Electricity Magnetism
Key Concepts
| # | Concept | Explanation |
|---|---|---|
| 1 | Ohm’s Law | V = I × R (Voltage = Current × Resistance). Works only when temperature is constant. |
| 2 | Series Resistance | R_total = R₁ + R₂ + … (Current same, voltage divides). |
| 3 | Parallel Resistance | 1/R_total = 1/R₁ + 1/R₂ + … (Voltage same, current divides). |
| 4 | Power Formulas | P = VI = I²R = V²/R (unit: watt). 1 kWh = 3.6 × 10⁶ J. |
| 5 | Joule’s Law | Heat H = I²Rt (joules). Fuse wire rating uses this. |
| 6 | Right-Hand Thumb Rule | Thumb = current direction, fingers = magnetic field lines around straight wire. |
| 7 | Fleming’s Left-Hand Rule | F (thumb), B (fore-finger), I (middle-finger) → motor effect. |
| 8 | Faraday’s Law | Induced e.m.f. ∝ rate of change of magnetic flux → basis of generator & transformer. |
15 Practice MCQs
1. The resistance of a 100 W, 200 V bulb is
A) 100 Ω B) 200 Ω C) 400 Ω D) 50 ΩAnswer: C
Solution: R = V²/P = 200²/100 = 400 Ω
Shortcut: Remember “Watt = V²/R” → R = V²/P
Tag: Power formula
2. Three 6 Ω resistors in parallel give equivalent resistance
A) 18 Ω B) 2 Ω C) 3 Ω D) 0.5 ΩAnswer: B
Solution: 1/R = 1/6 + 1/6 + 1/6 = 3/6 ⇒ R = 2 Ω
Trick: n equal R in parallel → R_eq = R/n
Tag: Parallel resistance
3. A 2 A current flows through a 5 Ω resistor for 10 s. Heat produced is
A) 50 J B) 100 J C) 200 J D) 500 JAnswer: C
Solution: H = I²Rt = 2² × 5 × 10 = 200 J
Tag: Joule’s law
4. Device which works on both AC & DC is
A) Transformer B) Dynamo C) Electric iron D) Induction coilAnswer: C
Solution: Heating devices (iron, heater) give same heat for same V-rms irrespective of AC/DC.
Tag: Applications
5. Magnetic field inside a long straight solenoid is
A) Zero B) Uniform & parallel to axis C) Non-uniform D) RadialAnswer: B
Tag: Solenoid field
6. Unit of magnetic flux is
A) Tesla B) Weber C) Henry D) GaussAnswer: B
Tag: Units
7. A step-up transformer has 100 primary turns and 500 secondary turns. If primary voltage is 220 V, secondary voltage is
A) 44 V B) 1100 V C) 22 V D) 550 VAnswer: B
Solution: V₂/V₁ = N₂/N₁ ⇒ V₂ = 220 × 500/100 = 1100 V
Trick: “Turns ratio = Voltage ratio”
Tag: Transformer
8. Lenz’s law is a consequence of conservation of
A) Charge B) Energy C) Momentum D) MassAnswer: B
Tag: Lenz law
9. If current direction is north, the magnetic field above the wire is
A) East B) West C) Up D) DownAnswer: B (use RH-thumb rule)
Tag: Magnetic field direction
10. A 1 kW heater used for 2 h consumes energy in kWh equal to
A) 0.5 B) 1 C) 2 D) 3Answer: C
Trick: Energy (kWh) = kW × h
Tag: Energy billing
11. The colour code for 470 Ω ±5 % resistor is
A) Yellow-Violet-Brown-Gold B) Yellow-Violet-Brown-Silver C) Yellow-Violet-Red-Gold D) Yellow-Violet-Black-GoldAnswer: A
Trick: 4-7-×10¹ = 470, Gold = ±5 %
Tag: Resistor colour code
12. A fuse wire has
A) High melting point, low resistance B) Low melting point, high resistance C) Low melting point, low resistance D) High melting point, high resistanceAnswer: B
Tag: Fuse properties
13. The force on a charge q moving velocity v perpendicular to field B is
A) qvB B) zero C) qvB/2 D) qvB sin 45°Answer: A (θ = 90° ⇒ sin 90° = 1)
Tag: Magnetic force
14. Which material is best for making permanent magnet?
A) Soft iron B) Steel C) Copper D) AluminiumAnswer: B
Tag: Magnetic materials
15. If both the number of turns & length of a solenoid are doubled keeping current same, magnetic field
A) Doubles B) Halves C) Remains same D) Becomes 4 timesAnswer: C (B = μ₀nI, n = N/l constant)
Tag: Solenoid field
Speed Tricks
| Situation | Shortcut | Example |
|---|---|---|
| Equal resistors in parallel | R_eq = R ÷ n | Three 9 Ω → 3 Ω |
| Power at home | 1 kW × 1 h = 1 unit = ₹5-8 | 100 W tubelight 10 h → 1 unit |
| Bulb brightness | P ∝ 1/R (same V) | 60 W glows brighter than 100 W in series! |
| Transformer quick check | V₂ = V₁ × (N₂/N₁) | 1100 V from 220 V with 5:1 → step-up |
| Fuse rating | I = P/V | 1 kW, 220 V heater → 1000/220 ≈ 4.5 A fuse |
Quick Revision
| Point | Detail |
|---|---|
| Ohm’s law valid for | Metallic conductors at constant temperature |
| EMF unit | Volt (same as potential difference) |
| Kirchhoff’s 1st law | ΣI_in = ΣI_out (conservation of charge) |
| MCB full form | Miniature Circuit Breaker |
| Dynamo converts | Mechanical → Electrical energy (AC) |
| Motor converts | Electrical → Mechanical energy |
| Core of transformer | Laminated soft iron (reduces eddy loss) |
| Earth wire colour | Green (or Green-Yellow) |
| SI unit of resistivity | Ohm-metre (Ω m) |
| Safety device in household circuit | Fuse / MCB / Earthing |
BETA
