How do I find the total Resistivity if using AWG 18?

[JeeebeZ]

Pretty easy Circuit question...

So, I am in my first year ENGE Class second week in, and I am given this diagram

Homework Statement

Basically I am given this Diagram (attachment)

And told to find the total Resistivity if using AWG 18

So, i got

P=800W
E=100V
R=E^2/P = 15.125 Ohms

Homework Equations

So for the Resistivity R = p l/a.

The Attempt at a Solution

But I am completely unsure how to get p l & a.

This isn't even a assignment question it was a example done in class on the board, but my instructor just randomly says everything and doesn't explain anything.

So when alls said and done I have

R = 4.13
I = 110/23.5= 4.7 Amps

But absolutly no clue as to how to get it :(

 

[Astronuc]

There should be a table in one's text regarding the sizes of wires in the American Wire Gauge (AWG) - http://en.wikipedia.org/wiki/American_wire_gauge

See if this example helps - http://hyperphysics.phy-astr.gsu.edu/hbase/electric/resis.html#c2

 

[JeeebeZ]

So,
p = resistivity of copper = 10.37 CM
l = length of wire = 200m / 0.3048 = 656.17 ft
a = area of 18 AWG = 1624 CM

R = 10.37 * 656.17 / 1624 = 4.18

So then to get I i got

I = E / R
I = E / R1 + R2 + R3
I = 110V / 4.18 + 4.18 + 15.125
I = 110/23.485
I = 4.68

Is there any way i could have done this without having to convert the meters to feet?

 

[Astronuc]

How did one get area, a = = 1624 CM

cm is unit of length, and resistivity of copper is 1.68 x10^-8 Ωm
Ref: http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/rstiv.html#c1

I read the table for AWG 18 and find area = 0.823 mm² and resistance is 20.948 Ω/km.

 

[JeeebeZ]

cm is a unit of length
CM is circular mil

you said they should all be in my text. Everything in my text is based off CM, i don't know why but it is >_<

 

[Bradracer18]

A circular mil(or CM) is diameter is mills squared

OR

1 mil = 0.001 in.

That's what my book says...

Your resistivity(or rowe) should be in Ohms * CM / ft.

 

[JeeebeZ]

yah that sounds right, i just don't know how to make symbols on here so well... i didnt @_@

 

[Astronuc]

cm is a unit of length
CM is circular mil

Ah, my mistake. I should have recognized it, but its a unit I now use seldom. 1624 cmil is correct.

If resistivity is given in SI/metric units, one does not have to use feet. The problem in the US is that most engineering is still taught with English/British units, and there are sometimes mixed units.

I use both at work, but prefer metric.

 

[Bradracer18]

I agree...prefer metric to work problems with...but still understand english better, when someone yells out a distance or figure or something...