Heat and Temperature

Heat and Temperature

Heat

Heat is that form of energy that flows from one body to another due to the difference in temperature between them.

• The amount of heat contained in a body depends upon the mass of the body.

• If work performed is W and heat produced is H, then W = JH, where J is a constant called the Mechanical Equivalent of Heat.

• Its value is 4.186 joules/calorie, meaning that if 4.186 joules of work are performed, 1 calorie of heat is consumed.

Units of Heat

• C.G.S. unit: calorie = amount of heat required to raise the temperature of 1 g of pure water by 1°C.

• International calorie: heat required to raise 1 g of water from 14.5°C to 15.5°C.

• F.P.S. unit: B.Th.U (British Thermal Unit) = heat required to raise 1 pound of water by 1°F.

Relations Between Different Units

• 1 B.Th.U = 252 calories

• 1 calorie = 4.186 joules

• 1 Therm = 10⁵ B.Th.U

• 1 pound calorie = 453.6 calorie

Temperature

Temperature is the physical cause that decides the direction of flow of heat.

• Heat flows from a body at a higher temperature to a body at a lower temperature.

Measurement of Temperature

Thermometer

• A device that measures the temperature of a body is called a thermometer.

Scales of Temperature Measurement

• Two fixed points:

  1. Freezing point of water (Lower Fixed Point, LFP)

  2. Boiling point of water (Upper Fixed Point, UFP)

Absolute Zero

• Celsius: -273.15°C

• Fahrenheit: -459.6°F

• Rankine: -218.4°R

Relations Between Temperature Scales

(C + 0)/100 = (F - 32)/180 = (R + 0)/80 = (K - 273)/100 = (Ra - 492)/180

Conversion Scale

ΔC/5 = ΔF/9 = ΔR/4 = ΔK/5 = ΔRd/9

• Celsius was initially known as centigrade. The 

• Kelvin scale does not use the degree symbol

Range of Thermometers

• Mercury thermometer: -30°C to 350°C

• Alcohol thermometer: F.P. of alcohol -115°C

• Constant volume gas thermometer: -200°C to 500°C (with H₂), below -200°C to -268°C (with He), above 1000°C to 1600°C (with N₂ and glazed porcelain bulb)

• Platinum resistance thermometer: -200°C to 1200°C

• Thermocouple thermometer: -200°C to 1600°C

• Total Radiation Pyrometer: Measures the temperature of glowing bodies based on radiation; cannot measure below 800°C

Specific Heat Capacity

The specific heat capacity of a material is the heat required to raise the temperature of a unit mass by 1°C.

• SI unit: Joule/kg·K

• One calorie raises 1 g of water by 1°C (4200 J/kg·K)

Specific Heat Capacities of Materials

Specific Heat of Gases

• Depends on the conditions of heating

• Two significant types:

  1. Constant volume (sᵥ)

  2. Constant pressure (sₚ)

Molar Heat Capacity

• Heat required to raise 1 mole of gas by 1°C:

  1. Cᵥ: at constant volume

  2. Cₚ: at constant pressure

Relations

• Adiabatic exponent γ depends on the atomicity of the gas.

• Cₚ / Cᵥ = sₚ / sᵥ = γ

• Cₚ = M * sₚ, Cᵥ = M * sᵥ (M = molar mass of gas)

• Cₚ - Cᵥ = R in SI units

• sₚ - sᵥ = R / M in SI units

Final Thoughts

Heat is energy that flows from a hotter body to a cooler one, depending on the body’s mass. The mechanical equivalent of heat shows 4.186 joules of work produce 1 calorie.

Heat is measured in calories, BTU, and joules, and thermometers track temperature on scales like Celsius, Fahrenheit, and Kelvin, with absolute zero as the lowest point. Specific heat capacity tells how much heat is needed to raise a substance’s temperature, with water at 4200 J/kg·K. For gases, heat capacity varies at constant volume or pressure, and molar heat capacity is per mole.

Understanding heat and temperature explains everyday phenomena, from cooking and climate to industrial processes.