Maarg Manthan · Topic 8.5

Concepts of Electromagnetism

Science & Technology › Physics – Mechanics, Waves and Thermodynamics · Topic 8.5

Electromagnetism is the study of electric and magnetic forces and fields and the link between them. Electric charges exert forces on each other, as stated by Coulomb's law, and moving charges form an electric current. Ohm's law states that the current through a conductor is proportional to the voltage across it, V equals I times R, where R is resistance. Hans Christian Oersted found in 1820 that a current produces a magnetic field, and Michael Faraday found in 1831 that a changing magnetic field produces an electric current, called electromagnetic induction. James Clerk Maxwell brought electricity, magnetism and light together in a set of equations in the 1860s and predicted electromagnetic waves. Motors, generators, transformers and electromagnets all use these principles.

5 min read · UPSC Prelims · Science & Technology · Physics – Mechanics, Waves and Thermodynamics

At a Glance

Ohm’s law V = I R
Oersted 1820; current gives magnetism
Faraday 1831; induction
Maxwell Unified electricity, magnetism, light
Units Ampere, volt, ohm, tesla
Exam link UPSC Prelims, Science & Technology: Physics – Mechanics, Waves and Thermodynamics

Where Does It Fit?

This is the fifth topic of the macro on physics. It builds on the forces and energy topics and explains the basis of power systems and electromagnetic waves. Previous topic: The Electromagnetic Spectrum. Next topic: Heat and Thermodynamic Principles.

What Are the Key Laws and Discoveries?

Law or discovery	Statement
Coulomb’s law	The force between two charges is proportional to the product of the charges and inversely proportional to the square of the distance
Ohm’s law	Georg Ohm, 1827. Current is proportional to voltage at constant temperature. Resistance depends on material, length and area
Oersted’s experiment	1820. A current-carrying wire deflects a compass needle, so electricity produces magnetism
Ampere’s work	Parallel currents attract or repel each other. The ampere is the unit of current
Faraday’s law of induction	1831. A changing magnetic flux in a coil induces an electromotive force
Lenz’s law	The induced current opposes the change that causes it, an expression of conservation of energy
Maxwell’s equations	Describe how electric and magnetic fields arise and change, and predict electromagnetic waves travelling at the speed of light, as in The Electromagnetic Spectrum

How Do Electromagnetic Devices Work?

Electromagnet: a coil with a soft iron core becomes a strong magnet when current flows, used in cranes, bells and relays.
Electric motor: a current-carrying coil in a magnetic field feels a force and turns, as given by Fleming’s left-hand rule. Used in fans, pumps and electric vehicles.
Generator: rotating a coil in a magnetic field induces a current, as given by Fleming’s right-hand rule. Power plants use turbines to turn generators.
Transformer: uses induction to raise or lower AC voltage, which lets power be transmitted at high voltage with less loss.
AC and DC: alternating current changes direction, as in household supply of 50 hertz in India at about 230 volts, while direct current flows in one direction, as from batteries.
Loudspeaker and microphone: use coils and magnets to change between electrical and sound signals.

What Else Should You Remember?

Series and parallel circuits: in series, the same current flows through all components and resistances add. In parallel, the voltage is the same across each and household appliances are connected this way.
Joule heating: the heat produced equals current squared times resistance times time, used in heaters, irons and fuses.
Safety devices: a fuse or circuit breaker cuts the circuit in overload, and the earth wire gives a safe path for leakage current.
Power and bill: power equals voltage times current, and electricity bills are charged in units, each one a kilowatt-hour, as in Forms of Energy.
Magnetic field and Earth: the Earth acts like a bar magnet, and a compass needle points along its field. The tesla is the unit of magnetic field.
Magnetic materials: ferromagnetic materials such as iron, nickel and cobalt are strongly attracted, while others are weakly paramagnetic or diamagnetic.
Superconductors: have zero resistance, as in Superconductivity: History, Classification and Properties.
Static electricity and lightning: charges that build up on clouds discharge as lightning, and lightning conductors protect tall buildings.
Exam pattern: scientists and their discoveries, Fleming’s rules and circuit questions.
Electric current: current is the flow of charge, measured in amperes, and in metals it is carried by electrons moving opposite to the conventional current direction.
Resistivity: the resistivity of a material is its property, and copper and aluminium are used for wires because they have low resistivity, while nichrome is used in heaters because of its high resistivity.
Electromagnetic waves: Maxwell showed that changing electric and magnetic fields travel together as waves, which Hertz produced in 1887.

Why Does It Matter for the Exam?

Law questions: who stated each law and what it says.
Device questions: principles of motors, generators and transformers.
Policy link: power transmission, smart grids and electric mobility.
Safety questions: fuses, earthing and lightning protection.
Common errors: confusing Fleming’s left- and right-hand rules or saying that a transformer works on direct current.

As an analytical point, electromagnetic induction made large-scale electricity possible. Every power plant, from coal to wind, depends on a coil turning in a magnetic field.

Why Is This Topic Important?

Facts Scientists and laws asked
Devices Motor and generator
Safety Fuse and earth

What Came Next?

The next topic covers heat and thermodynamic principles. Next topic: Heat and Thermodynamic Principles.

Key People and Terms

Coulomb Unit of charge
Ampere Unit of current
Ohm Unit of resistance
Induction Producing current by changing magnetic field
Tesla Unit of magnetic field
Transformer Changes AC voltage

Exam Corner

Points to Remember

Ohm’s law: V = I R.
Oersted 1820; Faraday 1831.
Motor: left-hand rule; generator: right-hand rule.
Indian supply: 50 Hz.
Transformer works on AC only.

Do Not Confuse With

Motor and generator Electrical to mechanical versus the reverse
AC and DC Alternating versus direct
Fuse and earth wire Cuts overload versus carries leakage

Memory Hook

Oersted current to magnet, Faraday magnet to current; motor left, generator right; Maxwell joined light to this.

Mains Angle

Questions link electromagnetism with the power sector. These points add depth.

Laws Ohm, Faraday
Devices Motor, generator, transformer
Grid Transmission

A Question You May Face

An original practice question, not a past paper question.

“Explain electromagnetic induction and discuss its applications in the generation and transmission of electricity.”

How to Answer

Discoveries Oersted, Faraday.
Induction Principle.
Devices Generator.
Grid Transformer.

GS Relevance

Prelims: laws and scientists, units, devices, domestic circuits. GS Paper 3: science and technology, energy.

Frequently Asked Questions

What is Ohm's law?

Ohm's law, stated by Georg Ohm in 1827, says that the current flowing through a conductor is directly proportional to the voltage across it at constant temperature, which is written V equals I times R. The constant R is the resistance of the conductor, measured in ohms.

What is electromagnetic induction?

Electromagnetic induction is the production of an electromotive force, and so a current, in a coil when the magnetic field through it changes. Michael Faraday discovered it in 1831, and it is the principle behind generators, transformers and induction cooktops.

What is the difference between an electric motor and a generator?

An electric motor converts electrical energy into mechanical energy, because a current-carrying coil in a magnetic field experiences a force and rotates. A generator does the opposite, converting mechanical energy into electrical energy by rotating a coil in a magnetic field to induce a current.

Why are transformers used in power transmission?

Transformers raise the voltage of alternating current for long-distance transmission, which cuts the current and so the heat loss in the lines, and then lower it again for homes and factories. They work by electromagnetic induction between two coils and cannot work with direct current.

How do fuses and earth wires protect us?

A fuse has a thin wire that melts and breaks the circuit when the current is too high, preventing fire and damage. The earth wire connects metal parts of an appliance to the ground, so that leakage current flows into the earth and not through a person who touches the appliance.

PYQ Practice — Statement Analysis

1 Ohm's law relates voltage, current and resistance.

True

V = I R.

2 A transformer can step up direct current voltage.

False

It works with alternating current.

3 Faraday discovered electromagnetic induction.