phy B1 thermal energy transfers

particulate model of matter

1. matter consists of small particles (atoms/molecules) → interaction (separation, forces and movement) between particles depends on state

2. separation + forces + movement → shape/volume/density/compression

3. internal energy of system of particles consists of KE and intermolecular PE

4. intermolecular PE: intermolecular forces

   • ideal gas negligible since intermolecular forces negligible

   • liquids and solids have negative PE

5. temperature: related to average KE of particles by average E_k = 3/2k_bT (only for monoatomic gases) → total E_k = average E_k X n X N_a

6. different gases have different mass → for same temperature, same avg KE but diff avg speed

thermal equilibrium

• two objects at same temperature

• zero net heat transfers

transfer of thermal energy

• flows from object of higher temperature to lower temperature

• types

  • conduction

  • convection

  • radiation

conduction

• in solids: lattice vibration

  • one end heated, atoms vibrate more vigorously, collide with neighbouring atoms → transfer thermal energy

  • continues until atoms at other end gain energy

• in metals: free electron diffusion (in addition to lattice vibration)

  • electrons move freely, transfer thermal energy

• in gases: poor thermal conductivity because atoms are far apart

• thermal conductivity k: rate of flow of heat (ΔQ/Δt) through a material per unit area (A) per unit temperature gradient (ΔT/Δx where x is length)

  • for two objects next to each other, regardless of temperature, ΔQ/Δt is the same

convection

• due to density changes in fluid when heated

  • expands when heated, less dense, rises → cold is denser, sinks → creates convection current

thermal capacity

• quantity of heat needed to produce a unit rise in temperature in a body (J K^-1)

specific heat capacity

• quantity of heat needed to produce a unit rise in temperature for a unit mass of the material (J kg^-1 K^-1)

• water is around 4.2kJ kg^-1 K^-1

specific latent heat

heat required to change the phase of a unit mass of a substance without a change in temperature → heat converted into intermolecular potential energy

(units J kg^-1)

source of error that affects value of temperature of flame of bunsen burner (other than heat lost to surroundings)

• some water vaporised the instant that copper is transferred. rise in temperature should be higher, calculated temperature of flame is lower than expected.

radiation

• thermal energy transferred via EM waves (mainly IR) → produced by a source due to its temperature

• dark dull bodies emit and absorb radiation better than bright shiny bodies

• black body

  • theoretical perfect absorber/emitter

  • absorbs all radiation of every wavelength falling on it

  • example of setup: enclosure wit dull black interior walls and a small hole → any radiation that enters has low chance of escaping, all radiation will be absorbed after multiple reflections

wien’s displacement law

• as temperature increases, energy emitted in each band of wavelengths increases (brightness increases)

• at each temperature, energy radiated is a maximum for certain wavelength which decreases with increased temperature

inverse square law

• intensity is directly proportional to inverse square of distance from point source