How can the voltage across a wire change when you only alter its length?

Picture a circuit with a constantan wire, a power pack, an ammeter and a voltmeter. The supply voltage across the whole circuit is 9V. The ammeter is placed to measure the current in the circuit, while the voltmeter measures the voltage across the constantan wire. The length of the constantan wire is increased. Of course, its resistance would increase proportionally. This would mean that the current would decrease while the voltage remained the same, no? I've done an experiment in which the voltage varied. I don't understand why, as I would think that the supply voltage would always equal the voltage across the constantan wire (as long as its the only component in the circuit).How can the voltage across a wire change when you only alter its length?
The answer is in your last statement: %26quot;as long as its the only component in the circuit%26quot;. The fact is the wire is only one of three resistances in your circuit; you also have the resistance of the ammeter (and the wires connecting it to the circuit) and the output impeadance of the power pack (and the wires connected to it).



So, your actual circuit is really a voltage divider. The voltage across the constantan wire is going to be:



OC_Voltage x R_C / (R_C + R_E)



Where OC_Voltage is the open circuit voltage of the power pack, R_C is the resistance of the Constantan, and R_E is the resistance of everything else.How can the voltage across a wire change when you only alter its length?
Ohms law state V=IR or voltage equals current times resistance. When you change the length of the wire, you change not only the resistance, but the distance over which potential difference(voltage) is acting.How can the voltage across a wire change when you only alter its length?
V=IR

if you increase the length of a wire, the resistence will also increase and thus the voltage will decrease, but just a little.How can the voltage across a wire change when you only alter its length?
I see what you mean. In this simple circuit you would have a constant source, either a current or a voltage one. In your case you have a voltage source, which should be constant regardless of the current and/or resistance.



You should check if your voltage source is set to a fixed current; if you’re not able to verify this visually, check if the current stays the same as you change the length of the wire. If so, then you have source that is biased for current within a “vicinity” of voltage.



Most of the times, this is done to protect electronic components from drastic changes in voltages and keep the current steady.

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