Electricity and Magnetism

Prerequisites

Motion and Force, for force and energy.
Light and Sound is not needed, but the idea of an electromagnetic wave links the two pages.
Energy Resources is the natural follow-up: how the electricity explained here is produced.

The Hook: From Korba’s Coal to Your Switchboard

Connect

Korba (कोरबा) is called the power capital of Chhattisgarh. Coal from giant open-cast mines such as Gevra is burnt in thermal power stations, the steam spins turbines, and generators turn that motion into electricity. Transformers raise its voltage for long-distance lines, lower it again near your village, and finally a fuse or MCB guards the wires in your house.

Every step in that chain is a law of electricity and magnetism. Why is power sent at very high voltage? Why is household wiring in parallel? Why does a metal-bodied cooler need a third, earth wire? This page answers those questions.

Why it matters: Prelims asks “which device works on which principle” and household-safety questions. Mains P-IV asks short explanations (fuse vs earthing, AC vs DC, transformers), often linked to energy and Chhattisgarh’s power sector.

Basic Quantities

Quantity	What it is	SI unit	Measured by
Electric charge	Property of matter; electrons carry negative charge	coulomb (C)	—
Current (विद्युत धारा), I	Rate of flow of charge	ampere (A)	Ammeter, connected in series
Potential difference (विभवांतर), V	Work done to move unit charge between two points; the “push”	volt (V)	Voltmeter, connected in parallel
Resistance (प्रतिरोध), R	Opposition to the flow of current	ohm (Ω)	—
Power, P	Rate of using electrical energy	watt (W)	—

By convention, current flows from the positive to the negative terminal. Electrons actually move the other way.

Ohm’s law

At constant temperature, the current through a conductor is proportional to the potential difference across it:

$$ V = IR $$

Resistance depends on: the material (its resistivity), the length (longer wire, more resistance), the thickness (thicker wire, less resistance) and the temperature (for metals, resistance rises with temperature).

Class of material	Examples	Use
Conductors	Silver (best), copper, aluminium	Copper for house wiring; aluminium for overhead transmission lines (light and cheap)
Insulators	Rubber, PVC, glass, porcelain, dry wood	Wire coatings, pylon insulators
Semiconductors	Silicon, germanium	Chips, solar cells, LEDs, transistors
Superconductors	Certain materials cooled below a critical temperature	Zero resistance; strong magnets in MRI machines and some maglev trains

Series vs parallel

Point	Series	Parallel
Current	Same through every component	Divides among branches
Voltage	Divides among components	Same across every branch
If one bulb fuses	All go off (the circuit breaks)	Others keep working
Total resistance	Adds up (increases)	Less than the smallest single resistance
Example	Old-style decorative bulb strings; a fuse is placed in series with the live wire	Household wiring

Why household appliances are connected in parallel: each appliance gets the full mains voltage, each can be switched on or off independently, and one failure does not cut the others.

Heating Effect of Current

When current flows through a resistance, electrical energy turns into heat. This is the Joule heating effect: the heat produced is proportional to $ I^{2}Rt $ (the square of the current, the resistance and the time).

Device	Material and reason
Heater, iron, toaster, geyser elements	Nichrome (nickel–chromium alloy): high resistivity and does not oxidise easily when red-hot
Filament bulb	Tungsten: very high melting point (about 3,400 °C). Bulbs are filled with an inert gas such as argon or nitrogen to lengthen filament life
Fuse	A wire of low melting point (traditionally a tin–lead alloy) that melts and breaks the circuit when the current is too high

Most of the energy in a filament bulb is wasted as heat. LEDs give the same light for a small fraction of the power, which is why governments have pushed LED adoption (for example, through the UJALA LED distribution scheme launched in 2015).

Electric Power and the “Unit” on Your Bill

$$ P = VI = I^{2}R $$

1 kilowatt (kW) = 1,000 W.
The commercial unit of electrical energy is the kilowatt-hour (kWh), the “unit” on an electricity bill.
1 kWh = 1,000 W × 3,600 s = $ 3.6 \times 10^{6} $ joules.

Worked example

A 1,500 W room heater runs 2 hours a day for 30 days.

Energy = 1.5 kW × 2 h × 30 = 90 kWh = 90 units. At ₹6 per unit, the cost is ₹540 (illustrative tariff only).

Domestic Electric Circuits and Safety

Supply in India

AC (alternating current) at a nominal 230 V and 50 Hz (Indian Standard IS 12360). NCERT textbooks round the voltage to 220 V; both appear in MCQs, and the frequency is 50 Hz either way.
At 50 Hz, the current reverses direction 100 times a second (once every 1/100 s).
For comparison, the USA uses about 120 V at 60 Hz.

The three wires

Wire	Role	Colour (older code, used in NCERT)	Colour (current IS 732 practice)
Live (phase)	Carries the supply voltage	Red	Brown
Neutral	Return path, near zero potential	Black	Light blue
Earth	Safety wire to a metal plate buried deep in moist ground	Green	Green-and-yellow

Black means neutral in the old code but is a phase colour in the new one. Older houses use the old colours, so you will meet both.

Safety devices

Device	Protects against	How
Fuse	Overloading and short circuits	Melts when the current exceeds its rating; fitted in series in the live wire; must be replaced
MCB (miniature circuit breaker)	The same faults	Trips automatically and can be reset with a switch
Earthing	Electric shock from appliances with metal bodies (cooler, fridge, iron, washing machine)	If the live wire touches the metal body, current flows safely to earth instead of through a person, and usually blows the fuse or trips the MCB
RCCB / ELCB (residual-current device)	Leakage current and shock	Detects any imbalance between live and neutral currents and cuts the supply in a fraction of a second; devices for personal protection are typically rated at 30 mA

Fault	Meaning
Short circuit	Live and neutral wires touch directly (damaged insulation). Resistance falls almost to zero and the current becomes very large
Overloading	Too many appliances on one circuit or socket, or a voltage surge, so the current exceeds the safe limit

Shock facts in one box

It is the current through the body that harms. Wet skin has much lower resistance than dry skin, so never touch switches with wet hands.
A bird on a single live wire is safe: both its feet are at the same potential, so no current flows through it. Touching the wire and the ground (or two wires) completes a circuit.
In a thunderstorm, a car with a metal body is a safe shelter because charge stays on the outer metal surface. A lone tree or open field is dangerous. Lightning conductors on tall buildings carry the discharge safely to the ground. The Damini app, from IITM Pune under the Ministry of Earth Sciences, gives lightning warnings.

AC vs DC

Point	AC (alternating current)	DC (direct current)
Direction	Reverses periodically (50 times per second each way in India)	One direction only
Sources	Power-station generators	Cells, batteries, solar panels
Transformers	Work with AC, so voltage can be stepped up or down easily	Do not work with DC
Long-distance transmission	Standard choice	Used in special HVDC links for very long distances
Everyday use	Household supply	Mobile phones, laptops, electronics (adaptors and chargers convert AC to DC); inverters convert DC from batteries or solar panels back to AC

Magnetism (चुंबकत्व)

Magnets and Earth’s magnetic field

Every magnet has a north and a south pole. Like poles repel, unlike poles attract. Cutting a magnet gives two smaller magnets; a single isolated pole is never found.
A freely suspended magnet points roughly north–south: this is the principle of the magnetic compass.
Earth behaves like a giant magnet. Its magnetic poles do not coincide with its geographic poles, and the magnetic pole near the geographic North Pole is actually a magnetic south-type pole (that is why it attracts the compass’s north pole).
Earth’s magnetic field deflects many charged particles from the Sun. Particles funnelled near the poles light up the upper air as auroras.
Magnetic field lines run from north to south outside a magnet, and they never cross.

Magnetic effect of current: electromagnets

In 1820, Hans Christian Oersted found that a current-carrying wire deflects a compass needle: electric current produces a magnetic field.

A coil of wire (a solenoid) carrying current behaves like a bar magnet.
With a soft iron core, it becomes a strong electromagnet that loses its magnetism as soon as the current stops. Steel keeps its magnetism, so it is used for permanent magnets.
Uses of electromagnets: cranes that lift scrap iron, electric bells, relays, loudspeakers, MRI scanners, and maglev (magnetic levitation) trains that float above the track.

Electric motor: electricity → motion

A current-carrying coil placed in a magnetic field experiences a force and rotates. Fleming’s left-hand rule gives the direction of the force.

Uses: fans, mixers, washing machines, water pumps, coolers, and the traction motors of electric vehicles and electric locomotives.

Electromagnetic induction: motion → electricity

In 1831, Michael Faraday showed that a changing magnetic field through a coil produces a current in it. This is electromagnetic induction (विद्युत चुंबकीय प्रेरण), and it is the principle behind almost all the world’s electricity.

Device	How induction is used
Generator (dynamo)	A coil is rotated in a magnetic field (by steam, water or wind turbines) and current is induced. Fleming’s right-hand rule gives its direction. Converts mechanical energy into electrical energy
Transformer	Changes AC voltage using two coils on an iron core. Step-up at the power station (high voltage, low current, so less loss as heat in the lines); step-down at substations before supply to homes. Works only on AC
Induction cooktop	A changing magnetic field induces currents that heat the base of the vessel. It needs a vessel with a ferromagnetic base, such as iron or steel
Others	Wireless phone charging, metal detectors at airports, electric guitar pickups

Why transmit at high voltage?

For the same power ($ P = VI $), a higher voltage means a smaller current. The heat lost in the lines depends on $ I^{2}R $, so lowering the current cuts losses sharply. That is why power from Korba’s stations travels on high-voltage lines and is stepped down near consumers.

How electricity is generated: one chain

graph LR
    A["Energy source
coal, water, wind, nuclear"] --> B["Turbine spins"]
    B --> C["Generator
(electromagnetic induction)"]
    C --> D["Step-up transformer"]
    D --> E["High-voltage lines"]
    E --> F["Step-down transformer
at substation"]
    F --> G["Homes: 230 V, 50 Hz AC"]

Solar photovoltaic cells are the exception: they turn light directly into DC using semiconductors, with no turbine or generator, and an inverter converts the DC to AC.

Memory Tricks

Ohm's law triangle: 'VIR'

Write V on top and I R below. Cover what you want: V = I × R, I = V ÷ R, R = V ÷ I.

Fleming's rules: 'Left Motor, Right Generator'

Motor = Left hand (think “Move Left”). Generator = Right hand (Generate Right). Both use thumb = motion, forefinger = field, middle finger = current.

Who discovered what

Oersted 1820: current makes magnetism (O for “One-way: electricity to magnet”). Faraday 1831: magnetism makes current (F for “Flip it back”).

Meters: 'Ammeter in a line, Voltmeter on the side'

Ammeter in series (in line with the current). Voltmeter in parallel (across the component, on the side).

Don’t Confuse

This	vs	That
Electric motor: electrical → mechanical, Fleming’s left-hand rule	vs	Generator: mechanical → electrical, Fleming’s right-hand rule
Fuse / MCB: protect against overloading and short circuits	vs	Earthing: protects people from shock via metal bodies
Short circuit: live touches neutral	vs	Overloading: too much total load
Step-up transformer: raises voltage, at power stations	vs	Step-down transformer: lowers voltage, near consumers
Soft iron: temporary magnet (electromagnets)	vs	Steel: permanent magnet
Nichrome: heating elements	vs	Tungsten: bulb filaments
Ammeter: in series	vs	Voltmeter: in parallel

Common Mistakes

MCQ traps

“A transformer can step up DC voltage.” False. Transformers work only with AC, because they need a changing magnetic field.
“A fuse is connected in parallel with the appliance.” False. It is in series, in the live wire.
“Earthing protects against overloading.” False. It protects against shock from leakage to metal bodies. Overloading is handled by fuses and MCBs.
“Household appliances are wired in series to save wire.” False. They are in parallel so each gets the full voltage and works independently.
“High voltage is used in transmission to increase the current.” Reversed. High voltage lowers the current and so reduces $ I^{2}R $ losses.
“Earth’s geographic and magnetic poles are at the same place.” False. They are apart, and the magnetic pole near the geographic North Pole is a south-type magnetic pole.

Practice

All questions below are original practice questions written in CGPSC style. They are not previous-year questions.

Level 1: Foundation

The commercial unit of electrical energy, the “unit” on an electricity bill, is the ______.
Electromagnetic induction was discovered by ______ in ______.
The heating element of an electric iron is usually made of ______.
A fuse is connected in ______ with the ______ wire.

Answers

Kilowatt-hour (kWh), equal to $ 3.6 \times 10^{6} $ J.
Michael Faraday, 1831.
Nichrome.
Series; live (phase).

Level 2: Prelims MCQs

Q1. Consider the following statements about domestic electric circuits in India:

Household appliances are connected in parallel.
The earth wire protects users from electric shock if the live wire touches the metal body of an appliance.
The household supply is direct current at 50 Hz.

Which of the statements given above is/are correct?

(a) 1 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1 and 2 only

Answer and explanation

(d) 1 and 2 only. Parallel wiring gives each appliance the full voltage and independent switching. The earth wire carries leakage current safely to the ground. Statement 3 is wrong: household supply is alternating current at 50 Hz. Frequency has no meaning for steady direct current.

Q2. Match List I (Device) with List II (Principle or effect):

List I	List II
A. Electric generator	1. Heating effect of current
B. Electric motor	2. Electromagnetic induction
C. Electric fuse	3. Force on a current-carrying conductor in a magnetic field
D. Electromagnet in a crane	4. Magnetic effect of current

(a) A-2, B-3, C-4, D-1
(b) A-3, B-2, C-1, D-4
(c) A-2, B-3, C-1, D-4
(d) A-2, B-1, C-3, D-4

Answer and explanation

(c) A-2, B-3, C-1, D-4. A generator works by electromagnetic induction. A motor works because a current-carrying coil feels a force in a magnetic field. A fuse melts because of the heating effect. An electromagnet uses the magnetic effect of current discovered by Oersted.

Q3. Electric power is transmitted over long distances at very high voltage mainly because:

(a) high voltage increases the current in the lines
(b) a lower current at high voltage reduces energy lost as heat in the lines
(c) transformers work only at high voltage
(d) high voltage makes the electricity direct current

Answer and explanation

(b). For the same power, a higher voltage needs a smaller current, and the heat lost in the lines depends on the square of the current. Step-up transformers at the power station and step-down transformers near consumers make this possible.

Q4. A 2,000 W geyser is used for 1 hour every day for 30 days. The energy consumed is:

(a) 6 units
(b) 20 units
(c) 60 units
(d) 600 units

Answer and explanation

(c) 60 units. Energy = 2 kW × 1 h × 30 days = 60 kWh, and 1 kWh is one “unit”. Option (d) comes from forgetting to convert watts to kilowatts somewhere in the working.

Q5. Which of the following materials is used for the filament of an incandescent bulb, and why?

(a) Nichrome, because it has high resistivity
(b) Copper, because it is a good conductor
(c) Tungsten, because it has a very high melting point
(d) Tin–lead alloy, because it has a low melting point

Answer and explanation

(c) Tungsten. A filament must glow white-hot without melting, and tungsten melts at about 3,400 °C. Nichrome is for heating elements, copper for wiring and a low-melting alloy for fuse wire.

Level 3: Mains Answer Writing

Q1. (60 words) Distinguish between a fuse and earthing as safety measures in domestic wiring.

Model answer outline

Fuse (or MCB): in series in the live wire; melts or trips when current exceeds the safe limit; protects wiring and appliances from overloading and short circuits.
Earthing: connects metal bodies of appliances to the ground; diverts leakage current; protects people from shock.
Conclusion: They complement each other; an RCCB adds protection against small leakage currents.

Q2. (150–200 words) Explain the principle of electromagnetic induction. How is it applied in the generation and transmission of electricity? Illustrate with reference to Chhattisgarh.

Model answer outline

Intro: Faraday (1831): a changing magnetic field through a coil induces an electric current in it.
Generation: Turbines driven by steam (coal, nuclear), falling water or wind rotate coils in magnetic fields inside generators, converting mechanical energy into electrical energy. Solar PV is an exception: it produces DC directly from light.
Transmission: Step-up transformers (also based on induction, and working only with AC) raise the voltage so that the current, and hence $ I^{2}R $ loss, is low. Step-down transformers at substations bring it to the 230 V household supply.
Other applications: induction cooktops, metal detectors, wireless charging.
Chhattisgarh angle: Korba, the state’s “power capital”, has large coal-based thermal power stations fed by mines such as Gevra. Its electricity reaches distant consumers through high-voltage lines and transformers. The state also has hydro capacity (for example, at Hasdeo Bango) and growing solar capacity, which uses inverters to feed the AC grid.
Conclusion: Electromagnetic induction is the foundation of the modern grid. Cleaner sources and efficient transmission are the path forward.

Quick Revision Table

Fact	Detail
Ohm’s law	$ V = IR $ at constant temperature
Units	Current: ampere; voltage: volt; resistance: ohm; power: watt
Ammeter / voltmeter	Series / parallel
Household wiring	Parallel
Supply in India	230 V nominal (NCERT: 220 V), 50 Hz AC
1 kWh	One “unit” = $ 3.6 \times 10^{6} $ J
Nichrome	Heating elements
Tungsten	Bulb filament; melting point about 3,400 °C
Fuse	Low-melting wire, in series in the live wire
Earthing	Protects against shock from metal bodies
Wire colours	Old: red / black / green; current: brown / blue / green-yellow
Oersted, 1820	Current produces a magnetic field
Faraday, 1831	Electromagnetic induction
Motor	Electrical → mechanical; Fleming’s left-hand rule
Generator	Mechanical → electrical; Fleming’s right-hand rule
Transformer	AC only; step-up for transmission, step-down for supply
Soft iron vs steel	Electromagnet vs permanent magnet
Korba	“Power capital” of Chhattisgarh

Key Takeaways

V = IR; households are wired in parallel; ammeters go in series, voltmeters in parallel.
Fuse/MCB guard against overloading and short circuits; earthing guards people against shock.
Bills count kWh: 1 unit = $ 3.6 \times 10^{6} $ J. India’s supply is 230 V, 50 Hz AC.
Oersted linked electricity to magnetism (1820); Faraday reversed it with induction (1831).
Motor: left hand. Generator: right hand. Transformers need AC, and high-voltage transmission cuts losses.

What’s Next

Physics in Everyday Life: Quick Revision, the whole chapter on one page
Energy Resources, covering thermal, hydro, nuclear and solar power
Minerals, Energy and Industries of Chhattisgarh, for Korba and the state’s power sector
Computers and Information Technology, where semiconductors drive electronics
Light and Sound, the previous topic

Fact	Detail
Ohm’s law	\( V = IR \) at constant temperature
Units	Current: ampere; voltage: volt; resistance: ohm; power: watt
Ammeter / voltmeter	Series / parallel
Household wiring	Parallel
Supply in India	230 V nominal (NCERT: 220 V), 50 Hz AC
1 kWh	One “unit” = \( 3.6 \times 10^{6} \) J
Nichrome	Heating elements
Tungsten	Bulb filament; melting point about 3,400 °C
Fuse	Low-melting wire, in series in the live wire
Earthing	Protects against shock from metal bodies
Wire colours	Old: red / black / green; current: brown / blue / green-yellow
Oersted, 1820	Current produces a magnetic field
Faraday, 1831	Electromagnetic induction
Motor	Electrical → mechanical; Fleming’s left-hand rule
Generator	Mechanical → electrical; Fleming’s right-hand rule
Transformer	AC only; step-up for transmission, step-down for supply
Soft iron vs steel	Electromagnet vs permanent magnet
Korba	“Power capital” of Chhattisgarh

Prerequisites

The Hook: From Korba’s Coal to Your Switchboard

Basic Quantities

Ohm’s law

Series vs parallel

Heating Effect of Current

Electric Power and the “Unit” on Your Bill

Domestic Electric Circuits and Safety

Supply in India

The three wires

Safety devices

AC vs DC

Magnetism (चुंबकत्व)

Magnets and Earth’s magnetic field

Magnetic effect of current: electromagnets

Electric motor: electricity → motion

Electromagnetic induction: motion → electricity

How electricity is generated: one chain

Memory Tricks

Don’t Confuse

Common Mistakes

Practice

Level 1: Foundation

Level 2: Prelims MCQs

Level 3: Mains Answer Writing

Quick Revision Table

Key Takeaways

What’s Next

Keep going in this chapter