Physics in Bihar Police Constable is not the same as Class 12 Board physics. You are not being tested on derivations or numerical precision. You are being tested on whether you know what a concept is, what its unit is, and how it behaves in everyday situations. The exam has consistently asked about heat transfer, latent heat, units of physical quantities, and basic energy concepts.
Think of the physics section as "fact physics" — the kind your science teacher summarized on the last day before the exam. You need clean, accurate one-liners stored in your head.
Here is a grounding analogy for heat transfer that ties the three modes together:
Imagine you are sitting near a bonfire on a cold Bihar winter night. Your back is cold, your face is hot. How is the heat reaching your face? Through radiation — electromagnetic waves traveling through the air (and even through vacuum if needed). Now, the hot air above the fire rises and cooler air rushes in — that circular movement of fluid is convection. If you accidentally touch the metal rod sitting in the fire, the heat travels through the rod into your hand — that is conduction. Three modes, one bonfire.
This analogy is your anchor. Every question about heat transfer modes comes back to this bonfire picture.
For energy and work: work is done only when a force causes displacement in the direction of the force. Carrying a bag horizontally does zero work against gravity — though it is tiring, physics does not care about your effort, only about force times displacement. Power is simply how fast that work gets done. These distinctions — especially the "no medium needed" for radiation and "unit of heat is Joule not calorie in SI" — are the exact points where Bihar Police PYQs have placed their answer choices.
Conduction (चालन): Heat moves through a solid material from the hotter end to the cooler end, particle by particle, without any bulk movement of the material itself. Metals are good conductors; wood and plastic are insulators. The key condition: the medium must be in physical contact.
Convection (संवहन): Heat transfer in fluids (liquids and gases) through bulk movement of the fluid itself. Hot fluid becomes less dense, rises; cooler fluid sinks and replaces it, creating a convection current. Land breeze, sea breeze, and boiling water are all convection examples.
Radiation (विकिरण): Heat transfer through electromagnetic waves. No medium is required — radiation works perfectly in vacuum. This is the only mode that can transfer heat through empty space. The Sun heats the Earth through radiation across 150 million km of vacuum. Dark/black surfaces are better absorbers and emitters of radiation; shiny/white surfaces reflect more.
The exam loves asking: "which mode needs no medium?" The answer is always radiation.
Latent heat (गुप्त ऊष्मा) is the heat energy absorbed or released during a phase change without any change in temperature. This is counterintuitive — you keep adding heat to boiling water at 100°C and the temperature stays at 100°C. All that energy is going into breaking the bonds between water molecules to convert liquid into steam.
Specific Latent Heat of Vaporization of Water: 2260 kJ/kg
This means to convert 1 kg of water at 100°C into steam at 100°C, you need 2260 kilojoules. This value has appeared directly in Bihar Police PYQs. Memorize it as 2260.
Specific Latent Heat of Fusion of Ice: approximately 336 kJ/kg (to convert ice at 0°C into water at 0°C).
When you mix two quantities of water at different temperatures (with no heat lost to surroundings), the final temperature always lies between the two initial temperatures. It is never equal to the sum, never below the lower temperature, never above the higher temperature. The exact value depends on the masses and specific heats — but for water mixed with water, the mixing rule is:
T_final = (m₁T₁ + m₂T₂) / (m₁ + m₂)
For the PYQ case — mixing water at 40°C with water at 50°C — without knowing the masses you can still confidently say the result is between 40°C and 50°C.
This is a pure memorization zone. Bihar Police regularly asks for SI units of various quantities. Key ones:
| Quantity | SI Unit | Symbol | |---|---|---| | Heat (ऊष्मा) | Joule | J | | Force (बल) | Newton | N | | Power (शक्ति) | Watt | W | | Pressure (दाब) | Pascal | Pa | | Work (कार्य) | Joule | J | | Energy (ऊर्जा) | Joule | J |
The trap: "calorie" is a commonly heard unit for heat but it is not the SI unit. The SI unit is Joule. Dyne is a unit of force in CGS system, not SI. Joule/second equals Watt (unit of power, not heat). Joule-second is the unit of angular momentum or action.
Work = Force × Displacement × cos(θ), where θ is the angle between force and displacement.
Sound is a mechanical wave — it requires a medium to travel. It cannot travel through vacuum. This directly contrasts with radiation (electromagnetic waves) which travels through vacuum.
Speed of sound in air at room temperature: approximately 343 m/s. Speed of sound is highest in solids, lower in liquids, lowest in gases.
Frequency determines pitch (shrillness); amplitude determines loudness. The human ear hears frequencies between 20 Hz and 20,000 Hz. Below 20 Hz: infrasound. Above 20,000 Hz: ultrasound.
Gravitational acceleration (g) on Earth's surface: 9.8 m/s² (often approximated as 10 m/s² in calculations).
Pressure = Force / Area. Increasing area decreases pressure (why snowshoes work). Pascal's Law: pressure applied to an enclosed fluid is transmitted equally in all directions (basis of hydraulic machines).
Atmospheric pressure at sea level: 101,325 Pa ≈ 1 atm ≈ 760 mm of Hg.
When a question asks which mode needs NO medium: eliminate Conduction (needs solid medium) and Convection (needs fluid medium). Only Radiation is left. This takes 4 seconds vs reading all options carefully for 20 seconds. Standard method: 20s. Elimination: 4s.
Latent heat of vaporization of water = 2260 kJ/kg. Pattern trick: 2-2-6-0. Notice 2+2 = 4, 6+0 = 6 — not mathematically meaningful, but the digit pattern "two-two-six-zero" is very distinct. If an option says 2130, 3360, or 1528, you can instantly reject them because none start with "22". Standard method: trying to recall from a list of 4 numbers — 30s. Pattern recall: 5s.
For any mixing question with two temperatures: the answer is always between the two. Never at the sum (90°C in the PYQ is clearly the sum of 40+50 — eliminate it immediately). Never below the lower temperature. This eliminates 2 wrong options in 3 seconds without any calculation. Standard method: writing the mixing formula and solving — 45s. Between Rule: 3s.
When asked the unit of heat: Joule/second = Watt (power, not heat), Joule-second = angular momentum (not heat), Dyne = force (CGS, not heat). Only "Joule" alone = heat. Memorize the trap answers: if you see Joule with any operator attached (per second, times second), it is NOT the unit of heat. Eliminates 3 distractors in 5 seconds.
Sound needs a medium (mechanical wave). Light/radiation needs no medium (electromagnetic wave). Remember: "Sound is shy — it needs a medium to speak. Light is bold — it crosses vacuum." In any question pairing sound and radiation as options for "travels through vacuum," radiation wins every time. 3-second recall vs 15-second reasoning.
When you see a physics question in the Bihar Police exam, run through this decision tree:
Step 1 — Is it a "which mode/method" question? If about heat transfer: apply CRC elimination (Conduction needs solid, Convection needs fluid, Radiation needs nothing).
Step 2 — Is it a unit question? Match the quantity to its SI unit from your memorized table. Check for traps: "Joule with an operator" is never the answer for heat.
Step 3 — Is it a mixing/combination question? Apply the Between Rule — the result is always between the two given values.
Step 4 — Is it a specific value question (latent heat, speed of sound, g)? Recall the exact memorized value. For vaporization of water: 2260. For g: 9.8 or 10.
Step 5 — Is it a conceptual definition? Identify the one option that matches the definition from your concept notes. Eliminate options that describe a different phenomenon.
Do not spend more than 45 seconds on any single physics question. If you cannot identify the answer in 45 seconds, mark your best guess and move on.
Why this question: This is the most commonly asked heat transfer question — "no medium required" is the exact differentiator that has appeared multiple times across Bihar/UP Police papers.
Solving path: Apply CRC elimination. Conduction (चालन) needs a solid medium — eliminate. Convection (संवहन) needs a fluid medium — eliminate. Rूपांतरण (transformation) is not a heat transfer mode — eliminate. Radiation (विकिरण) works in vacuum — correct. Time: 6 seconds.
Why this question: Temperature mixing questions appear with a deliberate trap — the sum of temperatures (40+50 = 90°C) is always given as an option to catch students who confuse mixing with adding.
Solving path: Immediately eliminate 90°C (that is the sum, physically impossible for a mixture). Eliminate "less than 40°C" (mixing cannot cool below the cooler temperature). Choose "between 40°C and 50°C" — this is always the correct framing for two-temperature mixing. Time: 8 seconds.
Why this question: The three-modes question is a pure recall question that tests whether you know all three correct terms together, and specifically whether you can distinguish "convection" from "transformation" (रूपांतरण is a distractor).
Solving path: You need Conduction + Radiation + Convection. Scan options for this exact triplet. Option C gives चालन, विकिरण, संवहन — that is the correct three. Any option containing रूपांतरण (transformation) or अवशिष्ट (residual) is automatically wrong because neither is a heat transfer mode. Time: 10 seconds.
Why this question: Specific latent heat values are direct-recall questions. The exam gives you four plausible-looking numbers — your only defense is having memorized 2260.
Solving path: You either know it is 2260 kJ/kg or you do not. No shortcut replaces knowing this value. Recall pattern "two-two-six-zero." Select 2260 kJ/kg. Time: 5 seconds if memorized.
Why this question: Unit questions are the fastest marks on the paper — but only if you have cleared the "Joule vs Joule/second vs Joule-second vs Dyne" confusion beforehand.
Solving path: Joule-second = unit of action/angular momentum — eliminate. Joule/second = Watt = unit of power — eliminate. Dyne = unit of force in CGS — eliminate. Joule alone = SI unit of heat (and work and energy) — select. Time: 8 seconds.
Selecting "calorie" as the SI unit of heat. Calorie is an older, non-SI unit. The SI unit is Joule. Whenever the question says "मात्रक" (unit) without specifying CGS, assume SI and select Joule.
Confusing convection with conduction. Conduction is through solids by direct contact. Convection is through fluid movement (bulk flow). The words sound different in Hindi too — चालन (conduction) vs संवहन (convection) — do not swap them.
Selecting 90°C for the mixing question. 90 = 40 + 50, which is what happens when you add temperatures instead of mixing. You cannot mix two water samples and get a temperature higher than the hotter sample.
Thinking sound travels through vacuum. Sound is mechanical — it absolutely requires a medium. Light and radiation are electromagnetic — they do not require a medium. This distinction is tested regularly.
Mixing up latent heat values. 2260 kJ/kg is for vaporization (liquid to gas). Fusion (ice to water) is approximately 336 kJ/kg. If the question says "वाष्पीकरण" (vaporization), the answer is 2260, not 336.
Selecting "transformation/रूपांतरण" as a heat transfer mode. It is not a recognized mode of heat transfer. When you see it as an option, eliminate it immediately — it is always a distractor in these papers.