# Finding New Resistance of a Length of Wire

1. Mar 31, 2015

### B3NR4Y

1. The problem statement, all variables and given/known data
A length of wire, length l, is clamped at its midpoint. It is then stretched on the right side, with the left side unchanged, to a length of l (the right side has a length l). The original Resistance of the wire was R, what is the new resistance of the wire?

2. Relevant equations
R is proportional to length

3. The attempt at a solution
Using the fact that R is proportional to length (it wants R' in terms of R, so I am ignoring resistivity and such), I conclude that R' = 1.5*R, which is wrong. I got 1.5 by saying the left side is length l/2 and the rightside is length l so l+l/2 = 1.5l. So R'=1.5*R

2. Mar 31, 2015

### Dick

The resistance of the wire is proportional to length if the wire has the same diameter along it's length. When you stretch the wire the stretched part will get thinner. You need to take that into account.

3. Apr 1, 2015

### B3NR4Y

How would I do that? Should I set up a ratio of diameter to length that should remain constant?

4. Apr 1, 2015

### BvU

Yes. Not a simple ratio like d/l or so. Think of what does remain constant.

5. Apr 1, 2015

### B3NR4Y

Ah, is it cross sectional area that changes? So a ratio of length to cross sectional area, since resistivity remains constant?

6. Apr 1, 2015

Yep