Physics — Basic Principles
Overview
Physics forms a crucial component of the General Science section in TNPSC Group II/IIA Prelims. Questions typically test fundamental concepts rather than complex calculations — expect direct factual questions on laws, units, phenomena, and everyday applications.
The syllabus spans six domains: mechanics (motion and forces), electricity, magnetism, heat, light, and sound. Focus on understanding definitions, SI units, famous laws and their discoverers, and real-life applications. Numerical problems, when they appear, involve simple substitution into standard formulas.
Key Concepts
- Newton's Three Laws of Motion: First Law (inertia — objects resist change in motion), Second Law (F = ma — force equals mass times acceleration), Third Law (action-reaction pairs are equal and opposite). These explain everything from seatbelt safety to rocket propulsion.
- Work, Energy and Power: Work = Force × Distance (in direction of force). Energy is capacity to do work; Power is rate of doing work. Kinetic energy depends on motion (½mv²); Potential energy depends on position (mgh).
- Electric Current and Ohm's Law: Current (I) is flow of charge measured in Amperes. Ohm's Law states V = IR (Voltage = Current × Resistance). Resistance is measured in Ohms (Ω).
- Magnetic Effect of Current: A current-carrying conductor produces a magnetic field around it (Oersted's discovery). This principle underlies electromagnets, electric motors, and generators.
- Heat Transfer Mechanisms: Conduction (through solids by molecular vibration), Convection (through fluids by actual movement), Radiation (through vacuum via electromagnetic waves). All three can occur simultaneously.
- Reflection and Refraction of Light: Reflection obeys equal angles of incidence and reflection. Refraction (bending of light) occurs when light changes medium; governs lenses, prisms, and optical instruments.
- Sound Wave Properties: Sound needs a medium; cannot travel in vacuum. Speed in air ≈ 343 m/s at 20°C. Sound travels fastest in solids, slowest in gases. Frequency determines pitch; amplitude determines loudness.
- Electromagnetic Spectrum: Radio waves → Microwaves → Infrared → Visible light (VIBGYOR) → Ultraviolet → X-rays → Gamma rays. Wavelength decreases and frequency increases left to right.
Formulas / Key Facts
| Concept | Formula / Fact |
|---|---|
| Speed | Speed = Distance / Time |
| Acceleration | a = (v − u) / t, where u = initial velocity, v = final velocity |
| Force | F = ma (Newton's Second Law) |
| Momentum | p = mv; conserved in collisions |
| Work | W = F × d × cos θ (θ = angle between force and displacement) |
| Kinetic Energy | KE = ½mv² |
| Potential Energy | PE = mgh |
| Power | P = Work / Time; SI unit is Watt (W) |
| Ohm's Law | V = IR |
| Electrical Power | P = VI = I²R = V²/R |
| Refractive Index | n = Speed of light in vacuum / Speed of light in medium |
| Mirror Formula | 1/f = 1/v + 1/u |
| Lens Power | P = 1/f (in metres); unit is Dioptre (D) |
| Frequency-Wavelength | v = fλ (wave speed = frequency × wavelength) |
| Heat Energy | Q = mcΔT (mass × specific heat × temperature change) |
Must-remember SI Units: Force — Newton (N), Energy/Work — Joule (J), Power — Watt (W), Current — Ampere (A), Voltage — Volt (V), Resistance — Ohm (Ω), Frequency — Hertz (Hz), Temperature — Kelvin (K).
Worked Examples
Example 1 — Newton's Second Law A 5 kg object accelerates at 4 m/s². Find the force applied.
Solution: F = ma = 5 × 4 = 20 N
Example 2 — Ohm's Law Application A circuit has a 12 V battery and 4 Ω resistance. Calculate current.
Solution: I = V/R = 12/4 = 3 Amperes
Example 3 — Lens Power A convex lens has focal length 25 cm. Find its power.
Solution: Convert to metres: f = 0.25 m P = 1/f = 1/0.25 = 4 Dioptres (positive for convex lens)
Common Mistakes
- Confusing mass and weight: Mass (kg) is amount of matter and constant everywhere; Weight (N) is gravitational force and varies with location. Weight = mg.
- Thinking sound travels in vacuum: Sound requires a material medium. Light (electromagnetic wave) travels in vacuum; sound does not. This is why astronauts use radio communication.
- Mixing up convex and concave properties: Convex lens converges light, used to correct hypermetropia (farsightedness). Concave lens diverges light, used for myopia (nearsightedness). Remember: Convex = Converging.
- Forgetting unit conversions in lens formula: Focal length must be in metres when calculating power in Dioptres. A 50 cm focal length lens has power 2 D, not 0.02 D.
- Confusing frequency and amplitude effects on sound: Higher frequency = higher pitch (shriller sound). Greater amplitude = louder sound. These are independent properties.
Quick Reference
- Newton's Laws: Inertia → F=ma → Action-Reaction
- Ohm's Law: V = IR; Power = VI
- Light: Reflection (mirrors), Refraction (lenses); speed in vacuum = 3 × 10⁸ m/s
- Sound: Needs medium; speed order: Solid > Liquid > Gas
- Heat transfer: Conduction (solids), Convection (fluids), Radiation (no medium needed)
- EM Spectrum order: Radio, Micro, IR, Visible, UV, X-ray, Gamma (increasing frequency)
- Concave mirror and Convex lens: Both converging; used in torches, headlights, magnifying glasses