Notes Magnets - Science & Pedagogy Paper 2 for CTET & TET Exams - CTET & State TET

Magnets

A piece of iron or another material that exhibits the properties of magnetism, such as attracting objects containing iron, is called a magnet. The ancient Greeks discovered magnetite (also called lodestone), a naturally occurring magnetic mineral.

Key observable properties of magnets:

• A magnet is dipolar: it has two ends called poles - a north (N) pole and a south (S) pole.
• Like poles repel; unlike poles attract: two north poles or two south poles push each other away, whereas a north and a south pole pull towards each other.
• Freely suspended magnet aligns north-south: a magnet hung freely (unrestricted) tends to align itself along the magnetic north-south direction of the Earth.

Magnetic and Non‐Magnetic Materials

Magnetic materials are those that are attracted by a magnet. Common examples are iron, nickel and cobalt.

Non‐magnetic materials are not attracted by a magnet; for example, wood and paper.

Natural and Artificial Magnets

Natural Magnets

A natural magnet is found in nature. Lodestone (magnetite) is the classical example of a natural magnet.

Artificial Magnets

Artificial magnets are made by humans. They are classified into two broad kinds:

• Temporary magnets: materials that behave like magnets only while placed in a magnetic field and lose their magnetism when the external field is removed. Examples include some weakly magnetic minerals and soft iron pieces used temporarily as magnets.
• Permanent magnets: materials that retain their magnetism for a long time. Examples include refrigerator magnets, magnets used in headphones and microphones, and magnets made of steel alloys, ferrites, and rare‐earth materials.

Magnetic Compass

A magnetic compass is an instrument used to find directions. It works because a small magnetised needle, when freely suspended, aligns itself along the Earth's magnetic field and thus points approximately towards geographic north and south.

Magnetic Field

The magnetic field of a magnet is the region around the magnet in which magnetic effects can be detected. The magnetic field is represented pictorially by magnetic field lines (also called magnetic lines of force).

Important properties of magnetic field lines:

• Field lines emerge from the north pole and enter the south pole of a magnet.
• Field lines are denser near the poles where the magnetic field is strongest and spread out away from the poles.
• Field lines never intersect one another.
• Field lines form continuous closed loops (they pass inside the magnet from south to north and outside from north to south).
• The strength of the magnetic field at a point is proportional to the density (closeness) of the field lines.
• At any point, the magnetic field direction is tangent to the magnetic field line at that point.

Magnetic Field Due to a Current‐Carrying Conductor

Hans Christian Ørsted observed that a magnetic needle is deflected when placed near a current‐carrying conductor. This shows that an electric current produces a magnetic field around the conductor.

• The magnitude of the magnetic field produced by a straight current‐carrying conductor is directly proportional to the current (I) flowing and inversely proportional to the distance (r) from the conductor.
• For a long straight conductor, the magnetic field varies approximately as B ∝ I / r (qualitative relation used at school level).

Right‐hand thumb rule (right‐hand grip rule): if a person holds a current‐carrying wire in the right hand with the thumb pointing in the direction of the conventional current, the fingers wrapped around the wire show the direction of the magnetic field lines encircling the wire.

Electromagnet

An electromagnet is a magnet in which the magnetic field is produced by an electric current. Typically, a coil of insulated wire (a solenoid) is wound around an iron core; when current flows through the coil the iron core becomes magnetised and acts as a magnet. Electromagnets are widely used because their magnetism can be switched on and off by controlling the current.

How an Electric Bell Works

An electric bell contains an electromagnet formed by a coil of wire wound around an iron piece. An iron strip carrying a hammer is placed close to this electromagnet and a contact screw is arranged so that the strip can touch it.

• When the iron strip touches the contact screw, the circuit is complete and current flows through the coil, making it an electromagnet.
• The electromagnet attracts the iron strip and pulls the hammer to strike the gong, producing a sound.
• When the iron strip is pulled, it breaks contact with the screw and the current stops; the electromagnet loses its magnetism and the strip returns to its original position.
• The strip again touches the contact screw; the circuit is completed and the process repeats rapidly, causing the hammer to strike the gong repeatedly and the bell to ring.

Fleming's Left‐Hand Rule

Fleming's left‐hand rule is used to find the direction of the force on a current‐carrying conductor placed in a magnetic field (application: electric motors). If the thumb, forefinger (first finger) and middle finger of the left hand are stretched mutually perpendicular to one another, then:

• The forefinger points in the direction of the magnetic field.
• The middle finger points in the direction of the conventional current.
• The thumb gives the direction of the force (or motion) experienced by the conductor.

Electric Motor

An electric motor converts electrical energy into mechanical energy. It works on the principle that a current‐carrying conductor placed in a magnetic field experiences a force. In a simple direct‐current (D.C.) motor, a rectangular coil is placed between the poles of a magnet and current passed through the coil produces forces on its sides that make the coil rotate. The rotating coil is attached to a shaft so the shaft turns and does useful mechanical work.

The continuous rotation is achieved by reversing the direction of current in the coil every half turn using a commutator (in D.C. motors).

Electromagnetic Induction

Electromagnetic induction is the process by which an electric current or voltage is produced in a conductor when the magnetic environment of the conductor changes. In simple words, whenever there is a change in magnetic flux through a circuit, an induced emf (and possibly induced current) appears in the circuit. This principle is the basis of electric generators and many other devices.

Fleming's Right‐Hand Rule

Fleming's right‐hand rule is used to find the direction of the induced current when a conductor moves in a magnetic field (application: electric generators). If the thumb, forefinger and middle finger of the right hand are stretched mutually perpendicular to one another, then:

• The thumb represents the motion (direction of movement) of the conductor.
• The forefinger represents the magnetic field direction.
• The middle finger gives the direction of the induced current.

Electric Generator

An electric generator converts mechanical energy into electrical energy using the principle of electromagnetic induction. When a conductor (or coil) experiences a change in magnetic flux because of relative motion between the conductor and a magnetic field, an emf is induced and electricity can be obtained.

Two main types of generators are:

• A.C. generator: produces alternating current (A.C.), where the current changes direction periodically.
• D.C. generator: produces direct current (D.C.), where the current flows in a single direction (commutators are used to obtain DC output from rotating coils).

Domestic Electric Circuits

In typical household supply in our region, the mains supply is A.C. with a frequency of 50 Hz and a nominal voltage often referred to as 220 V in school-level text (practical nominal voltages may vary by region and standards).

Three wires are commonly used in domestic wiring:

• Live (phase) wire: usually insulated with red plastic covering in many school examples; it carries the alternating potential and is the source of current to appliances.
• Neutral wire: usually insulated with black plastic covering in many school examples; it completes the circuit and is close to earth potential.
• Earth (ground) wire: usually insulated with green plastic covering; it is a safety conductor that provides a path for stray current to reduce the risk of electric shock.

Important protective devices in domestic circuits:

• Fuse: a safety device with a metal wire that melts when excessive current flows, thereby opening the circuit and preventing damage due to short circuit or overload.
• MCB (Miniature Circuit Breaker): an automatic switch that trips to cut off current in case of overcurrent or short circuit; it can be reset after the fault is removed.

Applications and Everyday Examples

• Electromagnets are used in electric bells, loudspeakers, MRI machines (medical imaging), cranes for lifting scrap iron, and relays.
• Motors are used in fans, mixers, washing machines, electric vehicles and many household appliances.
• Generators are used in power stations, portable generators, and as alternators in automobiles to charge batteries.
• Magnetic compasses are used for navigation in ships and for orientation in many instruments.

Summary

Magnets have two poles and produce a magnetic field represented by continuous field lines. Materials such as iron, nickel and cobalt are magnetic; others like wood and paper are not. Electric currents produce magnetic fields and changing magnetic fields induce electric currents (electromagnetic induction). These principles form the basis of important devices such as electromagnets, electric bells, motors and generators, which are widely used in daily life and industry. Domestic circuits use live, neutral and earth wires along with protective devices such as fuses and MCBs for safety and reliable supply.