# Calculating area

1. Apr 12, 2005

### suppo

Im a first year electrical apprentice and I cant work out the area for resistance. The formula is R = PL/A :eg 20mm by 9mm conductor the answer has to be in meters squared can you show me the formula please?

2. Apr 12, 2005

### dextercioby

Is that a rectangle,or an ellipse???I think you mean

$$R=\rho\frac{l}{S}$$

Daniel.

3. Apr 12, 2005

### chroot

Staff Emeritus
Are you just asking how to find out how many meters-squared an area 20 mm by 9 mm is?

If so, just convert each length into meters (0.020 m x 0.009 m) and multiply them.

- Warren

4. Apr 13, 2005

### suppo

A question I have for homework is find the resistance of a 500 meter length of copper conductor having a cross sectional area of 20mm by 9mm? I know the resistivity of copper is 1.72 x10-8 and L is given 500, the A I think is 1.8x10-4. So R=PL/A !.72x10-8 x 500 / 1.8x10-4. The answer I get is 4.7 but the answer I'm supposed to get is 0.133 . Can you tell mr where Ive gone wrong?

5. Apr 13, 2005

### dextercioby

Again,check your arithmetics.The surface is okay.

Daniel.

6. Apr 13, 2005

### Gokul43201

Staff Emeritus
I get 0.0477 ohms (this number is also within 10% of a number generated from the american wire gauge tables, so I believe it is correct).

Are you sure you have written down the dimensions correctly ?

7. Apr 14, 2005

### suppo

The actual question is. A copper conductor is 200 meters long and has a rectangular cross section of 12mm by 8mm. It's resistance is 0.01 ohm. Find the resistance of 500 meters of a copper conductor having a cross section of 20mm by 9mm?

8. Apr 14, 2005

### dextercioby

That's something else.

$$R=\rho\frac{l}{S}$$

Use this for the first conductor to find $\rho$ and then use this resistivity to find the resistance of the 2-nd conductor.

Daniel.

9. Apr 16, 2005

### suppo

I did try that Daniel but I still cant arrive at the answer the course notes gives . I think my calculations for area isn't right. Is 20mm by 9mm 20+9=29 x 10-3squared =2.9x10-5 or is it 20x9=180x10-3squared 1.8x10-4. Or is there another way?

10. Apr 16, 2005

### dextercioby

$$20 \mbox{mm}\cdot 9 \mbox{mm}=180 (\mbox{mm})^{2}=1.8\cdot 10^{-4}\mbox{m}^{2}$$

That's the area.

Daniel.

11. Apr 16, 2005

Thanks mate!