V = I R · R = ρ L / A · Change the battery and the wire, and watch the charges, meters and heat.
Voltage V0 V
Current I0 A
Resistance R0 Ω
Heat produced P = I²R0 W
V = I R Ohm's law: at constant temperature, the current through a conductor is directly proportional to the potential difference across it: I ∝ V.
Raise the battery voltage and press Record reading a few times. The points lie on a straight line through the origin. Its slope (V / I) is the resistance R, measured in ohm (Ω): 1 Ω = 1 V / 1 A.
The ammeter is joined in series, the voltmeter in parallel across the wire.
R = ρ L / A ρT = ρ20 [1 + α (T − 20 °C)] Length: double L and electrons pass twice as many ions, so R doubles (R ∝ L). Area: a thicker wire gives more paths side by side, so R halves when A doubles (R ∝ 1/A). Material: resistivity ρ differs: copper 1.7 × 10⁻⁸, aluminium 2.8 × 10⁻⁸, iron 1.0 × 10⁻⁷, nichrome 1.1 × 10⁻⁶ Ω m. Nichrome is used in heaters. Temperature: hotter ions vibrate more and block electrons more often, so R increases for metals.
Kamran Afsar Khan · Professor of PhysicsElectric Current