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CAPF AC Electricity & Magnetism

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This page covers CAPF AC Electricity & Magnetism with complete concept notes, 4 graded practice MCQs, key points and exam-specific tips. Free to study.

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Concept Notes

Electricity & Magnetism— Rules & Concept

Core ConceptRead this first — the foundation of the topic

Electricity and Magnetism are interconnected phenomena that form the backbone of modern technology. Understanding their relationship is crucial for RRB Group D exams. Electricity is the flow of electric charge through conductors. When current flows through a wire, it creates a magnetic field around it. This is called electromagnetism. Similarly, when a magnet moves near a conductor, it generates electricity. This two-way relationship is the foundation of motors and generators.

Key RulesCore rules you must know cold
1

Moving magnetic field creates electric current (Faraday's Law)

2

Current-carrying conductor creates magnetic field (Oersted's Law)

3

Direction follows Fleming's Left Hand Rule (motors) and Right Hand Rule (generators)

Formula BlockMemorise — at least one formula appears in every paper
Electric Power: P = V × I = I²R = V²/R
Ohm's Law: V = I × R
Electrical Energy: E = P × t = VIt
Resistance in Series: R_total = R₁ + R₂ + R₃
Resistance in Parallel: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃
Exam PatternsWhat examiners ask — read before attempting PYQs
Common question types include

• Unit conversions (kWh to Joules) • Ohm's law applications • Power calculations in household appliances • Fleming's rules for motor/generator direction • Magnetic field direction around current-carrying conductors Shortcut for Power Calculations: For household appliances, remember: Higher wattage = Higher power consumption = Higher electricity bill

Quick formula

Monthly cost = (Wattage × Hours used × Days × Rate per unit) ÷ 1000

Worked ExampleSolve this step-by-step before moving on
1
Step 1

Calculate total energy = 100W × 5h × 30 days = 15,000 Wh = 15 kWh

2
Step 2

Monthly cost = 15 × ₹3 = ₹45 Answer: ₹45 Worked Example 2: Find current through a 60W, 220V bulb using Ohm's law.

1
Step 1

Use P = VI, so I = P/V

2
Step 2

I = 60W ÷ 220V = 0.27A

3
Step 3

Find resistance using V = IR, so R = V/I = 220 ÷ 0.27 = 815Ω Answer: Current = 0.27A, Resistance = 815Ω Magnetism Quick Rules: • Like poles repel, unlike poles attract • Earth's magnetic north is actually geographic south • Iron, Nickel, Cobalt are magnetic materials • Magnetic field lines go from North to South outside the magnet Fleming's Left Hand Rule (Motor): Thumb = Force direction, First finger = Field direction, Middle finger = Current direction. Remember as 'FBI' - Force, Field, Current. Common

Exam TrapsCommon mistakes students make — avoid these

#1: Students confuse kWh (energy unit) with kW (power unit). Remember: kWh = kW × hours. Energy is what you pay for in electricity bills, not power.

Always convert watts to kilowatts by dividing by 1000 when calculating electricity costs. Electromagnetic Applications: • Electric motor converts electrical energy to mechanical energy • Generator converts mechanical energy to electrical energy • Transformer changes voltage levels using electromagnetic induction • Electromagnets are used in speakers, MRI machines, and cranes Safety in Electricity: Earth wire (green) provides safety path for leakage current. Fuses and circuit breakers protect against overload. Never touch electrical appliances with wet hands as water conducts electricity.

Key Points to Remember

  • Moving magnetic field generates electric current (Faraday's Law)
  • Current-carrying conductor creates magnetic field around it
  • Power formula: P = VI = I²R = V²/R
  • Ohm's Law: V = I × R (most important formula)
  • kWh is energy unit, kW is power unit - never confuse these
  • Fleming's Left Hand Rule: FBI - Force, Field, Current
  • Like magnetic poles repel, unlike poles attract
  • Series resistance: R_total = R₁ + R₂ + R₃
  • Parallel resistance: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃
  • Monthly electricity cost = (Wattage × Hours × Days × Rate) ÷ 1000

Exam-Specific Tips

  • Earth behaves like a huge magnet with magnetic south pole near geographic north pole
  • Standard household voltage in India is 220V AC
  • Iron, Nickel, and Cobalt are the three main magnetic materials
  • 1 kWh = 3.6 × 10⁶ Joules (important conversion)
  • Magnetic field lines never intersect each other
  • Electric fuse works on heating effect of current
  • AC changes direction 50 times per second in India (50 Hz frequency)
  • Green wire is earth wire, red/brown is live wire, black/blue is neutral wire
Practice MCQs

Electricity & Magnetism — Practice Questions

4graded MCQs · easy to hard · full solution & trap analysis

All MCQs →
Practice 1easy

A wire of resistance 5 Ω is connected to a 10 V battery. What is the current flowing through the wire?

Practice 2medium

A wire of resistance 12 Ω is connected to a 6 V battery. If the same wire is cut into three equal pieces and all three pieces are connected in parallel to the same 6 V battery, what is the equivalent resistance of the parallel combination?

Practice 3medium

A student connects a resistor of 15 Ω in series with another resistor of 10 Ω, and applies a potential difference of 50 V across the combination. Calculate the current flowing through the circuit.

Practice 4hard

A copper wire of resistance 5 Ω is connected to a 10 V battery. The wire is then replaced with another copper wire of the same length but twice the cross-sectional area. If the new wire is connected to the same 10 V battery, what is the ratio of the power dissipated in the new wire to the power dissipated in the original wire?

60-Second Revision — Electricity & Magnetism

  • Remember: P = VI for all power calculations, convert watts to kilowatts
  • Formula: Monthly cost = (Wattage × Hours × Days × Rate) ÷ 1000
  • Trap: kWh is energy (bill units), kW is power - don't mix them
  • Fleming's Left Hand Rule: FBI sequence for motor direction
  • Quick fact: Earth's magnetic north is geographic south pole
  • Ohm's Law: V = IR - memorize this triangle method
  • Series adds resistance, parallel reduces total resistance
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