Electricity and Magnetism
Electricity and Magnetism
Electricity questions are mostly small numericals: Ohm's law, resistors in series and parallel, power and the bill in kWh. Magnetism questions ask about electromagnetic induction, the motor and the generator, and the hand rules. Learn the formulas and the device-to-principle table.
1. Current Electricity
Ohm's law: V = IR
Series: R = R₁ + R₂ + R₃ (same current)
Parallel: 1/R = 1/R₁ + 1/R₂ + 1/R₃ (same voltage)
Power P = VI = I²R = V²/R; energy in kWh = kW × hours
| Quantity | Unit and instrument |
|---|---|
| Current | Ampere; measured by an ammeter connected in series |
| Potential difference | Volt; measured by a voltmeter connected in parallel |
| Resistance | Ohm; depends on length, area, material and temperature |
| Resistivity | Ohm metre; silver is the best conductor, copper is used in wires |
| Household supply in India | 220 V AC at 50 Hz |
| Fuse | Thin wire of low melting point in series; melts on overload |
| Heating effect | H = I²Rt; used in heaters, irons and bulbs (tungsten filament) |
2. Magnetism and Electromagnetism
| Idea | Fact |
|---|---|
| Magnetic field lines | Go from north to south outside a magnet; never cross |
| Oersted | A current-carrying wire produces a magnetic field |
| Electromagnetic induction | Faraday: a changing magnetic field induces current |
| Fleming's left-hand rule | Direction of force on a current-carrying conductor; electric motor |
| Fleming's right-hand rule | Direction of induced current; generator |
| Electric motor | Electrical energy to mechanical energy |
| Generator (dynamo) | Mechanical energy to electrical energy |
| Transformer | Changes AC voltage; works only on AC, by mutual induction |
| Electromagnet | Soft iron core in a coil; used in cranes, bells and MRI |
✗ An ammeter is connected in parallel | ✓ An ammeter is in series; a voltmeter is in parallel
✗ A generator works on Fleming's left-hand rule | ✓ A generator follows the right-hand rule; the motor follows the left-hand rule
हिंदी नोट: ओम के नियम के अनुसार V = IR। श्रेणी क्रम में प्रतिरोध जुड़ते हैं, समानांतर क्रम में उनके व्युत्क्रम जुड़ते हैं। 1 किलोवाट घंटा = 1 यूनिट बिजली। मोटर विद्युत ऊर्जा को यांत्रिक ऊर्जा में और जनित्र यांत्रिक ऊर्जा को विद्युत ऊर्जा में बदलता है।
Exam Pointer: No NTPC shift was verified for these patterns in this run; they are tagged syllabus-based. Electricity numericals and motor-generator questions appear across RRB papers.
Pariksha Pattern: Every Way NTPC Asks This Topic
Pattern 1: Ohm's law and resistance combinations
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. Three resistors of 2 Ω, 3 Ω and 6 Ω are connected in parallel. Find the equivalent resistance.
1/R = 1/2 + 1/3 + 1/6 = 3/6 + 2/6 + 1/6 = 1, so R = 1 Ω.
Answer: 1 Ω
EXAMATLAS LEVEL
Q. The same three resistors are joined in series across a 22 V battery. Find the current and the voltage across the 6 Ω resistor.
Series resistance = 2 + 3 + 6 = 11 Ω, so I = 22/11 = 2 A. Voltage across 6 Ω = 2 × 6 = 12 V.
Answer: 2 A; 12 V
Pattern 2: Electric power, energy and the bill
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. A 1,000 W heater runs for 3 hours. How many units of electricity does it use?
Energy = 1 kW × 3 h = 3 kWh = 3 units.
Answer: 3 units
EXAMATLAS LEVEL
Q. A bulb is rated 100 W, 220 V. Find its current and resistance. If four such bulbs run 5 hours a day for 30 days at ₹6 per unit, what is the cost?
I = P/V = 100/220 = 5/11 A, about 0.45 A. R = V²/P = 48,400/100 = 484 Ω. Energy = 0.4 kW × 5 × 30 = 60 kWh, so cost = 60 × 6 = ₹360.
Answer: About 0.45 A, 484 Ω; ₹360
Pattern 3: Electromagnetic induction, motor and generator
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. Who discovered electromagnetic induction?
Michael Faraday.
Answer: Michael Faraday
EXAMATLAS LEVEL
Q. Match the device with its energy change and rule: electric motor, generator. Why does a transformer not work on DC?
A motor turns electrical energy into mechanical energy (left-hand rule). A generator turns mechanical energy into electrical energy (right-hand rule). A transformer needs a changing magnetic flux, which steady DC cannot give.
Answer: Motor: electrical to mechanical, left hand; generator: mechanical to electrical, right hand; DC gives no changing flux
Pattern 4: Safety devices and heating effect
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. How is a fuse connected in a circuit, and what kind of wire is it made of?
In series with the live wire; it is a thin wire of low melting point that melts when the current is too high.
Answer: In series; low melting point wire
EXAMATLAS LEVEL
Q. What are the voltage and frequency of household AC supply in India, and why is tungsten used for bulb filaments?
220 V at 50 Hz. Tungsten has a very high melting point (about 3,400 °C), so it glows white-hot without melting.
Answer: 220 V, 50 Hz; very high melting point
60-Second Revision
- V = IR; series add; parallel add reciprocals.
- P = VI = V²/R; 1 unit = 1 kWh.
- Ammeter in series, voltmeter in parallel, fuse in series.
- Motor: left hand, electrical to mechanical; generator: right hand, mechanical to electrical.
Next Step: Physics done. Try the physics questions in the ExamAtlas RRB NTPC 2026 mock tests; unit and formula questions are the quickest marks in General Science.