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# Confusing DC electricity question watch

1. This question has been confusing me for hours yet I don't think it's even meant to be that hard.
I can do the first part (I3=I1+I2) that all makes sense.

The answer to part B is 10V, and I have no idea how to get there. How do voltages work in parallel when the batteries have different voltages?

Can someone also help explaining part C as well? Thank you!
2. (Original post by fliilip)

This question has been confusing me for hours yet I don't think it's even meant to be that hard.
I can do the first part (I3=I1+I2) that all makes sense.

The answer to part B is 10V, and I have no idea how to get there. How do voltages work in parallel when the batteries have different voltages?

Can someone also help explaining part C as well? Thank you!
Have you done Kirchhoff's Voltage law?

The voltages around a loop have to sum to zero... including the loop made up of battery V1, R1 and R3
if R1 = 0 then the PD across R1 = 0
so where does the anticlockwise 10V in that loop come from to sum the clockwise 10V from the battery from.

Part C requires you to find the PD across R2... which shouldn't be difficult given PD across R3 is known and the loop V2, R2, R3 also sums to zero.
3. (Original post by Joinedup)
Have you done Kirchhoff's Voltage law?
The voltages around a loop have to sum to zero... including the loop made up of battery V1, R1 and R3
if R1 = 0 then the PD across R1 = 0
so where does the anticlockwise 10V in that loop come from to sum the clockwise 10V from the battery from.

Part C requires you to find the PD across R2... which shouldn't be difficult given PD across R3 is known and the loop V2, R2, R3 also sums to zero.
thanks for reply. No I don't remember ever covering that, i'll read up on it thanks.But for part B why doesn't the 12 volts effect the voltage across R3 at all?
4. (Original post by fliilip)
thanks for reply. No I don't remember ever covering that, i'll read up on it thanks.But for part B why doesn't the 12 volts effect the voltage across R3 at all?
Just to re-state the questions set up... V1 is an ideal voltage source (with zero internal resistance), it can pass any amount of current to keep its terminals at a PD of 10V
other things apart from R3 in the LHS loop:

conductors... zero resistance therefore zero voltage exists across them at any current
a variable resistor set to zero... as above.

KVL is a statement of the conservation of energy - if you want to have a different PD across R3 you'll have to explain how you destroyed (or created) energy...

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