Temperature — Set 1

Correct Answer: A. Zeroth law of thermodynamics

• **Zeroth law of thermodynamics** = If A is in thermal equilibrium with B, and B with C, then A and C are also in thermal equilibrium — this defines temperature comparison. • **Three-body equilibrium** — The zeroth law was named after the first and second laws were already established, but logically precedes them as the basis of thermometry. • 💡 Wrong-option analysis: Second law of thermodynamics: relates to entropy and direction of heat flow; Third law of thermodynamics: concerns behavior near absolute zero; First law of thermodynamics: states conservation of energy.

Correct Answer: B. Average kinetic energy of particles

• **Average kinetic energy of particles** = Temperature is a measure of the average translational kinetic energy of molecules in a substance — higher temperature means faster random motion. • **½mv² (per molecule)** — In an ideal gas, T ∝ (2/3)×(average KE), so doubling temperature doubles average kinetic energy. • 💡 Wrong-option analysis: Total heat content of a body: heat is energy transferred, not stored; Total internal energy of a body: includes potential energy too, not just temperature; Total mass of the body: mass is unrelated to temperature.

Correct Answer: B. Kelvin

• **Kelvin** = The kelvin (symbol K) is the SI base unit of thermodynamic temperature, starting from absolute zero. • **273.15 K** — The ice point of water equals 273.15 K, linking kelvin to the familiar Celsius scale. • 💡 Wrong-option analysis: Joule: the unit of energy, not temperature; Degree Fahrenheit: a non-SI unit used mainly in the USA; Degree Celsius: a derived unit, not the SI base unit.

Correct Answer: B. Heat is energy transferred due to a temperature difference

• **Heat is energy transferred due to a temperature difference** = Heat flows from a hotter body to a cooler one because of a temperature gradient — it is not stored in a body. • **Q = mcΔT** — The heat transferred depends on mass, specific heat capacity, and temperature change. • 💡 Wrong-option analysis: Heat is stored inside a body as a fixed amount: incorrect — heat is energy in transit; Heat is the same as temperature: temperature is a state property; heat is energy transfer; Heat is a measure of coldness: coldness is not a physical quantity — it is the absence of heat.

Correct Answer: D. Kelvin scale

• **Kelvin scale** = The kelvin scale is an absolute thermodynamic temperature scale that starts at absolute zero (0 K), avoiding negative values for thermodynamic calculations. • **0 K = −273.15°C** — Absolute zero is the theoretical lower limit of temperature where molecular motion reaches its minimum. • 💡 Wrong-option analysis: Fahrenheit scale: a relative scale, not absolute; Celsius scale: a relative scale with 0°C at ice point; Reaumur scale: an obsolete relative scale with 80 divisions between ice and steam points.

Correct Answer: A. 298 K

• **298 K** = Convert Celsius to kelvin using T(K) = T(°C) + 273.15; so 25 + 273.15 ≈ 298 K. • **273.15** — This constant represents the difference between the kelvin and Celsius scales, equal to the ice-point temperature in kelvin. • 💡 Wrong-option analysis: 275 K: would correspond to about 1.85°C; 248 K: would correspond to about −25°C; 325 K: would correspond to about 51.85°C.

Correct Answer: D. It does not wet glass and forms a distinct meniscus

• **It does not wet glass and forms a distinct meniscus** = Mercury is a non-wetting liquid — it does not adhere to glass, so its meniscus is convex and easy to read accurately. • **Meniscus shape** — Non-wetting liquids form a convex meniscus; wetting liquids form a concave one (like water). • 💡 Wrong-option analysis: It easily wets the glass surface: wetting would make reading harder with a concave meniscus; It has very low density: mercury is actually very dense (~13.6 g/cm³); It has a very high specific heat: mercury has a relatively low specific heat, which helps it respond quickly to temperature changes.

Correct Answer: A. 32°F

• **32°F** = Using F = (9/5)×C + 32, substituting C = 0 gives F = 32°F — the Fahrenheit freezing point of water at standard pressure. • **F = (9/5)C + 32** — This conversion formula shows a 9:5 scaling ratio plus a 32-unit offset between Fahrenheit and Celsius. • 💡 Wrong-option analysis: 0°F: is −17.8°C, not 0°C; 100°F: is about 37.8°C; 212°F: is 100°C, the boiling point of water.

Correct Answer: C. Alcohol thermometer

• **Alcohol thermometer** = Ethyl alcohol remains liquid down to about −114°C, far below mercury's freezing point of −39°C, making it suitable for very low temperatures. • **−39°C** — Mercury freezes near −39°C; below that temperature a mercury thermometer becomes unusable. • 💡 Wrong-option analysis: Bimetallic thermometer: less accurate for precise low temperatures; Mercury thermometer: useless below −39°C as mercury solidifies; Thermocouple only: thermocouples work but alcohol thermometers are simpler and cheaper for the same range.

Correct Answer: C. Ice point 0°C and steam point 100°C

• **Ice point 0°C and steam point 100°C** = The Celsius scale was calibrated using two reproducible fixed points at standard pressure — 0°C for ice-water equilibrium and 100°C for boiling water. • **100 equal divisions** — The scale between 0°C and 100°C is divided into 100 equal parts, each one degree Celsius. • 💡 Wrong-option analysis: Melting point of iron and boiling point of mercury: not traditional Celsius calibration points; Triple point of water and critical point of water: used in modern thermometry but not the traditional Celsius fixed points; Freezing point of alcohol and boiling point of alcohol: not standard calibration reference points.