# A Level Physics Electricity Question Help

#1
I don't understand why the answer to this MCQ is D - could someone explain please?

I thought it would be C, I get why it is 1.4V when closed but why is it 4.2V when open?
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1 month ago
#2
The voltmeter is now in series with the other resistor (two, parallel). It has a high resistance so ...
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#3
(Original post by mqb2766)
The voltmeter is now in series with the other resistor (two, parallel). It has a high resistance so ...
yeah and so if its in series with components how can it have any reading? i thought a voltmeter has to always be in parallel with a component in order for it to record a potential difference
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1 month ago
#4
(Original post by fluffypoopies)
yeah and so if its in series with components how can it have any reading? i thought a voltmeter has to always be in parallel with a component in order for it to record a potential difference
It would be worth reading a bit about voltmeter/ammeters. A voltmeter will have a high resistance so the current dioesn't flow through it and instead goes through the component its in parallel with, as youd want. However, if you put it in series with another resistor, it simply acts as a high value resistor and so "alll" the potential difference will be across the voltmeter. An ammeter has the opposite property (low resistance, in series) as you want the current to flow through it. If you put it in parallel, "all" the current would flow through it and would act like a short across that part of the circuit.
Last edited by mqb2766; 1 month ago
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1 month ago
#5
Switch open=> no current flow. Hence no pd across the 40 ohms resistor. Thats is to say the right side of the voltmeter can be taken to be connected to the -ve terminal of battery and the left side of the voltmeter is connected to the ve terminal of batt.
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#6
(Original post by mqb2766)
It would be worth reading a bit about voltmeter/ammeters. A voltmeter will have a high resistance so the current dioesn't flow through it and instead goes through the component its in parallel with, as youd want. However, if you put it in series with another resistor, it simply acts as a high value resistor and so "alll" the potential difference will be across the voltmeter. An ammeter has the opposite property (low resistance, in series) as you want the current to flow through it. If you put it in parallel, "all" the current would flow through it and would act like a short across that part of the circuit.
Ohhh that makes a lot of sense now - thank you for this.
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#7
(Original post by Ethanity007)
Switch open=> no current flow. Hence no pd across the 40 ohms resistor. Thats is to say the right side of the voltmeter can be taken to be connected to the -ve terminal of battery and the left side of the voltmeter is connected to the ve terminal of batt.
Ahh yes I understand now. Thank you.
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