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Voltmeter that has a p.d. across it but no amperes or ohms?

2014-05-07 21_18_20-SmoothDraw 4.0.1.png

With the above question, why does the voltmeter not have a current across it or a resistance? It has to, if it has a voltage across it because the p.d. is the product of resistance and current at a point in a circuit?

Is it just because all voltmeters have a huge resistance so they stop the flow of current? But why is 4V going into it?
(edited 9 years ago)
Reply 1
Avatar for tory88
tory88
18
A voltmeter has a massive resistance, which means that nearly all current flows across the component it is measuring. Why do you think it has zero resistance?
Reply 2
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halpme
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7
Original post by tory88
A voltmeter has a massive resistance, which means that nearly all current flows across the component it is measuring. Why do you think it has zero resistance?


What happens to the 4V? It's just dissipated through the resistance?
Reply 3
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tory88
18
Original post by halpme
What happens to the 4V? It's just dissipated through the resistance?


Voltage is what drives the current, and so is the same across all branches of a parallel circuit. So the voltage doesn't dissipate. The reason that it is there with a small current is due to the large resistance however - current is the flow of electrons, so for a large resistance it takes a larger voltage to push even a few electrons through.

Does that make sense?
Reply 4
Avatar for halpme
halpme
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7
Original post by tory88
Voltage is what drives the current, and so is the same across all branches of a parallel circuit. So the voltage doesn't dissipate. The reason that it is there with a small current is due to the large resistance however - current is the flow of electrons, so for a large resistance it takes a larger voltage to push even a few electrons through.

Does that make sense?


Yes. So you're saying the 4V is pushing only a negligible amount of electrons through which corresponds to the puny current through that huge resistance?
Original post by halpme
What happens to the 4V? It's just dissipated through the resistance?


The energy is dissipated in the resistor and lamp parallel combination not the voltmeter.

The voltmeter is measuring the potential difference across the parallel combination of the resistor and the lamp.

Since voltage is defined as Joules per Coulomb of charge, pd, is a measure of how much energy is converted in the resistances beteen the measuring points.

4V means 4 Joules of energy are converted for every Coulomb (1 Ampere = 1 Coulomb per second) of charge that flows in the resistances.
Reply 6
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tory88
18
Original post by halpme
Yes. So you're saying the 4V is pushing only a negligible amount of electrons through which corresponds to the puny current through that huge resistance?


Yeah, that's right. An important thing to remember is that the 4V corresponds to the P and Q components as well.

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