Heat Questions and Answers – Page 8

Gas thermometers use the pressure or volume changes of gases to measure temperature. Option A (Nitrogen gas thermometer) is incorrect — nitrogen gas thermometers are not the standard reference for the -200°C to 500°C range. Nitrogen liquefies at -196°C, limiting its use as a gas thermometer below that range. Option B (Helium gas thermometer) is incorrect — helium gas thermometers are used for measuring very low temperatures, especially below -200°C, because helium remains gaseous at extremely low temperatures (boils at -269°C). Option C (Hydrogen gas thermometer) is CORRECT — according to the study material, hydrogen gas thermometers are used in the temperature range of -200°C to 500°C. Hydrogen has a wide gaseous range making it ideal for this broad temperature span. Option D (Oxygen gas thermometer) is incorrect — oxygen liquefies at -183°C, making it unsuitable for gas thermometry below that temperature.

The freezing point (solidification point) of mercury is critically important for understanding thermometer applications. Option A (0°C) is incorrect — 0°C is the freezing point of water, not mercury. Option B (-39°C) is CORRECT — Mercury freezes at -39°C. This means mercury thermometers cannot be used to measure temperatures below -39°C, because mercury will solidify and stop working. Option C (-114°C) is incorrect — -114°C is the freezing point of ethanol (alcohol), not mercury. Students often confuse mercury and alcohol freezing points. Alcohol freezes at -114°C while mercury freezes at -39°C. Option D (-273°C) is incorrect — -273.15°C (or 0 Kelvin) is absolute zero, the lowest theoretically possible temperature. This is not the freezing point of mercury. Alcohol: freezing -114°C, boiling 78°C.

Mercury's boiling point is an important physical property tested frequently in competitive exams. Option A (100°C) is incorrect — 100°C is the boiling point of water (at standard atmospheric pressure), not mercury. Confusing water and mercury boiling points is a common mistake. Option B (212°C) is incorrect — 212°F is the boiling point of water in Fahrenheit scale. It does not correspond to mercury's boiling point. Option C (357°C) is CORRECT — Mercury boils at 357°C (630 K approximately). This high boiling point makes mercury excellent for high-temperature thermometers. Mercury remains in liquid state from its freezing point (-39°C) to its boiling point (357°C) — a range of 396°C. This wide liquid range is why mercury is preferred in laboratory thermometers for general use. Option D (450°C) is incorrect — 450°C is higher than mercury's actual boiling point of 357°C. No such standard thermodynamic property equals 450°C for mercury.

Alcohol thermometers are specially suited for extreme cold environments. Option A (Tropical countries) is incorrect — tropical countries have warm to hot climates. Mercury thermometers work well in these regions since temperatures rarely fall below -39°C (mercury's freezing point). Alcohol thermometers are not specifically needed here. Option B (Polar or extremely cold regions) is CORRECT — alcohol thermometers are used in polar regions (like Antarctica, Arctic Circle) and countries that experience extremely low temperatures (such as Russia, Canada, Iceland, Greenland). This is because alcohol freezes at approximately -114°C, far below mercury's freezing point of -39°C. So alcohol thermometers can measure temperatures in polar regions where mercury would freeze. Option C (Desert regions) is incorrect — deserts have high daytime temperatures, so alcohol thermometers (limited upper range) are not specifically needed. Option D (Coastal regions) is incorrect — coastal regions have moderate climates, and there is no specific need for alcohol thermometers there.

Liquid thermometers use specific liquids that show significant change in volume with temperature change. Option A (Water) is incorrect — water is not suitable for thermometers because it has a narrow liquid range (0°C to 100°C), a high surface tension, it is transparent making it hard to read, and most importantly it does not contract or expand uniformly. Option B (Alcohol or Mercury) is CORRECT — two main liquids are used in thermometers: Mercury and Alcohol. Mercury is used for high temperature ranges because it remains liquid from -39°C to 357°C and is opaque (easy to read). Alcohol is used for low temperature measurements because it freezes at around -114°C. Option C (Glycerin) is incorrect — glycerin is a viscous liquid with a high boiling point, not used in standard thermometers due to its high viscosity and non-uniform expansion. Option D (Kerosene) is incorrect — kerosene is flammable and does not expand uniformly with temperature change, making it unsuitable for thermometers. Mercury and alcohol thermometers are the most common types tested in competitive exams.

A thermometer is the instrument used to measure temperature. Option A (Barometer) is incorrect — a barometer is used to measure atmospheric pressure, not temperature. Option B (Thermometer) is CORRECT — a thermometer (Thermo meter) measures temperature. Option C (Hygrometer) is incorrect — a hygrometer measures humidity (moisture content in air), not temperature. Option D (Anemometer) is incorrect — an anemometer measures wind speed and direction.

Option A (2.186 J) is incorrect — this is not the correct value. The mechanical equivalent of heat is not 2.186 J/cal. Option B (3.186 J) is incorrect — this value is also wrong. No standard thermodynamic relationship gives 3.186 J per calorie. Option C (4.186 J) is CORRECT — 1 calorie = 4.186 joules. Option D (5.186 J) is incorrect — this value is too high. The correct value is 4.186 J/cal, not 5.186.

The British Thermal Unit (BTU) is a traditional unit of heat. Option A (100 calories) is incorrect — 100 calories is not the equivalent of 1 BTU. Option B (186 calories) is incorrect — 186 is not the correct conversion. Some students confuse 4.186 (mechanical equivalent of heat) with BTU conversion, but these are entirely different values. Option C (252 calories) is CORRECT — 1 BTU is defined as the amount of heat required to raise the temperature of 1 pound of water by 1°F. This equals exactly 252 calories (or approximately 1055 joules). Option D (425 calories) is incorrect — 425 calories is not the equivalent of 1 BTU.

The relationship between kilocalorie and joule is fundamental in thermodynamics. Option A (4.186 J) is incorrect — this is the value of 1 calorie (not 1 kilocalorie) in joules. 1 calorie = 4.186 joules. Option B (418.6 J) is incorrect — this is not a standard conversion value. Option C (4186 J) is CORRECT — 1 kilocalorie (kcal) = 1000 calories, and since 1 calorie = 4.186 joules, therefore 1 kcal = 1000 × 4.186 = 4186 joules. This can also be written as 4.186 × 10³ joules. Option D (41860 J) is incorrect — this is 10 times the correct answer. This conversion (1 kcal = 4186 J or 4.186 × 10³ J) is frequently asked in competitive exams. The value J = 4.186 joules/calorie is called the mechanical equivalent of heat and was first determined by James Prescott Joule through paddle-wheel experiments.

The CGS (Centimeter-Gram-Second) system unit of heat is the Calorie. Option A (Joule) is incorrect — Joule is the SI unit of heat, not the CGS unit. Though Joule is the internationally accepted unit, CGS system uses calorie. Option B (Watt) is incorrect — Watt is the unit of power (rate of energy transfer), not heat energy itself. 1 Watt = 1 Joule per second. Option C (Calorie) is CORRECT — In the CGS system, calorie is the standard unit of heat. One calorie is defined as the amount of heat required to raise the temperature of 1 gram of water by 1°C (from 14.5°C to 15.5°C). Option D (Kelvin) is incorrect — Kelvin is the SI unit of temperature, not heat. It is important to distinguish between the measurement of heat energy and temperature. Students preparing for SSC, UPSC, or Railway exams must remember: CGS unit = Calorie, SI unit = Joule, and the relationship 1 calorie = 4.186 joules.