Temperature — Set 5

Correct Answer: C. -459.67°F

• **−459.67°F** = Absolute zero is 0 K = −273.15°C; converting to Fahrenheit: F = (9/5)(−273.15) + 32 = −491.67 + 32 = −459.67°F. • **−459.67°F = 0 K** — On the Rankine absolute scale (Fahrenheit-based), absolute zero is 0 °R = −459.67°F. • 💡 Wrong-option analysis: −273.15°F: that is the Celsius value, not Fahrenheit; 0°F: corresponds to about −17.8°C, not absolute zero; −100°F: corresponds to about −73.3°C, far above absolute zero.

Correct Answer: A. -39°C

• **−39°C** = Mercury freezes at approximately −38.83°C (≈ −39°C); below this temperature it solidifies and cannot be used as a thermometric liquid. • **−38.83°C** — Above this freezing point, mercury remains liquid and expands uniformly, making it suitable for thermometry in the range −39°C to 357°C. • 💡 Wrong-option analysis: −100°C: mercury is already solid well before −100°C; −10°C: mercury is still liquid at −10°C; 0°C: mercury is definitely liquid at 0°C.

Correct Answer: D. Pressure of the gas

• **Pressure of the gas** = In a constant-volume gas thermometer, the volume is fixed; as temperature changes, the gas pressure changes proportionally (P ∝ T in kelvin), allowing temperature measurement. • **P/T = constant (constant V, n)** — From the ideal gas law PV = nRT, at constant V and n: P = (nR/V)T, so P varies linearly with T. • 💡 Wrong-option analysis: Volume of the gas: volume is held constant in this thermometer; Color of the gas: gas color is generally not a reliable thermometric property; Mass of the gas: mass does not change with temperature in a sealed container.

Correct Answer: A. No net heat flows between them

• **No net heat flows between them** = Thermal equilibrium means two bodies in contact have reached the same temperature, so there is no net heat transfer between them. • **Zeroth law implication** — Thermal equilibrium is the basis of the zeroth law: if A is in equilibrium with B, and B with C, then A and C are in equilibrium too. • 💡 Wrong-option analysis: They must be made of the same material: different materials can reach thermal equilibrium; They must have the same mass: equal mass is not required for thermal equilibrium; They must be at 0°C: thermal equilibrium can occur at any temperature.

Correct Answer: C. 122°F

• **122°F** = Using F = (9/5)×C + 32: (9/5)×50 + 32 = 90 + 32 = 122°F — a moderately hot temperature. • **50°C = 122°F** — At 50°C the air feels very hot; this temperature is above the safe human body comfort level. • 💡 Wrong-option analysis: 100°F: corresponds to about 37.8°C, not 50°C; 82°F: corresponds to about 27.8°C; 212°F: that is 100°C, the boiling point of water.

Correct Answer: C. Very hot furnaces without contact

• **Very hot furnaces without contact** = An optical pyrometer measures extremely high temperatures (above ~700°C) by comparing the brightness of emitted light from a hot object to a calibrated filament — no physical contact needed. • **Above 700°C** — Below this temperature, the thermal glow is in infrared and not bright enough in visible light for an optical pyrometer; other radiation thermometers cover lower ranges. • 💡 Wrong-option analysis: In a glass of water: water temperatures do not require a pyrometer; Inside a refrigerator: very low temperatures need different sensors; In the human mouth: clinical thermometers are used, not pyrometers.

Correct Answer: C. 273 K

• **273 K** = The freezing point of water at standard pressure is 0°C; converting to kelvin: 0 + 273.15 ≈ 273 K. • **273 K = 0°C** — This is the ice point, one of the fundamental reference temperatures in thermometry. • 💡 Wrong-option analysis: 100 K: corresponds to −173°C, far below freezing; 373 K: corresponds to 100°C, the boiling point of water; 0 K: that is absolute zero, not the freezing point.

Correct Answer: D. A change of 10 K equals a change of 10°C

• **A change of 10 K equals a change of 10°C** = The kelvin and Celsius scales have identical step sizes — a temperature difference of 10 K is exactly equal to a temperature difference of 10°C. • **ΔT(K) = ΔT(°C)** — Only the zero reference differs; every temperature interval is numerically the same in both units. • 💡 Wrong-option analysis: A change of 10 K equals a change of 20°C: the steps are equal, not doubled; A change of 1 K equals a change of 2°C: incorrect — 1 K = 1°C; A change of 10°C equals a change of 5 K: incorrect — the steps are equal.

Correct Answer: B. 263 K

• **263 K** = Convert −10°C to kelvin: −10 + 273.15 ≈ 263 K — a sub-zero temperature still well above absolute zero. • **−10°C = 263 K** — Even negative Celsius values give positive kelvin values because 0 K is −273.15°C, far below most encountered temperatures. • 💡 Wrong-option analysis: 10 K: corresponds to −263°C, an extremely low cryogenic temperature; 283 K: corresponds to +10°C, not −10°C; −10 K: negative kelvin values are physically undefined.

Correct Answer: D. Platinum resistance thermometer

• **Platinum resistance thermometer** = This instrument measures temperature by detecting the change in electrical resistance of a platinum wire, which increases linearly with temperature over a wide range. • **Wide range: −200°C to +850°C** — Platinum resistance thermometers (PRTs) are highly accurate and stable, used as reference standards in scientific and industrial applications. • 💡 Wrong-option analysis: Thermocouple: measures temperature via the Seebeck effect (emf at a junction), not resistance; Alcohol thermometer: measures temperature by liquid expansion, not electrical resistance; Bimetallic thermometer: measures temperature by differential thermal expansion of metals, not resistance.