in 📓 Notes

# Physics

## Voltage

The voltage is the difference in electrical potential between two points. It’s measured in Volts, $V$.

## Capacitor

• A capacitor can store electrical charge.
• Buffers voltage changes.
• Builds up charge on the two sides of an insulator.

## Capacitance

Capacitance refers to the ability of a capacitor to store charge. Represented in Farads, $F = A^2 s^4 kg^{−1} m^{−2}$.

• ⬆ area –> ⬆ capacitance

### Parallel Plate Capacitor

The formula for the capacitance of a parallel plate capacitor is as follows:

$$C=\epsilon\frac{A}{d}$$

Where:

• $A$ is the area of overlap of the two plates in $m^2$.
• $d$ is the separation between the plates in $m$.
• $\epsilon$ is the permittivity.

## Resistance

Resistance is how much a conductor opposes the [current][#current]. The unit is Ohm, $\Omega$.

## Conductance

Conductance is the inverse of resistance, representing the ease with which an electric current passes. The unit is Siemens, $S = \Omega^{-1}$.

$C = \frac{1}{R}$

## Current

Current is the flow of charge. Usually represented by $I$. Measured in Amperes.

## Ohm’s Law

$$I = \frac{V}{R}$$

Where:

## Time Constant

• How much time it takes to charge 63%

$$\tau=RC$$

• $R$ in Ω cm
• $C$ in F

## Length Constant

• Distance it takes for a potential change to fall off ~37%.

$$\lambda=\sqrt{\frac{R_{membrane}}{R_{axial}}}$$

• $R_{membrane}$ in Ω cm
• $R_{axial}$ in Ω / cm

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