2.3 Transfer of Thermal Energy (Cambridge (CIE) O Level Physics): Flashcards
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What is meant by a good thermal conductor?
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What is meant by a good thermal conductor?
A good thermal conductor is a solid which easily transfers heat.
Define a thermal insulator.
A thermal insulator (bad thermal conductor) is a solid which does not transfer heat well.
Almost all ______ are good thermal conductors.
Almost all metals are good thermal conductors.
Give two examples of materials that are bad thermal conductors.
Examples of bad thermal conductors include a wool blanket and layers of cardboard or paper. Non-metals and organic materials (from plants or animals) are often poor thermal conductors.
Why does a bare foot placed on a tile feel colder than a foot placed on a rug, even though both are at the same temperature?
The tile is a better thermal conductor, so it transfers heat away from the foot faster, making the foot feel cold. The rug is a poor conductor (insulator), so it does not transfer heat away from the foot as quickly.
A rod, half wood and half metal, is wrapped in paper and heated at the joint. Why does the paper touching the metal stay undamaged, while the paper touching the wood chars?
Metal is a good conductor, so heat is conducted away from the paper into the metal, keeping the paper cool. Wood is a good insulator (poor conductor), so heat is not conducted away from the paper, allowing it to get hot enough to char.
Ball bearings are stuck with wax to four different metal strips at equal distances from a centrally heated point. How does this apparatus allow the relative thermal conductivities of the strips to be compared?
The strips are heated at the centre. Heat is conducted along each strip at a different rate depending on the metal. The wax on the better conductor melts sooner, dropping its ball bearing faster. Timing how long each ball bearing takes to drop ranks the metals by thermal conductivity.
Define conduction.
The transfer of thermal energy through a substance (mainly solids) from hotter to cooler regions, as particles vibrate and collide (and, in metals, free electrons collide too), without the particles permanently changing position.
Define free electrons.
Electrons in a metal that are not bound to a particular atom, allowing them to move throughout the structure and transfer energy by colliding with atoms and ions.
Describe how atomic vibrations transfer thermal energy through a solid (a process that occurs in both metals and non-metals).
Atoms/ions at the hotter end vibrate more than those at the cooler end. As they vibrate, they collide with neighbouring atoms/ions, transferring energy from atom to atom, until thermal equilibrium is reached throughout the substance.
Why do metals conduct thermal energy faster than non-metals?
Metals contain free electrons, which vibrate and move more at the hotter end and collide with atoms and ions, transferring extra energy. This happens in addition to atomic vibration, speeding up conduction.
When two solids of different temperatures are in contact, thermal energy is transferred from the ______ object to the cooler object.
When two solids of different temperatures are in contact, thermal energy is transferred from the hotter object to the cooler object.
True or False?
Conduction is the main method of thermal energy transfer in liquids and gases.
False.
Conduction is the main method of thermal energy transfer in solids. In liquids and gases, convection is the main method.
Define convection.
The transfer of thermal energy through a fluid (a liquid or a gas), by the movement of the fluid itself, as warmer, less dense regions rise and cooler, denser regions sink.
Explain, in terms of density, why a heated fluid rises (for example above a radiator near the floor).
Heating makes the molecules push apart, so the fluid expands and becomes less dense than the surrounding fluid. The warmer, less dense fluid rises, and cooler fluid moves in to take its place, setting up a convection current.
Explain, in terms of density, what happens when a fluid is cooled (for example by an air conditioning unit high on a wall).
The molecules move closer together, so the fluid contracts and becomes more dense than the surrounding fluid. The cooler, denser fluid falls, displacing warmer fluid, which is then cooled in turn — setting up a convection current.
How is potassium permanganate (purple dye) used to demonstrate a convection current in a beaker of water?
Crystals of potassium permanganate are placed to one side of the beaker. When that side is heated, the crystals dissolve, and the coloured water rises with the warmed water, tracing out the path of the convection current.
A convection current forms because heated fluid becomes ______ dense and rises, while cooler fluid sinks to take its place.
A convection current forms because heated fluid becomes less dense and rises, while cooler fluid sinks to take its place.
True or False?
Convection can occur in solids.
False.
Convection can only occur in fluids (liquids and gases) because it relies on particles being free to move from place to place; particles in a solid are not free to move like this.
What is wrong with the statement "heat rises" when describing convection?
Heat itself does not rise — it is the hot gas or liquid that rises, because heating makes it expand and become less dense than its surroundings.
Define thermal radiation.
The transfer of thermal energy by electromagnetic waves in the infrared part of the spectrum. All objects emit thermal radiation, and hotter objects emit more.
Define thermal equilibrium.
The state reached when a body absorbs and emits thermal radiation at the same rate, so its temperature remains constant.
What three factors affect the amount of thermal radiation emitted by a surface?
Its temperature (hotter = more), its colour and texture (darker, matt = more; shinier, lighter = less), and its surface area (greater surface area = more).
Why are black, matt surfaces good absorbers and good emitters of thermal radiation?
Their colour and texture make them efficient at absorbing incoming radiation, and once warmed they are equally efficient at emitting it — this is why laptop chargers and car radiators are often coloured black, to help them cool down.
Why do shiny surfaces make poor absorbers and poor emitters of thermal radiation?
Shiny surfaces reflect most incident radiation rather than absorbing it, and correspondingly emit very little themselves — this is why they take longer to cool down than a dark, matt surface.
A body will heat up if it absorbs thermal radiation at a ______ rate than it emits it.
A body will heat up if it absorbs thermal radiation at a greater rate than it emits it.
True or False?
Thermal radiation is the only method of thermal energy transfer that can occur through a vacuum.
True.
Thermal radiation travels as electromagnetic waves and does not need a medium of particles — this is how thermal energy reaches the Earth from the Sun through the vacuum of space.
In the experiment investigating thermal radiation from flasks of different colours, what is the independent variable and what is the dependent variable?
Independent variable: the colour of the flask surface (black, dull grey, white, silver). Dependent variable: the temperature of the water in each flask, measured over time.
Give three variables that must be controlled in the experiment investigating how surface colour affects radiation of heat.
Any three of: identical flasks (except for colour), the same volume of hot water in each flask, the same starting temperature of the water, and the same time interval between readings.
How does this experiment show that any difference in cooling rate between the flasks is due to thermal radiation, rather than conduction or convection?
Heat loss by conduction and convection is the same for every flask, since only the surface colour differs between them. Therefore, any difference in the rate of cooling between the flasks must be due to differences in radiation.
In this experiment, what determines the wavelength of the thermal radiation emitted by each flask, and what determines the amount (intensity) emitted?
Only the temperature of the flask's contents determines the wavelength distribution of the radiation emitted. The flask's surface area and colour affect the amount (intensity) of radiation emitted, not its wavelength.
Give two ways of reducing error when carrying out this experiment.
Any two of: make sure all flasks start at the same temperature; use a data logger connected to a digital thermometer for more accurate readings; carry out the experiment in pairs, so that starting the stopwatch and inserting the thermometer can be coordinated.
Why is a heatproof mat placed under each flask in this experiment?
To protect the surface underneath and prevent heat loss from the bottom of the flask, which would otherwise introduce an error unrelated to the surface colour being tested.
In this experiment, four identical flasks are painted ______, dull grey, white and silver to compare how their surface affects radiation of heat.
In this experiment, four identical flasks are painted black, dull grey, white and silver to compare how their surface affects radiation of heat.
True or False?
In this experiment, the thermometer used has a resolution of 0.1 °C.
False.
The thermometer has a resolution of 1 °C. (The stopwatch used has a resolution of 0.01 s.)
Why does a shiny, light-coloured emergency blanket help to keep an accident survivor warm?
A light, shiny surface emits much less infrared radiation than a dark, matt surface. Using a shiny blanket means less thermal radiation is lost from the patient's body, helping to keep them warm.
A mug of hot tea loses thermal energy by conduction, convection and radiation at the same time. Identify where each process occurs.
Conduction: from the tea to the sides of the (solid) cup, and from the cup to the surface it rests on. Convection: from the surface of the tea to the air directly above it. Radiation: from the hot sides of the cup in all directions to the surrounding air.
Explain why fibreglass loft insulation reduces the rate at which a house loses thermal energy.
Air trapped between the glass fibres is a poor conductor, and because it is trapped it cannot form convection currents — giving the insulation a much lower thermal conductivity than the roof material. Using several thick layers further decreases the rate of cooling.
What does it mean for a cooling drink to reach thermal equilibrium with its surroundings?
Thermal equilibrium is the state reached when the drink, its cup, and the surrounding air have all reached the same temperature, so there is no further net transfer of thermal energy between them.
Trapping air between the fibres of loft insulation prevents it from circulating, which stops heat loss by ______.
Trapping air between the fibres of loft insulation prevents it from circulating, which stops heat loss by convection.
True or False?
In real situations, thermal energy is usually transferred by only one process (conduction, convection or radiation) at a time.
False.
In practice, all three processes usually occur at once, transferring energy from hotter to cooler regions — for example, a hot drink cools by conduction, convection and radiation simultaneously.
Give three everyday examples of thermal energy transfer by radiation.
Examples of thermal energy transfer by radiation:
Heating from sunlight
Using an infrared thermometer to measure temperature
Using a thermal imaging camera
Using night vision
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