How do voltmeters measure work done over a component? In my physics class, we were shown the picture below to measure the current and potential difference through a component. We were told that the resistance of the ammeter is as low as possible, and the resistance of the voltmeter as high as possible so that the electrons go through the component instead of the voltmeter. We were also told that the voltmeter measures the change in energy the electrons have before and after going through the component.

This has confused me as if the resistance of the voltmeter is so high that hardly any electrons pass through it, how can it measure how much energy the electrons are carrying? The few that do go through the voltmeter have surely avoided the component altogether, so how could you get a reading for how much energy has been transferred by the component?

• It measures the difference between terminals, not the current passing through it. Feb 11 '19 at 17:46
• The voltmeter doesn't really directly measure "the change in energy the electrons have before and after going through the component". What it measures is the voltage difference across the component, which you can think of as effectively being a measurement of the electronic "pressure" across the component. And knowing the voltage difference across the component is not in itself enough information to determine how much power is being dissipated by the component. You also need to know the current through the component, which is why the ammeter is in the circuit.
– user93237
Feb 11 '19 at 18:51
• @SamuelWeir That seems more like an answer than a comment.
– rob
Feb 12 '19 at 0:55