9 — Thermal processes Flashcards
Thermal equilibrium
Thermal equilibrium describes a state in which 2 or more subjects hv the same tempt n there is no net transfer of energy betw them.
Conduction
Conduction is a process of energy transfer where energy is transferred thru the passing on of vibrational motion from one particle to another without any flow of the material medium.
Good thermal conductors
R materials that can transfer energy quickly by conduction
Thermal insulators
R materials that transfer energy slow by conduction
Convection
is a process of energy transfer by means of convection currents of a fluid (liquid or gas) due to a difference in density
Radiation
Is the process of energy transfer by electromagnetic waves. It does not require a medium.
Applications of conduction in non-metals
Hairdryers, instant water heaters, thermal wear, double glazed windows
Applications of convection
Electric kettles, hot air balloons, sea breeze, land breeze
Applications of radiation
Greenhouses, space blankets
Energy transfer answer formula
X at a higher tempt than y. Energy is transferred from x to y. As energy in internal store of x decreases, tempt of x decreases and energy in internal store of y increases, tempt of y increases to the same as x; thermal equilibrium is reached.
Convection Answ formula
Liquid/air expands due to heating via conduction -> warmer, less dense, rises to top -> surrounding denser n cooler liquid sinks in place of rising liquid ->convection currents carry warmer air/liquid away from flame and cooler air towards it.
Conduction in metals Answ formula
Metals contain delocalised free-moving electrons, transferring energy via collision w other electrons n atoms, to _.
Conduction in non-metals Answ formula
When heated, particles of substance near heat source gains heat, is more energetic, vibrate more vigorously about their fixed positions, transfers thermal energy thru collision w neighbouring particles to the other end of substance.
Convection in liquids
Liquid near heat source heated -> volume increases -> warm water less dense and rises -> cooler water sinks and gets heated by heat source -> convection current established, allowing energy transfer throughout the substance.
Name characteristics of objects that r good emitters and absorbers of radiation.
S. Tempt: high
S. Colour & texture: black, rough surfaces
S. Area: large
Name characteristics of good reflectors of radiation
S. Tempt: low
S. Colour & texture: shiny, silver, smooth surface
S. Area: small
Metal and rubber r good conductors of heat since they’re both solids. Dya?
No. While heat is being conducted thru vibrations n collisions of molecules in both solids, heat is conducted much faster in metals due to free-moving electrons. Rate of conduction is much faster in metals than in solids -> metals r btr conductors of heat.
Metal + wooden rod rolled in paper heated by a flame. Describe wut will happen.
Paper attached to wooden rod will be burnt but not the one attached to metal rod. The wooden rod is a poor conductor and will not be able to conduct heat quickly enuf before the paper is burnt. Conversely, the metal rod can conduct heat away from the paper b4 it is burnt since it is a better conductor of heat.
Explain why you can hold your fingers beside candle flame without harm but not directly over flame.
Air surrounding flame heats up, expands and density decreases, rise up and finger above candle is scalded. However, beside the flame, convection current is established, constantly cooling the air surrounding flame, preventing us from scalding.
Ice at bottom of beaker containing water. Explain w reasons where tempt will be higher.
Tempt of water will be higher at the top. Heat is being conducted away from water at the bottom of cylinder to ice. Cooler and denser water remains at the bottom, thus no convection occurs. Thus water at top have a higher tempt than bottom.
Heat radiation affecting tempt change
Blac surface is a better absorber of radiation, polished surface is a better reflector of heat. Thus, black x absorbs more heat which translates to a higher tempt change.
Explain features of solar heating panel that maximise heating of water
Metal tubes r good conductors of heat
Black surface of tubes is good absorber of radiation
Wide arrangement of pipes increase s.a. -> more heat absorbed.
Why is it easier to compress a gas than solid
Molecules of gas r further apart and intermolecular forces of attraction r weaker -> more distance for molecules to move closer tgt.
Evidence for motion of gas
Brownian motion. Smoke particles in constant and random motion.
State why thermal energy is transferred by conduction thru solids
Energy transfer by conduction occurs most effectively thru solids as particles r closely packed tgt in fixed positions.
Lower heat capacity vs higher heat capacity graph
Steeper gradient for tempt rise: lower heat capacity
For the same mass, same period of time, Y takes a longer time to decrease in temperature than Z. This indicates that a higher amount of energy needs to be lost by Y compared to Z for the same amount of fall in temperature.
