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Circuits

Ohm's law

Ohm's law: the current through a conductor is proportional to the voltage across it, V = IR, at a fixed temperature. A 1200 Ω bulb filament on 220 V draws I = 220 / 1200 = 0.18 A.

Ohm's law

V = IR at a fixed temperature. Pick what to find and type the other two. Add a time to get the heat, H = I²Rt, and the energy in kWh.

Current I0.18 A
Power40.33 W
  1. I = V / R = 220 / 1200 = 11/60 ≈ 0.18 A
  2. Power: P = VI = 220 × 0.18 = 121/3 ≈ 40.33 W. The same as I²R and V²/R.
Derivation

Why it works

  1. Connect a nichrome wire, an ammeter in series and a voltmeter across it, with 1, 2, 3 and 4 cells in turn.
  2. Each time note V and I. The ratio V/I stays the same.
  3. So V ∝ I, or V = IR. The constant R is the resistance, in ohms: 1 Ω = 1 V/A.
  4. The V-I graph is a straight line through the origin, and its slope is R.
  5. The law holds while the temperature stays the same. A hot filament's resistance rises.
Questions

Ohm's law: quick answers

What is Ohm's law?

At a constant temperature, the current through a conductor is directly proportional to the potential difference across it: V = IR.

What is 1 ohm?

A conductor has a resistance of 1 Ω when a potential difference of 1 V across it drives a current of 1 A through it.

Do all conductors obey Ohm's law?

No. Metals at a steady temperature do. A diode, a filament as it heats up and semiconductors do not, so their V-I graph is not a straight line.

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