Study Notes: Physics — SSC CHSL
Overview
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
- Force = mass × acceleration; SI unit is newton (N).
- Weight = mass × gravitational acceleration (W = mg); g ≈ 9.8 m/s² on Earth.
- Work = Force × Displacement × cos θ; SI unit is joule (J).
- Kinetic Energy = ½mv²; Potential Energy (gravitational) = mgh.
- Power = Work / Time; SI unit is watt (W).
- Ohm's Law: V = IR, where V is voltage (volt), I is current (ampere), R is resistance (ohm).
- Electrical Power = VI = I²R = V²/R; unit is watt.
- Speed of Light in Vacuum = 3 × 10⁸ m/s (approximately 300,000 km/s).
- Refractive Index (n) = Speed of light in vacuum / Speed of light in medium.
- Mirror Formula: 1/f = 1/v + 1/u, where f = focal length, v = image distance, u = object distance.
- Lens Formula: 1/f = 1/v − 1/u (sign convention matters).
- Magnification = Image height / Object height = v / u.
- Wavelength (λ) × Frequency (ν) = Speed of light (c).
- Charge of electron = −1.6 × 10⁻¹⁹ coulombs; mass ≈ 9.1 × 10⁻³¹ kg.
- 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).
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.