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# Electricity power/energy Q

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1. https://onedrive.live.com/redir?resi...nt=photo%2cJPG

I'm not sure how 2a is different from 2b.

The mark scheme uses E = VIt = Pt for part A (1200J)

For part B, it uses E(lost) = I^2t = Pt for part B (46J)

So using the power formula, P = VI = I^2R = V^2/R, why aren't the answers the same?

Kind regards,

Kevin
2. (Original post by TheKevinFang)
https://onedrive.live.com/redir?resi...nt=photo%2cJPG

I'm not sure how 2a is different from 2b.

The mark scheme uses E = VIt = Pt for part A (1200J)

For part B, it uses E(lost) = I^2t = Pt for part B (46J)

So using the power formula, P = VI = I^2R = V^2/R, why aren't the answers the same?

Kind regards,

Kevin
I'm not too sure what you're asking, but the energy transferred by the battery will not all be lost in the connecting wires and some will be lost in the motor instead.

So you have 1200J of energy produced by the battery, which is (a). But (b) just asks how much is lost in the connecting wires and, by the conservation of energy, will not be 1200J, as energy will be lost in other places too.
3. (Original post by kingaaran)
I'm not too sure what you're asking, but the energy transferred by the battery will not all be lost in the connecting wires and some will be lost in the motor instead.

So you have 1200J of energy produced by the battery, which is (a). But (b) just asks how much is lost in the connecting wires and, by the conservation of energy, will not be 1200J, as energy will be lost in other places too.
Hi,

Thanks for the reply. Why is the energy lost for (b) I^2R? Could you explain please?
4. (Original post by TheKevinFang)
Hi,

Thanks for the reply. Why is the energy lost for (b) I^2R? Could you explain please?
Well energy = power * time

Power = IV = I^2R = V^2/R.

If you know all three, you can use any of them and you'll get the same answer, but in this case you don't know all three. You don't know the voltage across the connecting wires (you know the voltage across the battery).

You only know the amount of current that flows through the wires and the resistance of them, so P=I^2R is most appropriate here. Then you multiply this by the time.

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