# Watts calculation help

## Homework Statement

An electrical device consumes 600 W of power, when connected to 220 V voltage.

## Homework Equations

Calculate the power consumption of the device, when it is connected in 160 V voltage. Give the answer in 1 W precision.

## The Attempt at a Solution

I tried and found I=2.72 amps, R=80.88 Ohms, and W=316.51

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Looks good.You need to round up the final answer and for problems of this type it's probably best to do the rounding up at the end.

tiny-tim
Homework Helper
hi chawki!
chawki said:
I tried and found I=2.72 amps, R=80.88 Ohms, and W=316.51
yeees

but now do it the quick way (ie, without bothering to find I or R)

when you vary V, what is Q proportional to?

Oh please Tiny-Tim...don't tease me on this one too

317?

tiny-tim
Homework Helper
write out all the equations (for W V I and R) …

eliminate I (since you don't know what I is), that should give you just W V and R …

then assume that R is constant …

how does W depend on V ?

W=V^2 / R
But what does it have to do if it is proportional to V^2

tiny-tim
Homework Helper
if W = V2/R

and if V is decreased from 220 to 160 (and R stays the same) …

how much is W increased?

i give up

Is it 317watts ?

It is said to give the answer in 1watt precision

Last edited:

since there is .51 the answer would be 317watts?

ehild
Homework Helper

Yes. And even more so if you do not make that rounding error in the current.

ehild

We need to round also the current??

ehild
Homework Helper

You rounded it to 2.72. It is wrong. It was 600/220=2.727272~2.727 A. The resistance is 220/2.727=80.67 ohm.

You should use at least one more significant digits in the intermediate results than in the final result.

ehild