Introduction
Physics is the most fundamental of the natural sciences. The subtopic "Physics — mechanics, heat, light, sound, electricity" forms the backbone of the BPSC General Studies (Paper I) Science section. Over the years, BPSC has asked 61 questions directly sourced from this subtopic in the available question pools (2018–2025), making it one of the most frequently tested areas within Science. The examiners do not demand university-level depth; rather, they test conceptual clarity, everyday applications, units, fundamental laws, and simple numerical ability. Questions range from definitions (e.g., "Frequency is measured in hertz" – BPSC 2018) to short calculations (e.g., "Two trains cross each other in 9 seconds" – BPSC 2019) and phenomenon recognition (e.g., "Photoelectric cell converts light into electric energy" – BPSC 2023).
The syllabus explicitly lists "mechanics, heat, light, sound, electricity". However, the PYQs reveal a hidden pattern: magnetism, astrophysics (pulsars, comets, solar constant), and modern physics (photoelectric effect, relativity) also appear under this umbrella, because they intersect with electricity and light. Therefore, a comprehensive preparation must go beyond the five labels.
This chapter is designed to teach you everything you need: from zero understanding to confident problem-solving. We will build concepts from first principles – defining every piece of jargon before using it, using analogies, and embedding all 61 PYQ lessons seamlessly. You will learn not just the answers but why each wrong choice is wrong and how to spot the trap. The structure follows the BPSC test pattern: definitions → formulas → applications → numerical tricks.
By the end of this chapter, you will be able to:
- Identify the correct unit, device, or law for any given physical quantity.
- Solve simple numerical problems on motion, force, work, energy, and electricity.
- Distinguish between scalar/vector, contact/non-contact forces, conduction/convection/radiation, and series/parallel circuits.
- Predict what BPSC is likely to ask next based on the trend of repeated concepts and unexplored gaps.
Core Concepts & Foundations
This section builds the conceptual toolkit. Every term defined here will be used repeatedly in the deep-dive sections and worked examples.
Mechanics: The branch of physics dealing with motion (kinematics) and the forces causing motion (dynamics). It includes concepts of displacement, velocity, acceleration, force, work, energy, and power.
Scalar Quantity: A physical quantity that has only magnitude (size) and no direction. Examples: mass, temperature, speed, pressure, energy. BPSC tests this frequently – e.g., "Pressure is a scalar quantity" (BPSC 2019).
Vector Quantity: A physical quantity that has both magnitude and direction. Examples: force, velocity, acceleration, displacement, torque. "Speed is not a vector quantity" (BPSC 2021).
Speed vs. Velocity: Speed is distance covered per unit time (scalar); velocity is displacement per unit time (vector). The person who first defined speed was Galileo (BPSC 2021). The person who first defined motion was Aristotle (BPSC 2022).
Force: A push or pull that can change the state of motion of an object. Measured in newtons (N). Contact forces (friction, impact, tension) require physical touch; non-contact forces (gravitational, magnetic, electrostatic) act at a distance. "Magnetic force is a non-contact force" (BPSC 2024).
Newton’s Laws of Motion: (i) Inertia – an object at rest stays at rest, in motion stays in motion unless acted upon. (ii) F = ma – force causes acceleration proportional to mass. (iii) Action–reaction – every action has an equal opposite reaction.
Work: The product of force and displacement in the direction of the force. Work = Force × Displacement (W = Fd). Unit: joule (J). Work done on a plough: 140 N × 15 m = 2100 J (BPSC 2025).
Energy: The capacity to do work. Forms: kinetic (motion), potential (position), thermal, electrical, etc. The principle of conservation of energy states that energy cannot be created or destroyed, only transformed. A freely falling body has decreasing potential energy but increasing kinetic energy – total remains constant, so the decrease in potential energy is not a violation of conservation (BPSC 2025).
Friction: A contact force that opposes relative motion. When an object moves at constant velocity, the net force is zero – applied force equals frictional force. "If 200 N is applied and the box moves at constant velocity, friction is 200 N" (BPSC 2025).
Pressure: Force per unit area. Unit: pascal (Pa) = N/m² = kg/cm² (both accepted in BPSC – see Q5 and Q37). Pressure is a scalar.
Temperature and Heat: Temperature is a measure of the average kinetic energy of molecules; heat is energy transferred due to temperature difference. The Celsius and Fahrenheit scales are linear: °F = (9/5)°C + 32. So 40°C = 104°F (BPSC 2018). Note: 50°C = 122°F (correct fact, though one PYQ key wrongly gave 104°F – ignore the error).
Heat Transfer: Three modes – Conduction (through matter, requires medium), Convection (fluid movement), Radiation (via electromagnetic waves, does NOT require medium). "Heat transfer without a medium is radiation" (BPSC 2024). The worst conductor of heat among metals is mercury (BPSC 2020). Mercury is a liquid metal with poor thermal conductivity.
Light and Optics: Light is an electromagnetic wave. In a vacuum it travels at 3×10⁸ m/s. When it enters a medium, speed and wavelength change, but frequency remains unchanged (BPSC 2020). Refractive index = speed in vacuum / speed in medium. Light slows down most in denser media: glass (slowest), then water, then air, then vacuum fastest (BPSC 2021).
Lenses: Convex lens (converging) and concave lens (diverging). A magnifying glass uses a convex lens (BPSC 2022, 2021). For a plane mirror, focal length is infinity (BPSC 2025) because the mirror surface is flat – no convergence.
Sound: A mechanical (longitudinal) wave that requires a medium. "Sound wave in air is longitudinal" (BPSC 2019). Speed is greatest in solids (steel), less in liquids, least in gases (BPSC 2025: steel > water > air/hydrogen).
Electric Current: Flow of charge. Measured by ammeter (BPSC 2018, 2019). AC current is produced by a dynamo (BPSC 2023). A bad conductor of electricity among liquids? The correct answer is "None of the above" (BPSC 2023) – salted water, orange juice, lemon juice all conduct due to ions; pure water is a poor conductor.
Resistance: Opposition to current flow. Ohm’s law: V = IR, so R = V/I (BPSC 2024). Unit: ohm (Ω). In a short circuit, resistance drops to near-zero, so current increases heavily (BPSC 2025).
Electric Power: Rate of electrical energy consumption. P = VI = I²R = V²/R. "IR² is not a correct expression for power" (BPSC 2025). Unit: watt (W). 1 watt = 1 volt × 1 ampere.
Magnetic field and Magnetism: Unit of magnetic field is tesla (T). 1 T = 1 N/(A·m) (BPSC 2024). Paramagnetic materials have unpaired electrons and are weakly attracted by a magnetic field. Platinum is paramagnetic; iron and nickel are ferromagnetic (strongly attracted) – BPSC 2022, 2021.
Astrophysical Terms: Light-year is a unit of distance (BPSC 2020). Pulsars are rotating neutron stars (BPSC 2023). A comet has a highly elongated elliptical orbit (BPSC 2024). The solar constant (energy received per unit area per second at Earth’s distance) is about 1.4 kW/m² (BPSC 2024). Sun is the largest celestial body in our solar system (BPSC 2022).
Modern Physics: Einstein’s Nobel Prize was for the photoelectric effect (BPSC 2018). Photoelectric cell converts light energy into electric energy (BPSC 2023). Faraday constant is a universal constant (96,485 C/mol) – BPSC 2020.
These foundations are the building blocks. Each deep-dive section will elaborate on the most frequently tested clusters.