)= IR, P/I, (sqrt)PR

Current(I)=E/R, P/E, (sqrt)P/R

Resistance(R)= E/I, E(squared)/P, P/I(squared)

Herbymcduff

**broken link removed** a good artical on ohm's law.

MOD EDIT: raybo,Please try to by polite!

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hmmm, i dont know what makes your posts more irritating;

how you jam everything together and make it hard to read,
or how you just whine and scream at people for anything and everything.

help me out on that one. what do you think?

People are passionate about the things they love. (Or passionate about hating the things they hate.)

i always used the triangle way

_v__
I(x)R

and cover over the one you want to find

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I never understood the triangle method. It was always easier for me just to remember V=IR and use algebra which has been burned into my brain. THere is another triangle method I used though...but I forget what it was and what it was used for. dknguyen said:
I never understood the triangle method. It was always easier for me just to remember V=IR and use algebra which has been burned into my brain. THere is another triangle method I used though...but I forget what it was and what it was used for. if you look at the bad pick i drew, with a divide being the line under v, then just cover over the one you want to find eg cover over the v in that drawing and whats left is IxR cover the I and you got V/R etc captaincaveman said:
if you look at the bad pick i drew, with a divide being the line under v, then just cover over the one you want to find eg cover over the v in that drawing and whats left is IxR cover the I and you got V/R etc i dont think there is any need of some trialngular method to remember the ohm's law.probably the easiest and the simplest of all the formulae

dknguyen said:
I never understood the triangle method. It was always easier for me just to remember V=IR and use algebra which has been burned into my brain. THere is another triangle method I used though...but I forget what it was and what it was used for. i too go with him.i never understood it.its just complicating an easy thing

I think what he says in the triangle method is some what same as ohms law. What he meant by covering is that you write down the triangle method and just cover the one you want to find and the remaining is the formula to find it. For example if you want to find V then cover V and the rest is your formula. I think he touches the nose the other way round. Anyway thanks CAPTAIN.

the triangle method is how i was taught and remember it, also use a triangle for power, amps and voltage.

Its my preferred way, but im sure everyone is taught or learns it differently.

But I Think Ohms Law Is Not V=ir But It Is The Statement That Voltage Is Directly Proportional To Resistance Because It Is Said That Semiconductors Do Not Obey Ohms Law But Obey V=ir Can Someone Explain This

Semi conductors obey ohms law the same as every other material in the universe does. All materials have varying conductances at different temperatures and under various circumstances. The semi conductors popularized in electronics just do so to a significantly greater degree than 'common' materials do.

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Semi-conductors are not conductors which do not conduct very well.

I know that I am slightly misquoting you. Sorry, just trying to stress the point.

Semiconductors have a totally different characteristic to a resistor.
Ohms law dosn't apply.

Regards
Eric

Yes it's totally different than a resistor, but it still follows Ohm's law. PN junctions due to their molecular interactions cause otherwise conductive materials to exhibit 'virtual insulation' behaviors due to internally created electric fields which prevent electron flow between the two layers under various circumstances and to difference degrees. No they don't behave linearly, but at any given instant of time a semi conductor has definable ohmic state. Basically in a semi conductor the V and I parts of the circuit cause a variable R. It's dynamic, not fixed, but in the end it still follows ohms law, there's just a whole lot of other math tacked on with it.

You are quite right, I am sure your explanation makes it clear to a student.

I was concerned that a student/newbie would think he could use a ohm meter
etc, to measure the 'resistance' of his semi's.

Once had a univ student on a mid summer work experience, he was trying to find the labs 'megger' [insulation tester].
Asked why, he said he wanted to check the gate input resistance of some FET's he was using, he had been
told it could be megohms!.

Regards
EricG

That's a good way of turning a mosfet into a smoke generator, single shot of course =)

hi all
v=IR is always valid for any material for instance. that is
V(t) = r I(t) hold for any material : r is a function of time
in other words
:delta:V = r :delta:I

Nice JavaScript Ohms Law Calculator 