Overview
Physics studies the fundamental laws governing matter, energy, motion, and forces. For TNPSC Group 4, physics questions focus on school-level (Class 8–10) concepts — especially motion, optics, electricity, and sound. Approximately 15–20 science questions appear in the Group 4 paper.
Motion and Force
Newton's Three Laws of Motion
| Law | Statement | Example |
|---|---|---|
| 1st Law (Inertia) | Object at rest stays at rest; object in motion stays in motion unless external force acts | Passenger lurches forward when bus brakes suddenly |
| 2nd Law (F = ma) | Force = Mass × Acceleration | Heavier object needs more force to accelerate |
| 3rd Law (Action-Reaction) | Every action has equal and opposite reaction | Rocket propulsion; swimming |
Key Terms
- Speed = Distance ÷ Time (scalar — no direction)
- Velocity = Displacement ÷ Time (vector — has direction)
- Acceleration = Change in velocity ÷ Time
- Momentum = Mass × Velocity (conserved in collision)
TNPSC Tip: The SI unit of force is Newton (N). Unit of work = Joule (J). Unit of power = Watt (W).
Light (Optics)
Reflection of Light
Light bounces off surfaces following two laws: 1. Angle of incidence = Angle of reflection 2. Incident ray, reflected ray, and normal are in the same plane
| Mirror Type | Shape | Image | Uses |
|---|---|---|---|
| Plane mirror | Flat | Virtual, erect, same size | Dressing mirrors |
| Concave mirror | Curves inward | Real/virtual depending on position | Torches, headlights, dental mirrors |
| Convex mirror | Curves outward | Always virtual, erect, diminished | Vehicle rear-view mirrors |
Refraction of Light
Light bends when it passes from one medium to another. This is refraction.
- Light bends toward the normal when entering a denser medium (air → water)
- Light bends away from the normal when entering a less dense medium (water → air)
- Total Internal Reflection — light reflects completely inside a denser medium when angle exceeds critical angle → used in optical fibres
Lenses
| Lens | Shape | Effect | Uses |
|---|---|---|---|
| Convex (converging) | Thicker at centre | Converges light, forms real image | Cameras, microscopes, eye correction for hypermetropia |
| Concave (diverging) | Thinner at centre | Diverges light, forms virtual image | Eye correction for myopia (short-sightedness) |
Dispersion — Rainbow
White light splits into 7 colours (VIBGYOR) when passing through a prism: Violet, Indigo, Blue, Green, Yellow, Orange, Red
TNPSC Tip: Red has the longest wavelength; Violet has the shortest. Red light deviates least in a prism.
Sound
- Sound is a mechanical wave — it needs a medium to travel (cannot travel in vacuum)
- Speed of sound in air ≈ 343 m/s at 25°C
- Sound travels faster in solids > liquids > gases
- Frequency = number of vibrations per second → unit: Hertz (Hz)
- Human hearing range: 20 Hz to 20,000 Hz
- Below 20 Hz = Infrasound (used by elephants and whales)
- Above 20,000 Hz = Ultrasound (used in medical imaging, sonar)
Echo
An echo is the reflection of sound. Minimum distance for echo = 17.2 metres (sound must take at least 0.1 seconds to return).
Heat and Temperature
| Concept | Explanation |
|---|---|
| Conduction | Heat transfer through solids (contact) — metals are good conductors |
| Convection | Heat transfer through fluids (liquid/gas movement) — sea breeze, monsoon winds |
| Radiation | Heat transfer without medium — sunlight reaching Earth |
| Specific Heat Capacity | Energy needed to raise 1 kg of substance by 1°C — water has very high SHC |
- 0°C = 273 K = 32°F
- Normal human body temperature = 37°C = 98.6°F
Electricity
Basic Concepts
- Electric Current (I) = Flow of electric charge — unit: Ampere (A)
- Voltage / EMF (V) = Potential difference — unit: Volt (V)
- Resistance (R) = Opposition to current flow — unit: Ohm (Ω)
Ohm's Law
V = IR
| Term | Formula | Unit |
|---|---|---|
| Current | I = V/R | Ampere (A) |
| Voltage | V = IR | Volt (V) |
| Resistance | R = V/I | Ohm (Ω) |
| Power | P = VI = I²R | Watt (W) |
Series vs Parallel Circuits
| Series | Parallel | |
|---|---|---|
| -- | -------- | ---------- |
| Current | Same throughout | Different in each branch |
| Voltage | Divided | Same across each branch |
| If one component fails | Whole circuit breaks | Others continue |
| Use | Christmas lights (old style) | Home electrical wiring |
TNPSC Tip: Home electrical appliances are connected in parallel — each device gets full voltage and can be switched independently.
Magnetism
- Magnetic poles — every magnet has a North and South pole; like poles repel, unlike poles attract
- Earth's magnetic field — Earth behaves like a giant bar magnet; geographic North pole is near magnetic South pole
- Electromagnet — coil of wire carrying current creates magnetic field; used in motors, generators, MRI machines
- Electromagnetic induction — moving a magnet near a coil generates electricity (Michael Faraday) → basis of electric generators
Key Instruments and Their Uses
| Instrument | Measures |
|---|---|
| Thermometer | Temperature |
| Barometer | Atmospheric pressure |
| Hygrometer | Humidity |
| Lactometer | Purity of milk |
| Seismograph | Earthquake intensity |
| Ammeter | Electric current |
| Voltmeter | Voltage (connected in parallel) |
| Galvanometer | Small electric currents |
Important Scientists
| Scientist | Contribution |
|---|---|
| Isaac Newton | Laws of motion, universal gravitation |
| Albert Einstein | Theory of relativity, E = mc² |
| Michael Faraday | Electromagnetic induction |
| James Watt | Steam engine improvements; unit of power named after him |
| C.V. Raman | Raman Effect (light scattering) — Nobel Prize 1930; first Asian Nobel in science |
Previous Year TNPSC Questions (Pattern)
1. Q: Which mirror is used in vehicle rear-view mirrors? → Convex mirror 2. Q: The SI unit of force is → Newton 3. Q: Sound cannot travel through → Vacuum 4. Q: Ohm's Law relates → Voltage, Current, and Resistance 5. Q: C.V. Raman received the Nobel Prize for → Raman Effect 6. Q: The instrument used to measure atmospheric pressure → Barometer