Unit 3 - Lesson 11: Heat Transfer (Conduction, Convection, and Radiation) Flashcards
Why are fossil fuels bad for the environment?
Fossil fuels produce a lot of CO2. They are also going to run out, so reducing their use is essential.
What three methods can heat be transferred with?
Conduction - Heat transferred between particles in a solid.
Convection - Heat transferred between particles in liquids & gases.
Radiation - Heat transferred by Infrared waves. Can move through solids, liquids & gases.
Some objects emit heat (thermal energy) and others absorb it. Give an example for an object that emits heat and an object that absorbs heat.
An object that emits heat: Radiators.
An object that absorbs heat: Solar Panels.
The bigger the temperature difference is between regions, the (faster/slower) the heat is transferred between the thermal energy stores of the object and its surroundings.
The bigger the temperature difference is between regions, the faster the heat is transferred between the thermal energy stores of the object and its surroundings.
Fill in the blank. Heat during conduction and convection is the result of the …………………….. of molecules (or any particle).
Heat during conduction and convection is the result of the MOVEMENT of molecules (or any particle).
Explain why something large and cold can still have more heat energy than something small and hot.
Heat energy is transferred by conduction or convection through the movement of particles. A large object will have more particles than a smaller one. So, even though each of these particles has less heat energy (because they’re not moving as much), the sheer number of them gives the large object a greater total amount of heat energy.
What type of heat transfer does the statement describe? Convection, radiation or radiation?
The movement of currents (of molecules or atoms) through a liquid or gas.
Convection
Gliders are aircraft that don’t have engines. They work by using natural (convection/conduction). They are pushed up by (hot/cold) air near the Earth’s surface (heated by the sun’s radiation).
Gliders are aircraft that don’t have engines. They work by using natural convection. They are pushed up by hot air near the Earth’s surface (heated by the sun’s radiation).
You’re on a mountain expedition. Your guide has given you hot chocolate in a thermos flask. Explain why the drink stays warm.
A thermos is one bottle inside another. The gap between the two bottles is a vacuum (free space i.e. doesn’t have anything in it). Because there are no molecules in a vacuum, none of the heat can escape the thermos by conduction. This isn’t a perfect system, though, because a little bit of heat radiation does escape. IR radiation can move through a vacuum.
Air is a poor conductor of heat. Use this fact to explain why a big puffer jacket helps keep you warm.
The padding is full of air pockets. Air is a poor conductor of heat, so the heat gets trapped in the jacket (next to you). The pockets of air are small, meaning convection is prevented.
Why do all objects emit heat?
All objects emit heat because their particles are moving. The more movement, the more heat.
Does an object that’s hotter than its surroundings emit more or less IR than it absorbs (as it cools down)?
An object hotter than its surroundings emits MORE IR than it absorbs (as it cools down) .
Does an object that’s colder than its surroundings emit more or less radiation than it absorbs (as it warms up)?
An object colder than its surroundings absorbs MORE radiation than it emits (as it warms up). I.E. It emits less radiation than it absorbs.
What is thermal radiation?
All objects emit heat because their particles are moving. The more movement, the more heat. Some of this heat is emitted as infrared (an EM wave). In a vacuum or space, all of it is transferred by radiation. An object hotter than its surroundings emits more IR than it absorbs (as it cools down). An objects colder than its surroundings absorbs more radiation than it emits (as it warms up). Put your hands in front of a fire or feel the heat from the sun and you’re feeling thermal radiation (IR).
What is conduction?
The process where vibrating particles transfer energy from their kinetic energy store to the kinetic stores of neighbouring particles i.e. heat makes particles vibrate. These particles make particles next to them move.
What does heat travel through when it comes to conduction?
Heat travels through and between touching objects (solids).
Why do metals conduct heat very fast?
Metals conduct heat very fast because the free electrons can easily move. Metals have a unique structure with lots of free electrons (they’re not bound to an atom).
When does convection occur?
Convection occurs when the more energetic particles move from the hotter region to the cooler region and transfer energy as they do so.
Is hot air less or more dense than cool air?
Hot air is less dense than cool air.
What are convection currents?
Hot air rises and cool air sinks down to replace it. These movements are called convection currents and often create a circular motion in a room.
Why is hot air less dense?
Hot air is less dense because it has more kinetic energy. All particles move. The more they move, the further apart they can get so hot air expands. This is how we get things turn from liquid to gas. The particles move so much they break away from the others. Hot air has more energy. Its particles move further apart and the density of the air decreases (there’re less particles per given area of space). As they cool, they get closer together and denser.
Why can’t convection happen in solids?
Because the particles can’t move (apart from vibrating which they do in conduction).
Give examples of convection.
Boiling water in a kettle & heat rising from a radiator or fire.
What’s a cavity wall? What happens if there’s no insulation in the cavity?
A cavity wall is when two walls have a gap in the middle. It can be filled with an insulating material or left as air. If there’s no insulation in the cavity, heat is trapped because air is a bad conductor.
How is heat spread around the cavity?
By convection.
What material can the cavity be filled with?
Glass fibre insulating wool, which also traps air. This is covered in aluminium foil to reflect heat.
What is the glass in the UK specially designed to increase the effect of?
Glass in the UK is specially designed to increase the greenhouse effect (light and UV rays entering the house from the sun, but IR from inside not getting out).
What does double and triple glazing windows do?
It traps heat between the layers of glass.
What do immersion heaters provide?
Hot water for washing, as well as pass hot water through the radiators to then heat the air.
How do immersion heaters work?
- Energy is transferred from the heater coils to the thermal energy store of the water by conduction.
- Particles near the coils are given more energy so they start moving faster. The water expands and gets less dense.
- This reduction in density means hotter water tends to rise above the denser, cooler water.
- As the hot water rises, it displaces (moves) the colder water out of the way, making it sink down towards the heater coils.
- This cold water is then heated by the coils and rises, etc.
Be ready to see convection in diff. types of heater scenarios. The same applies. Hot air rises and cool air sinks to replace it.
What is the movement of heat called?
A convection current.
What is convection most efficient in?
Round or square containers because they allow the convection currents to work best.
What is thermal conductivity?
Thermal Conductivity - Describes how well an object transfers heat energy by the shaking (or vibration) of particles i.e. conduction. Materials (such as metals) with a high thermal conductivity transfer energy between particles quickly.
How does the colour and material of an object affect how heat is reflected and absorbed?
Darker, matt materials absorb radiation, whereas lighter, shiny ones reflect it.
Core Practical - Investigating how well different materials emit heat.
What is a Leslie cube?
What is the method for investigating how well different materials emit heat?
A Leslie cube is a cube, where each vertical face (there’re 4) has a different material on its surface.
1. Place the Leslie cube on a heat proof mat.
2. Fill it with boiling water from the top.
3. Let it heat up a little then hold a thermometer next to each side. All 4 sides should have the same temperature.
4. Use an IR detector as a set distance from one face and record the amount of IR radiation it detects.
5. Repeat step 4 for each face. There should be more IR from the black surface or matt surface.
6. Black matt surfaces are poor heat reflectors. White and shiny surfaces are good heat reflectors.
How can we investigate how well a material absorbs heat, rather than emit it?
Stick ball bearings to the back of 2 different surfaces with wax and see which falls off first when the surfaces are placed equal distance from a Bunsen burner. Black matt surfaces are good heat absorbers. White shiny surfaces are poor absorbers of heat.
Design a lounge. Include an interior and exterior to maximise heat efficiency. Make the house cool in summer & warm in winter by not letting heat in or out. Include:
- Labels for conduction, radiation and convection in action.
- Descriptions of why certain surfaces, materials or designs were used to maximise heat efficiency.
Roof insulated with glass fibre insulating wool.
Double or triple glazing windows to trap heat in the air cavity. It can’t escape easily because air is a poor conductor.
Radiators.
Lots of soft furnishings to trap heat.
Sealed doors, windows and floors. We want to keep convection currents inside the buildings.
Exterior walls painted white, not black.
Convection currents would move up from radiators and other heating objects and travel in currents through the air. The warm currents would rise to the roof and the cool sink down to replace it. Convection currents could leave the roof if it wasn’t properly insulated.
Radiation would be emitted from radiators and the sun.
Conduction would be heat through objects, so walls, windows and furniture.
- You burn your hand on a hot tea mug. What process radiates the heat to your hand?
- Fill in the blank. The bigger the temperature difference, the …… energy is transferred.
- Select the correct option. Hotter water is (less/more) dense than cooler water.
- Fill in the blank. Convection is the movement of heat in fluids and/or …….
- Fill in the blank. Convection currents are changes in ……..
- You add cold milk to hot water and don’t stir. Name a process that allows the milk to still mix into the water.
- Name the only type of heat transfer that does not begin with the letter C.
- Fill in the blank. Infrared radiation is another way of saying t…….. radiation. Add the t to your answer.
- Which mug cools faster? White or black?
- Conduction.
- Faster.
- Less.
- Air.
- Density.
- Convection.
- Radiation.
- Thermal.
- Black
a) Why do we see objects?
b) Do white objects reflect all light?
c) Why do black objects appear black?
d) Do light coloured objects absorb less OR more radiation?
e) Do dark coloured objects absorb less OR more radiation?
f) Does a perfectly black object absorb ALL radiation?
a) We see objects because of the reflection of light.
b) Yes. White is made of all colours of the visible light spectrum.
c) Black objects absorb all light. Because none is reflected back at your eyes, it appears black.
d) Light coloured objects absorb less radiation.
e) Dark coloured objects absorb more radiation.
f) Yes. A perfectly black object absorbs all radiation.
Why will a black object at the same temperature as its surroundings absorb all incident (incoming) radiation & emit the exact same amount?
Objects hotter than their surroundings emit more heat than they absorb; objects that’re cooler than their surrounding absorb more heat than they emit. The RATE they do this depends on their colour. When an object is at the same temperature as its surroundings, radiation is absorbed at the same rate it’s emitted.
What is an object that emits perfectly called?
A black body. I.e. It emits at all wavelengths equally.
Define thermal conduction.
Heat is transferred between particles in a solid by collisions. i.e. the movement of heat through a solid.
Here is an experiment to demonstrate conduction:
1. Attach beads/pins at regular intervals halfway along a metal bar using wax.
2. Hold the metal bar in a clamp stand, and, using a Bunsen burner, heat the side of the bar with no beads.
3. Energy will be transferred along the bar by conduction and the temperature will increase.
4. The beads closest to the heat source should fall off first because the wax holding them in place is melting.
a) Explain why this experiment cannot demonstrate convection.
b) How would you make sure your experiment was accurate?
a) Convection is the result of changes in density. As air or liquid is heated, it becomes less dense & rises. Cool, denser air sinks to replace it. These convection currents need space to move. Heat couldn’t be transferred through the bar as it is a solid & doesn’t change density and thus cannot perform convection because the particles in the solid don’t move enough. However, the heat from the Bunsen burner flame would produce convection currents in the air surrounding the bar.
b) By following the method carefully. EG, making sure the pins are at regular intervals. Also, by using the same amount of wax to stick each of the pins on.
Leslie cubes can be used to show how different materials emit IR radiation. Describe a safety precaution that should be taken when carrying out the Leslie cube experiment.
Be careful with boiling water when pouring into the top of the cube.
A customer isn’t convinced that your suggestion for double glazing will save them money. Explain why it will using the terms conduction, convection, air and conductor.
Double glazing works by trapping heat in a cavity between two layers of glass. The heat is trapped because the space is small and air is a bad conductor, preventing heat flow. As the cavity is small, it dramatically reduces convection currents.
Glass is also a poor conductor, but glass used in windows is made of many fused layers of glass to make it even more insulative.
Here is an experiment to demonstrate convection currents:
OBSERVING CONVECTION CURRENTS USING COLOURED CRYSTALS
1. Place some purple potassium permanganate crystals in a beaker of cold water. It’s best to put 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.
a) A salesperson is trying to sell you an immersion heater for your home. He says it’ll improve energy efficiency, but you’re not convinced. The heater is a triangular shape. Explain why the heater will not be very good.
b) Suggest why potassium permanganate is a good compound to use.
a) Convection is most efficient in round or square containers because they allow the convection currents to work best. A triangular heater will not allow convection currents to flow as freely.
b) It’s purple and so easier to see.
Define convection.
Heat transferred between particle in liquids and gases. As the fluid (fluid is a word to describe liquid or gas) heats, it expands, gets less dense and therefore rises. Cool, denser fluid sinks to replace it.
Give a formula for calculating energy efficiency as a percentage.
Efficiency = Useful Energy Output/Total Energy Output x 100
Describe the principle of conservation of energy.
“Energy is not created or destroyed in any process”
Are ocean currents usually due to radiation, convection or conduction? Explain your answer.
Convection. Ocean currents are often the result of areas of different density water moving by convection currents. Some areas of our oceans are hotter than others for various reasons.
IR radiation is reflected just like light & sound waves. Shiny, white objects reflect radiation very well. Matt, black objects don’t reflect radiation very well. They absorb radiation.
a) Explain why a firefighter might wear a shiny, silver suit.
b) Explain why some solar panels are black and matt.
c) Explain why a cavity wall filler may have a coating of foil.
a) To reflect any heat from a fire, whilst they’re trying to put it out.
b) To absorb more solar (from the sun) IR radiation.
c) The foil is placed on one side of the cavity (facing in towards the room). This means any heat that radiates in to the cavity is bounced back off the foil and doesn’t pass outside.
Explain how wind on your skin can cause convection currents as your sweat evaporates.
Warm sweat evaporates off the skin. It’s warmer than the air it surrounds so it rises. This brings cool air back down to your skin surface to replace it.
You eat food with 120 J of energy. Of this, 90 J helps your body to run, 10 J goes to keeping you warm and the rest is wasted. Draw a Sankey Diagram to represent this.
https://i.pinimg.com/736x/56/79/26/56792671192b4d48ce39e2fffe5f6e5c.jpg
During an on-shore/sea breeze, the land heats up quicker than the sea. Cool dense air from the sea moves to replace the hotter air on land. Explain how off-shore/land breezes work using this information.
At night, land cools down more quickly than the sea. The cool air travels towards the sea to replace the warm air rising up from it.
