Monday, May 26, 2014

electrostatics - Why do capacitors in a dielectric have the same charge as without the dielectric



If you imagine a dielectric of height $d$ and dielectric constant $\kappa$ placed evenly in a capacitor of height $D$. We define $\Delta x = \frac{D-d}{2}$.



The equivalent capacitance is now


$$\begin{align} \frac{1}{C_{eq}} &= 2 \frac{\Delta x }{\epsilon_0 A} + \frac{d}{\kappa \epsilon_0 A} \\ &= \frac{D-d}{\epsilon_0 A} + \frac{d}{\kappa \epsilon_0 A} \\ &= \frac{\kappa(D-d)+d}{\kappa\epsilon_0 A} \end{align} $$


At this point


$$C_{eq} = \frac{\kappa\epsilon_0 A}{\kappa(D-d)+d}$$


Based on prior knowledge, it should be that the charge on the equivalent capacitor is the same as the charge on the original, non dielectric capacitor


In other words:


$q = CV = C_{eq}V$ and voltage hasn't changed.


But $CV \ne C_{eq}V$!


What am I missing?



Answer




My friend, you spoke too soon. If you find the new voltage,


$$V^\prime = \frac{q\kappa(D-d)x + qd}{\epsilon_0 A \kappa} = \frac{q}{\epsilon_0A\kappa}\bigg(\kappa(D-d)+d\bigg)$$


Where $$C_{eq} = \frac{\kappa \epsilon_0 A}{k(D-d)+d}$$


Multiply out $C_{eq}V^\prime$ everything cancels and you are left with:


$$q^\prime = C_{eq} V^\prime = q$$


Therefore $q$ is constant wrt introduction of dielectrics.


Clarification Regarding $\kappa$


In the derivation we implicitly assume that $E^\prime = \frac{E_0}{\kappa}$, the same $\kappa$ from $\kappa = \frac{C^\prime}{C}$. The textbook justifies this by writing that such is true. That we are dealing with the same kappa in both of the last two equations is quite startling – what is the motivation for making the assumption that $E^\prime = \frac{E_0}{\kappa}$.


Doing a little bit of contextualization work, we can postulate that $\kappa$ was originally defined as $$\kappa = \frac{q}{q-q^\prime}$$ where $q^\prime$ is the charge induced. With this being true, we go through a set of simultaneous steps beginning with


$$E_0= \frac{q}{\epsilon_0 A} \qquad E^\prime = \frac{q-q^\prime}{\epsilon_0 A} = \frac{q}{\epsilon_0 A \kappa } = \frac{E_0}{\kappa} $$



Leading to


$$V_0= \frac{qd}{\epsilon_0 A} \qquad V^\prime = \frac{qd}{\epsilon_0 A} \kappa$$


Plugging the expressions for $V_0$ and $V^\prime$ into the definition of capacitance $C = \frac{q}{V}$ we obtain that


$$\frac{C^\prime}{C} = \kappa$$


This last broad statement about capacitance emerges from the definition of $\kappa$ as it relates the the microscopic concern of induced charges on the dielectric.


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