Electricity and Magnetism
Overview
Electricity and Magnetism is a high-scoring topic in Railway NTPC and Group D exams. It covers electric current, voltage, resistance, Ohm's Law, power, magnetic effects of current, electromagnetic induction, and practical devices. Questions frequently test formulas, units, SI units of quantities, and real-life applications like motors, generators, and transformers. Mastering this chapter ensures 3-5 easy marks in every Railway exam.
Typed Notes
Electric Current, Voltage & Resistance
Electric current is the flow of electric charge; SI unit is Ampere (A). Voltage (potential difference) is the work done per unit charge; SI unit is Volt (V). Resistance opposes current flow; SI unit is Ohm (Ω). Ohm's Law states V = IR, valid at constant temperature. Resistors in series: R_total = R1 + R2 + R3. Resistors in parallel: 1/R_total = 1/R1 + 1/R2 + 1/R3. Household wiring uses parallel combination so each appliance gets the same voltage. Exam tip: In series, current is same; in parallel, voltage is same.
Electric Power, Energy & Heating Effect
Electric power P = VI = I²R = V²/R; unit is Watt (W). Energy consumed = Power × Time; commercial unit is kilowatt-hour (kWh), also called 1 unit. 1 kWh = 3.6 × 10⁶ J. Joule's Law of Heating: Heat produced H = I²Rt. This heating effect is used in electric bulbs, heaters, fuses, and geysers. A fuse wire has high resistance and low melting point — it melts and breaks the circuit during overload, protecting appliances. MCB (Miniature Circuit Breaker) is the modern replacement for fuses.
Magnetic Effect of Current & Electromagnetic Induction
A current-carrying conductor produces a magnetic field around it — discovered by Oersted (1820). The direction of the magnetic field is given by the Right-Hand Thumb Rule (Maxwell's Corkscrew Rule). An electromagnet is a temporary magnet made using a current-carrying coil around a soft iron core. Fleming's Left-Hand Rule gives the direction of force on a current-carrying conductor in a magnetic field — principle of the Electric Motor. Faraday's electromagnetic induction (1831) states that a changing magnetic field induces an EMF — principle of the Electric Generator. Fleming's Right-Hand Rule gives the direction of induced current in a generator.
Electric Motor, Generator & Transformer
Electric Motor converts electrical energy into mechanical energy; uses Fleming's Left-Hand Rule; works on DC or AC. Electric Generator converts mechanical energy into electrical energy; uses Fleming's Right-Hand Rule; AC generator is called alternator. Transformer works only on AC and transfers electrical energy between circuits using mutual induction. Formula: Ns/Np = Vs/Vp = Ip/Is. Step-up transformer: Ns > Np, voltage increases. Step-down transformer: Ns < Np, voltage decreases. India's household AC supply is 220 V, 50 Hz. Power is transmitted at high voltage to reduce energy loss (P_loss = I²R).
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Ohm's Law: V = IR — Voltage (Volts) = Current (Amperes) × Resistance (Ohms); SI unit of resistance is Ohm (Ω).
- 02Electric Power P = VI = I²R = V²/R; SI unit is Watt (W); 1 kWh = 3.6 × 10⁶ Joules (1 unit of electricity).
- 03Fleming's Left-Hand Rule → Electric Motor (force on conductor); Fleming's Right-Hand Rule → Electric Generator (induced EMF direction).
- 04Faraday's Laws of Electromagnetic Induction: induced EMF is proportional to rate of change of magnetic flux; basis of generators and transformers.
- 05Transformer works on AC only; Step-up increases voltage (decreases current); Step-down decreases voltage (increases current); Ns/Np = Vs/Vp.
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
A 60 W bulb is used for 5 hours daily for 30 days. Calculate the electrical energy consumed in kWh.
Three resistors of 2Ω, 3Ω, and 6Ω are connected in parallel. Find the equivalent resistance.
What is the difference between an electric motor and an electric generator in terms of energy conversion and the rule used?
A transformer has 500 turns in the primary coil and 2500 turns in the secondary coil. If input voltage is 220 V, find the output voltage. Is it step-up or step-down?
Why is household wiring done in parallel combination rather than series combination?
Electricity and Magnetism covers Ohm's Law (V=IR), power (P=VI), heating effect (H=I²Rt), and the unit of electrical energy (kWh). Oersted discovered magnetic effect of current; Faraday discovered electromagnetic induction. Motors use Fleming's Left-Hand Rule; generators use Fleming's Right-Hand Rule. Transformers work only on AC. India's supply is 220V, 50Hz. These facts and formulas are directly tested in Railway NTPC and Group D papers.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.The SI unit of electric charge is:
Q2.Which law states that the induced EMF in a circuit is proportional to the rate of change of magnetic flux?
Q3.Fleming's Left-Hand Rule is used to find the direction of force in which device?
Q4.India's standard domestic AC supply frequency is:
Mark karo — dashboard par progress dikhega.