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Power dissipation

Discussion in 'General Electronics Chat' started by kvi037, May 17, 2013.

1. kvi037 Thread Starter New Member

May 17, 2013
17
0
Hi,

I'm struggling with this task for my preperation for a exam next week.

An AC voltage of amplitude 150 volts is impressed across a pure resistance of 100 ohms. Calculate the current and the power dissipation when the frequency of the voltage is 60 cycles/sec.

2. MrChips Moderator

Oct 2, 2009
19,132
6,148
I already gave you some tips.

Why are you posting the same question again?

3. kubeek Expert

Sep 20, 2005
5,585
1,094
Since the load is purely resistive the frequency doesn´t matter at all.
Amplitude means peak voltage, so you need to calulate the average voltage, then the equation is P=V*V/R and I=V/R
The current you calculated is completely wrong, VR/R is still V, not I. (unless VR means Vavg as in average voltage, then it would be true)

Last edited: May 17, 2013
4. kubeek Expert

Sep 20, 2005
5,585
1,094
Also, this is completely wrong too. V0*sin (wt) is the instantenous voltage at a certain point in time t, so where did the t go in the second part? It should be Io*R sin(wt) = Io R sin (2*pi*60*t) so if you wanted to know the voltage at t=260ms this equation would be useful.

5. MrChips Moderator

Oct 2, 2009
19,132
6,148
I think when he writes VR he is referring to V across resistor R, i.e. V with subscript R.

Sloppy notation.

6. kvi037 Thread Starter New Member

May 17, 2013
17
0
Thanks for reply. So you can just say. I = 1.5A, and that the power disspation is
P = IVcos 1 =225 W

Is that correct?

7. kubeek Expert

Sep 20, 2005
5,585
1,094
No, the average current is 150*0.707/100=1.06A and the power dissipation is average voltage times average current = 112W. And no cosinus nor sinus in there.

kvi037 likes this.
8. kvi037 Thread Starter New Member

May 17, 2013
17
0
Aha, thanks a lot for the help! 9. MrChips Moderator

Oct 2, 2009
19,132
6,148
Power dissipation = 112W is correct.

But do not say average voltage times average current.

The correct answer is rms voltage times rms current.

Average voltage and rms voltage are not the same.

10. kvi037 Thread Starter New Member

May 17, 2013
17
0
Thanks for all the replies 