TRANSFER OF THERMAL ENERGY
thermal equilbm: when temp diff in any obj, heat will flow from hot to cold region until the 2 regions have equal temp, ie temp grad decreases to zero (the 2 regions w/ same temp: in thermal equilbm)
Mechanism of thermal conduction
(i)Electron diffusion (metals only, need free electrons)
-when there is a temp gradient in metal, conduction electrons gain E at hotter region > diffuse to cold region > E passed through collisions to lattice ions & conduction electrons in cooler region
(ii)Lattice vibration (metals & non-metals, conductors & non-conductors)
-atoms in solid have KE; hot region: vib KE high > E passed to neighbouring molecules w/ less energy due to coupling of atoms by intermolecular forces of attraction
electron diffusion faster: electrons light & move at high speed > E transfer is quick
lattice vib: slow

good elec cond > good heat cond
good heat cond > may or may not be good elec cond

Steady state temp distribution graph

heat escapes from sides of unlagged metal bar
greater heat flow at T_h > larger temp grad at hot end

for perfect heat insulation > no heat escapes through sides > rate of heat flow same at all pts > temp grad const

Q/t = A(T_h – T_c)/x => dQ/dt = -A/dx
: thermal conductivity (higher: more heat conveyed)

Convection
most fluids expand when heated => density decreases > hotter region rises, cooler region falls (due to buoyancy F exerted in hot fluid by surrounding cooler fluid being greater in weight) => form convection currents

Radiation
-flow of heat from one place to another by means of electromagnetic waves (infra-red radn from hot bodies)
-doesn't need matter to transfer heat, can occur in a vacuum
-since heat transfer is by means of em waves, radn has em wave properties
-when radn falls on obj: some radn -reflected, -transmitted, -absorbed (radn absorbed raises obj temp)
-emitted by all objects above abs zero (consists mainly of infra-red, at high temp: visible light + uv emitted)
-rate of emission of radn depends on material, surface, temp (main factor = temp)
-rate ∝ T⁴ (T: abs temp)

Testing radiation absorbing

dull, black surface: better absorber & emitter of radn
Testing radiation emitting

shiny, white surface: better reflector of radn
when surface that is good absorber of radn is hot => good emitter
black body: perfect radn absorption & best possible emitter

rate of radiating E depends on temp, surface, area

Vacuum flask

-conduction: felt pad, glass walls, stopper
-convection: stopper
-radiation: silver surface
vacuum: eliminates heat loss by conduction & convection

The Greenhouse