Temperature — Set 3

Correct Answer: B. Two fixed points

• **Two fixed points** = A thermometer scale is traditionally defined using two reproducible fixed points — the ice point (lower fixed point) and the steam point (upper fixed point). • **Ice point 0°C and steam point 100°C** — These two reference temperatures allow the scale between them to be divided into equal intervals. • 💡 Wrong-option analysis: Four fixed points: thermometers need only two to define a linear scale; One fixed point: one point defines a reference but not a complete scale; Three fixed points: three would over-determine a linear scale.

Correct Answer: A. Different metals expand by different amounts when heated

• **Different metals expand by different amounts when heated** = A bimetallic strip consists of two metals bonded together with different coefficients of thermal expansion; when heated, the unequal expansion causes the strip to bend. • **Bending direction** — The strip bends toward the metal with a smaller expansion coefficient, as the other side expands more and forces a curve. • 💡 Wrong-option analysis: Metals change color with temperature: color change occurs in some indicators but is not the working principle of a bimetallic strip; Only one metal expands while the other never expands: both metals expand, just by different amounts; Metals become superconductors when heated: superconductivity occurs at very low temperatures, not when heated.

Correct Answer: C. -273.15°C

• **−273.15°C** = Absolute zero is the lowest theoretical temperature, defined as 0 K on the kelvin scale; converting to Celsius gives 0 − 273.15 = −273.15°C. • **0 K = −273.15°C** — At absolute zero, molecules possess minimum possible energy; this limit is unattainable in practice. • 💡 Wrong-option analysis: −459.67°C: that is the Fahrenheit value of absolute zero (−459.67°F), not Celsius; −100°C: corresponds to 173.15 K, far above absolute zero; 0°C: the ice point, which is 273.15 K — nowhere near absolute zero.

Correct Answer: C. -40°

• **−40°** = Setting C = F in the conversion formula F = (9/5)C + 32 and solving gives C = F = −40° — the unique point where Celsius and Fahrenheit coincide. • **Algebraic derivation: C = (9/5)C + 32 → −40 = C** — Rearranging: C − (9/5)C = 32 → (−4/5)C = 32 → C = −40. • 💡 Wrong-option analysis: 100°: on Celsius = 212°F, not equal; −10°: −10°C = 14°F, not equal; 32°: 32°C = 89.6°F, not equal.

Correct Answer: B. A change of 1 K equals a change of 1°C

• **A change of 1 K equals a change of 1°C** = The kelvin and Celsius scales have identical step sizes — a temperature interval of 1 K is exactly the same magnitude as 1°C. • **ΔT(K) = ΔT(°C)** — Only the zero points differ: 0 K = −273.15°C; but any temperature difference is numerically the same in both units. • 💡 Wrong-option analysis: A change of 1 K equals a change of 10°C: incorrect — steps are equal; A change of 1°C equals a change of 2 K: incorrect — steps are equal; A change of 1 K equals a change of 1°F: 1 K = 1°C ≠ 1°F (Fahrenheit step is smaller).

Correct Answer: D. 37°C

• **37°C** = Using C = (F − 32) × 5/9: (98.6 − 32) × 5/9 = 66.6 × 5/9 = 37°C — the normal human body temperature. • **98.6°F = 37°C** — This conversion is clinically important; a doctor measuring a patient's temperature in Fahrenheit can convert to the Celsius standard. • 💡 Wrong-option analysis: 50°C: would be 122°F, not 98.6°F; 30°C: would be 86°F, not 98.6°F; 40°C: would be 104°F — a high fever temperature.

Correct Answer: D. Constant volume gas thermometer

• **Constant volume gas thermometer** = A gas thermometer measures temperature by tracking the pressure of a fixed volume of gas — pressure is proportional to absolute temperature, directly realizing the thermodynamic scale. • **P ∝ T (constant V)** — This relationship follows from the ideal gas law PV = nRT; at constant V and n, P/T = constant. • 💡 Wrong-option analysis: Bimetallic strip thermometer: a mechanical thermometer, less accurate for fundamental standards; Liquid crystal strip thermometer: uses color change, not suitable for precision thermometry; Clinical mercury thermometer: limited range and less precise than a gas thermometer.

Correct Answer: D. Its zero corresponds to absolute zero

• **Its zero corresponds to absolute zero** = The kelvin scale is an absolute scale; 0 K is absolute zero (−273.15°C), the theoretical minimum temperature. • **0 K = −273.15°C** — No object can be cooled below absolute zero; the kelvin scale can never go negative. • 💡 Wrong-option analysis: It uses degree symbol like °K: kelvin is written as K (no degree symbol); It has no fixed relation to Celsius: T(K) = T(°C) + 273.15 is the exact relation; Its zero is at the freezing point of water: 0 K ≠ 0°C; 0°C = 273.15 K.

Correct Answer: C. T = t + 273.15

• **T = t + 273.15** = To convert Celsius temperature t to kelvin T, add 273.15: T(K) = t(°C) + 273.15. • **Offset = 273.15** — This constant equals the kelvin equivalent of the ice point; it shifts the Celsius scale up to start from absolute zero. • 💡 Wrong-option analysis: T = t − 273.15: subtracting would give negative kelvin for most ordinary temperatures, which is impossible; T = 273.15 × t: multiplication gives wrong units and values; T = t ÷ 273.15: division produces meaningless results.

Correct Answer: B. Because boiling point depends on external pressure

• **Because boiling point depends on external pressure** = At lower pressures (e.g., high altitude), water boils below 100°C; at higher pressures it boils above 100°C — 100°C is only exact at 1 atm (101.325 kPa). • **Pressure–boiling point relationship** — A pressure cooker raises pressure above 1 atm, allowing water to reach above 100°C before boiling, cooking food faster. • 💡 Wrong-option analysis: Because water is not a liquid: water is indeed a liquid below its boiling point; Because temperature cannot be measured accurately: modern thermometers are very accurate; Because 100°C is defined only for ice: 100°C is the steam point, not an ice point.