Discussion in 'Homework Help' started by bhuvanesh, Feb 23, 2015.

1. ### bhuvanesh Thread Starter Member

Aug 10, 2013
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signal cannot be both energy and power at the same time ,why?. Need justification.

2. ### WBahn Moderator

Mar 31, 2012
20,256
5,759
What is the definition of an energy signal?

What is the definition of a power signal?

What is the energy of a power signal?

What is the power of an energy signal?

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3. ### bhuvanesh Thread Starter Member

Aug 10, 2013
268
2
energy signal
signal does not lose(or dissipate or transfer) its energy.example any non periodic signal except u(t)

power signal
signal which lose its energy(or dissipate or transfer). Example sin (ac current lose its electric energy as heat while passing through ressistor or all periodic signal are power signal )

energy of power signal is infinity because power is energy transfer in time and energy running sum of power of adding all power gives infinity

power of energy is zero ,because dividing finite energy by infinite period give zero

4. ### ErnieM AAC Fanatic!

Apr 24, 2011
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I have no idea what your signal mean.

As far as your energy vs power goes, while your math is correct your conclusion is meaningless. Sometimes we turn things off so a finite amount of energy can supply some non zero power.

Mar 31, 2012
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6. ### bhuvanesh Thread Starter Member

Aug 10, 2013
268
2
i got some stuffs form the link above ,still i have confusion, i sate what are those

1)
could you explain this briefly
consider step signal ,here the energy is infinite over all time but the power is not finite over all time . It is finite only for 0 to infinity and not for minus infinity to 0 still we say it has power signal .why?

7. ### WBahn Moderator

Mar 31, 2012
20,256
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What is the average power of ANY signal that has finite energy over all time?

How you are coming to the conclusion that the power in a step signal is infinite from minus infinity to 0?

8. ### bhuvanesh Thread Starter Member

Aug 10, 2013
268
2
zero, since energy divided by entire period is zero
power is energy per time and its constant for all period .if i say the sine wave has power 2(it mean 2 units of energy spent per unit time OVER ENTIRE PERIOD).and in unit step there is no signal for minus infinity to zero so how could we say power for that .am i making any sense

9. ### WBahn Moderator

Mar 31, 2012
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If a signal is zero, what is the power in that signal? Hint: It is NOT infinite!

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10. ### bhuvanesh Thread Starter Member

Aug 10, 2013
268
2
if a signal is zero over finite period then energy is zero and so power is also zero.am i right?

11. ### WBahn Moderator

Mar 31, 2012
20,256
5,759
Yes.

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12. ### bhuvanesh Thread Starter Member

Aug 10, 2013
268
2
okay then come back to our question power should be constant over entire period but power of u(t) is 1/2 we know that but you itself said power of u(t) is 0 for period minus infinity to zero then how a signal has two power

13. ### WBahn Moderator

Mar 31, 2012
20,256
5,759
Where are you getting this notion that power should be constant? A power signal is one in which AVERAGE POWER is FINITE. Consider a sine wave. The power is fluctuating at twice the frequency of the waveform and varies from zero to some maximum value. But the AVERAGE POWER is FINITE, meaning that it is NOT INFINITE.

If the AVERAGE POWER of u(t) over all time is 0.5, is that finite or infinite? If it is finite, then u(t) is a power signal.

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Aug 10, 2013
268
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