Calculating Joules from Capacitance: Answers & Tips

  • Thread starter Thread starter RED119
  • Start date Start date
  • Tags Tags
    Capacitance
AI Thread Summary
The discussion revolves around calculating energy stored in capacitors and the implications of connecting them in parallel. A user initially calculated the energy from 18 capacitors but arrived at a significantly lower value than expected, prompting questions about the calculations. It was clarified that when capacitors are connected in parallel, they all share the same voltage, and their capacitance adds up, but the applied voltage must not exceed the lowest voltage rating among them to avoid damage. The formula for energy storage, E = 0.5 * C * V^2, was confirmed, with C representing capacitance. Proper understanding of voltage ratings and charge analysis is crucial for safe capacitor usage in circuits.
RED119
Messages
29
Reaction score
3
So I read a page where someone said they had put 18 400v 3900uF capacitors in parallel and that this came out to 5600 joules... Now I'm sure I am doing something wrong but when I did it out I only got 28.08 joules...
I used this website: http://www.rapidtables.com/calc/electric/Volt_to_Joule_Calculator.htm
plugged 400 in for volts and 0.0702 in for coulombs, if someone can let me know what I am doing wrong here please let me know.

Second little question, i know if you put capacitors in parallel you just add the Farads, but if they all have different volts, say one has 20v 3000F, the other has 400v 90uF, what would that end up being in parallel?
 
Physics news on Phys.org
5600 J looks like the correct answer. Use: energy = CV2/2.

If you put the capacitors in parallel, by definition they will all assume a single voltage across all connected capacitors. The parallel capacitance is independent of the voltage.
[edit] Sorry--if you mean what is the resulting voltage, you have to do a charge analysis.
 
Last edited:
Thanks so much for the reply, is the C in that equation coulombs or capacitance?
 
Welcome. C is capacitance. 'q' is usually represents the charge.
 
hi ya'welcome to PF :)

RED119 said:
Second little question, i know if you put capacitors in parallel you just add the Farads, but if they all have different volts, say one has 20v 3000F, the other has 400v 90uF, what would that end up being in parallel?

just remember tho, you cannot mix voltage values ... that is, the applied voltage to the capacitors must not exceed the lowest rated capacitor voltage value

eg. as in your example of a 20V and a 400V cap, the circuit voltage shouldn't exceed 20V else that lower voltage rated capacitor will fail

but having a 50V, 100V etc rated cap in a 20V circuit isn't a problemDave
 
The voltage rating of a capacitor is the voltage that the capacitor can withstand before being destroyed. At this moment I forget whether that is peak or RMS voltage, but it is general practice to overrate capacitors in a circuit. So if one capacitor in a network of paralleled capacitors is rated 20 volts, you can't apply voltage greater than 20 volts without risking damage to the one rated at 20V.
 
This is from Griffiths' Electrodynamics, 3rd edition, page 352. I am trying to calculate the divergence of the Maxwell stress tensor. The tensor is given as ##T_{ij} =\epsilon_0 (E_iE_j-\frac 1 2 \delta_{ij} E^2)+\frac 1 {\mu_0}(B_iB_j-\frac 1 2 \delta_{ij} B^2)##. To make things easier, I just want to focus on the part with the electrical field, i.e. I want to find the divergence of ##E_{ij}=E_iE_j-\frac 1 2 \delta_{ij}E^2##. In matrix form, this tensor should look like this...
Thread 'Applying the Gauss (1835) formula for force between 2 parallel DC currents'
Please can anyone either:- (1) point me to a derivation of the perpendicular force (Fy) between two very long parallel wires carrying steady currents utilising the formula of Gauss for the force F along the line r between 2 charges? Or alternatively (2) point out where I have gone wrong in my method? I am having problems with calculating the direction and magnitude of the force as expected from modern (Biot-Savart-Maxwell-Lorentz) formula. Here is my method and results so far:- This...
Back
Top