Magnetism and Electromagnetic Induction Notes

Magnetism is the force with which magnets attract iron and push or pull other magnets. Electromagnetism is the deep connection between electricity and magnetism: an electric current creates a magnetic field, and a changing magnetic field can create an electric current. This second effect, called electromagnetic induction, is the principle behind generators in power plants that supply electricity to our homes, transformers that change voltage, induction cooktops, wireless chargers and many sensors. Questions on magnets, Fleming's rules, motors, generators and transformers are common in SSC CGL, CHSL, MTS, railway (RRB NTPC, Group D), defence and state PCS exams, and the applications appear in UPSC Prelims science and technology questions. This note explains each idea in simple steps.

Quick Facts at a Glance

• Like poles repel, unlike poles attract.
• Hans Christian Oersted (1820) discovered that an electric current produces a magnetic field.
• Michael Faraday (1831) discovered electromagnetic induction.
• Electric motor converts electrical energy into mechanical energy; generator converts mechanical energy into electrical energy.
• Transformer works only on alternating current (AC) and changes voltage levels.
• India's household supply is AC at about 220–230 V and 50 Hz.

1. Magnets and Their Properties

A magnet is a material that produces a magnetic field. Natural magnets, such as lodestone (magnetite, Fe₃O₄), were known to ancient civilisations. Artificial magnets are made from iron, steel, nickel, cobalt and special alloys such as alnico and neodymium-iron-boron.

• Every magnet has two poles, north and south. Even if a magnet is broken, each piece has both poles; isolated single poles (monopoles) have not been found.
• A freely suspended magnet always comes to rest in the north-south direction. This is the principle of the magnetic compass.
• Like poles repel and unlike poles attract. Repulsion is the surest test of a magnet, because a magnet can attract an unmagnetised iron piece too.
• Magnets lose their magnetism when heated strongly, hammered or dropped repeatedly.

Types of Magnetic Materials

2. Magnetic Field and Field Lines

The region around a magnet where its force can be felt is its magnetic field. We represent it with magnetic field lines, which can be seen by sprinkling iron filings around a bar magnet.

• Field lines emerge from the north pole and enter the south pole outside the magnet; inside the magnet, they go from south to north. So they form closed loops.
• Field lines never intersect, because the field cannot have two directions at the same point.
• Where lines are closer together, the field is stronger, as near the poles.
• The SI unit of magnetic field strength (flux density) is the tesla (T); a smaller unit, the gauss, equals 10⁻⁴ T.

3. The Earth as a Magnet

The Earth behaves like a giant bar magnet, mainly because of electric currents in its molten outer core (the geodynamo). The Earth's geographic north pole is close to its magnetic south pole, which is why the north pole of a compass needle points north. The magnetic poles slowly wander and have even reversed many times over geological history. The Earth's magnetic field forms the magnetosphere, which shields us from charged particles of the solar wind; when these particles are guided towards the poles, they create the beautiful auroras.

4. Magnetic Effect of Electric Current

In 1820, Hans Christian Oersted noticed that a compass needle moved when placed near a wire carrying current. This showed that electricity produces magnetism.

• Right-hand thumb rule: If you hold a current-carrying straight wire in your right hand with the thumb pointing in the direction of current, your curled fingers show the direction of the magnetic field lines around it.
• Solenoid: A long coil of wire. When current flows through it, it behaves like a bar magnet with a uniform field inside.
• Electromagnet: A solenoid with a soft iron core. It is a temporary magnet that works only while current flows, and its strength can be increased by increasing current or the number of turns. Electromagnets are used in electric bells, cranes that lift scrap iron, loudspeakers, MRI machines and maglev trains.

5. Force on a Current-Carrying Conductor and the Electric Motor

A current-carrying conductor placed in a magnetic field experiences a force. The direction of this force is given by Fleming's left-hand rule: stretch the thumb, forefinger and middle finger of the left hand mutually at right angles. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of current, then the thumb shows the direction of motion (force).

The electric motor works on this principle. A rectangular coil placed between the poles of a magnet experiences forces in opposite directions on its two sides, which makes it rotate. A split-ring commutator reverses the current direction every half turn so that the coil keeps rotating in the same direction. Motors are used in fans, washing machines, mixers, pumps, electric vehicles and trains.

6. Electromagnetic Induction

In 1831, Michael Faraday discovered that when the magnetic field through a coil changes, an electric current is induced in the coil. This can happen by moving a magnet into or out of a coil, moving the coil in a magnetic field, or changing the current in a nearby coil.

• Faraday's law: The induced electromotive force (EMF) is proportional to the rate of change of magnetic flux through the coil. Faster change means a larger current.
• Lenz's law: The induced current flows in a direction that opposes the change that produced it. This follows from the law of conservation of energy.
• Fleming's right-hand rule: Used for generators. With the thumb, forefinger and middle finger of the right hand at right angles, the thumb shows the motion of the conductor, the forefinger the magnetic field, and the middle finger the direction of induced current.

Memory aid: Left hand for Motors ("LM") and Right hand for Generators ("RG").

7. The Electric Generator

A generator converts mechanical energy into electrical energy using electromagnetic induction. A coil (armature) is rotated in a magnetic field by a turbine driven by steam, water, wind or gas. As the coil rotates, the magnetic flux through it changes continuously, inducing a current. An AC generator uses slip rings and produces alternating current; a DC generator uses a split-ring commutator and produces direct current. Almost all power stations, whether thermal, hydroelectric, nuclear or wind, use AC generators.

8. AC vs DC

9. The Transformer

A transformer changes the voltage of alternating current using mutual induction between two coils (primary and secondary) wound on an iron core. A step-up transformer has more turns in the secondary coil and increases voltage; a step-down transformer has fewer turns in the secondary and decreases voltage. Electricity is generated at moderate voltage, stepped up to very high voltages (hundreds of kilovolts) for transmission to reduce energy losses, and then stepped down near homes to about 230 V. A transformer does not work with DC, because DC does not produce a changing magnetic field.

10. Everyday Applications of Electromagnetic Induction

• Induction cooktops: A changing magnetic field induces currents directly in the metal base of the vessel, heating it.
• Wireless phone chargers: Coils in the charger and phone transfer energy through induction.
• Metal detectors at airports detect metal objects through changes in magnetic fields.
• Electric guitar pickups, microphones and card readers convert motion or magnetic patterns into electrical signals.
• Regenerative braking in electric vehicles and modern trains uses the motor as a generator to recover energy while slowing down.

11. Practice Questions

1. Which material is used as the core of an electromagnet? Answer: Soft iron.
2. What is the SI unit of magnetic flux density? Answer: Tesla.
3. Which device changes the voltage of AC? Answer: Transformer.
4. Copper is which type of magnetic material? Answer: Diamagnetic.

Related Notes

• Everyday Science: Heat, Sound and Electricity
• Physics Basics: Motion, Force, Gravitation and Light
• Renewable Energy in India Notes
• All General Science Notes

Frequently Asked Questions

Who discovered that electric current produces a magnetic field?

Hans Christian Oersted, in 1820.

Who discovered electromagnetic induction?

Michael Faraday, in 1831.

What is the surest test of a magnet?

Repulsion, because only a magnet can repel another magnet.

Do magnetic field lines intersect?

No. They never intersect, because a point cannot have two field directions.

What is an electromagnet?

A temporary magnet made by passing current through a coil wound on a soft iron core.

Which rule gives the direction of force on a current-carrying conductor?

Fleming's left-hand rule, used for electric motors.

Which rule is used for generators?

Fleming's right-hand rule.

What does an electric motor do?

It converts electrical energy into mechanical energy.

What does a generator do?

It converts mechanical energy into electrical energy using electromagnetic induction.

Why does a transformer not work on DC?

Because DC does not produce a changing magnetic field, which is needed for induction.

What is the frequency of AC supply in India?

50 hertz.

What does Lenz's law state?

The induced current always opposes the change in magnetic flux that produced it.