# Find resistance with length in a circuit

• logic111
If they want resistivity, they need to vary the length and measure the voltage and current for each length, and then use the formula R=\frac{\rho l}{A} to create a graph of length vs. resistance.

#### logic111

1. I need to find the resistance of unknown resistor by varying its lenght. I have a voltmeter, and ammeter (the voltage is to supposed to stay constant, with the current changing). From this I am supposed to create a graph, but teh only graphs I know are V by I to get R, but is V is constant the slope of the line won't work. Anyone have any ideas how I can make the graph?

2. The only equations I've been taught are V=IR and P=IV.

3. I'm really stuck, any help would be great

HINT:What values are changing there?

the values changing are length (manipulated) and current (responding). But if I plot those, the slope is A/M, which isn't the same as resistance.

Is this formula helpful:$$R=\frac{\rho l}{S}$$

Forum went down,so here is the correct formula:

$$R=\frac{\rho l}{A}$$

where

$$l$$ is the length

$$A$$ is the cross sectional area

$$\rho$$ is the resistivity of the material.

Last edited:
If I understood correctly,$$V$$ is not given there?

And are you sure you understood everything completely?

logic111 said:
1. I need to find the resistance of unknown resistor by varying its lenght. I have a voltmeter, and ammeter (the voltage is to supposed to stay constant, with the current changing). From this I am supposed to create a graph, but teh only graphs I know are V by I to get R, but is V is constant the slope of the line won't work. Anyone have any ideas how I can make the graph?

2. The only equations I've been taught are V=IR and P=IV.

3. I'm really stuck, any help would be great

Something sounds not right with this question. How can one vary the length of a common resistor?
Are they asking for resistance or resistivity? If they want the resistance, just measure voltage and current, there is no need to vary the length or take more than one measurement.