4.3: Thermal Physics

4.3: Thermal Physics

1. Solids, liquids and gases

   • Solids - Packed in neat arrangement of molecules/ions (crystal shapes). Fixed positions.
   • Liquids - Packed similar to solids. Particles can slide past each other, so a little more space than solids (but water molecules packed more closely than ice crystals in water)
   • Gases - Vastly increase spacing, no arrangement 'chaotic'.

2. Specific heat capacity

   • S.H.C - Is the quantity of thermal energy required to raise the temperature of a unit mass of a substance by a unit temperature rise.
     • Experiement - metal block - drilled two holes
       • One hole for heat and other for temperature sensor/
         • Plot temperature rise against time, then W=VIt, over blah blah blah... x 1/gradient of graph.

3. Brownian motion

   • Conservation of energy
     • Particles with high KE, will collide with particles of lower KE, therefore high KE particles lose KE and particles with low KE initially with gain KE. Eventually, KE will be the same throughout. (e.g. hot tea mixed with milk)
   • Experiment: particles of smoke dust.
     • Brownian motion - The random movement of small particles (e.g. pollen or smoke particles) when suspended in a liquid or gas.
     • Continuous molecular bombardment (perpetual motion)

4. Kinetic theory and gas pressure

   • The total volume of the molecules is negligible compared with volume of container.
     • No intermolecular forces exists except during collisions.
       • Gravitational force on particles is negligible.
         • Collisions between molecules and walls of a container are elastic.
           • A gas consists of large number of molecules moving in rapid, random motion.
   • F = 2mv x v/2d = mv²/d
   • p = 1/3(ρv²)

5. Internal energy

   • The internal energy of a body is the sum of the random distributions of KE's and PE's of all the molecules in the body.
   • Ideal gas - A gas which has internal energy in the form of only random KE.
   • Factors affecting internal energy
     • Temperature - Rise in T, cause molecules travelling more rapidly, so KE rises, therefore, internal energy rises.
     • Pressure - If P falls with no change in T, internal energy unaltered in an ideal gas. If not ideal, molecules attract one another, gas expands so work done on molecules to pull them apart. So higher PE, higher IE.
     • State - No KE chnge BUT random PE changes considerably. (e.g. water boiling as a liquid to gas or solid to liquid).
     • Non-affecting include speed or g.

6. Temperature

   • Thermal equilibrium - Two objects at the same temperature (zero resultant energy).
   • Absolute scale of temperature, does not depend on any property of a substance. Starts from zero at absolute zero (minimum internal energy, 0K ).
   • Celsius = n-273.15 conversion from Kelvin.
     • Triple point = 273.16, where solid, liquid and gas states exist simultaneously.

7. Latent heat

   • S.L.H of fusion - Of a substance is the quantity of energy per unit mass required to chnage it at constant temperature from a solid into a liquid.
     • S.L.H of vaporisation - Quanity of energy per unit mass, required to change a liquid into a gas at constant temperature.
       • A vapour can be changed back to a liquid by applying pressure, but gas cannot with P alone.

8. Boyle's Law

   • The volume of a fixed mass of gas is inversely proportional to the pressure exerted on it, provided the temperature is kept constant.
     • Experiment - Long tube, closed at one end and containing air above some oil. The pressure on the oil can be increased by means of a pump, and the pressure exerted on air by oil is indicated by pressure gauge. The volume of air in tube can be measured from a scale behind the tube.
       • Boyle's law breaks if: the gas is close to boiling point or very high pressure used.
   • For a fixed mass of an ideal gas at constant pressure, its volume V is proportional to the ideal gas temperature T (Kelvin).
     • V proportional to T.
     • Ideal gas equation
   • Boltzmann constant
     • The Boltzmann constant is the gas constant for one molecule.