# Physics Question

Discussion in 'MCAT: Medical College Admissions Test' started by mm6284, May 9, 2007.

1. ### mm6284 2+ Year Member

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How do you know when to use which:
P= (V^2)/R or P=(i^2) *R
for the power of a resistor?
I expected to get the same answers for both equations but for some reason it didn't work when I was doing ek probs

and also... does anyone know why Vreal<Videal and moderately high pressures.. and can we still sub this lower Vreal into Van der Waals equation?

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3. ### CATallergy 2+ Year Member

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the two equations are the same (V=IR, so plug IR for V in your first equation, and you'll see that they're the same)

to your second question, the space occupied by the particles becomes significant during high pressure, so the effective volume is lower. you probably won't have to use an equation for real gases unless they give you the formula and explain in a passage, but you should generally know what is going on.

4. ### bluesTank Zombie 5+ Year Member

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Use V^2/R if you have a constant voltage, and I^2 R if you have a constant current throughout...I believe that is the determinant. I have also wondered that, and I think thats what my physics instructor said.

5. ### CATallergy 2+ Year Member

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uh, no. these formulas all assume constancy (hence, no calculus required), and if you are looking at the power dissipated across a resistor, I and V have to be specifically the current and voltage across that resistor.

The two equations are exactly the same - there is absolutely no difference between them, if voltage is not constant, then current wouldn't be either.

shrike, pls confirm

6. ### scottj72 2+ Year Member

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Well, i believe it is simply identifying the varibles involved. With these two you need to figure out if voltage or current is used. But there are other formulas for power as well.

P= (V^2)/R
P- power(watts)
V-voltage(volts)
R-Resistance(ohms)

P=(i^2) *R
P- power(watts)
I-Current(amps)
R-Resistance(ohms)

As for the Vreal<Videal you need to keep in mind the assumptions of an ideal gas:
1.The volume of the molecules is considered negligible compared to
the volume of the container in which they are held.
2.We also assume that gas molecules move randomly, and collide in
completely elastic collisions.
3.Attractive and repulsive forces between the molecules are therefore
considered negligible.
At a moderate high pressures the volume of the molecules is still negligible compared to the volume of the free space. However, this changes at very high pressures and Vreal > Videal.
In reality the intermolecular forces are always involved.Now the particles are closer together and attractive/repulsive forces do occur to a greater degree. So now molecules deviate from their straight line path and take longer to reach the walls of container. So fewer collisions and lower velocity mean a lower pressure than ideally expected. This would ten translate to a lower volume than idealy expected.
The Van der Waals equation:
P + an2/V2)(V - nb) = nRT
uses Videal - nb(constant) but that value equals Vreal
Vreal = Videal - nb

Maybe I helped.