Ohm's Law — Resistance, resistivity, conductivity
Current Electricity
19
JEE Qs
8%
Hard
60
min
Clearly distinguish between the macroscopic form (V=IR) and the microscopic form (J=σE) of Ohm's Law and understand their respective conditions of applicability.
🧮 Key Formulas
✅ Key Points for JEE
- 1Ohm's Law (V=IR) is applicable only to ohmic devices, typically for metallic conductors, and under constant physical conditions like temperature.
- 2Resistance (R) is a property of a specific conductor, depending on its material (resistivity ρ), length (L), and cross-sectional area (A).
- 3Resistivity (ρ) is an intrinsic property of the material itself, independent of the conductor's geometry. Its reciprocal is conductivity (σ).
- 4The microscopic form of Ohm's Law, J = σE, is more fundamental and universally valid, relating current density (J) to the electric field (E) and material conductivity (σ).
- 5Resistance and resistivity of most materials (especially metals) are temperature-dependent, increasing with temperature (positive temperature coefficient α).
⚠️ Common Mistakes
- ✕Confusing resistance (R) with resistivity (ρ) and conductivity (σ) — R is for a specific object, while ρ and σ are material properties.
- ✕Applying Ohm's Law (V=IR) blindly to non-ohmic devices (e.g., diodes, transistors) or assuming resistance is constant irrespective of temperature.
- ✕Incorrectly using units (e.g., area in cm^2 instead of m^2, leading to significant errors in calculations involving resistivity).
📝 Practice Questions
See allQ47.The value of current I in the electrical circuit as given below, when potential at A is equal to the potential at B, will be ____ A.
Q27.Given below are two statements : Statement-I : The equivalent emf of two nonideal batteries connected in parallel is smaller than either of the two emfs. Statement-II : The equivalent internal resistance of two nonideal batteries connected in parallel is smaller than the internal resistance of either of the two batteries. In the light of the above statements, choose the correct answer from the options given below. (1) Both Statement-I and Statement-II are false (2) Statement-I is false but Statement-II is true (3) Both Statement-I and Statement-II are true (4) Statement-I is true but Statement-II is false
Q34. Sliding contact of a potentiometer is in the middle of the potentiometer wire having resistance Rp = 1Ω as shown in the figure. An external resistance of Re = 2Ω is connected via the sliding contact. The electric current in the circuit is : (1) 0.9 A (2) 1.35 A (3) 0.3 A (4) 1.0 A 2025 (22 Jan Shift 1) JEE Main Previous Year Paper
Q45.Which of the following resistivity ( ρ ) v/s temperature (T) curves is most suitable to be used in wire bound standard resistors? (1) (2) (3) (4)
Q47.The net current flowing in the given circuit is_______ A.
Q30.Refer to the circuit diagram given in the figure. which of the following observations are correct? A. Total resistance of circuit is 6Ω B. Current in Ammeter is 1 A C. Potential across AB is 4 Volts. D. Potential across 2025 (23 Jan Shift 1) JEE Main Previous Year Paper CD is 4 Volts E. Total resistance of the circuit is 8Ω. Choose the correct answer from the options given below: (1) A, B and D Only (2) A, B and C Only (3) A, C and D Only (4) B, C and E Only
NCERT Chapters
- Class 12 Physics Ch 3: Current Electricity