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

  • 1
    \$\begingroup\$ Capacitance in this context. (In other contexts, the speed of light, but not here.) \$\endgroup\$ Feb 3, 2021 at 15:27

1 Answer 1


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.

  • \$\begingroup\$ Thank you! that's the missing piece I needed \$\endgroup\$
    – Dyskord
    Feb 3, 2021 at 2:10
  • \$\begingroup\$ would C be in farads in this equation? assuming E is in joules? \$\endgroup\$
    – Dyskord
    Feb 3, 2021 at 2:13
  • 1
    \$\begingroup\$ @Dyskord yes. That would be correct. \$\endgroup\$ Feb 3, 2021 at 2:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.