Physics Concepts for TNPSC Group I
Laws of Motion — Newton's Three Laws
Newton's Laws of Motion are among the most commonly tested physics topics in TNPSC prelims.
First Law (Inertia): An object at rest stays at rest, and an object in motion stays in motion unless acted upon by an external force. Example: a passenger jerks forward when a bus brakes suddenly.
Second Law: F = ma. Force equals mass times acceleration. Doubling the force doubles the acceleration; doubling the mass halves the acceleration for the same force.
Third Law: Every action has an equal and opposite reaction. Example: a rocket moves forward because it pushes exhaust gases backward.
Light — Reflection, Refraction, and Spectrum
| Property | Reflection | Refraction |
|---|---|---|
| Definition | Light bouncing off a surface | Light bending when passing between media |
| Law | Angle of incidence = angle of reflection | Snell's Law: n₁ sin θ₁ = n₂ sin θ₂ |
| Example | Mirror image | Pencil appearing bent in water |
Dispersion: White light splits into 7 colours (VIBGYOR) when passing through a prism. Violet bends most; red bends least. This is because each colour has a different wavelength and refractive index.
Total Internal Reflection: When light moves from a denser to rarer medium at greater than the critical angle, it reflects entirely back. This is the principle behind optical fibre cables used in telecommunications.
Electricity — Basic Concepts
Current (I): Flow of electric charge. Unit: Ampere (A). 1 Ampere = 1 Coulomb of charge per second.
Voltage (V): Electric potential difference. Unit: Volt (V).
Resistance (R): Opposition to current flow. Unit: Ohm (Ω).
Ohm's Law: V = IR
Power: P = VI = I²R = V²/R. Unit: Watt (W).
Series circuit: Same current through all components; resistance adds up (R_total = R₁ + R₂ + ...).
Parallel circuit: Same voltage across all components; reciprocal of total resistance = sum of reciprocals (1/R_total = 1/R₁ + 1/R₂ + ...).
Heat and Thermodynamics
Conduction: Heat transfer through direct contact (solids). Metals are good conductors; wood, plastic are insulators.
Convection: Heat transfer through fluid movement (liquids and gases). Sea breeze and land breeze are convection examples.
Radiation: Heat transfer without a medium. The Sun transfers heat to Earth through radiation across vacuum.
Specific Heat Capacity: Heat needed to raise 1 kg of a substance by 1°C. Water has one of the highest specific heats (4,186 J/kg·K) — that is why it is used as a coolant and why coastal areas have moderate climates.
Sound
Sound is a longitudinal mechanical wave — it requires a medium and cannot travel through vacuum. Speed of sound in air: ~343 m/s at 20°C. Speed increases in water (~1,480 m/s) and in solids (~5,000 m/s for steel).
Frequency: Number of vibrations per second. Unit: Hertz (Hz).
- Infrasound: < 20 Hz (earthquakes, elephants)
- Audible range (humans): 20 Hz – 20,000 Hz
- Ultrasound: > 20,000 Hz (used in sonar, medical imaging)
Chemistry Concepts for TNPSC Group I
Acids, Bases, and Salts
pH Scale: 0–14. Below 7 = acid. 7 = neutral. Above 7 = base.
| Substance | pH |
|---|---|
| Hydrochloric acid (HCl) | ~1 |
| Lemon juice | ~2 |
| Vinegar | ~3 |
| Pure water | 7 |
| Sodium bicarbonate | ~8 |
| Soap | ~9–10 |
| Caustic soda (NaOH) | ~13 |
Indicators: Litmus (red in acid, blue in base), phenolphthalein (colourless in acid, pink in base), methyl orange (red in acid, yellow in base).
Neutralisation: Acid + Base → Salt + Water. Example: HCl + NaOH → NaCl + H₂O.
Chemical Reactions — Types
Combination: A + B → AB (e.g., 2Mg + O₂ → 2MgO)
Decomposition: AB → A + B (e.g., CaCO₃ → CaO + CO₂ on heating — used in cement industry)
Displacement: A + BC → AC + B (e.g., Fe + CuSO₄ → FeSO₄ + Cu — iron displaces copper because iron is more reactive)
Double displacement: AB + CD → AD + CB (e.g., AgNO₃ + NaCl → AgCl↓ + NaNO₃ — white precipitate forms)
Oxidation-Reduction (Redox): Oxidation = loss of electrons / gain of oxygen. Reduction = gain of electrons / loss of oxygen. They always occur together.
Metals and Non-Metals
| Property | Metals | Non-Metals |
|---|---|---|
| State at room temp | Mostly solid | Solid, liquid, or gas |
| Conductivity | Good (electricity and heat) | Poor (except graphite) |
| Lustre | Shiny | Dull |
| Malleability | Yes | No (brittle if solid) |
| Reaction with O₂ | Forms metallic oxides (basic) | Forms non-metallic oxides (acidic) |
Reactivity Series (high to low): K > Na > Ca > Mg > Al > Zn > Fe > Ni > Sn > Pb > H > Cu > Hg > Ag > Au
A metal displaces any metal below it in the series from its salt solution.
Periodic Table Essentials
Groups: Vertical columns. Elements in a group have the same number of valence electrons and similar chemical properties.
Periods: Horizontal rows. Atomic number increases left to right. Atomic radius decreases; electronegativity increases across a period.
Key groups to remember:
- Group 1 (Alkali metals): Li, Na, K, Rb, Cs — highly reactive, react vigorously with water
- Group 17 (Halogens): F, Cl, Br, I — highly electronegative, form salts with metals
- Group 18 (Noble gases): He, Ne, Ar, Kr, Xe — inert, full valence shells
- Transition metals (3–12): include Fe, Cu, Zn, Mn — used industrially
Everyday Chemistry
| Substance | Chemical name | Use |
|---|---|---|
| Baking soda | Sodium bicarbonate (NaHCO₃) | Cooking, fire extinguisher |
| Washing soda | Sodium carbonate (Na₂CO₃) | Cleaning agent |
| Bleaching powder | Ca(OCl)Cl | Disinfectant, textile bleaching |
| Plaster of Paris | Calcium sulphate hemihydrate | Casts, moulding |
| Vinegar | Acetic acid (CH₃COOH) | Food preservative |
| Table salt | Sodium chloride (NaCl) | Food, electrolytes |
| Rust | Iron(III) oxide (Fe₂O₃) | Corrosion product |
| Limestone | Calcium carbonate (CaCO₃) | Building material, cement |
Board Exam Tips
- Most Group I science questions are school-level (Samacheer Kalvi 6–10). Do not over-prepare at degree level.
- Formulae for F=ma, V=IR, P=VI are directly tested with numerical substitution.
- Reactivity series and pH scale are frequently asked in two-mark MCQ patterns.
- Everyday chemistry (baking soda uses, bleaching powder formula, rust formation) appears almost every year.
- Revision method: make one A4 sheet of formulas and chemical names. Test yourself from memory three days before the exam.