• Support PF! Buy your school textbooks, materials and every day products Here!

Watts calculation help

  • Thread starter chawki
  • Start date
  • #1
510
0

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
 

Answers and Replies

  • #2
2,463
97


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.
 
  • #3
tiny-tim
Science Advisor
Homework Helper
25,832
250
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?
 
  • #4
510
0


Oh please Tiny-Tim...don't tease me on this one too :confused:
 
  • #5
510
0


317?
 
  • #6
tiny-tim
Science Advisor
Homework Helper
25,832
250
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:
 
  • #7
510
0


W=V^2 / R
But what does it have to do if it is proportional to V^2
 
  • #8
tiny-tim
Science Advisor
Homework Helper
25,832
250
if W = V2/R

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

how much is W increased? :smile:
 
  • #9
510
0


i give up
 
  • #10
510
0


To Dadface...
Is it 317watts ?
 
  • #11
510
0


It is said to give the answer in 1watt precision
 
Last edited:
  • #12
510
0


since there is .51 the answer would be 317watts?
 
  • #13
ehild
Homework Helper
15,477
1,854


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

ehild
 
  • #14
510
0


We need to round also the current??
They asked only the answer to be rounded...
 
  • #15
ehild
Homework Helper
15,477
1,854


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
 

Related Threads on Watts calculation help

  • Last Post
Replies
3
Views
9K
  • Last Post
Replies
8
Views
2K
  • Last Post
Replies
3
Views
1K
  • Last Post
Replies
9
Views
1K
Replies
7
Views
5K
  • Last Post
Replies
1
Views
6K
Replies
1
Views
17K
Replies
1
Views
2K
  • Last Post
Replies
2
Views
981
  • Last Post
Replies
2
Views
2K
Top