# Physics - resistivity

1. Mar 2, 2008

### mysqlpress

1. The problem statement, all variables and given/known data

Here's the problem.
The resistance of a piece of constantan wire of radius 0.6mm is reduced to 1/3 its original resistance when a unifrom layer of copper is deposited on it. Find the thickness of the layer of copper
(Resistivity of constantan = 49*10^-8 ohm m
resistivity of copper =1.7*10^-8 ohm m

2. Relevant equations

I do think that it is related to
R =pl/A where p is the resistivity , l = length of wire
A = cross-section area

3. The attempt at a solution
I've tried ... but don't know how to work out
...
Thanks

2. Mar 2, 2008

### Staff: Mentor

Hint: The two layers are in parallel.

3. Mar 2, 2008

### mysqlpress

Yup.. and
Should I treated the cross-section area of copper as ( cross-section area of a big wire - cs a of a small wire) ? Since it is deposited .. I think on the surface of the constantan..covered it out...

4. Mar 2, 2008

Sure.

5. Mar 2, 2008

### mysqlpress

Okay, Thanks for your guide~~ :)