Thermal Energy Flashcards
what is heat
form of energy that flows from a region of higher temperature to a region of lower temeprature
SI unit of heat
Joule (J)
definition, SI unit
differences between heat and temperature
heat:
1) refers to the amt. of thermal energy being transferred from hotter to colder regions
2) SI unit is Joule (J)
temperature:
1) refers to how hot or cold an object is
2) SI unit is Kelvin (K)
what is heat
form of energy that flows from a region of higher temperature to a region of lower temperature
SI unit of heat
Joule (J)
definition, SI unit
difference between heat and temperature
heat:
1) refers to the amount of thermal energy being transferred from a hotter region to a colder region
2) SI unit: Joule (J)
temperature:
1) refers to how hot or cold an object is
2) SI unit: Kelvin (K)
why does the liquid level in the glass tube fall initially before rising when heated? (for that 1 famous diagram from this chapter)
glass of test-tube gets heated first, causing it to expand and increase the volume of test-tube, leading to initial fall
after a while, water in test-tube gains heat from ___, causing water to expand and since water expands more than glass, liquid level rises
why does the liquid level in the glass tube rise initially before falling when cooled? (for that 1 famous diagram from this chapter)
glass of test-tube gets cooled first, causing it to contract and decrease the volume of test-tube, leading to initial rise
after a while, water in test-tube loses heat from ___, causing water to contract and since water contracts more than glass, liquid level falls
what is the order of magnitude of expansion and intermolecular forces in solids, liquids and gases
magnitude of expansion:
gases (largest), solids (least)
intermolecular forces:
solids (strongest), gases (weakest)
why do solids expand the least and gases expand the most when heated
intermolecular forces is strongest in solids, followed by liquids then gases
what happens to the volume, mass and density of an object when heated and cooled
when heated, volume increases, mass remains the same, density decreases
when cooled, volume decreases, mass remains the same, density increases
3 points
why does ice float on water
volume of water decreases as temperature decreases towards 4°C
volume starts to increase as temperature decreases from 4°C to 0°C
volume is lowest at 4°C and hence, density is highest
what is the importance of ice floating on water
allows aquatic life to survive during winter as the pond/lake does not freeze completely
negative effects of expansion and contraction + solutions
1) concrete road
2) railway tracks
3) bridges
4) electric cables and telephone wires
5) laboratory glassware
6) water pipes
concrete road:
thermal expansion forces on very hot days will cause roads to crack
provide expansion gaps
railway tracks:
can bend or twist due to expansion during hot weather
provide sliding joints/expansion gaps
bridges:
can buckle under forces of expansion and contraction
build rollers into it/provide expansion gaps
electric cables and telephone wires:
may become taut or snap when contracting in cold weather
string them loosely from pole to pole
laboratory glassware:
when boiling water poured into thick glass, it might crack due to uneven expansion
use pyrex (expands very little when heated)
water pipes:
unusually high pressures and leaks may occur when transporting hot water or steam
build expansion loops or joints into them
describe the 2 benefits of expansion and contraction
1) fixing axel into wheels
cool the axel for it to contract before fitting it into the wheel and allowing it to expand
due to the forces of expansion, there is a very tight fit between the axel and wheel such that it will not come loose easily
2) hot riveting
used to join smaller sheets of steel in ships, and shorter steel bars in cranes and bridges
insert hot rivet through holes of 2 metal pieces which are to be fastened
the other end of the rivet is hammered to form a new head
when rivet cools, it contracts and pulls the metal pieces tightly together
what are bimetallic strips
formed by riveting or welding 2 different metals that expand at different rates together (e.g. brass and iron; brass expands more than iron)
what 3 processes does heat transfer by
conduction
convection
radiation
what is conduction
process of thermal energy transfer through a medium without any flow of the medium
what are good thermal conductors
materials that allow heat to flow through easily and have high thermal conductivity (e.g. silver, gold, copper)
what are thermal insulators (poor thermal conductors)
materials that do not allow heat to flow through easily and have low thermal conductivity (e,g, wood, plastic, glass, vacuum)
a piece of paper around a wooden pole chars while a piece of paper aroud a copper pole does not burn; why?
heat is conducted away from the paper quickly by the metal and the paper around the metal rod never gets hot enough to burn
materials, speed
difference between molecular vibration and free electron diffusion
molecular vibration:
1) applies to all materials
2) slower process
free electron diffusion:
1) applies only to materials with free electrons (e.g. metals)
2) faster process
*free electron diffusion occurs in metals on top of molecular vibration and thus transfers heat easier
3 points
describe molecular vibration
when heated, particles at heated end gain energy and vibrate faster and more vigorously
they collide with neighbouring particles more often, passing some of their energy to them
these neighbouring particles will vibrate faster and more vigorously and the energy is passed along to the next neighbouring particles, and subsequently the rest of the object
3 points
why are liquids and gases poorer conductors of thermal energy relative to solids
solids have particles closely packed together compared to liquids and gases as its intermolecular forces are stronger
liquids and gases are poorer conductors of thermal energy relative to solids for the same volume as there are fewer particles to collide against
because the distance between particles in liquids and gases are larger than solids
2 points
describe free electron diffusion
metals have many free electrons which are free to move about
free electrons gain kinetic energy and move more rapidly; they collide with atoms in cooler parts of the metal and pass on their energy in the process
why is vacuum the best thermal insulator
there are no particles in vacuum and thus no conduction of heat can occur
what is convection
transfer of thermal energy by means of convection currents in a fluid due to a difference in density
4 points
describe convection currents in gases
the gas above the heat source gets heated and expands
as the hot gas is less dense than the surrounding cooler air, it rises
the cooler, denser gas from the surroundings is drawn in downwards to take the place of the hot gas which escaped, and is then heated by the heat source, becoming less dense and rising
circulation of hot gas and cool gas sets up a convection current
(smouldering paper used to make movement of air current visible)
4 points
describe convection currents in liquids
the liquid at the bottom is heated by the heat source and expands
the hot liquid becomes less dense and rises
the cooler, denser surrounding liquid sinks to the bottom and is then heated by the heat source, becoming less dense and rises
circulation of hot liquid and cool air sets up a convection current
(potassium permanganate crystals used to dissolve in water to make movement of water current visible)
2 points (day), 3 points (night)
describe land and sea breeze
day (sea to land):
air over the land becomes warm, expands and rises as its density decreases
cooler, denser air from over the sea blows into the land to take the place of the warm air rising above the land
night (land to sea):
land loses heat more quickly than the sea
air over the sea is warmer and less dense and rises
cooler, denser air from above the land blows out into the sea to take the place of the warm air rising above the sea
how do you eliminate conduction and convection
create a vacuum
3 points
difference between conduction and convection
conduction:
1) occurs mainly in solids
2) heat can flow in any direction
3) energy is transferred through the solid without the solid itself moving
convection:
1) occurs mainly in fluids
2) heat flows upwards
3) hot fluid moves freely and carries energy from one place to another
what is radiation
transfer of thermal energy in the form of infrared radiation without the aid of a medium
1 point
difference between radiation and conduction or convection
can take place in a vacuum and its speed in vacuum is light speed
describe net heat loss (radiation)
when an object is at a higher temperature than its surroundings, it will radiate more infrared radiation than it absorbs, resulting in a net heat loss
describe net heat gain (radiation)
when an object is at a lower temperature than its surroundings, it will absorb more infrared radiation than it emits, resulting in a net heat gain
explain why there is no net heat loss or gain when an object is of the same temperature as its surroundings
rate of emission of infrared radiation is equal to rate of absorption of infrared radiation
what are 3 factors affecting rate of emission/absorption of radiation
1) temperature
higher the temperature, greater the rate of infrared radiation emission
2) surface area
greater the surface area, faster the rate of absorption and emission of infrared radiation
3) nature of the surface
a black and dull surface absorbs infrared radiation very well (good absorber)
a shiny and silvery surface reflects infrared radiation very well (poor absorber)
*if an object is a good absorber of infrared radiation, it is also a good emitter of infrared radiation
*when answering a question, DO NOT describe an object to be BOTH a good emitter and absorber
describe heat transfer of a thermal flask
1) plastic stopper/cap
2) trapped air
3) vacuum
4) shiny/light-coloured outer case
5) silvered walls
6) double glass walls
plastic stopper/cap:
reduce heat gain/loss by conduction as plastic is a poor conductor of heat
reduce heat loss/gain by convection as they prevent surrounding air from getting into contact with fluids in flask
trapped air:
reduce heat gain/loss by conduction as air is a poor conductor of heat
vacuum:
prevents heat gain/loss by conduction/convection as both processes cannot take place in a vacuum
shiny/light-coloured outer case:
reduces heat gain/loss by radiation as shiny/light-coloured surfaces are good reflectors but poor absorbers and radiators of heat
silvered walls:
reduces heat gain/loss by radiation as silvered surfaces are good reflectors but poor absorbers and radiators of heat
double glass walls:
reduce heat gain/loss by conduction as glass is a poor conductor of heat
