Heat Questions and Answers – Page 5

Bad conductors (also called insulators or poor conductors) are materials that do not allow heat to pass through them easily. Option A (Copper) is incorrect — copper is an excellent conductor of heat, second only to silver among metals. Copper is widely used in heat exchangers, cooking vessels, and electrical wiring because of its high thermal and electrical conductivity. Option B (Aluminium) is incorrect — aluminium is a good conductor of heat. It is used in cooking utensils, heat sinks in electronics, and aircraft bodies because of its good thermal conductivity and light weight. Option C (Iron) is incorrect — iron is a good conductor of heat (though less so than copper and aluminium). Iron is used in cooking pans and industrial machinery. All metals, including iron, are generally good conductors of heat. Option D (Cotton) is CORRECT — cotton is a bad conductor of heat. The image specifically lists bad conductors (कुचालक) as: wood (लकड़ी), glass (काँच), silica (सिलिका), cotton (कपास), wool (ऊन), and similar materials. Cotton traps air between its fibres, reducing heat conduction. This is why cotton and woollen clothes keep us warm — they insulate the body from the cold outside.

Conductors of heat are materials through which heat passes easily and rapidly. Option A (Wood) is incorrect — wood is a bad conductor (insulator) of heat. This is why wooden handles are used on cooking utensils, as wood does not allow heat to flow to the hand. Wood has very low thermal conductivity. Option B (Glass) is incorrect — glass is also a bad conductor of heat. This is why double-glazed windows trap air between glass panes to reduce heat transfer. Glass wool is used as thermal insulation in buildings. Option C (Silver) is CORRECT — all metals are good conductors of heat, and silver is the BEST thermal conductor among all metals. Other good conductors include copper, aluminium, iron, and other metals. According to the image, good conductors include all metals, water, and the human body. The high thermal conductivity of metals is due to free electrons that carry heat energy rapidly through the material. Option D (Asbestos) is incorrect — asbestos is a thermal insulator, used specifically because it does NOT conduct heat. Asbestos is used in fireproof clothing, heat shields, and insulation boards. It is classified as a thermal insulator (ऊष्मारोधी) alongside ebonite.

Heat can be transferred from one place to another by different mechanisms. Option A (2) is incorrect — heat transfer does not occur by just 2 methods. While students sometimes think only of conduction and convection, there is an essential third method. Option B (3) is CORRECT — heat is transferred by exactly 3 methods: (1) Conduction — transfer of heat through direct contact without actual movement of molecules from one place to another. (2) Convection — transfer of heat through the actual movement of heated molecules/fluid from one place to another. (3) Radiation — transfer of heat without any material medium, in the form of electromagnetic waves. All three methods are important for competitive exams. Option C (4) is incorrect — there is no fourth standard method of heat transfer in classical physics. Some advanced texts mention 'advection' as a sub-type of convection but in standard competitive exam curriculum, only 3 are recognized. Option D (5) is incorrect — five methods of heat transfer do not exist in standard physics. The three methods (conduction, convection, radiation) are universally accepted in all syllabi for government competitive exams.

The linear expansion coefficient measures how much a material expands per unit length per degree of temperature rise. Option A is CORRECT — α = ΔL / (L₀ × Δθ), where ΔL is the increase in length, L₀ is the original (initial) length, and Δθ is the temperature rise. Rearranged: ΔL = L₀ × α × Δθ, which gives the actual increase in length. The final length after expansion: L = L₀(1 + αΔθ). Option B is incorrect — placing original length in the numerator and increase in length in the denominator inverts the correct formula. Option C is incorrect — multiplying temperature rise with length increase in the numerator gives wrong dimensions. α is dimensionless per degree, not [length²·temperature/length]. Option D is incorrect — this formula would give units of (length × temperature)/length = temperature, which is not the unit of α.

Thermal expansion coefficients describe how a material expands when heated. Option A (α : β : γ = 1 : 2 : 3) is CORRECT — for isotropic solids (materials that expand uniformly in all directions): α is the linear expansion coefficient (expansion per unit length per degree), β = 2α is the area (superficial) expansion coefficient (expansion per unit area per degree), and γ = 3α is the volume (cubical) expansion coefficient (expansion per unit volume per degree). This relationship is derived mathematically from the geometry of expansion: if a length increases by α per degree, a 2D area increases by approximately 2α per degree, and a 3D volume increases by approximately 3α per degree. Option B (1:3:6) is incorrect — these ratios do not correspond to any standard thermal expansion relationship. Option C (2:3:4) is incorrect — these are not the standard ratios for thermal expansion coefficients.

Gases are unique in that they have two distinct specific heats, unlike solids and liquids which have only one. Option A (Cv and Cp) is CORRECT — gases have two specific heats: (1) Specific heat at constant volume (Cv) — the heat required to raise the temperature of 1 mole of gas by 1°C while keeping the volume constant. (2) Specific heat at constant pressure (Cp) — the heat required to raise the temperature of 1 mole of gas by 1°C while keeping the pressure constant. At constant pressure, the gas expands and does work against external pressure, so more heat is needed. The ratio γ = Cp/Cv is called the adiabatic index or heat capacity ratio and is approximately 1.4 for diatomic gases like air (N₂, O₂). Option B (Cs and Ck) is incorrect — these are not standard symbols for specific heats of gases in thermodynamics. Option C (Cf and Cb) is incorrect — no such standard thermodynamic quantities exist. Option D (Ch and Cl) is incorrect — these have no standard meaning in thermodynamics related to specific heats of gases.

The specific heat of water is one of the most important values in physics and chemistry. Option A (0.5 kcal/g/°C) is incorrect — 0.5 cal/g/°C (not kilocalorie) is the specific heat of ice. Multiplied by 1000, 0.5 kcal/g/°C would be enormously high and physically meaningless for water. Option B (1 kcal/g/°C) is incorrect — 1 kcal/g/°C = 1000 cal/g/°C, which is not the specific heat of water. The specific heat is 1 cal/g/°C (not kilocalorie). Option C (1 calorie/gram/°C) is CORRECT — the specific heat of water is 1 cal/g/°C. This is the highest specific heat value among all common liquids and most substances. It means that 1 gram of water requires 1 calorie to raise its temperature by 1°C. Water's high specific heat is the reason why: (1) oceans moderate coastal climates, (2) water is used as a coolant in engines, (3) hot water bottles remain warm for long periods. Option D (4 cal/g/°C) is incorrect — 4 is associated with joules (1 cal = 4.186 J), not with the specific heat value in cal/g/°C. The specific heat of water in cal units is 1 cal/g/°C, not 4.

This question is directly linked to the definition of calorie and the specific heat of water. Option A (0.5 calorie) is incorrect — 0.5 cal/g/°C is the specific heat of ice, not water. Ice requires only half the heat that liquid water requires to achieve the same temperature rise per gram. Option B (1 calorie) is CORRECT — by definition, 1 calorie is the amount of heat required to raise the temperature of 1 gram of water by 1°C (specifically from 14.5°C to 15.5°C at standard pressure). This means the specific heat of water is 1 cal/g/°C, which is the highest specific heat among common substances. This high specific heat makes water an excellent coolant for engines and a moderating agent for climate near oceans and seas. Option C (4 calories) is incorrect — 4 calories is approximately equal to 4 × 4.186 ≈ 16.7 joules. While 1 calorie ≈ 4.186 joules, the amount needed to heat 1g of water by 1°C is 1 calorie, not 4 calories. Option D (10 calories) is incorrect — 10 calories would raise 1 gram of water by 10°C, not 1°C. Using Q = mcθ: Q = 1 × 1 × 1 = 1 calorie.

The SI unit of specific heat is derived from the basic formula c = Q/(mθ). Option A (cal/g/°C) is incorrect — this is the CGS unit of specific heat, not the SI unit. While 1 cal/g/°C = 4186 J/kg/K, the calorie-based unit is not part of the SI system. Option B (Joule × kilogram⁻¹ × Kelvin⁻¹) is CORRECT — the SI unit of specific heat is J·kg⁻¹·K⁻¹, which can also be written as J/(kg·K) or Joule per kilogram per Kelvin. This is the standard unit used in all scientific and engineering calculations. Option C (Watt/kg) is incorrect — Watt is the unit of power (J/s), not heat energy. Watt/kg does not represent specific heat. Option D (Joule/second) is incorrect — Joule/second is the unit of power (Watt), not specific heat. Specific heat involves energy per unit mass per unit temperature, not energy per unit time.

Option A (Speed of light) is incorrect — the speed of light is denoted by 'c' in Einstein's equation E = mc², where c = 3 × 10⁸ m/s. However, in the heat formula Q = mcθ, 'c' does NOT represent speed of light. Option B (Charge of electron) is incorrect — the charge of an electron is denoted by 'e' (= 1.6 × 10⁻¹⁹ coulombs), not 'c'. Option C (Specific heat of the substance) is CORRECT — in Q = mcθ, 'c' stands for the specific heat capacity of the substance. It is defined as the amount of heat required to raise the temperature of 1 gram (or 1 kg) of a substance by 1°C (or 1 K). The SI unit of specific heat is J·kg⁻¹·K⁻¹ (joule per kilogram per kelvin). The specific heat of water is 1 cal/g/°C = 4186 J/kg/K. Option D (Coefficient of expansion) is incorrect — coefficients of thermal expansion are denoted by α (linear), β (area), and γ (volume). They represent dimensional change per unit temperature change and are not the same as specific heat.