Mechanical Physics - Thermal Properties of Matter

Thermal Properties of Matter

• Heat, temperature, thermal expansion

• Thermal expansion of −

  • Solids

  • Liquids

  • Gases

• Anomalous expansion of water

• Specific heat capacity

• Cp, Cv – calorimetry

• Change of state

• Latent heat capacity

• Heat transfer −

  • Conduction

  • Convection

  • radiation

• Thermal conductivity

• Qualitative ideas of Blackbody radiation

• Wein's displacement Law

• Stefan's law

• Greenhouse effect

SUMMARY

1. Heat is a form of energy that flows between a body and its surrounding medium by virtue of temperature difference between them. The degree of hotness of the body is quantitatively represented by temperature.

2. A temperature-measuring device (thermometer) makes use of some measurable property (called thermometric property) that changes with temperature. Different thermometers lead to different temperature scales. To construct a temperature scale, two fixed points are chosen and assigned some arbitrary values of temperature. The two numbers fix the origin of the scale and the size of its unit.

3. The ideal gas equation connecting pressure (P), volume (V) and absolute temperature (T) is : PV = μRT where μ is the number of moles and R is the universal gas constant.

4. In the absolute temperature scale, the zero of the scale is the absolute zero of temperature – the temperature where every substance in nature has the least possible molecular activity. The Kelvin absolute temperature scale (T ) has the same unit size as the Celsius scale (Tc ), but differs in the origin : Tc = T – 273.15

5. The latent heat of fusion (Lf ) is the heat per unit mass required to change a substance from solid into liquid at the same temperature and pressure. The latent heat of vaporisation (Lv ) is the heat per unit mass required to change a substance from liquid to the vapour state without change in the temperature and pressure.

6. The three modes of heat transfer are conduction, convection and radiation.

POINTS TO PONDER

1. The relation connecting Kelvin temperature (T ) and the Celsius temperature tc ;  T = tc + 273.15 and the assignment T = 273.16 K for the triple point of water are exact relations (by choice). With this choice, the Celsius temperature of the melting point of water and boiling point of water (both at 1 atm pressure) are very close to, but not exactly equal to 0 °C and 100 °C respectively. In the original Celsius scale, these latter fixed points were exactly at 0 °C and 100 °C (by choice), but now the triple point of water is the preferred choice for the fixed point, because it has a unique temperature.

2. A liquid in equilibrium with vapour has the same pressure and temperature throughout the system; the two phases in equilibrium differ in their molar volume (i.e. density). This is true for a system with any number of phases in equilibrium.

3. Heat transfer always involves temperature difference between two systems or two parts of the same system. Any energy transfer that does not involve temperature difference in some way is not heat.

4. Convection involves flow of matter within a fluid due to unequal temperatures of its parts. A hot bar placed under a running tap loses heat by conduction between the surface of the bar and water and not by convection within water.