# Current and resistance

Tags:
1. Feb 21, 2017

### IDK10

1. The problem statement, all variables and given/known data
The question wants to know why th current should be small through a wire when using an ohmmeter.

2. Relevant equations
None

3. The attempt at a solution
I can't think of a solution, and looking online doesn't give any answer.

2. Feb 21, 2017

### kuruman

How do you think an ohmmeter measures "ohms"? What information online did you get about that? There is a very relevant equation equation involved.

3. Feb 21, 2017

### IDK10

All I get is the resistance = voltage/current. That's it. Everything was from BBC Bitesize, nothing else came up.

4. Feb 21, 2017

### kuruman

That's all you need. An ohmmeter measures voltage V across, for a known current I through the unknown resistor and displays the ratio. You provide the resistor. What, do you think, provides the V and the I once you have have hooked up the resistor and turned the selection switch to "Ohms"? Can you draw a simple circuit diagram with your external resistor also drawn in?

5. Feb 21, 2017

### IDK10

Its not that sort of circuit, its an ohmmeter, with two wires coming out and connecting to a piece of wire. No exterior battery or resistor. Well the wire acts as a variable resistor, as the crocodile clips connecting the wire to the ohmmeter are moved in increments of 10cm, but no actual resistors were used.

Last edited: Feb 21, 2017
6. Feb 21, 2017

### kuruman

I didn't say anything about an external battery. How do you think the instrument "knows" what the resistance you connect to it is? What do you think could be inside? Where does the current that "should be small enough" come from?

7. Feb 21, 2017

### IDK10

1. Voltage/Current
2. No idea, something to calculate, an ammeter, and a voltmeter (not just to calculate resistance, but it also allows you to view the amperes and volts)
3. The electrons )but all I need to know is why the current has to be small)

8. Feb 21, 2017

### kuruman

You will not be able to answer this question without some basic understanding of how an ohmmeter works. I suggest that you google "How does an ohmmeter work?" and read through some of the links that come up. Then you will be in a better position to understand what I am asking and eventually to answer your original question.

9. Feb 22, 2017

### IDK10

OK, tbh, its actually a multimeter, but on the ohmmeter setting.

10. Feb 22, 2017

### kuruman

Yes, indeed. I indicated as much in post#4. They are all multimeters, but we call them voltmeters when we use them to measure voltage, ammeters when we use them to measure current and ohmmeters when we use them to measure resistance. Do you think you can answer my questions now?

11. Feb 22, 2017

### IDK10

What I found was 'Most ohmmeters work by passing a (usually small) current through the sample and measuring the voltage drop - thus finding the resistance by Ohm's law V=IR. Any internal resistance of the ohmmeter should be well known and included in the calculation (usually automatically).', so is it to give a large enough resistance so that, when the internal resistance is taken into account, the final resistance is significant.

12. Feb 22, 2017

### kuruman

Good. Can you now draw a simple basic circuit of the innards of the ohmmeter and find an expression for the measured resistance? Include the internal resistance and ask yourself (and answer) the question, "What if the current is not small and the internal resistance is not that well known or even not known at all?"
I don't understand what this means. Draw the circuit to focus on what you wish to say.

13. Feb 22, 2017

### IDK10

Finding a diagram is going to be hard. I have no idea of what the inside of an ohmmeter looks like, so I can't find a diagram of the inside of one. We also don't need to know about the inside of one.

14. Feb 22, 2017

### kuruman

I am not asking you to find a diagram. I am asking you to draw one that shows the basic elements you need for the measurement. Start with the resistor you want to measure. Draw it. What else do you need to have a current through the resistor? Draw that too. Can you do that?

15. Feb 22, 2017

### IDK10

I can try, keep in mind that the resistor, isn't an actual resistor, but a metal wire, which acts like a variable resistor.

16. Feb 22, 2017

### kuruman

I think you can safely assume that the wire has a fixed length which means a fixed non-zero resistance for answering this question. If the resistance varied while you are trying to measure it, you would be wasting your time.

17. Feb 22, 2017

### IDK10

The box at the top is the ohmmeter, the circle is the dial and the smaller box is the display screen. The two red lines are the connecting wire. The line with the line through it is the one being moved along the black line. The black line is the wire that is being used as the resistor

18. Feb 22, 2017

### IDK10

The resistance would vary while measuring it, we record the resistance of different lengths of wire.

19. Feb 22, 2017

### Staff: Mentor

Perhaps you could approach the question from an alternative perspective. What might be the possible negative consequences of using large currents to measure resistance (either small or large)?

20. Feb 22, 2017

### IDK10

Apparently, the answer was to stop the wire from heating up, and deforming the wire, which would vhange its cross-sectional area, and then therefore change its resistivity.