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# electricity power question Watch

1. http://www.scribd.com/mobile/doc/50654250?width=768

I'm stuck on question 4. The correct answer is B.
P=IV
V=IR

So P=I2R

If the new lamp has twice the resistance, it has to have twice the power. So WHY isn't the correct answer C? I need an explanation so I can understand why B is right and not C.

Thank you
2. Voltage is constant at 230V.

P=IV. P=VI
V=IR therefore V/R=I so P=V^2/R
Double R, half the power
3. (Original post by pratstercs)
Voltage is constant at 230V.

P=IV. P=VI
V=IR therefore V/R=I so P=V^2/R
Double R, half the power
I guess it is better to substitute I=V/R into P=VI.

Thanks a lot.
4. (Original post by krisshP)
I guess it is better to substitute I=V/R into P=VI.

Thanks a lot.
It's not necessarily better, you just need to remember that the current will change as the resistance changes
5. (Original post by pratstercs)
It's not necessarily better, you just need to remember that the current will change as the resistance changes
Thanks

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