Physics — TNPSC Group IV Study Notes
Overview
Physics forms a significant portion of the General Science section in TNPSC Group IV examination. Questions are set at SSLC (10th standard) level, testing fundamental concepts rather than complex derivations.
Success in this section requires understanding basic definitions, SI units, simple formulas, and their direct applications. The examiner typically tests whether you can identify correct units, apply straightforward formulas to numerical problems, and distinguish between related concepts (like mass vs weight, or speed vs velocity). Memorising key values—speed of light, sound in air, normal human body temperature—gives quick marks.
Focus your preparation on concept clarity over mathematical complexity. Most questions involve single-step calculations or direct recall of facts and units.
Key Concepts
- SI Units form the foundation: The International System has seven base units—metre (length), kilogram (mass), second (time), ampere (current), kelvin (temperature), mole (amount), candela (luminous intensity). All derived units come from these.
- Scalar vs Vector quantities: Scalars have only magnitude (speed, mass, time, temperature). Vectors have both magnitude and direction (velocity, acceleration, force, displacement).
- Newton's Three Laws govern motion: First law defines inertia, second law gives F = ma, third law states action-reaction pairs are equal and opposite.
- Heat is energy in transit: Heat flows from higher to lower temperature. Temperature measures degree of hotness; heat measures total thermal energy transferred.
- Light travels fastest in vacuum: 3 × 10⁸ m/s. It slows in denser media, causing refraction. Mirrors reflect; lenses refract.
- Sound needs a medium: Cannot travel through vacuum. Travels fastest in solids, slowest in gases. Speed in air at 20°C is approximately 343 m/s.
- Electric current is charge flow: Measured in amperes. Ohm's law relates voltage, current, and resistance. Power determines energy consumption.
- Work-Energy relationship: Work done equals force times displacement in force's direction. Energy is capacity to do work; both measured in joules.
Formulas / Key Facts
| Concept | Formula / Fact |
|---|---|
| Speed | Speed = Distance ÷ Time |
| Velocity | Velocity = Displacement ÷ Time (vector) |
| Acceleration | a = (v − u) ÷ t |
| Force | F = m × a (Newton's second law) |
| Weight | W = m × g (g ≈ 9.8 m/s² or 10 m/s² for calculation) |
| Momentum | p = m × v |
| Work | W = F × d × cos θ |
| Kinetic Energy | KE = ½ × m × v² |
| Potential Energy | PE = m × g × h |
| Density | Density = Mass ÷ Volume |
| Pressure | Pressure = Force ÷ Area |
| Ohm's Law | V = I × R |
| Electric Power | P = V × I = I²R = V²/R |
| Electrical Energy | E = P × t (in kWh for billing) |
| Heat | Q = m × c × ΔT (c = specific heat capacity) |
| Mirror Formula | 1/f = 1/v + 1/u |
| Lens Power | P = 1/f (f in metres, P in dioptres) |
| Speed of Light | 3 × 10⁸ m/s in vacuum |
| Speed of Sound in Air | ≈ 343 m/s at 20°C |
| Normal Body Temperature | 37°C or 98.6°F |
| Absolute Zero | 0 K = −273°C |
Worked Examples
Example 1: Force Calculation A body of mass 5 kg accelerates at 4 m/s². Find the force applied.
Solution: F = m × a F = 5 × 4 = 20 N
Example 2: Electrical Energy Consumption A 100 W bulb runs for 10 hours daily for 30 days. Calculate energy consumed in kWh.
Solution: Power = 100 W = 0.1 kW Time = 10 × 30 = 300 hours Energy = P × t = 0.1 × 300 = 30 kWh
Example 3: Ohm's Law Application A 12 V battery drives 2 A current through a resistor. Find resistance.
Solution: V = I × R R = V ÷ I = 12 ÷ 2 = 6 Ω
Example 4: Kinetic Energy Find KE of a 2 kg object moving at 10 m/s.
Solution: KE = ½ × m × v² KE = ½ × 2 × 10² = ½ × 2 × 100 = 100 J
Common Mistakes
- Confusing mass and weight → Mass (kg) is constant everywhere; weight (N) varies with gravity. On Moon, mass stays same but weight reduces to 1/6th.
- Mixing speed and velocity → Speed is scalar (50 km/h). Velocity is vector (50 km/h northward). A body moving in a circle at constant speed has changing velocity.
- Wrong unit conversions → 1 km = 1000 m, 1 hour = 3600 seconds. To convert km/h to m/s, multiply by 5/18. To convert m/s to km/h, multiply by 18/5.
- Forgetting that sound cannot travel in vacuum → Light travels in vacuum; sound does not. Astronauts cannot hear each other directly in space.
- Confusing series and parallel circuits → In series: same current, voltages add, total resistance increases. In parallel: same voltage, currents add, total resistance decreases.
- Using wrong sign convention in optics → For mirrors: real images have positive v, virtual images have negative v (using standard convention). Know that convex mirrors always form virtual, diminished images.
Quick Reference
- SI unit of force is Newton (N); 1 N = 1 kg⋅m/s²
- g = 9.8 m/s² (use 10 m/s² for quick calculations)
- 1 kWh = 1000 W × 1 hour = 1 unit of electricity
- Convex lens: converging; Concave lens: diverging
- Sound range: Infrasonic < 20 Hz; Audible 20–20,000 Hz; Ultrasonic > 20,000 Hz
- Fuse wire: low melting point, high resistance—protects circuits from overload