9 Physics - Energy and Matter Flashcards
What is thermal expansion?
When you heat up something the particles speed up, the vibrations take up more space, making it expand in all directions.
Given the same temperature, which state expands the most?
Gases expand the most, then liquids, then solids
Why does a gas not necesarilly expand when heated?
The volume of a gas depends on the container it is in
Give real life examples of expansion
Gap left on a bridge
Glass dish put in an oven would crack
Thermometer
Bimetal strip (pg 105)
Water when cooled (solidified) expands (pg 105)
Bimetal thermostat (when temp rises, the bimetal strip bends and the contacts separate, current to heater is cut off, when temp. falls the opposite happens)
Real life examples of contraction
Overhead cables are left slack, to allow for contraction that would happen on a cold day
How are gas temperature and volume related?
At a fixed pressure, as the temp increases, the volume increases
directly proportional
How does heat travel through solids?
Thermal conduction - one end of an object is heated, the heat is transferred to the colder end, as faster particles bump onto the slower ones, making them move
What are poor conductors called?
insulators
Give examples of conductors and insulators
Conductors:
-Metals
-Silicon
Graphite
Insulators:
- Glass
- Water
- Plastics
- Rubber
- Wood
- Materials containing trapped air (wool, plastic foam, polystyrene)
Examples of conduction in real life
- A pan might have a copper bottom but a plastic handle so you don’t burn your hand
- Birds fluff up their feathers to trap more air
- It´s safer to pick hot dishes with a dry cloth than a wet one because it has air trapped in it
Insulating materials in a house
- Plastic foam lagging round a hot water storage tank
- Glass or mineral wool insulation in the loft
- Wall cavity filled with plastic foam, or wool
- Double-glazed windows: 2 sheets of glass with air in between
How does heat travel through liquids and gases?
Convection
Experiments of convection in liquid and gas
Liquid: Potassium permanganate in one side of beaker filled with water, heat that side of beaker, p.p. moves up and to the other side
Gas: In a box, have cold ice in one side and burning tissue and warm rocks on the other, smoke will move up, to the cold side, then down, and repeat
Real life examples
Room with radiator
Windy by the sea
Hot water system
Refrigerator
Why is it windy by the sea?
During the day the sand is hotter, the war air rises (less dense), it moves towards the sea (less pressure), cools again (more dense), falls, creating wind towards coast
How is heat that travels through a vacuum called?
Thermal radiation
Mainly infrared waves
What are surfaces that take in or send out thermal radiation called?
Sending out : emitters
Taking in : absorbers
Good emitters are also good absorbers
Which surfaces are good/bad emitters/absorbers?
MATT BLACK WHITE SILVER
emitters best———————————-worst
reflectors worst———————————-best
absorbers best———————————-worst
(note that silver is a bad emitter and absorber because it reflects most of the thermal radiation away, while matt doesn’t)
How do you compare absorbers?
2 metal plates with diff surfaces are placed at the same distance from a radiant heater. To find which absorbs more, measure the rise in temperature
(pg. 112)
How do you compare emitters?
Metal cube filled with boiling water, heats the surfaces to the same temp. Thermal radiation detector is placed in turn at the same distance from each surface and the metre readings are compared
How does a greenhouse work?
The suns´s thermal radiation passes easily through glass or plastic, the ground inside warms up and heats the air. Hot air cannot escape
Real life examples of thermal radiation
- If you live in a desert you would want a white or silver car
- Radiators are painted white to look nicer but matt black would be a better choice as it is good at emitting heat
- Bears have black skin because it absorbs heat well, and white fur because it is a poor emitter, thick fur also traps air(insulator)
- Our body gives energy by radiation
- Sun heats the earth
Consequences of heat transfer
Convection: -diff temps in an oven, uncooked food
Convection: -can lead to burning
Radiation: heat lost to environment, requires constant source of energy
Example of all conduction, convection, and radiation
Bay Marie (hot water bath)
- Radiation: fire heats the pan
- Conduction: heat is absorbed by the pan
- Convection: heat from pan goes to the water, through air, and to the food
