Physics — Electricity and Magnetism

Electricity and Magnetism is one of the most frequently tested science topics in Kerala PSC exams. Questions are fact-based and formula-based. These notes cover all concepts from NCERT Class 10 Physics plus PSC-specific additions.

Basic Electrical Quantities

Quantity	Symbol	SI Unit	Measuring Instrument	Definition
Current	I	Ampere (A)	Ammeter (connected in series)	Rate of flow of electric charge
Voltage (Potential Difference)	V	Volt (V)	Voltmeter (connected in parallel)	Work done per unit charge
Resistance	R	Ohm (Ω)	Ohmmeter / Wheatstone Bridge	Opposition to current flow
Power	P	Watt (W)	Wattmeter	Rate of electrical energy consumption
Energy	E	Joule (J) or kWh	Energy meter (at homes)	Work done by electric current
Charge	Q	Coulomb (C)	—	Q = I x t

Ohm’s Law

Statement: At constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its ends.

Formula: V = I x R

Term	Meaning
V	Potential difference (Volts)
I	Current (Amperes)
R	Resistance (Ohms)

Ohm’s Law graph: V vs I is a straight line passing through origin (for ohmic conductors).

Non-ohmic conductors (do not follow Ohm’s law): diode, transistor, electrolyte, filament lamp (when heated).

Resistance and Resistivity

Factor	Effect on Resistance
Length (l)	R is directly proportional to length
Cross-sectional area (A)	R is inversely proportional to area
Material	Different materials have different resistivity
Temperature	For metals: resistance increases with temperature

Formula: R = ρ (l / A)

Where ρ (rho) = resistivity (unit: ohm-metre, Ω·m)

Resistivity of Common Materials

Material	Category	Resistivity
Silver	Conductor	Lowest resistivity among metals
Copper	Conductor	Second lowest; used in household wiring
Nichrome	Alloy	High resistivity; used in heating elements
Rubber	Insulator	Extremely high resistivity
Silicon	Semiconductor	Between conductor and insulator

Series and Parallel Circuits

Series Combination

Feature	Formula/Rule
Total resistance	R_total = R1 + R2 + R3 + …
Current	Same through all resistors
Voltage	Divided among resistors (V = V1 + V2 + V3)
If one component fails	Entire circuit breaks
Example	Old-style Christmas lights

Parallel Combination

Feature	Formula/Rule
Total resistance	1/R_total = 1/R1 + 1/R2 + 1/R3 + …
Current	Divided among branches (I = I1 + I2 + I3)
Voltage	Same across all resistors
If one component fails	Others continue to work
Example	Household appliances

Key fact: Total resistance in parallel is always LESS than the smallest individual resistance.

Electrical Power and Energy

Formula	Relation
P = V x I	Power = Voltage x Current
P = I² x R	Power in terms of current and resistance
P = V² / R	Power in terms of voltage and resistance
E = P x t	Energy = Power x Time

Practical Unit of Energy

Unit	Equivalent
1 kilowatt-hour (kWh)	3.6 x 10⁶ Joules
1 kWh	1 “unit” of electricity (as on electricity bill)
1 unit cost	Varies by state/slab (Kerala: ~₹3-8 per unit for domestic)

Heating Effect of Current (Joule’s Law)

H = I² x R x t (heat produced in Joules)

Application	Device
Electric heater	Nichrome wire coil
Electric iron	Nichrome heating element
Electric fuse	Melts when current exceeds rated value
Electric bulb	Tungsten filament (melting point ~3,400°C)

Household Electrical Wiring

Feature	Details
Supply voltage in India	230 V AC, 50 Hz (single phase domestic)
Three wires	Live (Red/Brown), Neutral (Black/Blue), Earth (Green)
Connection type	Parallel (so each appliance gets full 230 V)
Fuse/MCB	Connected in Live wire
Earth wire	Connected to metal body of appliance; safety against electric shock
Switch	Always in the Live wire
Ring main system	Used in modern houses; circuit loops back to distribution board

Safety Devices

Device	Function
Fuse	Wire of low melting point (tin-lead alloy); melts on overload
MCB (Miniature Circuit Breaker)	Trips automatically on overload; can be reset
ELCB / RCCB	Detects earth leakage/residual current; prevents electrocution
Earthing	Provides low-resistance path to ground for fault current

Magnetism — Basic Concepts

Concept	Details
Natural magnet	Magnetite (lodestone) — Fe₃O₄
Magnetic poles	North (N) and South (S); always exist in pairs (no magnetic monopoles)
Like poles	Repel
Unlike poles	Attract
Earth’s magnetism	Earth behaves as a giant magnet; geographic north is near magnetic south
Magnetic field lines	Go from N to S outside magnet; S to N inside magnet; never intersect

Electromagnetism

Oersted’s Experiment (1820)

Hans Christian Oersted discovered that a current-carrying conductor produces a magnetic field around it (deflected a compass needle).

Magnetic Field Due to Current

Configuration	Field Pattern	Rule
Straight wire	Concentric circles around wire	Right-hand thumb rule (thumb = current direction; curled fingers = field direction)
Circular loop (coil)	Similar to bar magnet at centre	Field strength increases with number of turns
Solenoid	Uniform field inside; like bar magnet outside	Right-hand rule for solenoid

Fleming’s Left Hand Rule (Motor Rule)

Used to find: Direction of force on a current-carrying conductor in a magnetic field

Finger	Represents
Forefinger (index)	Magnetic Field direction (N to S)
Middle finger	Current direction
Thumb	Thrust/Force/Motion direction

Application: Electric motor

Electric Motor

Feature	Details
Principle	Current-carrying conductor in magnetic field experiences a force (Lorentz force)
Energy conversion	Electrical energy to Mechanical energy
Key component	Split ring commutator — reverses current direction every half rotation
Other parts	Armature coil, permanent magnets, brushes, axle
Use	Fans, mixers, washing machines, electric vehicles

Electromagnetic Induction — Faraday’s Discovery

Michael Faraday (1831) discovered that a changing magnetic field induces an EMF (voltage) in a conductor.

Method	How EMF is Induced
Moving magnet near coil	Relative motion between magnet and coil
Moving coil in magnetic field	Conductor cuts magnetic field lines
Changing current in nearby coil	Mutual induction

Faraday’s Laws

Law	Statement
First Law	Whenever magnetic flux linked with a coil changes, an EMF is induced
Second Law	Magnitude of induced EMF is proportional to the rate of change of magnetic flux

Lenz’s Law

The direction of induced current is such that it opposes the change that causes it. (Conservation of energy principle.)

Fleming’s Right Hand Rule (Generator Rule)

Used to find: Direction of induced current

Finger	Represents
Forefinger	Magnetic Field
Thumb	Motion of conductor
Middle finger	Induced Current direction

Electric Generator (Dynamo)

Feature	Details
Principle	Electromagnetic induction (Faraday’s law)
Energy conversion	Mechanical energy to Electrical energy
AC Generator	Uses slip rings — produces alternating current
DC Generator	Uses split ring commutator — produces direct current
Power plants	Use AC generators (turbines driven by steam/water/wind)

Transformer

Feature	Details
Principle	Mutual induction
Function	Changes AC voltage level (step-up or step-down)
Works ONLY with	AC (not DC — DC has no changing flux)
Core	Laminated soft iron (reduces eddy current losses)

Type	Turns	Voltage	Current
Step-up	Secondary turns greater than primary	Increases voltage	Decreases current
Step-down	Secondary turns less than primary	Decreases voltage	Increases current

Formula: Vs/Vp = Ns/Np (Voltage ratio = Turns ratio)

Power Transmission

• Power plants generate at ~11,000 V
• Step-up to 132,000–400,000 V for long-distance transmission (reduces current, hence reduces I²R loss)
• Step-down at substations to 11,000 V, then to 230 V for domestic supply

Important Electrical Units — Summary

Quantity	Unit	Named After
Current	Ampere (A)	Andre-Marie Ampere
Voltage	Volt (V)	Alessandro Volta
Resistance	Ohm (Ω)	Georg Simon Ohm
Power	Watt (W)	James Watt
Capacitance	Farad (F)	Michael Faraday
Inductance	Henry (H)	Joseph Henry
Magnetic flux	Weber (Wb)	Wilhelm Eduard Weber
Magnetic field strength	Tesla (T)	Nikola Tesla
Frequency	Hertz (Hz)	Heinrich Hertz

PSC Expected Questions

1. SI unit of resistance — Ohm (Ω)
2. Ohm’s Law formula — V = IR
3. Household wiring connection — Parallel
4. Supply voltage in India — 230 V, 50 Hz
5. Fuse is connected in — Live wire
6. Discovered electromagnetic induction — Michael Faraday
7. Electric motor uses — Fleming’s Left Hand Rule
8. Generator uses — Fleming’s Right Hand Rule
9. Transformer works on — AC only (mutual induction)
10. 1 unit of electricity = 1 kWh = 3.6 x 10⁶ Joules
11. Oersted discovered — current produces magnetic field (1820)
12. Lenz’s Law — induced current opposes the cause