# Some basic questions

• Jul 19th 2009, 04:45 AM
Twig
Some basic questions
Hi!

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

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

Here is where I am confused.
Which is correct?

$dE=P\cdot dt$

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

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

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

Kinda want this cleared out.

Thx!
• Jul 19th 2009, 05:36 AM
mr fantastic
Quote:

Originally Posted by Twig
Hi!

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

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

Here is where I am confused.
Which is correct?

$dE=P\cdot dt$

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

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

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

Kinda want this cleared out.

Thx!

dE/dt = E', not P'.

$dE = P \cdot dt$ is the correct expression.
• Jul 19th 2009, 10:53 AM
Twig
Yes of course what was I thinking :)

Thanks man!