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# Cells opposing one another watch

1. Hi all,

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

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

Thanks for any help.
2. (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:

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

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.

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?
4. (Original post by Sir_Malc)

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.
5. So in this exam question:

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.
6. (Original post by Sir_Malc)
So in this exam question:

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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