- #1
etotheipi
Bit of a random question... capacitors can be a bit weird, in that if we connect one up to a source of EMF and do positive external work to separate the plates of the capacitor, the energy of the capacitor decreases (and instead the work you do plus the decrease in capacitor energy goes into the battery). So we get the strange property that if the electric field from one plate does positive work on the other plate, the potential energy of the plates actually increases (instead of decreasing, as we would expect from e.g. two point charges).
Specifically, we have that ##U = \frac{A\epsilon_0 V^2}{2x}## and that the force on the plate, obtained by multiplying the field from the other plate by the charge, is ##F_x = -\frac{A\epsilon_0 V^2}{2x^2}##. We might notice that for this setup we have that the electric force on the plate relates to the potential energy of the plates via$$F_x = \frac{dU}{dx}$$However, this seems like big a red-herring since for a conservative force we would usually expect the relation$$F_x = -\frac{dU}{dx}$$with an important negative sign. That just leads me to believe that the fact that ##F_x = \frac{dU}{dx}## is a bit of a fluke. I wondered if maybe I'm being a bit too judgemental here... but would you agree?
Specifically, we have that ##U = \frac{A\epsilon_0 V^2}{2x}## and that the force on the plate, obtained by multiplying the field from the other plate by the charge, is ##F_x = -\frac{A\epsilon_0 V^2}{2x^2}##. We might notice that for this setup we have that the electric force on the plate relates to the potential energy of the plates via$$F_x = \frac{dU}{dx}$$However, this seems like big a red-herring since for a conservative force we would usually expect the relation$$F_x = -\frac{dU}{dx}$$with an important negative sign. That just leads me to believe that the fact that ##F_x = \frac{dU}{dx}## is a bit of a fluke. I wondered if maybe I'm being a bit too judgemental here... but would you agree?