# Units

Discussion in 'Homework Help' started by boks, Dec 5, 2009.

1. ### boks Thread Starter Active Member

Oct 10, 2008
218
0
$-4,9\cdot10^{-6}m^3C^{-1}/1,71\cdot10^{-3}\Omega cm=-2,87\cdot10^3cm^2/Vs$

Is this right, or have I made a mistake with the units and/or exponentials?

Jul 7, 2009
1,577
142
Get yourself a copy of the GNU units program. It is one of the most-frequently-used programs on my computer. I especially like that I can go into the units.dat file and define my own custom units.

It indicates a units mistake. Back when dinosaurs roamed and one had to do such things manually, a table like that which was included in Resnick and Halliday (1960's editions) was very handy, as it gave all the common composite units in MKSA units, which enabled one to figure out such things. But the GNU units program is so much more convenient, has thousands of units, and doesn't make mistakes, as far as I know.

Code ( (Unknown Language)):
1.
2. You have: m3 C-1 / ohm cm
3. You want: cm2 / V s
4. conformability error
5.         m3 C-1 / ohm cm = 100 A^3 s^4 / kg
6.         cm2 / V s = 0.0001 A s^2 / kg
7.

3. ### steveb Senior Member

Jul 3, 2008
2,431
469
You are correct, sir.

Jul 7, 2009
1,577
142
Interesting...

Code ( (Unknown Language)):
1.
2. You have: 4.9e-6 m3/1.71e-3 C ohm cm
3. You want: cm2/V s
4.         4.9e-6 m3/1.71e-3 C ohm cm = 2865.4971 cm2/V s
5.
As steveb said, this indicates your conversion was correct. I should have written my first conversion given as

m3 C^(-1) / ohm cm

as the - was being interpreted as subtraction, not the exponent. My bad.

5. ### steveb Senior Member

Jul 3, 2008
2,431
469
That's the difficult thing, isn't it. Even when we find a good computer tool, there is always the doubt that there is a bug, or that we made a mistake in implementation. At the same time, we can make mistakes if we do it out by hand.

Personally, I find that I need to use both the computer tools and the manual methods to be sure I'm right. If they both agree, the odds of lingering mistakes are much reduced. Either one alone is risky. Even double or triple checking hand work is not foolproof. We tend to repeat the same mistakes.

That GNU units program looks like a nice tool for double checking.