Thermal Energy Flashcards
What three ways can heat energy be transferred?
conduction
convection
radiation
describe the matchstick demo
(conduction)
a matchstick is held up to a Bunsen Burner at the temperature of 5000C
the matchstick must be held 1cm away from the Bunsen Burener to ignite
this shows that gases are poor conductors
describe the wood-brass stick demo
(conduction)
a stick which is half wood and half brass is wrapped in paper
the stick is then waved through a bunsen burner
because metals are good at conducting heat, the brass conducts heat away from the centre - the paper does not get too hot and does not turn brown (get burnt)
because non-metals are bad conductors of heat, wood can’t conduct heat away from the centre - the paper gets too hot and turns brown (gets burnt)
describe the Igne-Haaze experiment
(conduction)
this experiment has several rodswith pins stuck on them using wax on a can
boiling hot water is poured into the can
the best conductors fall off the can in chronological order; copper, aluminium..
describe the test tube, ice, metal and gas demo
(conduction)
a test tube with ice at the bottom, a petal peice in water in the middle and a gas (formed when the test tube is heated) at the top
the top of the test tube is 1000C and the bottom is 00C
this shows that water is a poor conductor of heat as the ice did not melt
what is conduction?
the transfer of thermal energy through matter without the obvious movement of the matter itself
energy moves through the matter, but the matter does not move
in solids
are metals (generally) good or poor conductors?
good
are non-metallic substances (generally) good or poor conductors?
poor
list the following metals according to how well they conduct energy (start at the best conductor and end at the poorest conductor)
iron , lead, gold, copper, silver, aluminium
silver
copper
gold
aluminium
iron
lead
what are poor conductors also called?
insulators
what is the best insulator?
a vaccum
no energy can pass through a vacuum by conduction at all
(but energy can pass through a vaccum by radiation)
what is the second best insulator?
air
very little energy can pass through by conduction
give some examples of uses of poor conductors of energy
for handles of things that get hot - e.g. pots and pans, soldering irons, kettle
food containers - e.g. styrofoam containers for hamburgers (styrofoam is a good insulator)
keep foor warm longer by preventing energy from escaping
keep cold food cool onger by preventing energy from outside from coming in
give some examples of good conductors of energy
(things that need to get hot quickly)
cooking pots and pans
tips of soldering irons
electric irons
what is the most efficient transfer of energy?
convection
(although it tends to be on a large scale)
what happens to the particles in conduction?
they vibrate but they cannot move themselves
only energy is transferred
how does conduction work in solids (non-metal)?
in solids particles form a 3-D lattice structure
particles can vibrate but not move around
when a solid is heated the particles vibrate more
this causes them to collide with neighbouring particles
each collision with neighbouring particles shares KE between them
how does conduction work in metal solids (heat being conducted)?
metals have a sea of delocalised electrons
when metals are heated the electrons gain KE and collide with others, sharing their KE
in liquids and especially gases, the particles are further apart than in solids
when heated they have more KE
collisions result in energy being shared
describe how a plastic stopper on a vacuum will keep food hot (or cold)
plastic is an insulator
it will stop or reduce energy loss by conduction
describe how a vacuum will keep food hot (or cold)
you can’t have conduction or convection which prevents heat and energy from escaping or entering as there are no particles inbetween
describe how a thin silvered walls on a vacuum will keep food hot (or cold)
silver prevents heat loss by radiation but the thin walls mean that the heat radiation emmitted off the food/drink in the flask is heated back
describe the square tube filled with water in one corner demo
(convection)
a square-ish/circle tube is filled with water in one corner (starting by the arrow that says ‘heat’)
some purple dying cyrstals are places inside the tube (this does not affect the experiment but allows us to see what’s happening)
a bunsen burner on an orange flame is heated on the lower righ corner of the glass tube
by adding the prescence of heat, the particles move around faster and the spaces between them will expand
this makes the substance less dense
the water moves in the glass tube away from the heat
it hass less density so it goes up and around
explain convection and miners
convection can be used to ensure miners in mines get clean air
a bottom room and two plastic tubes are used to demonstrate this
a lit spplint is held over one tube (opening of mine)
the smoke enters, goes through the bottom compartment, reaches a candle (minera) and out the other tube
clean, cold air will come into the mines and reach the miners
as it gets hot and dirty, it will rise
clean air will fill the space
describe sea breeze
sea breeze occurs during the day
the land heats up faster than the sea
the air above the land is also heated and rises
the cooler air over the sea blows in as a sea breeze
describe land breeze
land breeze occurs during the nights
the land cools faster than the sea
the sea is warmer than the land and the air above the sea rises
the cooler air over tha land blows out as a land breeze
what is convection?
the transfer of thermal energy from one place to another by the movement of the heated material itself
in fluids and gases
what states does convection occur in?
liquids and gases only
not solids
briefly explain how convection works
part of a substance (liquid or gas only) is heated
particles in the substance gain KE and move around faster, making them spread out (expand)
this makes the substance less dense than the cooler substance
the hotter, less dense substance will rise
cooler, denser substance replace the hotter substance which has risen
explain why the heating element of an electric kettle is placed near the bottom
when the kettle is switched on, the water around the heating element is heated by the heating element
beacsue of convection, the warmer water will rise and the colder water sinks
the heating element must be placed at the bottom so that the colder water that sinks can be heated
in this way, all the water gets heated up by convection until it boils
what is radiation?
the transfer of thermal energy through a vacuum in the form of electromagnetic/infra-red waves
what does radiation not involve?
particles
what do all objects do?
absorb and emit (infra-red) radiation
what does the amount of radiation emmited depend on?
the surface area of an object
the temprature difference between the object and the surroundings
the colour/texture of the surface
what surfaces absorb heat easily and are good radiators (or emmiters)?
- dull black
- shiny black
what surfaces absorb heat badly and are poor radiators (or emmiters)?
- shiny silver/white
- dull silver/white
hot tea is poured into two similar containers - one black and the other silver
in which containers will tea cool down faster?
black
beacuse black surfaces emit heat faster than silver ones
Ah Chong and Ah Beng were walking under the hot sun
Ah Chong was wearing a white coloured shirt
Ah Beng was wearing a black coloured shirt
who will feel hotter?
Ah Beng
because black surfaced are good absorbers of heat
what happens in evaporation?
molecules of liquid are converted into gas molecules at a temperature below boiling point of the sunbstance
why do we sweat?
when evaporation takes place the liquid remaining is cooler than it was before
as sweat evaporateds your skin is left feeling cooler
conduction is …… a substance
through
convection is ….. a substance
in
radiation emmited …..
from a substance
where does heat go from your house (rough percentages)
roof
walls
windows
draughts
floors
roof - 25%
walls - 35%
windows - 10%
draughts - 15%
floors - 15%
What seven ways can you insultate a house?
loft insulation
hot water tank jacket
draight excluders
double glazing
cavity wall insulation
thick curtains
carpets/underlay
how does loft insulation work?
a thick layer of fibreglass wool laid out across the loft floor and ceiling reduces heat loss from the house by conduction (air and glass are a poor conductors) and convection (air is trapped)
how do draught excluders insulate a house?
brushes on your letter box prevent cold air entering and hot air escaping
strips of foam and plastic around doors and windows stop draughts of cold air blowing in, i.e. they reduce heat loss due to convection as they blcok currents of air
how does cavity wall insulation insulate a house?
foam squirted into the gap between the bricks stops convection currents being set up in the gap and radiation across the gap
the foam expands and solidifies - it has trapped air in it which prevents convection and conduction
the insulating foam and the air pockets trapped in it (air is an insulator) also reduces heat loss by conduction
how does double glazing insulate a house?
two layers of glass with a narrow air gap between reduce conduction and convection
there are two or thee layers of glass with either air or a vacuum between layers
metal strip enables vacuum
it is sealed so no conduction or convenction can occur
both air and vacuum are poor conductors - prevents (or reduces) conduction and convection
very expensive!
how does carpets/underlay insulate a house?
underlay is another layer under the carpet - extra layer of insulation
carpet is a bad conductor
air paricles trapped between fibres reduce conduction and convection
How do thick curtains insulate a house?
big bits of cloth over the window create an air gap between the room and the window, stopping hot air reaching the glass by convection
they also reduce heat loss by conduction and radiation
How does a hot water tank jacket insulate a house?
lagging such as fibreglass wool reduces conduction and convection
what do hotter object do?
(radiation)
emit more radiation
what is trapped air in terms of insulation?
cheap and effective
what is another term for thermal radiation?
infra-red radiation
how is a fridge insulated to prevent heat from enetering?
insulating materials inside the fridge walls cut down heat entering by conduction
magnetic strip around the door to form a tight seal when the door is closed prevents heat from the outside entering by convection
why is a freezer always places on top inside a fridge?
convection currents can cool down te fridge
the air around the freezer is cooled and sinks
this cold air cools the food in he compartment below
as a result, the air becomes less cold and rises
thus there is convection inside a fridge
what is cost efficiency
the amount of profit or loss you have made over a certain amount of time when you consider the initial outlay and annual saving
what is payback time
the number of years it takes to cover the initial outay
how does temperature affect the motion of particles in a solid?
the hotter the solid, the faster the particles vibrate because thermal energy is converted into kinetic energy
how does temperature affect the motions of particles in a liquid?
when a liquid is heated, the partciles start moving faster because thermal energy is converted into kinetic energy
this means there’s more distance between them - the liquid expands
Why are some metals better conductors than others?
metals with a higher density are better conductors
metals with more delocalised electrons are better conductors because there are a higher proportion of collisions
An object that’s hotter than its surrounding absorbs/emits more radiation that its abosrbs/emits (as it heats up/cools down)
An object that’s hotter than its surrounding emits more radiation that its abosrbs (as it cools down)
An object that’s cooler than its surrounding absorbs/emits more radiation that its abosrbs/emits (as it heats up/cools down)
An object that’s cooler than its surrounding absorbs more radiation that its emits (as it heats up)
The bigger the temperature difference between a body and it’s surroundings, the slower/faster heat is transferred
The bigger the temperature difference between a body and it’s surroundings, the faster heat is transferred
Explain the Immersion Heater example of convection
- Heat energy is transferred form the heater coils to the water by conduction (particle colisions)
- The particles near the coils get more energy, so they start moving around faster
- This means there’s more distance between them, i.e. the water expands and becomes less dense
- This reduction in density means the hotter water tends to rise above the denser, cooler water
- As the hot water rises it displaces the cooler water out of the way, making it sink towards the heater coils
- This cold water is then heated by the coils and rises - and so it goes on. You end up with convection currents going up, round and down, circulating the heat energy through the water
In what type of convection most efficient?
convection is mots efficient in roundish or squarish containers , because they allow the convection currents to work best
shallow, wide containers or tall, thin containers don’t work as well
Explain the radiator example of convection?
heating a room with a radiator relies on convection currents
- Air above the radiator is heates, becomes less desnse and rises
- Warm wair displaces cooler air
- Cool, denser air falls
- Cool air flows to fill the gap left by the rising, heated air
Why can’t convection work in solids?
the particles in solids cannot move so there cannot be a convection current
How do humans reduce their heat transfer?
- In the cold, the hairs on your skin ‘stand up’ to trap a thicker layer of insulating air around the body. This limits the amount of heat loss by convection
- Humans wear layers of clothes to reduce heat transfer. The pockets of air trapped in the clothes between layers mainly reduce heat tranfer by conduction and convection - air is an insulator
- Clothes also reduce heat loss by radiation from the body, as the material absorbs some heat radiated out by our bodies
