If you are preparing for an exam or just want to truly understand current electricity—not just memorize answers—you have come to the right place. Most MCQ sets give you the correct letter and stop there. This guide gives you the concept, the calculation, and the real-world takeaway for every single question.
Section 1: Basic Nature of Electric Current
Q1. The resistance of a conductor varies with:
(a) applied magnetic field (b) illumination (c) pressure (d) temperature
Correct Answer: (d) temperature
Explanation: For most conductors (metals), resistance increases as temperature increases. This is due to increased atomic vibrations that scatter electrons. Some materials (carbon, germanium) show decreased resistance with temperature—these have a negative temperature coefficient.
Real-world takeaway: This is why your phone or laptop slows down when hot—internal resistance increases, reducing efficiency.
Q2. Through a metallic conductor, electric current is due to the drift of:
(a) protons (b) neutrons (c) +ve ions (d) free electrons
Correct Answer: (d) free electrons
Explanation: In metals, the outer electrons of atoms are loosely held and form a “sea” of free electrons. When a voltage is applied, these electrons drift (slowly!) from negative to positive. Protons and neutrons are locked in the nucleus and do not move.
The common misconception: Electrons actually drift at about 0.1 mm/second. The electric signal travels near the speed of light, but the individual electrons crawl.
Q3. Through an electrolyte, electric current is due to the drift of:
(a) protons (b) free electrons (c) +ve and -ve ions (d) free electrons and holes
Correct Answer: (c) +ve and -ve ions
Explanation: In an electrolyte (salt water, acid, base), current is carried by both positive and negative ions moving in opposite directions. Positive ions (cations) move toward the cathode; negative ions (anions) move toward the anode.
Real-world takeaway: This is how batteries work and how electroplating deposits metal onto surfaces.
Q4. Current of 3.2 A in a conductor. Number of electrons crossing any section per second is:
(a) 2 × 10¹⁹ (b) 0.2 × 10¹⁹ (c) 20 × 10¹⁹ (d) 200 × 10¹⁹
Correct Answer: (a) 2 × 10¹⁹
Solution:
- Current (I) = 3.2 A = 3.2 coulombs/second
- Charge of one electron (e) = 1.6 × 10⁻¹⁹ C
- Number of electrons (n) = I / e = 3.2 / (1.6 × 10⁻¹⁹) = 2 × 10¹⁹ electrons per second
Real-world takeaway: That is 20,000,000,000,000,000,000 electrons flowing every second—yet the wire does not get lighter because the same number enters the other end.
Section 2: Material Properties & Temperature Effects
Q5. Which has a positive temperature coefficient of resistance?
(a) germanium (b) electrolyte (c) carbon (d) copper
Correct Answer: (d) copper
Explanation:
| Material | Temperature Coefficient | Behavior |
|---|---|---|
| Copper (metal) | Positive | Resistance ↑ as temperature ↑ |
| Carbon | Negative | Resistance ↓ as temperature ↑ |
| Germanium (semiconductor) | Negative | Resistance ↓ significantly as temperature ↑ |
| Electrolyte | Negative | Resistance ↓ as temperature ↑ |
Pro tip: Positive temperature coefficient metals are used in thermistors for overcurrent protection and in resistance thermometers (RTDs).
Q6. Conductance of some metals rises to infinity at low temperature is called:
(a) optical conductivity (b) thermal conductivity (c) magnetic conductivity (d) superconductivity
Correct Answer: (d) superconductivity
Explanation: Below a critical temperature (Tc), certain materials exhibit exactly zero electrical resistance. This is not just “very low”—it is truly zero. Current once started flows forever without a power source.
Real-world takeaway: Superconductors are used in MRI machines (for powerful magnetic fields), particle accelerators, and quantum computers. The challenge: most require cooling to liquid helium temperatures (4K, -269°C).
Q7. To deposit one litre of hydrogen at 22.4 atm from acidulated water, electricity needed is:
(a) 22.4 coulomb (b) 1 coulomb (c) 193000 coulomb (d) 96500 coulomb
Correct Answer: (d) 96500 coulomb
Explanation:
- One litre of hydrogen at STP (22.4 L = 1 mole) is not quite right here—the question specifies “22.4 atm” which is likely a typo or misprint.
- The key principle: Faraday’s laws of electrolysis state that 1 mole of electrons (6.022 × 10²³ electrons) carries 96,500 coulombs (1 Faraday).
- To produce 1 gram equivalent of hydrogen (1.008 g), you need 96,500 C.
Takeaway: 96,500 C is one Faraday—a fundamental constant in electrochemistry.
Section 3: Kirchhoff’s Laws & Basic Circuits
Q8. Kirchhoff’s 1st law (Junction rule) is based on conservation of:
(a) energy (b) momentum (c) mass (d) charge
Correct Answer: (d) charge
Explanation: Kirchhoff’s Current Law (KCL) states: total current entering a junction equals total current leaving. This is charge conservation—charge cannot accumulate at a junction.
Kirchhoff’s 2nd law (Loop rule) is based on conservation of energy.
Memory trick: 1st law = Current (charge). 2nd law = Voltage (energy).
Q9. Equivalent resistance of resistors in parallel is always:
(a) equal to sum (b) higher than highest (c) less than lowest (d) between lowest and highest
Correct Answer: (c) less than the lowest of component resistors
Formula: 1/R_eq = 1/R₁ + 1/R₂ + 1/R₃
Example: Two 10Ω resistors in parallel → R_eq = 5Ω, which is less than 10Ω.
Why: Adding parallel paths gives more space for current, reducing total opposition.
Q10. Cell (2V, negligible resistance) connected to 2Ω, 3Ω, 5Ω in series. Potential difference across 3Ω resistor is:
(a) 0.4V (b) 0.5V (c) 0.6V (d) 0.7V
Correct Answer: (c) 0.6V
Solution:
- Total resistance = 2 + 3 + 5 = 10Ω
- Current I = V/R = 2/10 = 0.2 A
- Voltage across 3Ω = I × R = 0.2 × 3 = 0.6 V
Concept: In series, current is same everywhere; voltage divides proportionally to resistance.
Q11. No potential difference applied across a metallic block → mean velocity of free electrons is:
(a) inversely proportional to T (b) directly proportional to T (c) directly proportional to √T (d) zero
Correct Answer: (d) zero
Explanation: With no applied voltage, electrons move randomly in all directions. The net drift velocity (average vector velocity) is zero. Their average speed is not zero—it’s related to temperature—but the question asks for mean velocity (direction considered), which is zero.
Key distinction: Velocity = vector (has direction). Speed = scalar (magnitude only).
Q12. Equivalent resistance of resistors in series is always:
(a) equal to sum (b) between lowest and highest (c) equal to mean (d) less than lowest
Correct Answer: (a) equal to the sum of the component resistors
Formula: R_eq = R₁ + R₂ + R₃
Why: Series connection forces current through each resistor sequentially, adding all oppositions.
Q13. Example of a non-ohmic resistance is:
(a) tungsten wire (b) carbon resistance (c) copper wire (d) diode
Correct Answer: (d) diode
Explanation: Ohmic materials follow Ohm’s law (V = IR) with constant resistance. Diodes have non-linear V-I characteristics—current increases exponentially after a threshold voltage (~0.7V for silicon).
Common non-ohmic devices: Diodes, transistors, thermistors (NTC/PTC), varistors (VDR).
Q14. Three resistors (2Ω, 3Ω, 5Ω) in parallel to 10V battery (negligible resistance). Potential drop across 3Ω is:
(a) 0V (b) 10V (c) 20V (d) 30V
Correct Answer: (b) 10V
Explanation: In a parallel circuit, each resistor is directly connected across the battery terminals. The voltage across each resistor equals the battery voltage (10V), regardless of resistance value.
Common student mistake: Thinking voltage divides in parallel. It does not—current divides, voltage is same.
Section 4: Resistance, Resistivity & Geometry
Q15. Specific resistance (resistivity) of a wire:
(a) varies with mass (b) varies with length (c) varies with cross-section (d) is independent of length, mass, area
Correct Answer: (d) is independent of length, mass, area of cross-section
Formula: Resistivity ρ = R × A / L
- R = resistance, A = cross-sectional area, L = length
Real-world takeaway: Resistivity is an intrinsic material property. It changes only with temperature and material composition, not with how much material you have.
Q16. 20 cm long wire, resistance 5Ω. Stretched uniformly to 40 cm. New resistance is:
(a) 2Ω (b) 20Ω (c) 200Ω (d) 2000Ω
Correct Answer: (b) 20Ω
Solution:
- When stretched to double length, cross-sectional area halves (volume constant: A₁L₁ = A₂L₂)
- Resistance R = ρL/A
- R₂/R₁ = (L₂/L₁) × (A₁/A₂) = 2 × 2 = 4
- New resistance = 5Ω × 4 = 20Ω
The 4× rule: When length doubles (area halves), resistance increases by factor of 4.
Q17. Aluminium wire drawn through dies to reduce diameter to half. New resistance is:
(a) 13× (b) 14× (c) 15× (d) 16×
Correct Answer: (d) sixteen times
Solution:
- Diameter halves → radius halves → cross-sectional area A ∝ r² → A becomes (1/2)² = 1/4 of original
- Volume constant: A₁L₁ = A₂L₂ → L₂/L₁ = A₁/A₂ = 4 (length quadruples)
- R ∝ L/A → R₂/R₁ = (L₂/L₁) × (A₁/A₂) = 4 × 4 = 16×
Memory trick: For diameter change, resistance scales as (d₁/d₂)⁴ = 2⁴ = 16.
Section 5: Cells, Internal Resistance & Networks
Q18. Primary cell: EMF = 2V, short circuit current = 4A. Internal resistance is:
(a) 0.3Ω (b) 0.4Ω (c) 0.5Ω (d) 0.6Ω
Correct Answer: (c) 0.5Ω
Solution:
- Short circuit current I_sc = EMF / r (where r = internal resistance)
- 4A = 2V / r
- r = 2V / 4A = 0.5Ω
Real-world takeaway: A car battery might have r ≈ 0.02Ω, allowing hundreds of amps for starting. A small 9V battery has r ≈ 1-2Ω, limiting short circuit current.
Q19. Two resistances in parallel: resultant = 6/5 Ω. One breaks → effective = 2Ω. Broken wire resistance was:
(a) 1Ω (b) 2Ω (c) 3Ω (d) 4Ω
Correct Answer: (c) 3Ω
Solution:
- Let resistances be R and X (R remains, X breaks)
- Parallel formula: 1/R_eq = 1/R + 1/X = 5/6
- After X breaks: R = 2Ω (only R remains)
- Substitute: 1/2 + 1/X = 5/6 → 1/X = 5/6 – 1/2 = 5/6 – 3/6 = 2/6 = 1/3
- Therefore X = 3Ω
Q20. Four wires (10Ω each) in a square. Equivalent resistance between opposite corners is:
(a) 0Ω (b) 10Ω (c) 20Ω (d) 30Ω
Correct Answer: (b) 10Ω
Solution:
- Square: label corners A, B, C, D. Opposite corners = A and C.
- Path 1: A→B→C (two 10Ω resistors in series = 20Ω)
- Path 2: A→D→C (two 10Ω resistors in series = 20Ω)
- These two paths are in parallel: 1/R_eq = 1/20 + 1/20 = 2/20 = 1/10
- R_eq = 10Ω
Visualization: Think of a square with resistors on each side. Between opposite corners, you have two parallel branches, each containing two resistors in series.
Section 6: Electrolysis & Conductivity in Liquids
Q21. Water can be made conducting by adding any of these except:
(a) CuSO₄ (b) NaCl (c) NH₄Cl (d) Sugar
Correct Answer: (d) Sugar
Explanation: Conductivity in water requires ions (charged particles).
- CuSO₄ → Cu²⁺ + SO₄²⁻ (ionic)
- NaCl → Na⁺ + Cl⁻ (ionic)
- NH₄Cl → NH₄⁺ + Cl⁻ (ionic)
- Sugar (C₁₂H₂₂O₁₁) → dissolves as neutral molecules, no ions → does not conduct
Real-world takeaway: Pure water is an insulator. “Tap water conducts” because of dissolved minerals (ions). Deionized water is used in laboratories for its high resistivity.
Q22. To deposit one gram equivalent of an element at an electrode, charge needed is:
(a) one statcoulomb (b) one absolute coulomb (c) one coulomb (d) 96500 coulomb
Correct Answer: (d) 96500 coulomb
Explanation: This is Faraday’s constant (F) – the charge carried by one mole of electrons.
- F = 96,485 C/mol ≈ 96,500 C
- One gram equivalent of any element requires 96,500 C of charge.
Examples:
- 1 gram equivalent of silver (107.87 g / 1 = 107.87 g) requires 96,500 C
- 1 gram equivalent of hydrogen (1.008 g) requires 96,500 C
- 1 gram equivalent of copper (63.55 g / 2 = 31.78 g) requires 96,500 C
Electroplating formula: Mass deposited = (I × t × equivalent weight) / 96,500
📊 Quick Answer Key (For Review)
| Q# | Answer | Q# | Answer | Q# | Answer |
|---|---|---|---|---|---|
| 1 | d | 9 | c | 17 | d |
| 2 | d | 10 | c | 18 | c |
| 3 | c | 11 | d | 19 | c |
| 4 | a | 12 | a | 20 | b |
| 5 | d | 13 | d | 21 | d |
| 6 | d | 14 | b | 22 | d |
| 7 | d | 15 | d | ||
| 8 | d | 16 | b |
Final Tips for Exam Success
- Memorize the constants:
- Electron charge = 1.6 × 10⁻¹⁹ C
- Faraday constant = 96,500 C
- Relationship: 1 A = 1 C/s
- Remember the key distinctions:
- Series: current same, voltage divides
- Parallel: voltage same, current divides
- Resistivity (ρ) is material property; resistance (R) depends on geometry
- Watch for common traps:
- “Mean velocity” vs. “average speed” (Q11)
- Parallel voltage vs. series voltage (Q14)
- Stretching wire: resistance scales with (L₂/L₁)² (Q16, Q17)
