How may thermal energy transfer take place?
- By conduction, convection and radiation
Describe the process of conduction
- Heat makes the particles vibrate more
- Particles collide with each other and pass their kinetic energy along
- Eventually the energy will be spread through the solid
Describe the process of convection
- Heat causes particles to gain kinetic energy
- Particles vibrate and spread out
- Particles become less dense
- Particles rise
- Particles cool and sink setting up a convection current
- Process repeats
White, shiny surfaces reflect radiation
- Dark, matt surfaces absorb radiation
- The hotter the substance, the more radiation that occurs
How does heat transfer occur in a vacuum?
- By radiation only
How does a vacuum flask prevent heat loss?
Shiny walls reflect infra-red radiation
- Plastic lid is a good insulator
- Vacuum prevents heat loss by conduction and convection
How may the rate of energy transfer be reduced at home?
- Loft insulation - fibreglass is a good insulator
- Cavity wall insulation - foam is a good insulator and prevents convection currents being
- Aluminium foil behind radiators - reflects IR
- Double glazed windows - vacuum between glass slows conduction and stops IR
does Energy tends to be transferred away from a hotter object to its cooler surroundings. ?
what is the three different ways for energy transfer by heating?
Energy can be transferred by heating through radiation, conduction or convection.
what is thermal radiation?
Thermal radiation is the transfer of energy by heating by infrared electromagnetic waves
what involves the transfer of energy by particles?
Conduction and convection are energy transfers that involve the transfer of energy by particles.
what is the main form of energy transfer by heating in solids?
Conduction is the main form of energy transfer by heating in solids
what is the main form of energy transfer by heating in liquids and gases?
Convection is the main form of energy transfer by heating in liquids and gases (see the next page).
can emission of thermal radiation occur in solids, liquids and gases?
Emission of thermal radiation occurs in solids, liquids and gases. Any object can both absorb and emit thermal radiation, whether or not conduction or convection are also taking place.
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page 81 section 7
The bigger the temperature difference, the faster energy is transferred between the thermal energy stores of a body and its surroundings.
is termal radiation also known as infrared radtion IR?
Thermal radiation can also be called infrared (IR) radiation, and it consists purely of electromagnetic waves of a certain range of frequencies. It’s next to visible light in the electromagnetic spectrum
does Thermal Radiation Involves Emission of Electromagnetic Waves
1) All objects are continually emitting and absorbing infrared radiation.
2) An object that’s hotter than its surroundings emits more radiation than it absorbs (as it cools down). And an object that’s cooler than its surroundings absorbs more radiation than it emits (as it warms up).
3) You can feel this radiation if you stand near something hot like a fire.
4) Some colours and surfaces absorb and emit radiation better than others —
Energy transfer by radiation happens constantly…
Conduction, convection and radiation are separate processes. But more than one of them can happen at once. So if you’re asked to think about how thermal energy is being transferred away from an object, make sure you think about everything that might be going on, both in the object and in its surroundings.
where does conduction mainly occur in ?
conduction occurs mainly in solids
how does conduction work in a solid?
In a solid, the particles are held tightly together. So when one particle vibrates, it collides with other particles nearby and the vibrations quickly pass from particle to particle.
This process continues throughout the solid and gradually some of the energy is passed all the way through, causing a rise in temperature at the other side of the solid. It’s then usually transferred to the thermal energy stores of the surroundings (or anything else touching the object).
how does thermal conduction work?
Thermal conduction is the process where vibrating particles transfer energy from their kinetic energy store to the kinetic energy stores of neighbouring particles.
1) Attach beads at regular intervals (e.g. every 5 cm) to one half of a long (at least 30 cm) metal bar using wax.
2) Hold the metal bar in a clamp stand. Using a Bunsen burner, heat the side of the bar with no beads attached from the very end.
3) As time goes on, energy is transferred along the bar by conduction and the temperature increases along the rod.
4) The wax holding the beads in place will gradually melt and the beads will fall as the temperature increases, starting with the bead closest to the point of heating. This illustrates conduction.
how does convection of heat work in liquids and gases?
1) Gases and liquids are usually free to move about — and that allows them to transfer energy by convection, which is a much more effective process than conduction.
2) This is how immersion heaters in kettles, hot water tanks and convector heaters work.
3) Convection simply can’t happen in solids because the particles can’t move (apart from vibrating — see page 103).
Convection occurs when the more energetic particles move from a hotter region to a cooler region — and transfer energy as they do.
give an example of how an immersion heater works?
1) Energy is transferred from the heater coils to the thermal energy store of the water by conduction (particle collisions).
2) 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.
3) This reduction in density means that hotter water tends to rise above the denser, cooler water.
4) As the hot water rises, the colder water sinks towards the heater coils.
5) 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 energy through the water.
6) Because the hot water rises (because of the lower density), you only get convection currents in the water above the heater. The water below it stays cold because there’s almost no conduction.
which shape of container is convection more efficient in?
Convection is most efficient in round-ish or square-ish containers, because they allow the convection currents to work best. Shallow, wide containers or tall, thin ones just don’t work quite so well.
CONVECTION CURRENTS are all about CHANGES IN DENSITY.
how You Can See Convection Currents Using Coloured Crystals
1) Place some purple potassium permanganate crystals in a beaker of cold water. Aim to put the crystals to one side of the beaker.
2) Using a Bunsen burner, gently heat the side of the beaker with the crystals at the bottom.
3) As the temperature of the water around the potassium permanganate crystals increases, they begin to dissolve, forming a bright purple solution.
4) This purple solution is carried through the water by convection, and so traces out the path of the convection currents in the beaker.
In convection, particles move from hotter areas to cooler areas… …
so the particles move, taking their energy with them. Don’t get this confused with conduction, where the particles can’t move from their fixed positions, but vibrate to transfer energy to neighbouring particles. Have a flick back to the previous page if you need to remind yourself about conduction.