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.

Worked Examples

**Example 1 (Mechanics)**: A 5 kg object is moving with a velocity of 10 m/s. What is its kinetic energy? *Solution*: KE = ½mv² = ½ × 5 × 10² = ½ × 5 × 100 = 250 J.

**Example 2 (Electricity)**: A bulb rated 60 W is connected to a 240 V supply. What is the current flowing through it? *Solution*: Power P = VI, so I = P/V = 60/240 = 0.25 A.

**Example 3 (Optics)**: A concave mirror has a focal length of 15 cm. An object is placed 30 cm from the mirror. Where is the image formed? *Solution*: Using 1/f = 1/v + 1/u → 1/15 = 1/v + 1/(−30) [u is negative by sign convention] 1/v = 1/15 + 1/30 = 2/30 + 1/30 = 3/30 = 1/10 So v = 10 cm. Image is real, inverted, and formed at 10 cm in front of the mirror.

Common Mistakes

• **Confusing mass and weight**: Mass is the amount of matter (kg); weight is the force due to gravity (N). Weight = mg, not just m.
• **Wrong application of Ohm's Law**: Students forget that Ohm's law applies only to ohmic conductors (constant resistance). Devices like diodes do not obey V = IR linearly.
• **Sign convention in mirrors/lenses**: Forgetting that distances measured against the direction of incident light are negative. For mirrors, object distance u is always negative; for real images v is negative (in front of mirror). Mixing up signs gives wrong image positions.
• **Mixing up concave and convex**: Concave mirror **converges** light (used in torches, shaving mirrors); convex mirror **diverges** (used in vehicle side mirrors). Similarly, concave lens diverges, convex lens converges.
• **Assuming all images are real**: Virtual images (formed by plane mirrors, convex mirrors, concave lenses for all object positions) cannot be projected on a screen. Real images can be projected.

Quick Reference

• **Newton's Second Law**: F = ma (force equals mass times acceleration).
• **SI unit of Energy**: Joule (J); 1 J = 1 N·m.
• **Ohm's Law**: V = IR (voltage = current × resistance).
• **Speed of Light**: 3 × 10⁸ m/s in vacuum; slows in denser media.
• **Concave mirror**: Converges light; forms real/virtual images depending on object position.
• **Convex lens**: Converges light; used in magnifying glasses, cameras.
• **Electron discoverer**: J.J. Thomson (1897); Neutron: James Chadwick (1932).
• **Alpha particles**: Helium nuclei (2 protons + 2 neutrons); Beta: electrons or positrons; Gamma: electromagnetic radiation.
• **Transformer works on**: Electromagnetic induction (AC only, not DC).
• **Refractive Index**: Always ≥ 1; higher for denser media (glass > water > air).

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**Exam Strategy**: Revise units and scientist–discovery pairs weekly. Practice matching applications (e.g., periscope → plane mirrors, electromagnet → current electricity). Many questions are one-liners testing definitions or direct fact recall. Don't over-invest in complex derivations; SSC CHSL Physics rewards clarity and breadth over depth.