# Homework Help: Non linear resistance, logs and graph

1. Dec 17, 2011

### greg997

Non linear resistance, voltage and current have the following relationship

I=VR^k, where k is a constant
I am given a few values for I and R
R= 2 then I= 5.8
R= 4 then I=15.7

I am supposed to take natural logs of both sides and plot on graph. And from graph to find k.

So then I have
1.757=ln(V)+k0.693
2.753=ln(V)+k1.386
and so on
And I have no idea how to procede and how to plot that.
Any help is welcome

2. Dec 17, 2011

### Frogeyedpeas

You can solve it as a system of equations to find V and K...

by setting K = K and creating a new variable J such that J = ln(v) and once you isolate the values of K and J respectively then you can take e^J to find V.

3. Dec 17, 2011

### Frogeyedpeas

But otherwise for your purposes just plot it as you would normally plot something

4. Dec 17, 2011

### greg997

I am sorry, that the problem. I do not know how to plot that.
can you please show me that?

5. Dec 17, 2011

### SammyS

Staff Emeritus
You have the 2 following equations and need to find k
1.757=ln(V)+k0.693

2.753=ln(V)+k1.386
Subtract one equation from the other solve for k.

If that doesn't make sense to you, then solve one of the equations for ln(V), in terms of k, and substitute that result for ln(V) in the other equation. Solve that for k.

6. Dec 17, 2011

### Staff: Mentor

This is so unusual that I'm inclined to think you may have made a mistake.

Are you examining one individual resistor here?

Are you sure this R shouldn't be a V?

In any case, do you know the value for k that you are supposed to arrive at?

7. Dec 18, 2011

### greg997

This is full description. And do not know how to plot the values I have calculated. No, I dont know the value of k I am supposed to arrive at.

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8. Dec 18, 2011

### SammyS

Staff Emeritus
Add two more rows to your table. One will contain ln(R) values and the other will contain ln(I).

Then graph those values, ln(R) on the vertical axis and ln(I) on the horizontal axis. So, this is like using ln(R) as y, and ln(I) as x.