📑 Topics on this page (Jump to Topic):
📌 1. Current, Ohm's Law & Resistance
- Current I = Q/t (ampere); electrons flow from negative to positive, conventional current from positive to negative. Electron charge 1.6 × 10⁻¹⁹ C.
- Ohm's law: V = IR (at constant temperature). An ammeter is connected in series (low resistance), a voltmeter in parallel (high resistance).
- Resistance R = ρL/A — proportional to length, inversely to cross-section. In metals resistance rises with temperature; in semiconductors it falls.
- Series: R = R₁ + R₂ (same current). Parallel: 1/R = 1/R₁ + 1/R₂ (same voltage) — home appliances are in parallel so each gets 220 V and others keep working if one is off.
- Silver is the best conductor, then copper, then aluminium. Superconductors have zero resistance at very low temperatures.
📌 2. Electric Power & Household Electricity
- P = VI = I²R = V²/R (watt). Energy = P × t; 1 unit = 1 kWh.
- Domestic supply in India is 220–230 V, 50 Hz AC; in the USA 120 V, 60 Hz.
- Fuse: wire of low melting point and high resistance (tin-lead alloy), always in series and on the live (phase) wire. MCBs are used today.
- Wire colours (old standard): red — live, black — neutral, green — earth. Earthing protects against shocks from metal-bodied appliances.
- A bulb filament is tungsten (m.p. ≈ 3422 °C) filled with inert gas (argon/nitrogen). A heater element is nichrome (high resistance, high m.p.). LEDs are the most efficient.
- A bird sits safely on a single live wire — negligible potential difference between its feet, no complete circuit.
📌 3. Magnetism & Electromagnetic Induction
- Like poles repel, unlike poles attract. A freely suspended magnet rests north–south. Repulsion is the sure test of magnetism.
- Oersted (1820): a current-carrying wire has a magnetic field around it. Faraday (1831): changing magnetic flux induces current — electromagnetic induction — basis of generators, transformers and induction cookers.
- Fleming's left-hand rule — motor (direction of force); right-hand rule — generator (direction of induced current).
- A transformer works only on AC; step-up raises voltage (less loss over long transmission), step-down lowers it. Electromagnets use soft iron; permanent magnets use steel/alnico/neodymium.
- The Earth is a huge magnet; near the geographic north lies the Earth's magnetic south pole. SI unit of magnetic field: tesla.
| Device | Function |
|---|---|
| Ammeter / voltmeter | Measure current / voltage |
| Galvanometer | Detects small current and its direction |
| Dynamo / generator | Mechanical → electrical |
| Motor | Electrical → mechanical |
| Rectifier | AC → DC |
| Inverter | DC → AC |
| Capacitor | Stores charge |
⚠️ Exam Traps:
- A fuse is connected in series, not parallel.
- A transformer does not work on DC.
- Household appliances are in parallel — not series.
📌 4. Practice MCQs
📝 Practice MCQs (prepared by SF Prep — not PYQs)
Q1. Ohm's law states: Practice
Correct answer: (B)
At constant temperature voltage is proportional to current.
At constant temperature voltage is proportional to current.
Q2. The filament of an electric bulb is made of: Practice
Correct answer: (C)
Tungsten has a very high melting point (≈3422 °C).
Tungsten has a very high melting point (≈3422 °C).
Q3. A fuse wire should have: Practice
Correct answer: (D)
It melts quickly on overload and breaks the circuit.
It melts quickly on overload and breaks the circuit.
Q4. The instrument used to measure electric current is the: Practice
Correct answer: (A)
An ammeter is connected in series.
An ammeter is connected in series.
Q5. Electromagnetic induction was discovered by: Practice
Correct answer: (D)
Faraday discovered it in 1831 — the basis of generators.
Faraday discovered it in 1831 — the basis of generators.
Q6. The frequency of domestic AC supply in India is: Practice
Correct answer: (A)
India: 220–230 V, 50 Hz.
India: 220–230 V, 50 Hz.
Q7. A transformer works on: Practice
Correct answer: (C)
Changing flux is produced only by AC.
Changing flux is produced only by AC.
Q8. Household appliances are connected in: Practice
Correct answer: (D)
Each gets full voltage and can be switched independently.
Each gets full voltage and can be switched independently.
Q9. An electric motor works on: Practice
Correct answer: (A)
Direction is given by Fleming's left-hand rule.
Direction is given by Fleming's left-hand rule.
Q10. Two 10 Ω resistors in parallel give an equivalent resistance of: Practice
Correct answer: (D)
1/R = 1/10 + 1/10 → R = 5 Ω.
1/R = 1/10 + 1/10 → R = 5 Ω.
Q11. Current drawn by a 1000 W heater at 220 V is about: Practice
Correct answer: (B)
I = P/V = 1000/220 ≈ 4.5 A.
I = P/V = 1000/220 ≈ 4.5 A.
Q12. A 100 W bulb used for 10 hours consumes: Practice
Correct answer: (B)
100 W × 10 h = 1000 Wh = 1 kWh.
100 W × 10 h = 1000 Wh = 1 kWh.
Q13. The heating element of a heater is made of: Practice
Correct answer: (C)
Nichrome (Ni-Cr) has high resistance, high melting point and resists oxidation.
Nichrome (Ni-Cr) has high resistance, high melting point and resists oxidation.
Q14. The device that converts AC to DC is the: Practice
Correct answer: (A)
A rectifier is made using diodes.
A rectifier is made using diodes.
Q15. The surest test of magnetism is: Practice
Correct answer: (B)
Attraction happens even with plain iron; repulsion only between two magnets.
Attraction happens even with plain iron; repulsion only between two magnets.
Q16. The best material for an electromagnet core is: Practice
Correct answer: (C)
Soft iron magnetises and demagnetises quickly.
Soft iron magnetises and demagnetises quickly.
Q17. When the temperature of a metallic conductor rises, its resistance: Practice
Correct answer: (C)
Electron collisions increase; the reverse happens in semiconductors.
Electron collisions increase; the reverse happens in semiconductors.
Q18. Why does a bird sitting on a live wire not get a shock? Practice
Correct answer: (B)
The circuit is not completed to earth or another wire.
The circuit is not completed to earth or another wire.
Q19. Which is NOT a formula for electric power? Practice
Correct answer: (A)
The correct form is I²R, not IR².
The correct form is I²R, not IR².
Q20. Who discovered the magnetic field around a current-carrying wire? Practice
Correct answer: (D)
Hans Christian Oersted in 1820.
Hans Christian Oersted in 1820.