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A level physics Electricity question (2)

Can someone please explain why it’s D ?
When the switch is open how can the the voltmeter measure anything there ?DE7B4774-0A8E-4DEE-B3FA-921E5694BAA1.jpeg
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(edited 1 month ago)
Voltmeters measure the potential difference across the ACTUAL voltmeter, not the Resistor in parallel.

Usually they are the same thing but not if the resistor section is disconnected.

So in the open switch case you can think of the voltmeter being in series with the battery, so there are 4.2V across it.
(edited 1 month ago)
When the switch is open, we can ignore the path through the 10 ohm resistor. I recommend drawing a circuit without the 10 ohm resistor and that shows that the battery, voltmeter and the "system" of resistors is in series. Another way to look at it is that the voltmeter in this case measures the voltage across the system of resistors. The two resistors have same voltage and considering our circuit is ideal, the voltage across the system will be 4.2.
Reply 3
Original post by mosaurlodon
Voltmeters measure the potential difference across the ACTUAL voltmeter, not the Resistor in parallel.
Usually they are the same thing but not if the resistor section is disconnected.
So in the open switch case you can think of the voltmeter being in series with the battery, so there are 4.2V across it.

Thank you that's helped a lot !! I never knew that Voltmeters measure the potential difference across the actual voltmeter
Reply 4
Original post by WordsFiddle
When the switch is open, we can ignore the path through the 10 ohm resistor. I recommend drawing a circuit without the 10 ohm resistor and that shows that the battery, voltmeter and the "system" of resistors is in series. Another way to look at it is that the voltmeter in this case measures the voltage across the system of resistors. The two resistors have same voltage and considering our circuit is ideal, the voltage across the system will be 4.2.

Thank you !!! your advice on drawing the circuit without the 10 ohm resistor made things so much easier to visualise

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