# The Ungerecht Family Tree

urp:physelec

## Electricity

### Circuits

Power = watts, current (I) = amps, volts (E or V), resistance (R) = ohms

• Ohm's Law e = ir
• p = ie = i²r

Resistors in parallel: 1/Rt = 1/R₁ + 1/R₂ + …

current in parallel: if have V, compute R for each path. For two paths, R₁ & R₂: I on R₁ = R₁/(R₁+R₂)

Voltage divider: Vout = R₂/(R₁+R₂) * Vtotal, where R₂ is resistor between Vout and ground

Joules = watts * seconds

### Capacitance

Q = CV , Q = charge (coulombs, always a positive value), C = capacitance (farads), V = voltage

Parallel plate capacitor

```C = epsilon (A/d), A = area, d = distance between plates
epsilon = permittivity of the dielectric
dielectric constant (over free space) = 8.84 * 10⁻¹² Farads/meter```

Electric field E = V/d (volts/meter)

```E = sigma/epsilon,  sigma = charge density
sigma = Q/A```

Coulomb's law: Find force between two charged objects

```F = k * q₁ * q₂ / r²
k = 9 * 10⁹ N m²/C²
r = distance in meters
C = coulombs
C = Q/V = Q/Ed = A*epsilon/d```

### Point Charges

Back to Physics page or Start page.