Signal energy & power

[RaVshesh]

If a layman asks me

What is energy and power of a signal??

What reply should one give...

 

[garyson02]

ENERGY SIGNAL:
x(t) (or x[n]) is said to be an energy signal (or sequence) if and only if 0 < E < infinity, and so P = 0.

POWER SIGNAL:
x(t) (or x[n]) is said to be a power signal (or sequence) if and only if 0 < P < infinity, thus implying that E = infinity.

 

[RaVshesh]

That is ok

But I want to know that energy of a signal is E=Integration(x(t)^2)

so what this value of E n joule represents

 

[albbg]

From physics the power is given by the product force*displacement (let's suppose only the case they are directed in the same direction). If we consider the motion of charges (electron) from a point A to B the displacement is represented by the current while the electric potential (voltage between A aand B) is the force that allows the motion of electrons, the the power will be P(t)=V(t)*I(t). If the voltage is applied to a resistor R, then we can write P=V(t)^2/R. This is the istantaneous power.
Speaking about signal often the power is referred to R=1.

But if I want to know the "effort" required to do a "job" I need to know how much time the power has to be applied. This is the energy; the product between average power and time.
For instance a MOV used to protect a line from overvoltage, has to be designed not only for a given amount of power, but also for "how many time it can absorb that power" ==> energy

This is equivalent to: I've to push my broken car; I need the power to make it moving but also I need the energy to push it for the time neccessary to reach an auto repair.