Heat and Temperature

Heat and temperature

• Heat is a form of energy that flows from a hotter to a colder body. Unit: joule (also calorie; 1 cal = 4.18 J).
• Temperature shows how hot or cold a body is. Unit: kelvin (SI), degree Celsius, degree Fahrenheit.
• K = C + 273 F = (9/5) C + 32 C / 5 = (F - 32) / 9
• Freezing point of water = 0 C = 32 F = 273 K; boiling point = 100 C = 212 F = 373 K.
• -40 degrees is the same on Celsius and Fahrenheit.
• Absolute zero = 0 K = -273 C, the lowest possible temperature.
• Normal human body temperature: 37 C = 98.6 F.

Thermometers

Mercury is used because it is a good conductor, expands evenly, and does not wet glass. Alcohol is used for very low temperatures. Never use a clinical thermometer to measure the boiling water.

Modes of heat transfer

• Good conductors: metals (silver best, then copper). Bad conductors (insulators): wood, plastic, air, wool, water (poor).
• Dark, rough surfaces absorb and radiate heat better; white, shiny surfaces reflect it. So white clothes in summer and dark clothes in winter.
• Woollen clothes keep us warm because trapped air is a poor conductor.
• Thermos flask (vacuum flask) reduces all three modes: vacuum (no conduction or convection), silvered walls (reflect radiation), cork or plastic stopper.
• Sea breeze blows by day (land is hotter, air rises, cool air comes from the sea); land breeze at night.

Expansion

• Solids, liquids and gases expand on heating. Gases expand most; solids least.
• Examples: gaps in railway tracks and bridges; iron rims fitted on cartwheels; overhead wires sag in summer.
• Anomalous expansion of water: water contracts from 0 C to 4 C, has its maximum density at 4 C, then expands. So lakes freeze from the top, and aquatic life survives below the ice.

Specific heat and latent heat

• Specific heat capacity is the heat needed to raise 1 kg by 1 degree C: Q = m c delta-T. Unit J/(kg K).
• Water has a very high specific heat (4200 J/kg K), so it heats and cools slowly; hence its use as a coolant and the mild climate near seas.
• Latent heat: heat absorbed or released during a change of state without any change in temperature.
  • Latent heat of fusion (ice to water): 336 kJ/kg.
  • Latent heat of vaporisation (water to steam): 2260 kJ/kg.
• Steam burns more than boiling water because of its extra latent heat.
• Evaporation causes cooling (sweating, earthen pots, wet cloth on forehead).
• Ice at 0 C cools better than water at 0 C (it absorbs the latent heat of fusion).