circular mills?

[angellla]

i have question need help!a conductor has a resistance of 00.475 ohms and a cross sectional area of 2500.000cmil.what is the length of this conductor (in meters)if its "rho factor is 42 (3.281feet=1 metre)the length in meters?

[Roger]

Angella I see no reason we cant help you solve your problem. If this is homework can you explain what it is you dont understand about finding the right equation and maybe we can work this out so we can all learn.

[angellla]

well i am pretty sure i have the equation r=pl/a transposed to l=r times a over p

[Roger]

You have the right formula so maybe its a units problem. What units are provided for resistance and rho?

[angellla]

l=(r).475 ohms and the cross section is 2500.000cirmils which i think it should be mm squared?and my rho is 42 times 1.66 times 10 to the power of negitave 8 ohm metre

[mdshunk]

Given that:

R is the resistance of the conductor in Ohms
A is the cross sectional area in m2
l is the length of the wire in meters
p is the resistivity of the material in Ohm(meters)

and given that:

R=pl/A

You are correct that l=(R*A)/p

Now, solving for l:

is impossible. You're missing the value for "p". This varies for each conductor type. For example, gold would have a lower value than lead or zinc, because it is a better conductor than they are.

[angellla]

i have p it is 1.66 times 10 to the power of 8 negitave never dealt with circular mills can i do it straight across the table now?


.475 times 2500.000 is that it for the top? divide by

42 times 1.66 times 10 to the power of 8 negitave?

[angellla]

my cross sectional area is in cirmils?HOW to convert

[angellla]

i think i got the right anwser 8.628 m?

[mdshunk]

Angela, you're stressing me out here. It's been a long time since junior high algebra. Anyhow, the circular mil is a measurement of the cross section of the conductor. One circular mil is equal to the cross section of a wire with a diameter of 0.001 inches.

It's value is:

pi/4 x 10 to the power of −6 inches squared

which can also be expressed as:

10 to the power of −6 circular inch

or it can be expressed as:

5.067074 × 10 to the power of −10 meters squared

Sounds like it's the math that tripping you up and not the electricity. To convert circular mils approximately to meters squared (m2), multiply by 5.066 x 10to the -10. Conversely, multiply by 1.974 x 10 to the 9th. To convert circular mils approximately to centimeters squared (cm2), multiply by 5.066 x 10 to the -6. Conversely, multiply by 1.974 x 10 to the 5th.

[Roger]

Angella you need to covert circular mils to square meters or It looks like your rho is 1.66 x 10 to -8 x 42 ohm meters. There are 1.66 x 10 to -8 ohm-meters per ohm-cmil/ft. So you have 69.72 cmil/ft. So the equation is
(.475 ohm)(250000cmil)/69.72= L(feet)......1703ft/3.28ft per meter gives you a conductor length of 519.280 meters.

Note: I think your area given is really 250 Kcmil or 250000 circular mils. Double check this.....2500.000 is a strange number. If you are correct then just substitute.

I'm not sure how you got your previous answer can you show us?

[mdshunk]

Yet another example in a long list of reasons why I hate the metric system. If the whole world would stop using the metric system and do things with proper measuring units like we do in the US, life would be much easier.