# Ohm's law power formula question

Discussion in 'Electronic Basics' started by ITSME.ULTIMATE, Feb 19, 2006.

1. ### ITSME.ULTIMATEGuest

Ok, using ohm's law, in a DC circuit, we can say that:

power=
(V^2)/R
I^2 * R
V * I

But what if you have a one ohm resistor?

Example, one ohm resistor with 0.25A going through.
V=I*R, so V= 0.25 x 1 = 0.25A

using the formula, power = V*I, you get 0.25W
but using the other formula (V^2)/R, you get 0.0625W
0.0625 != 0.25W

Let's try 4 ohm resistor with 0.5A flowing through:
V=I*R, V= 0.5 x 4 = 2V
P= V*I = 2 x 0.5 = 1W
P=(V^2)/R = (2^2)/4 =1W
P= (I^2) * R = (0.5 x 0.5) * 4 =1W

Why isn't it working and what did I not do right?

2. ### Dan HGuest

First example
V = .25, I = .25 so V x I = .25 x.25 = .0625

3. ### Ross MacGuest

Thats .25V
V does not equal 1 !
P=IE
P=.25^2=.0625W

You just messed up your variables......I assume you are a student and all I
can say is you REALLY need to know these two formula's inside and out. They
will serve you well into the future....good luck....Ross

4. ### John FieldsGuest

---

E = IR = 0.25A * 1R = 0.25V

_not_ 0.25A
---
---
No. In the firtst case:

P = EI = 0.25V * 0.25A = 0.0625 watt

and in the second,

E² 0.25A * 0.25A
P = --- = --------------- = 0.0625 watt
R 1R
---

---
Let's see:

E = IR = 0.5A * 4R = 2V

P = EI = 2V * 0.5A = 1 watt

and also:

E² 2V * 2V
P = --- = --------- = 1 watt
R 4R

and also:

P = I²R = 0.5A * 0.5A * 4R = 1 watt.

Those all come out right, so you made some transcription errors on

5. ### Pooh BearGuest

0.25 Volts actually !
No P=V * I = 0.25 * 0.25 = 0.0625W
Yes - correct. Your first instance was wrong.
No P=V * I = 2V * 0.5A = 1W

Graham

6. ### John FieldsGuest

---
No, he hasn't. He just made a simple error in the first part of his
post which he couldn't reconcile.

This is sci.electronics.basics, where anyone should be able to ask
any questions they might want to, no matter how stupid those
question might seem to be and, with the answers to those questions
in hand, walk away a little more informed than they were when they
first got here without feeling any guilt for their earlier
ignorance.

querant(s) into believing that your judgement of their submissions
matters because they're stupider than you are.

They may or not be true, but if they're asking for help and you
smugly deny it, then what's to be said in your favor?

7. ### Pooh BearGuest

Well, actually he got 'confused' about the value of I.

Ergo my response ! ;-)

Graham