# What is signal-energy?

What is energy in terms of signal processing? You can read here, that energy is

I just need a simple everyday-example for that energy. And yes, bringing an example like

$x(t)$ representing the potential of an electrical signal propagating across a transmission line

is still too complicated for me.

Pick up an apple and raise it one meter. You have just spent (approximately) one joule of energy. You had to spend energy because the action (raising the apple) goes against a force (Earth's gravity).

The same is true for electrons. An electrical signal is nothing but electrons moving in an electric field. Causing these electrons to move takes energy, exactly like moving a train or an apple.

In the formula you posted, $x(t)$ is a voltage causing electrons to move in a resistance of $1\,\Omega$, and $E_s$ is the energy required to make those electrons move around in the resistor, causing a current to develop.

• But what if I am thinking about signal as general function of t. Moreover, your explanation will fail in discrete time case – Jan Filip May 11 '18 at 5:48
• The "energy" of an abstract signal that is not associated with a physical voltage or current is just an abstract "measure" of the signal. You can think of it as "what energy would this signal have if it were a voltage". As for discrete signals, I interpret their energy as "the energy this signal will have when converted to continuous time". – MBaz May 11 '18 at 16:29
• Can you provide mote details about "when converted to continuous time"? I suppose disctete time signals as pulses with zero time duration How one can get energy of such signals in physical meaning? – Jan Filip May 18 '18 at 20:22
• @JanFilip The way I think about it is that discrete-time signals do not exist in the physical world -- they are just sequences of numbers. However, one can sample a physical signal and convert it to discrete. Many of the signal's properties, including its energy, are inherited by the discrete signal. In the same way, when discrete signal is interpolated and converted to continuous time, the CT signal inherits the energy of the DT signal (up to a constant factor). Let me know if this explanation helps. – MBaz May 18 '18 at 21:52