Study Notes: Physics — SSC CHSL
Overview
Physics questions in SSC CHSL typically account for 4–6 marks in the General Science section. The exam tests basic conceptual understanding rather than calculation-heavy problems. You need to know fundamental laws, units, SI units, key scientists and their discoveries, and real-world applications of physical principles.
The four major areas are **Mechanics** (motion, forces, work, energy), **Electricity** (current, resistance, circuits), **Optics** (light, mirrors, lenses) and **Modern Physics** (atomic structure, radioactivity, basic quantum concepts). Questions are direct: "What is the SI unit of force?" or "Who discovered the electron?" Rarely will you solve numerical problems; instead, you must recognize correct statements, match units to quantities, and identify applications like why a transformer works or how a periscope uses mirrors. Strong command of definitions, units and one-line laws will fetch full marks.
Key Concepts
- **Newton's Laws of Motion**: First law (inertia), second law (F = ma), third law (action-reaction). These govern all motion and force problems.
- **Work, Energy and Power**: Work is force × displacement in the direction of force. Energy is the capacity to do work. Power is the rate of doing work. Energy can neither be created nor destroyed (conservation law).
- **Current Electricity**: Current is the flow of charge (measured in amperes). Resistance opposes current flow. Ohm's law relates voltage, current and resistance: V = IR.
- **Mirrors and Lenses**: Concave mirrors converge light; convex mirrors diverge. Concave lenses diverge light; convex lenses converge. Real images can be projected on a screen; virtual images cannot.
- **Reflection and Refraction**: Reflection obeys angle of incidence = angle of reflection. Refraction is bending of light when it passes from one medium to another; governed by Snell's law.
- **Atomic Structure**: Atom has a nucleus (protons + neutrons) and electrons orbiting around it. Protons are positive, electrons negative, neutrons neutral.
- **Radioactivity**: Spontaneous emission of radiation from unstable nuclei. Types: alpha (helium nucleus), beta (electrons/positrons), gamma (high-energy photons).
- **SI Units and Dimensions**: Know the seven base units (meter, kilogram, second, ampere, kelvin, mole, candela) and common derived units (newton, joule, watt, volt, ohm).
Formulas / Key Facts
1. **Force** = mass × acceleration; SI unit is newton (N). 2. **Weight** = mass × gravitational acceleration (W = mg); g ≈ 9.8 m/s² on Earth. 3. **Work** = Force × Displacement × cos θ; SI unit is joule (J). 4. **Kinetic Energy** = ½mv²; Potential Energy (gravitational) = mgh. 5. **Power** = Work / Time; SI unit is watt (W). 6. **Ohm's Law**: V = IR, where V is voltage (volt), I is current (ampere), R is resistance (ohm). 7. **Electrical Power** = VI = I²R = V²/R; unit is watt. 8. **Speed of Light in Vacuum** = 3 × 10⁸ m/s (approximately 300,000 km/s). 9. **Refractive Index** (n) = Speed of light in vacuum / Speed of light in medium. 10. **Mirror Formula**: 1/f = 1/v + 1/u, where f = focal length, v = image distance, u = object distance. 11. **Lens Formula**: 1/f = 1/v − 1/u (sign convention matters). 12. **Magnification** = Image height / Object height = v / u. 13. **Wavelength** (λ) × Frequency (ν) = Speed of light (c). 14. **Charge of electron** = −1.6 × 10⁻¹⁹ coulombs; mass ≈ 9.1 × 10⁻³¹ kg. 15. **Half-life** is the time required for half of a radioactive substance to decay.