# Homework Help: Draw a circuit diagram to measure resistance of wire

1. Oct 19, 2015

### Janiceleong26

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

2. Relevant equations

3. The attempt at a solution

This is how I drew it (in pencil) , but there should be a rheostat connected in series with the power supply, why?i can already calculate the resistance straight away by V/I

2. Oct 19, 2015

### andrewkirk

There may be additional reasons, but a very basic one is that if you connected that circuit without additional resistance, such as from a rheostat, bad things would happen because you'd have a short circuit.

3. Oct 19, 2015

### andrevdh

A piece of wire has a very low resistance. That means that you are in effect short circuiting the power supply with the wire. The reostat adds additional resistance in the circuit to keep the current drawn from the power supply within the limits of the power supply, so it is more for practical than theoretical reasons that it is needed.

4. Oct 19, 2015

### Staff: Mentor

You need control over the voltage applied to the unknown resistance, else it is likely to be destroyed before you can take your measurements. Your circuit shows a fixed voltage source connected to the unknown R, and with R being unknown you can't know ahead of time whether this voltage will be safe for the unknown resistor. A rheostat set to maximum can offer protection, and its inclusion shows you have considered this angle.

Had you shown the voltage source to be variable, then the rheostat would probably not be considered a necessary inclusion.

The function highlighted by andrewkirk is also appropriate, because every voltage source has a limit to the current it can supply.

5. Oct 19, 2015

### andrevdh

You can vary the current in the wire by changing the reostat setting and thereby obtain a range of I, V values from which you could draw a graph and relate the gradient of the graph to the wire's resistance.

6. Oct 19, 2015

### Janiceleong26

Thanks! Got it