# Calculating resistance

1. Mar 31, 2016

### Enochfoul

Hi,

I am just starting out learning about DC curcuits and I have been presented with a formula in my learning materials to calculate the resistance of a 1km length of 0.6mm diameter copper wire.

The formula is attached as an image with the Answer 60Ω

My question is how would you enter this formula into a calculator as I can't seem to get the answer 60. Do I have to set the calulator to a specific setting? do we I have to break the calculation into different stages?

I don't know whether the format of my notes is messed up or its just me doing something wrong.

Thanks for the help

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2. Mar 31, 2016

### ProfuselyQuarky

I believe that it's a matter of order of operation (you know, putting parentheses in the proper places). I punched in the values and got 60.1252--which is the correct answer rounded.

3. Mar 31, 2016

### ProfuselyQuarky

How did you do it? Could you post what you typed into the calculator?

4. Mar 31, 2016

### Enochfoul

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5. Mar 31, 2016

### ProfuselyQuarky

That's odd. I did the same thing. This is basic arithmetic, so calculator settings should not really matter. What answer did you get? I see that your calculator is in degrees, but radians vs. degrees doesn't matter here at all. What are you're other settings?

6. Mar 31, 2016

### Enochfoul

Its set to Normal Mode in degrees

7. Mar 31, 2016

### ProfuselyQuarky

What was answer did you exactly get? From there, I can work backwards from your answer.

8. Mar 31, 2016

### Enochfoul

3.757825045

9. Mar 31, 2016

### ProfuselyQuarky

Hold on, I'm thinking . . .

10. Mar 31, 2016

### ProfuselyQuarky

The only thing I could think of was that perhaps your calculator was on a different base system, but I don't think so . . . These are my all settings:

Display Digits: Float 6
Angle: Degree
Exponential Format: Normal
Real or Complex: Real
Calculation Mode: Auto
Vector Format: Rectangular
Base: Decimal

Compare this with your calculator. And here's mine, for the sake of it:

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11. Mar 31, 2016

### Enochfoul

Im going to get a calculator app for my phone and give it another try. Thanks for your help at least I know Im not entering it in wrong

12. Mar 31, 2016

### ProfuselyQuarky

You know, I'll try that too. I really want to know what's wrong now!

13. Mar 31, 2016

### ProfuselyQuarky

14. Apr 1, 2016

### sophiecentaur

Ahh the youth of today. I would have thought it quite possible to put down the calculator and work out a simple sum like that on paper.
There's only a Pi (use three or four sig figs), a 60, a 1.7 and a 4 involved.

15. Apr 1, 2016

### ProfuselyQuarky

Don't worry, I don't know about the OP, but I still love paper and No. 2 pencils as much as you do (and if I'm not youth, I don't know who is)

16. Apr 1, 2016

### sophiecentaur

I get the impression that almost everyone is youth - relative to me (and a few other old PF gimmers)
So few people would even consider doing any arithmetic on paper, these days. It's refreshing to read that you would, though PQ.

17. Apr 1, 2016

### axmls

To the OP: In your calculator, you have entered $R = \rho \frac{(L/\pi D^2)}{4}$ instead of $R = \rho \frac{L}{\pi D^2 / 4}$. Notice that the first one gives $R = \rho \frac{L}{4 \pi D^2}$ and the second one (the correct one) gives $R = \rho \frac{4L}{\pi D^2}$

18. Apr 1, 2016

### Merlin3189

Although I'm in SophieCentaur's camp, I got as far as 1700/9π without anything. Then I did reach for a calculator. But when I got down to SC's comment, I tried that with pencil and back of envelope, just to see what happened, and with Pi at 22/7 it comes out at 60.1

But, since you are asking about calculator methods, I'd say your big mistake is to try to put it all into a single calculation. You CAN do that, but as far as I can see, even using a calculator, there is nothing to be gained in time nor accuracy, let alone mistakes, in combining the two calculations. Just work out A, then press reciprocal and enter the rest of your second formula, pausing only to note that 2.8x10-7 m2 for A is about right. If the second expression were more complex, just pop your first answer into a memory and use it wherever is convenient.

D sq x pi / 4 = recip x ρ x L = is one more keystroke than ρ x L / ( D sq x pi / 4 ) = but requires no thought nor planning and provides a small check along the way.

Even now Axmls has pointed out your possible error, I can't tell from your calculator display whether you did what he says or whether you put in something else. I can't read it as Axmls does. But he's probably correct, since your answer is a factor of 16 out.
If someone wrote that expression on paper as shown on your screen, I'd say it was wrong, but my reading of it does not give your answer.

19. Apr 1, 2016

### sophiecentaur

Goooood boy. If I could reach, I'd give you a gold star in your book.

20. Apr 3, 2016

### ProfuselyQuarky

(when I said youth, I meant <18)
Thanks, I just get annoyed when I see 8-year-olds and 9-year-olds with TI-89 calculators doing math homework

I just want to peer over and say, "Umm, 321 plus 447 is called mental math for a reason!!"