Physics — Magnetism, Electromagnets, Motors, Transformers, Galvanometer

Magnetism and electromagnetic devices are important topics in Kerala PSC science sections. Questions test properties of magnets, electromagnetic devices, and their working principles. Expect 1-3 questions per paper.

Basic Magnetism

Concept	Details
Magnet	Object that produces a magnetic field; attracts iron, cobalt, nickel
Natural magnet	Lodestone (magnetite, Fe3O4) — first known magnet
Properties	Attractive property, directive property (N-S alignment), like poles repel, unlike poles attract
Magnetic axis	Imaginary line joining the two poles of a magnet
Magnetic meridian	Vertical plane passing through the magnetic axis of a freely suspended magnet
Magnetic declination	Angle between geographic north and magnetic north at a place
Magnetic dip (inclination)	Angle between horizontal plane and the direction of Earth’s magnetic field
Dip at poles	90 degrees
Dip at equator	0 degrees

Types of Magnetic Materials

Type	Property	Examples	Behavior in Magnetic Field
Ferromagnetic	Strongly attracted; can be permanently magnetized	Iron, cobalt, nickel, steel, alnico	Align strongly with field; retain magnetism
Paramagnetic	Weakly attracted	Aluminium, platinum, chromium, oxygen, manganese	Weakly align with field; lose magnetism when field removed
Diamagnetic	Weakly repelled	Bismuth, copper, gold, silver, water, diamond	Align opposite to field; weakly repelled

Key Fact	Details
Curie temperature	Temperature above which a ferromagnetic material becomes paramagnetic (e.g., iron: 770 degrees C)
Hard magnetic material	Difficult to magnetize but retains magnetism (steel, alnico) — used for permanent magnets
Soft magnetic material	Easy to magnetize but loses magnetism quickly (soft iron) — used for electromagnets, transformers

Electromagnets

Feature	Details
Principle	Electric current through a conductor produces a magnetic field around it (discovered by Hans Christian Oersted, 1820)
Construction	Coil of insulated wire (solenoid) wound around a soft iron core
Why soft iron?	Easy to magnetize and demagnetize; strong temporary magnet
Strength depends on	Number of turns, current strength, nature of core material
Advantage over permanent magnet	Magnetic strength can be controlled; can be switched on/off

Applications of Electromagnets

Application	How Used
Electric bell	Electromagnet attracts hammer to strike bell; circuit breaks; hammer returns; repeats
Telephone receiver	Electromagnet vibrates a diaphragm to produce sound
MRI machine	Superconducting electromagnets create strong magnetic fields for body imaging
Maglev train	Electromagnetic levitation; train floats above track
Cranes (scrapyard)	Powerful electromagnets lift heavy iron/steel objects
Relay switches	Electromagnet controls a larger circuit switch
Hard disk drives	Data stored magnetically; read/write heads are electromagnets

Electromagnetic Induction

Concept	Details
Discovered by	Michael Faraday (1831)
Faraday’s Law	A changing magnetic field through a circuit induces an EMF (electromotive force) in the circuit
Lenz’s Law	The direction of induced current opposes the change that causes it (conservation of energy)
Fleming’s Right Hand Rule	For generators — thumb (motion), index (field), middle (induced current)
Fleming’s Left Hand Rule	For motors — thumb (motion/force), index (field), middle (current)

Galvanometer

Feature	Details
Purpose	Detects and measures small electric currents
Principle	Current-carrying coil in a magnetic field experiences a torque (force)
Construction	Rectangular coil between poles of a permanent magnet; attached to a pointer
Sensitivity	Very sensitive; can detect microamperes
Converted to ammeter	By connecting a low resistance (shunt) in parallel
Converted to voltmeter	By connecting a high resistance in series

Electric Motor

Feature	Details
Function	Converts electrical energy to mechanical energy (rotation)
Principle	A current-carrying conductor in a magnetic field experiences a force (Lorentz force)
Rule	Fleming’s Left Hand Rule
Key components	Armature (coil), Field magnet, Commutator (split rings), Brushes
Commutator function	Reverses the direction of current every half rotation to maintain continuous rotation
Types	DC motor, AC motor (induction motor, synchronous motor)

DC Motor Components

Part	Function
Armature	Rectangular coil that rotates
Field magnet	Provides steady magnetic field
Split ring commutator	Reverses current direction every half turn
Carbon brushes	Conduct current to the rotating armature via commutator
Axle	Shaft that delivers mechanical output

Applications of Electric Motor

Application	Type
Electric fan	AC induction motor
Washing machine	AC motor
Electric vehicles	DC brushless motor or AC induction motor
Mixer grinder	Universal motor (works on both AC and DC)
Hard disk drive	DC motor (spindle motor)

Electric Generator (Dynamo)

Feature	Details
Function	Converts mechanical energy to electrical energy
Principle	Electromagnetic induction (Faraday’s Law)
Rule	Fleming’s Right Hand Rule
Key components	Armature (coil), Field magnet, Slip rings (AC) or Commutator (DC), Brushes
AC generator	Uses slip rings — produces alternating current
DC generator	Uses split ring commutator — produces direct current

Motor vs Generator

Feature	Electric Motor	Electric Generator
Energy conversion	Electrical to Mechanical	Mechanical to Electrical
Rule	Fleming’s Left Hand	Fleming’s Right Hand
Input	Electric current	Mechanical rotation (turbine, wind, etc.)
Output	Rotation (mechanical work)	Electric current
Commutator	Reverses current for continuous rotation	Converts AC to DC (in DC generator)

Transformer

Feature	Details
Function	Changes (transforms) the voltage of alternating current (AC)
Principle	Mutual induction — changing current in one coil induces EMF in another coil wound on the same core
Construction	Two coils (primary and secondary) wound on a laminated soft iron core
Works on	AC only (not DC — DC does not produce changing magnetic field)
Efficiency	Can be over 95% (ideal transformer: 100%)

Types of Transformers

Type	Turns Ratio	Voltage	Current	Use
Step-up	Secondary turns are more than primary (Ns is greater than Np)	Increases voltage	Decreases current	Power stations (before transmission)
Step-down	Secondary turns are fewer than primary (Ns is less than Np)	Decreases voltage	Increases current	Substations, phone chargers, household use

Transformer Equation

Relationship	Formula
Voltage ratio = Turns ratio	Vs/Vp = Ns/Np
Power conservation (ideal)	Vp x Ip = Vs x Is
If voltage increases	Current decreases proportionally

Why Laminated Iron Core?

Reason	Details
Reduces eddy currents	Eddy currents are induced currents in the core that cause energy loss as heat
Lamination	Thin insulated sheets of soft iron reduce the path for eddy currents
Soft iron used because	High permeability, easy magnetization/demagnetization, low hysteresis loss

Power Transmission

Stage	Voltage	Device
Power station output	11,000-25,000 V	Generator
Before long-distance transmission	Stepped up to 132,000-400,000 V	Step-up transformer
At substation (city)	Stepped down to 33,000 V	Step-down transformer
At local distribution	Stepped down to 11,000 V	Step-down transformer
At consumer (household)	220-240 V (single phase)	Step-down transformer
Reason for high voltage transmission	Reduces current (P = VI), which reduces power loss (P_loss = I squared x R) in transmission lines

Earth’s Magnetism

Feature	Details
Earth behaves as	A giant bar magnet with magnetic poles near geographic poles
Geographic North Pole is near	Magnetic South Pole (and vice versa) — this is why compass north points to geographic north
Cause	Convection currents in Earth’s liquid outer core (iron-nickel)
Magnetic reversal	Earth’s magnetic poles have reversed multiple times in geological history

PSC Quick Recall

Question	Answer
First natural magnet known	Lodestone (magnetite)
Oersted discovered	Electric current produces a magnetic field (1820)
Faraday discovered	Electromagnetic induction (1831)
Motor converts	Electrical to mechanical energy
Generator converts	Mechanical to electrical energy
Motor uses which rule?	Fleming’s Left Hand Rule
Generator uses which rule?	Fleming’s Right Hand Rule
Transformer works on	AC only (not DC)
Step-up transformer	Increases voltage, decreases current
Laminated iron core reduces	Eddy currents
Soft iron used in electromagnet because	Easy to magnetize and demagnetize
Galvanometer to ammeter	Add low resistance (shunt) in parallel
Galvanometer to voltmeter	Add high resistance in series
Curie temperature of iron	About 770 degrees C
Diamagnetic material example	Bismuth, copper, gold
High voltage transmission reduces	Power loss (I squared R loss)