# What does C mean in terms of an energy calculation

I'm a bit of a noob when it comes to EE, so this might be a dumb question, but I find it super interesting and am learning as fast as I can

(1) I saw the following formula on another forum: "E = 0.5 * C * V^2 = 4 mJ"

Describing the energy that a gag shock pen would inflict on a human. I am confused how the op got to this conclusion. I read on a different site that energy = power * time where power equals V(voltage) * I(current), so I'm not sure where the 0.5, the C, and the second V comes in.

Does C stand for current? or is it some kind of constant? any explanation would be appreciated, Thanks!

(2) Also related follow up question, the thing I was initially trying to find out for curiosity and to kind of understand what a safe current is, is how much current a shock pen would have, assuming a voltage of about 50- 70 V

• Capacitance in this context. (In other contexts, the speed of light, but not here.) Feb 3, 2021 at 15:27

C in the equation

$$\E = \frac{CV^2}{2}\$$

refers to capacitance.

A capacitor with capacitance C which is charged to voltage V has

$$\E = \frac{CV^2}{2}\$$ energy stored in it.

• Thank you! that's the missing piece I needed Feb 3, 2021 at 2:10
• would C be in farads in this equation? assuming E is in joules? Feb 3, 2021 at 2:13
• @Dyskord yes. That would be correct. Feb 3, 2021 at 2:14