Originally Posted by

**Twig** Hi!

We know from physics that $\displaystyle E = P \cdot t $ , where E is the energy, P the power and t the time.

If we have $\displaystyle P=P(t)$ , that is, power as a function of time, we could integrate this expression from $\displaystyle t_{1} \mbox{ to } t_{2} $ , and get the total energy.

Here is where I am confused.

Which is correct?

$\displaystyle dE=P\cdot dt $

$\displaystyle dE=P'(t)\cdot dt $

Because usually, $\displaystyle \frac{dy}{dx}=y'(x) $ . This looks like the second one.

But i also makes sense to think about $\displaystyle dE $ as some "energy strip" , with height P and width dt.

Kinda want this cleared out.

Thx!