Thermal Energy Flashcards
(40 cards)
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
