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- Identify and explain some of the everyday applications and consequences of conduction, convection and radiation.
There are way too many everyday examples and applications to list and explain them all, so I’ll do what I think are the major examples of heat transfer. If they ask you about an unfamiliar example in the exam, you should still be able to explain it using logic and the information that you already know about conduction, convection and radiation.
Heating a room:
A heater warms up the air adjacent to it by conduction. This hot air becomes less dense, causing it to rise. It is replaced by the cold air in the room, which is then heated by the heater, and this rises, and so on. This is convection.
A sunny day:
On a sunny day, if you stand under the light of the sun, you start to feel warm. However, if you stand in the shade, you feel much cooler, despite the sun still being there. This is because the sun transfers some of its warmth to Earth by radiation – using IR waves. All the waves on the electromagnetic spectrum can only travel in straight lines, so you feel most of the warmth when you stand in the light, and when you’re in the shade, you only feel the small amount of warmth carried on the IR waves that have been reflected back towards you.
You might also feel the warmth of objects that are hot due to previously being in the sun. Either way, the warmth you feel is due to radiation.
Cooling a drink with ice:
When you add ice to a drink, heat energy is transferred to the ice from the drink by conduction (convection cannot occur in ice because it is solid). The ice slowly melts, and the cooler water of the melted ice sinks as the warmer water of the drink rises, again losing heat energy to the ice. This is convection. The drink continues to cool down until the ice is fully melted.
Refrigerators:
The cooling unit is usually at the top of the fridge. It cools the air around it, causing it to become denser. This air sinks, and the slightly warmer air at the bottom of the fridge rises. This repeats over and over in a continuous process. Convection occurs.
Remember when we discussed solar panels in Unit P3.2?
Well, solar panels:
In the diagram, Step 1 occurs because of radiation and conduction – the black surface absorbs the sun heat due to radiation, and that heat is transferred to the liquid circulating the solar panel by conduction.
In 3, the warm liquid heats up the water by transferring its heat energy to the water by conduction, and the entire water becomes warm (instead of just the water that the coils touch becoming warm) because of convection.
Dewar or vacuum flasks:
The layer of vacuum (empty space) between the layers of glass in the bulb prevents heat transfer by conduction and convection to the outside of the flask or heat transfer by conduction and convection from the outside of the flask to the inside.
The silvered surface of the double glass bulb ensures that most of the radiation is reflected, reducing heat transfer by radiation too (refer to Unit P6.3 if you want to be reminded of good and bad absorbers and emitters of radiation). The insulated support means that heat cannot be conducted easily via the supports either.
Separating the glass wall from the container surface also ensures that heat can’t be transferred from the glass bulb to the container surface by conduction.
Plastic is a bad conductor, so plastic lids will reduce heat transfer by conduction.
The lid is also tight, making the flask airtight, so air cannot carry heat into or out of the flask by convection.
There is a small mistake in the diagram – it is the glass bulb that is double walled (this allows the vacuum to be created between the walls), not the container of the flask.
Greenhouse walls:
Greenhouses are made of glass because they allow radiation to pass through it, but the shiny nature of glass also means that it reflects a lot of radiation.
Glass especially reflects IR radiation quite well, although it does let some through.
The sun’s light and heat enter the greenhouse through the glass, warming up everything inside. These things then radiate heat by IR waves, which are reflected by the greenhouse glass. This causes the inside of the greenhouse to stay warm.
The presence of walls also reduces heat loss by convection.
So in effect, the glass minimises heat loss by minimising the effect of both radiation and convection, allowing greenhouses to stay warmer longer, and allowing them to retain some warmth at night.
Notes submitted by Sarah.
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