Ohm’s Law, Resistance & Resistivity — A/L Physics

What must I remember about Ohm’s Law, Resistance & Resistivity?

• Ohm's Law: At constant temperature and physical conditions, electric current flowing through a metallic conductor is directly proportional to the potential difference across its terminals: V ∝ I => V = I R.
• Electrical Resistance (R): Opposition offered by a conductor to current flow: R = V / I (units Ohm Ω = V/A); Resistance of uniform wire: R = (ρ L) / A, where L is length, A is cross-sectional area, and ρ is resistivity (Ω·m).
• Electrical Resistivity (ρ) & Conductivity (σ): Resistivity ρ is an intrinsic material property independent of dimensions; Electrical conductivity σ = 1 / ρ (S/m = Ω⁻¹·m⁻¹).
• Temperature Dependence of Resistance: For pure metals: R_T = R₀ (1 + α ΔT) where α is positive temperature coefficient of resistance (resistance increases with heating due to enhanced lattice vibrational scattering).

Test yourself (1 minute)

1. Ohmic vs Non-Ohmic Devices

   Ohmic conductors exhibit linear I-V characteristic passing through origin (constant R); Non-Ohmic conductors exhibit non-linear I-V curve (e.g., filament lamp curves as filament heats up, semiconductor diode conducts only in forward bias, thermistor resistance drops with temperature).

2. Negative Temperature Coefficient (NTC)

   Semiconductors and electrolytes have NEGATIVE temperature coefficients of resistance (α < 0); resistance DECREASES with heating because thermal generation of charge carriers dramatically increases carrier density n.

3. Superconductivity

   Phenomenon wherein certain metals and alloys lose ALL electrical resistance (R = 0 identically) below a characteristic critical transition temperature T_c (e.g., Mercury at 4.2 K).

Practise Ohm’s Law, Resistance & Resistivity questions today

Source: NIE G.C.E. (A/L) Physics Resource Book - Unit 7: Current Electricity

What should I study first?

What comes next?

Next in the syllabus

Where does Ohm’s Law, Resistance & Resistivity fit in the syllabus?

Unit 7.2 · Year 2 · Term 1