Power consumption of 600W device when voltage drops from 220V to 160V

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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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Discussion



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.
 
hi chawki! :smile:
chawki said:
I tried and found I=2.72 amps, R=80.88 Ohms, and W=316.51

yeees :redface: …

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

when you vary V, what is Q proportional to?
 
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 ? :smile:
 


W=V^2 / R
But what does it have to do if it is proportional to V^2
 
if W = V2/R

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

how much is W increased? :smile:​
 


It is said to give the answer in 1watt precision
 
Last edited:


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

ehild
 


We need to round also the current??
They asked only the answer to be rounded...
 


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