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Really stuck on this electricity physics question. Please help! watch

    • Thread Starter

    The cell in the circuit has an emf of 2.0 V. When the variable resistor has a resistance of 4.0 Ω, the potential difference (pd) across the terminals of the cell is 1.0 V.
    What is the pd across the terminals of the cell when the resistance of the variable
    resistor is 12 Ω?
    [1 mark]

    The answer is 1.5 V

    I've got no Idea how they got there, please would someone help?
    •  Official Rep

     Official Rep
    Sorry you've not had any responses about this.

    Why not try posting in a specific subject forum- you might have more luck there.

    Here's a link to our subject forum which should help get you more responses.


    First we calculate the current of the circuit and resistance of the internal resistor:

    V/R = I = 1/4 = 0.25A

    Internal resistance = 1/0.25 = 4ohms

    Next we work out the output voltage with 12ohm variable resistor:

    Vout = Vint x R1/R1+R2 = 2 x 12/12+4 = 1.5V

    There you go.

    You don't even need to work out the current, i f the v across the cell is is same as the v across the resistor then they will have the same resistance because they're in series. So r is 4 ohms on both.
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