# Current and resistance

## Homework Statement

The question wants to know why th current should be small through a wire when using an ohmmeter.

None

## The Attempt at a Solution

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

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

How do you think an ohmmeter measures "ohms"? What information online did you get about that? There is a very relevant equation equation involved.
All I get is the resistance = voltage/current. That's it. Everything was from BBC Bitesize, nothing else came up.

kuruman
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All I get is the resistance = voltage/current. That's it.
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?

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?
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.

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kuruman
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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?

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?
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)

kuruman
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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.

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

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

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?
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.

kuruman
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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).
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?"
so is it to give a large enough resistance so that, when the internal resistance is taken into account, the final resistance is significant.
I don't understand what this means. Draw the circuit to focus on what you wish to say.

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.
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.

kuruman
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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?

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

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

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

gneill
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)?

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.