22. Important Formulas — Quick Reference

Coulomb's law

F = k q₁q₂ / r²

Electric current

I = Q / t

Ohm's law

V = I R

Resistance from dimensions

R = ρL / A

Resistors in series

R = R₁ + R₂ + R₃ + ...

Resistors in parallel

1/R = 1/R₁ + 1/R₂ + 1/R₃ + ...

Electric power

P = VI = I²R = V²/R

Electrical energy

Energy = P × t

1 kWh (1 unit)

1 kWh = 3.6 × 10⁶ J

Joule's law of heating

H = I² R t

Transformer voltage-turns ratio

Vs/Vp = Ns/Np