Current work, power, electrolysis

Work and power

The work and power of electric current, its heating effect, and the laws of electrolysis.

• Work of current: A = U·I·t.
• Power: P = UI = I²R = U²/R.
• Joule–Lenz law: Q = I²Rt.
• 1 kW·h = 3,6 MJ.

Formula	When it is convenient
P = UI	U and I are known
P = I²R	current and resistance are known
P = U²/R	voltage and resistance are known

Examples

• 220 V, 2 A, 10 s: A = 220·2·10 = 4400 J.
• R = 5 Ω, I = 3 A, t = 4 s: Q = 9·5·4 = 180 J.
• If a 1,5 kW heater runs for 2 hours, it consumes 1,5·2 = 3 kW·h of energy.

The kW·h is a unit of energy, not of power: to convert it to joules, multiply by 3,6·10⁶.

Electrolysis

Faraday's law: m = k·I·t, where k is the electrochemical equivalent. In other words: m = (M/(nF))·I·t, F = 96 500 C/mol.

1. Find the charge: q = I·t
2. Divide the charge by F: number of moles of electrons
3. Multiply by M/n: mass m

Electrolysis is used for electroplating, and for refining and obtaining metals.