Try: circle area, U = I × R, radius
Time constants, capacitor charging and current growth.
τ = RC
τ = L / R
I = I∞(1 − e⁻ᴿᵗ⁄ᴸ)
U꜀ = U(1 − e⁻ᵗ⁄ᴿᶜ)