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Which wave will generate more heat?

by samieee
Tags: square wave
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samieee
#1
Sep1-13, 11:07 AM
P: 71
Suppose there are some waveforms to be used as current input: i)sine wave ii)square wave iii) triangle wave iv)sawtooth wave and v)sawtooth wave with both the cycles(+ve half cycle and -ve half cycle) in positive Y axis. Which wave will generate more heat?

Actually, I faced this question in a recent examination. Though I ansewered (v), now I think the right answer would be square wave, as it grabs the maximum magnitude for more duration of time than the others. Am I right?

Thanks.
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milesyoung
#2
Sep1-13, 11:56 AM
P: 428
Assuming it's the waveform of the current through a resistor, you should have a look at how the RMS value of the waveform relates to the power dissipated in the resistor:
http://en.wikipedia.org/wiki/Root_me...ectrical_power

Then you can compare the RMS values of your different waveforms:
http://en.wikipedia.org/wiki/Root_me...mmon_waveforms
samieee
#3
Sep1-13, 01:03 PM
P: 71
So, from the comparison, I see that square wave has the higher value than any other rotating wave. DC shifted square wave has even higher value as it should have.

milesyoung
#4
Sep1-13, 02:29 PM
P: 428
Which wave will generate more heat?

Quote Quote by samieee View Post
So, from the comparison, I see that square wave has the higher value than any other rotating wave. DC shifted square wave has even higher value as it should have.
Right, so your intuition with regards to the square wave was spot on.

The instantaneous power delivered to the resistor is ##p(t) = R i(t)^2##. If you try to imagine the graph of ##p(t)## for your waveforms, it should confirm your idea of why its average value is highest in the case of the square wave.
Redbelly98
#5
Sep1-13, 04:19 PM
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Quote Quote by samieee View Post
Suppose there are some waveforms to be used as current input: i)sine wave ii)square wave iii) triangle wave iv)sawtooth wave and v)sawtooth wave with both the cycles(+ve half cycle and -ve half cycle) in positive Y axis. Which wave will generate more heat?
This question cannot really be answered as stated here. Are the amplitudes of all the waves supposed to be the same? That needs to be specified.
samieee
#6
Sep2-13, 04:07 AM
P: 71
Quote Quote by Redbelly98 View Post
Are the amplitudes of all the waves supposed to be the same? That needs to be specified.
Yes, amplitudes of all the waves are same.
Redbelly98
#7
Sep2-13, 09:21 AM
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Okay. So yes, it is the square wave. As milesyoung said, you need to consider [itex]i^2 R[/itex].
Windadct
#8
Sep3-13, 09:31 AM
P: 553
Also assuming the loads are the same.
KHMOU YOUSSEF
#9
Sep3-13, 10:39 AM
P: 6
All the signals have the same frequency?
Redbelly98
#10
Sep3-13, 05:52 PM
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Quote Quote by Windadct View Post
Also assuming the loads are the same.
Yes.

Quote Quote by KHMOU YOUSSEF View Post
All the signals have the same frequency?
That does not matter. The average of i2 is the same no matter what the frequency is. Just for example, for a sine wave, it is (1/2) the square of the amplitude, independent of the frequency.
rppearso
#11
Sep4-13, 12:55 AM
P: 104
The real question is could you make a saw tooth laser beam if you super imposed different frequencies on top of each other. Not sure how this could be accomplished but its an interesting thought.
Windadct
#12
Sep4-13, 07:42 AM
P: 553
Huh? Lasers operate at one frequency, i.e perfect sine wave oscillation, unless you mean to modulate the laser?
NascentOxygen
#13
Sep4-13, 11:34 AM
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Quote Quote by samieee View Post
Suppose there are some waveforms to be used as current input: i)sine wave ii)square wave iii) triangle wave iv)sawtooth wave and v)sawtooth wave with both the cycles(+ve half cycle and -ve half cycle) in positive Y axis. Which wave will generate more heat?
That's the beauty of the RMS value, it directly relates to the heating value (in a resistive load).
NascentOxygen
#14
Sep4-13, 11:40 AM
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Quote Quote by Windadct View Post
Huh? Lasers operate at one frequency, i.e perfect sine wave oscillation, unless you mean to modulate the laser?
Wouldn't frequency-doubling lasers allow you to generate some coherent harmonics?
mikeph
#15
Sep4-13, 02:42 PM
P: 1,212
or you could modulate the injection current to get a sawtooth amplitude at some frequency << lasing frequency


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