Let’s recall if you have a collection of charges, then there is potential energy stored in the collection of charges.
Potential is a scalar quantity.
Potential of a point charges
$V(r)=\frac{Q}{4 \pi \epsilon_0 r}$
So, that is a scalar quantity and potential of this point charge depends only on the distance of the point from the point charge.And the potential follows the principle of superposition. So, if you have multiple charges, then the total potential at any point is the sum of the potentials created by each and every individual charge.
Work done in moving a unit charge from A to B = $\left(V_0+d V\right)-V_0 =d V $
Work done = $ -\vec{E} \cdot \vec{dl}=-E dl \cos \theta $
$=-E_l \cdot dl $ $ -E_l dl =dV $
$\Rightarrow \quad E_l=-\frac{d V}{dl}$
$ E_x =-\frac{\partial V}{\partial x}$
$E_y =-\frac{\partial V}{\partial y}$
$E_z =-\frac{\partial V}{\partial z}$
$ \vec{E} =\hat{\imath} E_x+ \hat {j} E_y+\hat{\imath} E_z$
$=-\left(\hat{\imath} \frac{\partial V}{\partial x}+\hat{\jmath} \frac{\partial V}{\partial y}+ \hat{\imath} \frac{\partial V}{\partial z}\right)$
Example:
$r=\sqrt{x^2+y^2+z^2}$
$V(x, y, z)=\frac{Q}{4 \pi \epsilon_0 \sqrt{x^2+y^2+z^2}} $
$E_x=-\frac{\partial V}{\partial x}=-\frac{Q}{4 \pi \epsilon_0} \frac{1}{\left(x^2+y^2+z^2\right)^{3 / 2}}\left(-\frac{1}{2}\right) \cdot 2 x$
$=\frac{a}{4 \pi \epsilon_0\left(x^2+y^2+z^2) \right.} \frac{x}{\sqrt{x^2+y^2+z^2}}$
$E_x =\frac{Q}{4 \pi \epsilon_0 r^2} \frac{x}{r}$
$E_y =\frac{Q}{4 \pi \epsilon_0 r^2} \cdot \frac{y}{r}$
$E_z =\frac{Q}{4 \pi \epsilon_0 r^2} \cdot \frac{z}{r}$
$\vec{E} =\imath E_x+\hat{\jmath} E_y+\hat{\imath} E_z$ $=\frac{Q}{4 \pi \epsilon_0 r^2} \frac{\left(\hat{\epsilon} x+\hat{\jmath} y+\hat{\tau}_z\right)}{r}$
$\vec{E}=\frac{Q}{4 \pi \epsilon_0 r^2} \frac{\vec{r}}{r}=\frac{Q \vec{r}}{4 \pi \epsilon_0 r^2}$
We know V as a function of x, y and z, I can use these three relationships to calculate $ E_x, E_y and E_z$ and from there the total electric field $\vec E$.
Arbitrary relationship conductor.
No excess charges within the conductor.
$\sigma=\frac{Q}{4 \pi R^2}$
We have a conductor of arbitrary shape and I have a cavity.
This is the Gaussian surface enclosing the cavity and that Gaussian surface lies entirely within the conductor.
Gaussian Surface
$\oint \vec{E} \cdot \overrightarrow{d l}=0$
$\oint_C \vec{E} \cdot \vec{d l} \neq 0$
There can be no access charge on the inner surface
Conductor a spherical conductor with a spherical cavity cetered in the conductor
A charge -Q in place at the centerd of the cavity
a) Calculate the surface charge density on the inner & outer surface
b) Calculate the electric field every here
$ V_a=\frac{q_a}{4 \pi \epsilon_0 a} $
$V_b=\frac{q_b}{4 \pi \epsilon_0 b} $
$V_a=V_b $
-$ \Rightarrow \frac{q_a}{a}=\frac{q_b}{b} $
$ V(r)=\frac{Q}{4 \pi \epsilon_0 r} $
at r=R
$ V(R)=\frac{Q}{4 \pi \epsilon_0 R}$
$ q_a=\sigma_a \cdot 4 \pi a^2 $
$ q_b=\sigma_b \cdot 4 \pi b^2 $
$ \frac{\sigma_a \cdot 4 \pi a^2}{a}=\frac{\sigma_b \cdot 4 \pi b^2}{b} $
$ \sigma_a a=\sigma_b b $
Electric field = $ \frac{\sigma}{\epsilon_0} $
There will be some positive charges and they will there will be more positive charge accumulated here.
So, the positive charge density will increase.
$ E_a=\frac{\sigma_a}{\epsilon_0} $ ; $ E_b=\frac{\sigma_b}{\epsilon_0}$
$ E_a a=E_b, b $
$ \frac{E_b}{E_a}=\frac{a}{b} $
Plate area : A
Plate separation : d
Potential difference
$ V=E \cdot d =\frac{\sigma d }{\epsilon_0} $
$ \sigma=\frac{Q}{A} $
$V=Q \cdot \frac{d}{\epsilon_0 A}$
$ V=Q \cdot \frac{d}{\epsilon_0 A} $
$ C=\frac{\epsilon_0 A}{d} $ Capacitance
$ V=\frac{Q}{C} $
$ Q=C V $
C : Capacitance parameter
$ d=1 \mathrm{~mm} $
A=10 $ \mathrm{~cm}^2 $
Capacitance C= $ \frac{\epsilon_0 A}{d}$$ = \frac{8.85 \times 10^{-12} \times 10 \times 10^{-4}}{10^{-3}} = 8.85 \mathrm{pF} $
$ C=8.85 \times 10^{-12} \mathrm{~F} $
Potential difference $ V$
$ Q=C V =8.85 \times 10^{-12}(\mathrm{~F}) \times 1(\mathrm{V}) $ $ =8.85 \times 10^{-12} \mathrm{C} =8.85 \mathrm{kC} $