Heat Questions and Answers – Page 7

The Total Radiation Pyrometer collects all wavelengths of radiation from a hot body to measure its temperature. Option A (200°C to 1500°C) is incorrect — the lower end of 200°C is too low. Total Radiation Pyrometers require a minimum temperature of around 800°C to collect sufficient radiation for accurate measurement. 1500°C as the upper limit is also incorrect — the range extends much higher. Option B (500°C to 2000°C) is incorrect — again the lower limit of 500°C is wrong. At 500°C the radiation collected is insufficient for accurate readings. The upper limit of 2000°C is also short of the actual 3000°C range. Option C (800°C to 2700°C) is incorrect — this is the range of the Optical Pyrometer (Wien's law based), NOT the Total Radiation Pyrometer. The Optical Pyrometer uses Wien's radiation law and covers 800°C to 2700°C. Option D (800°C to 3000°C) is CORRECT — the Total Radiation Pyrometer, based on Stefan-Boltzmann law (E = σT⁴), can measure temperatures from 800°C to 3000°C. This slightly wider range (up to 3000°C) compared to the Optical Pyrometer (up to 2700°C) is its key advantage.

The Total Radiation Pyrometer is used for measuring very high temperatures using the total energy radiated by a body. Option A (Wien's displacement law) is incorrect — Wien's displacement law relates the peak wavelength of radiation to the absolute temperature. Wien's law is used by Optical Pyrometers, not Total Radiation Pyrometers. Option B (Newton's law of cooling) is incorrect — Newton's law of cooling describes the rate of heat loss from a body to its surroundings due to convection. Option C (Snell's law) is incorrect — Snell's law describes the refraction of light at an interface between two media. It is used in optics (lenses, prisms) but has no relation to radiation pyrometry. Option D (Stefan's law) is CORRECT — the Total Radiation Pyrometer is based on Stefan-Boltzmann law (Stefan's law): E = σT⁴. The pyrometer collects all the radiation (total, not wavelength-specific) from a hot body and focuses it on a thermocouple or bolometer. By measuring the total energy received, and using Stefan's law, the temperature is calculated. Range: 800°C to 3000°C — slightly wider than Optical Pyrometer.

The Optical Pyrometer (also called the disappearing filament pyrometer) has a specific operational temperature range. Option A (200°C to 1000°C) is incorrect — this range is too low for the Optical Pyrometer. At temperatures below 800°C, the visible light radiation from a body is insufficient for the disappearing filament method to work accurately. Option B (500°C to 1500°C) is incorrect — while the upper end (1500°C) is within the optical pyrometer's range, the lower end (500°C) is incorrect. The minimum operational temperature for Optical Pyrometers is 800°C because only at this temperature does a body begin to emit sufficient visible radiation for optical comparison. Option C (800°C to 2700°C) is CORRECT — the Optical Pyrometer can measure temperatures from 800°C to 2700°C. The lower limit is 800°C because that is when visible incandescence begins. Option D (1000°C to 5000°C) is incorrect — the upper limit of Optical Pyrometers is 2700°C, not 5000°C.

An Optical Pyrometer measures very high temperatures by analysing the radiation emitted by a hot body. Option A (Stefan's law) is incorrect — Stefan's law relates total energy radiation to T⁴. While Stefan's law is related to radiation, the Optical Pyrometer does not use total radiation. It specifically analyses the spectral (wavelength-specific) component of radiation. Total Radiation Pyrometers are based on Stefan's law, not Optical Pyrometers. Option B (Newton's law of cooling) is incorrect — Newton's law of cooling states that the rate of heat loss is proportional to the temperature difference between the body and surroundings. It applies to convective cooling at modest temperature differences, not to high-temperature pyrometry. Option C (Wien's radiation law) is CORRECT — Optical Pyrometers work on Wien's radiation law (also called Wien's distribution law). This law describes the spectral distribution of radiation from a black body. This method is called optical pyrometry or disappearing filament method. Option D (Planck's law) is incorrect — while Planck's law is the complete description of black body radiation, optical pyrometers specifically use Wien's approximation (valid at shorter wavelengths and higher temperatures) for practical measurement.

Wien's displacement law is fundamental to understanding how objects radiate heat and light. Option A (Pressure) is incorrect — Wien's displacement law has no relation to the pressure of the radiating body. Pressure affects gas behavior (via gas laws) but is not part of Wien's radiation law. Option B (Absolute temperature) is CORRECT — Wien's displacement law states that the wavelength at which a black body emits maximum radiation (λm) is inversely proportional to its absolute temperature (T). Mathematically: λm × T = b, where b is Wien's displacement constant = 2.898 × 10⁻³ m·K. So as temperature increases, the peak wavelength shifts to shorter wavelengths (from red/infrared to blue/violet). The Sun's peak wavelength (~500 nm) corresponds to its surface temperature (~5778 K). Option C (Mass) is incorrect — mass of a body has no direct relation to the peak wavelength of radiation. Wien's law deals with the spectral distribution of thermal radiation. Option D (Color) is incorrect — while color is related to wavelength, Wien's law is about the mathematical relationship between peak wavelength and absolute temperature, not color per se.

Stefan's law (also called Stefan-Boltzmann law) is one of the most important laws in heat radiation. Option A is CORRECT — Stefan's law states that the total energy radiated per unit surface area of a black body per unit time (E) is directly proportional to the fourth power of the absolute temperature (T⁴). Mathematically: E ∝ T⁴ or E = σT⁴, where σ (sigma) is the Stefan-Boltzmann constant = 5.67 × 10⁻⁸ W/m²K⁴. This law was proposed by Josef Stefan in 1879 and derived theoretically by Ludwig Boltzmann in 1884. Option B is incorrect — the relationship is with T⁴ (fourth power), not T² (square). The square relationship does not correctly describe black body radiation. Option C is incorrect — the relationship between radiation wavelength and temperature is described by Wien's Displacement Law (λm × T = constant), not Stefan's law. Option D is incorrect — Stefan's law applies to an ideal black body and does not depend on the color of the body in the simple sense. A black body absorbs all radiation regardless of color.

A thermoelectric thermometer (thermocouple) is a widely used temperature measuring device. Option A (Doppler effect) is incorrect — the Doppler effect relates to the change in frequency of waves when the source and observer are in relative motion. It has no direct relation to thermocouple thermometers. Doppler effect is used in radar, ultrasound, and astronomy. Option B (Seebeck effect) is CORRECT — a thermoelectric thermometer (thermocouple) works on the Seebeck effect, discovered by Thomas Johann Seebeck in 1821. The Seebeck effect states that when two dissimilar metals are joined at two junctions and the junctions are at different temperatures, an electromotive force (EMF) is generated. Common thermocouple pairs: copper-constantan, iron-constantan, platinum-rhodium. Option C (Photoelectric effect) is incorrect — the photoelectric effect describes the emission of electrons when light falls on a metal surface. Option D (Compton effect) is incorrect — the Compton effect describes the scattering of X-rays by electrons, discovered by Arthur Compton.

High-temperature gas thermometry requires gases with very high dissociation temperatures. Option A (Up to 500°C) is incorrect — 500°C is within the range of hydrogen gas thermometers (-200°C to 500°C). Nitrogen thermometers can measure much higher temperatures than this. Option B (Up to 1000°C) is incorrect — 1000°C is not the upper limit of nitrogen gas thermometers. Nitrogen thermometers can exceed this range. Option C (Up to 1500°C) is CORRECT — Nitrogen gas thermometers can measure temperatures up to 1500°C. Nitrogen (N₂) has a very high dissociation temperature (~3000°C) and a high boiling point (-196°C), giving it a very wide gaseous range. This makes nitrogen ideal for high-temperature industrial measurements up to 1500°C. Option D (Up to 2000°C) is incorrect — 2000°C exceeds the practical reliable range of nitrogen gas thermometers.

Very low temperature measurement requires gases that remain in gaseous state even at extremely low temperatures. Option A (Hydrogen gas thermometer) is incorrect — while hydrogen has a very low boiling point (-253°C), hydrogen gas thermometers are standardly used in the range of -200°C to 500°C, not specifically below -200°C. Also, at temperatures approaching -253°C, hydrogen itself starts to liquefy. Option B (Nitrogen gas thermometer) is incorrect — nitrogen liquefies at -196°C. Therefore, nitrogen cannot be used as a gas thermometer below -196°C, making it completely unsuitable for temperatures below -200°C. Option C (Helium gas thermometer) is CORRECT — helium is used for measuring temperatures below -200°C (i.e., in the cryogenic range). Helium has the lowest boiling point of any element at -269°C (4 K). Since it remains gaseous at near absolute zero temperatures, it is the ideal gas for cryogenic thermometry. Liquid helium itself is used to achieve near absolute zero temperatures in laboratories. Option D (Oxygen gas thermometer) is incorrect — oxygen liquefies at -183°C, making it completely useless as a gas thermometer at -200°C or below. Remember: Helium is the only noble gas with such an extremely low boiling point, making it uniquely suitable for cryogenic measurements.

Gas thermometers work on the principle that gas pressure or volume changes proportionally with temperature. Option A (Helium gas thermometer) is incorrect — helium thermometers are used for extremely low temperatures, especially below -200°C, because helium remains gaseous even near absolute zero (it boils at -269°C). Helium is used for the range below -200°C, not -200°C to 500°C. Option B (Nitrogen gas thermometer) is incorrect — nitrogen liquefies at -196°C, so it cannot be used as a gas thermometer below that temperature. Nitrogen thermometers are used for specific narrower ranges. Option C (Hydrogen gas thermometer) is CORRECT — hydrogen gas thermometers are used in the range of -200°C to 500°C. Hydrogen has a very wide gaseous range and expands uniformly with temperature, making it suitable for this broad measurement span. It is one of the most accurate gas thermometers for this range. Option D (Oxygen gas thermometer) is incorrect — oxygen liquefies at -183°C, making it unsuitable for gas thermometry at temperatures below -183°C.