Electric Current — Formula Sheet

I_av = ΔQ / Δt

I = nAev_d

V = IR ⟹ R = V/I

R = ρl/A

ρ = RA/l and σ = 1/ρ

R = R₀[1 + α(T − T₀)]

α = (R₂ − R₁) / [R₀(T₂ − T₁)]

R_series = R₁ + R₂ + R₃ + ...

1/R_parallel = 1/R₁ + 1/R₂ + 1/R₃ + ...

R_parallel = R₁R₂ / (R₁ + R₂) [for two resistors only]

E = V + Ir ⟹ V = E − Ir

I = E / (R + r)

Resistance (R) = V/I. It opposes the flow of current. SI unit: Ohm (Ω). 1 Ω = 1 V/A.

Series equivalent resistance

R_eq = R₁ + R₂ + R₃ + ... (always GREATER than any individual resistance).

Electric current = rate of flow of charge through a cross-section: I = ΔQ/Δt

SI unit of current is Ampere (A); 1 A = 1 C/s

Ohm's Law

V = IR, where R = resistance in ohms (Ω)

Temperature coefficient α = ΔR / (R₀ × ΔT); unit is per °C or per K