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    Hi all,

    I'm trying to understand e.m.f. with cells and wondered if the following diagrams are correct or not:

    Name:  scan0001.jpg
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    Do the voltages just add or subtract together to give a resultant voltage as I've drawn?

    Thanks for any help.
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    (Original post by Sir_Malc)
    Hi all,

    I'm trying to understand e.m.f. with cells and wondered if the following diagrams are correct or not:

    Name:  scan0001.jpg
Views: 25
Size:  78.5 KB

    Do the voltages just add or subtract together to give a resultant voltage as I've drawn?

    Thanks for any help.
    yeah - though I'd draw it a bit differently

    Name:  kvl1901.jpg
Views: 22
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    FWIW this shows Kirchoffs Voltage law - that the sum of all pd's around a loop is zero.

    you can decide to say clockwise is negative and go around the loop adding up
    -3
    +9
    -6
    equals 0 (you could equally validly pick any any start/end point or order)

    and obviously you'd get the same result if you decided clockwise was positive.

    it's useful if you know some of the pd's in a loop - because the unknown pds have to add up with the known pds to give a total of zero around the loop.
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    Thanks for your reply.

    Is the second circuit diagram basically how a battery charger works? You have a bigger voltage reversing the current flow to charge the smaller battery?
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    (Original post by Sir_Malc)
    Thanks for your reply.

    Is the second circuit diagram basically how a battery charger works? You have a bigger voltage reversing the current flow to charge the smaller battery?
    Well the basic idea there is to force charge back into the battery you're recharging with a reverse current... seeing opposing batteries doesn't imply one of the batteries is getting charged though, it might not be a rechargable type in which case it's just passing current and getting hot.
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    So in this exam question: Name:  Battery charger.png
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    The X point would attach to the positive terminal of the battery and the Y to the negative terminal. Thus producing a 'backwards' flow through the battery.
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    (Original post by Sir_Malc)
    So in this exam question: Name:  Battery charger.png
Views: 26
Size:  31.9 KB

    The X point would attach to the positive terminal of the battery and the Y to the negative terminal. Thus producing a 'backwards' flow through the battery.
    yeah
    so if you're jump starting a car you put the leads +ve to +ve and -ve to -ve
 
 
 
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