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
What is specific heat capacity?
Energy to increase 1 unit of mass of an object by one degree
some materials have a greater capacity of absorbing thermal heat than others
Equation for specific heat capacity
energy (Joules) = mass (Grams) x Specific het capacity (J G-1 K-1) x change in temperature (°K)
q = m c ^T
Specific heat capacity of water
c = 4200 J / Kg / K or 4.18 J / g / K
What is latent heat of fusion?
The amount of energy needed to melt 1 unit of mass
What is latent heat of evaporation?
The amount of energy needed to evaporate 1 unit of mass
Equation for latent heat
Q = m L
In our experiments for heat capacity, why was our C value much higher than the specific value?
We assumed that all the energy from the hot plate was going to the water, however a lot was lost to the environment, making our Q value higher, which increased everything else
some of the energy was also in the beaker
How could we fix that?
Having a lid
A container that doesn’t absorb energy (wood, styrofoam)
Why is a scald by steam at 100°C much more painful than one by water at 100°C?
Because when the steam converts into water it gives of latent heat. The steam has much more energy than the water
What is the equation for energy using power and time, and units?
power = energy x time
Power = Watts (W) Energy = Joules (J) Time = Seconds (s)
How does volume change when heated/cooled?
Increases when heated
Decreases when cooled
Heat gives energy to vibrate faster, bumping into particles near and moving them too
How does average kinetic energy of particles change when heated/cooled?
Increases when heated
Decreases when cooled
Heating adds kinetic energy (temperature)
How does density change when heated/cooled?
Decreases when heated
Increases when cooled
The volume increases but mass stays the same
Sketch a cooling curve from steam to ice
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Why does the temp of a substance not change when it changes state?
Because the extra energy being added is being used to break the bonds between particles
Diff between boiling and evaporation
boiling - at constant temp, at boiling pt
evaporation - when some particles near the top have enough energy to escape
http://www.bbc.co.uk/schools/gcsebitesize/science/aqa_pre_2011/energy/heatrev1.shtml
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What is a reaction that gives out energy called?
exothermic
What is a reaction that adds energy called?
endothermic
