I Electrostatic Influence and Series-Connected Capacitors

AI Thread Summary
In a series connection of capacitors, electrostatic induction causes opposite charges on every second plate, leading to a need for proof that all intermediate plate charges equal the charge of the first plate. The fundamental assumption of circuit theory states that the current in a series branch remains constant, allowing for the conclusion that all charges differ by a constant value. This holds true only when the wavelength of the current is significantly larger than the physical length of the series branch. A detailed analysis using a four-plate configuration illustrates that charges on facing surfaces must be equal and opposite, reinforcing the impossibility of certain charge distributions. Ultimately, the discussion emphasizes that the outer surfaces of the capacitor arrangement must maintain equal charge, regardless of the internal configuration.
reterty
Messages
29
Reaction score
2
As you know, "in the classics" the charges of the capacitor plates are equal in absolute value and opposite in sign. However, let us consider a series connection of capacitors. In this case, a charge of the opposite sign is induced on every second plate of each capacitor due to electrostatic induction. But how can we rigorously prove the strict equality of the absolute values of these charges to the charges of the first plates?
 
Physics news on Phys.org
Do you mean configuration as attached?
img20220223_17481649.jpg
 
No, we consider the case of capacitors in series connected with battery. The first plate of the first capacitor has charge q, whereas the second plate of the last capacitor has charge -q. Intermediate plates are charged through influence and it is necessary to prove that their absolute values are also equal to q
 
This is one of the basic assumptions of circuit theory, that the current in a series branch is everywhere the same:

$$I(t)=\frac{dQ_1}{dt}=\frac{dQ_2}{dt}=...=\frac{dQ_n}{dt}$$ from which equality by integration you can deduce that all the charges differ by a constant, this constant being the charge each plate has at the time t=0. If all the capacitors are uncharged at t=0, then their charges are equal at any time t.

The above assumption holds only if the wavelength of the current in the circuit is big in comparison with the physical length of the series branch.
 
  • Like
Likes Klystron, DaveE and vanhees71
reterty said:
Intermediate plates are charged through influence and it is necessary to prove that their absolute values are also equal to q
I think it is zero because no charge can enter into or go out from the intermediate part ,i.e. the second plate charged -q, the third plate charged q and the wire between non charged.
 
Here's another proof:
Say you have 4 unit-area plates stacked close together vertically. 4 plates, numbered 1 to 4, left to right. 3 gaps.

Number the surfaces 1 thru 8 left to right.

Put charge Q on plate 1 and -Q on plate 4. Assume the center two plates have zero charge.

Fact: the charges on each facing pair of surfaces (e.g. #2 & #3 or #4 & #5) must be equal and opposite, otherwise the divergence in the E field is non-zero which violates Maxwell.

Thus,
## \sigma_2 = \sigma_4 = \sigma_6 = -\sigma_3 =- \sigma_5 = -\sigma_7 ##.

But also, ## \sigma_1 + \sigma_2 = Q ## and
## \sigma_7 + \sigma_8 = -\sigma_2 + \sigma_8 = -Q ##.

Now put a + test charge inside plate 1. This charge will see a rightward force due to ## \sigma_1 ## and a leftward force force due to ## \sigma_8 ##. Note that ## \sigma_2 ## thru ## \sigma_7 ## forces cancel each other out.

That leaves us with
## \sigma_1 + \sigma_2 = Q ##
## \sigma_8 - \sigma_2 = -Q ##
and since the net force on the test charge must = 0,
## \sigma_1 - \sigma_8 = 0 ##.

OK look at these equations. They have no solution!
So the assumed charge distribution is impossible.

Matter of fact, no matter what charge you put on each plate, no matter how many plates, the outside 2 surfaces must have the same charge including sign!
 
Last edited:
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