Current Electricity: One-Page Formula Sheet

Current Electricity — formula sheet

Formulas

• R = ρL/A
• Series: R = ΣRᵢ; parallel: 1/R = Σ1/Rᵢ
• Power P = VI = I²R = V²/R
• Max power transfer when external R = internal r

Key ideas

• Drift velocity v_d = I/(neA)
• Kirchhoff's junction rule (charge) and loop rule (energy)
• Balanced Wheatstone bridge: P/Q = R/S
• Terminal voltage V = E − Ir when supplying current

Traps to avoid

• Sign errors in the loop rule
• Missing a balanced bridge that removes a resistor
• Forgetting resistance changes when a wire is stretched (R ∝ L²)

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