# Fall of the voltage [closed]

I know this is very simple task but I'm new to this so I just need someone to tell me if i solved it correctly. My english is not very good so I will use only P for power, R for resistance, I for current and U for voltage. The task is: In electric network that has voltage 220V, power of the device falls down from 1000W to 800W because of overload. I need to calculate fall of the voltage. The resistance stays constant. So i took $P=R*I^2$ and $I=\frac{U}{R}$ then $P=R*\frac{U^2}{R^2}$ i.e. $P=\frac{U^2}{R}$ If i calculate R i get $R=48.4 \Omega$. If I put that in second equation where $P=800W$ i get $U=196.7V$ so fall of the voltage would be $\Delta U =23.3V$ . I just need to know if this is correct. I am pretty new to this and there are no solutions in book so i need to check. Thank you.

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 Hi user9545, and welcome to Physics Stack Exchange! This is a site for conceptual questions about physics, not a place to get someone to check your work. If there is something specific that you think you've done wrong, or some concept that is giving you trouble in figuring out how to do a problem, you're welcome to ask about that. – David Zaslavsky♦ May 31 '12 at 6:51 I agree with the closing, but the answer is yes, you did everything right. The easy way to see this is scaling--- the power goes as the square of the voltage, so to lose 20% of the power, you lose 10% of the voltage, or about 22 volts. – Ron Maimon May 31 '12 at 7:36

## closed as too localized by David Zaslavsky♦May 31 '12 at 6:50

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