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I have read that power of a periodic signal is calculated for a single period (correct me if am wrong). So why can't we calculate energy of a periodic signal for a period and say it has finite energy. Why this is not the case ?

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  • \$\begingroup\$ We can, just by measuring power over a specific interval of time. \$\endgroup\$ – EM Fields Nov 24 '16 at 18:34
  • \$\begingroup\$ Can you elaborate by what you mean by "signal"? \$\endgroup\$ – Andy aka Nov 24 '16 at 20:20
  • \$\begingroup\$ I think what you mean is that power can be correctly calculated only if you know the entire period of a signal. That's the required information to calculate power. Whereas, power can only be calculated if you also know the energy transfer rate. A signal energy is proportional to Vrms^2, so to know it you need to calculate the signal voltage rms, which also means you also need to know the complete period of a signal. \$\endgroup\$ – PDuarte Nov 26 '16 at 18:41
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Your assumption is incorrect. One cycle of a periodic cycle does deliver some amount of energy. Power is energy per time, so the power the signal is delivering is the energy per cycle times the cycles per second. Of course you can calculate the reverse too.

For example, if you have a 3 kHz signal that is delivering 17 W, then each cycle is delivering (17 W)/(3 kHz) = 5.7 mJ.

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