Thermal physics is a huge topic in secondary school and JC physics that consists of concepts like transfer of thermal energy and thermal properties of matter. Often, students trip up because there are just so many concepts to remember, including formulae and application questions!
One of the biggest challenges when tackling thermal physics questions, especially in the realm of transfer of thermal energy, are the application questions. Knowing the concepts by heart isn’t enough – you need to truly understand it and recognise how it plays out in different real-life situations.
While some people can easily answer application questions using their ‘common sense’ and general knowledge, it is also worthwhile to be exposed to as many different types of scenarios so that you don’t get caught by surprise during the exams!
To help you with this, we will be running through multiple examples relating to transfer of thermal energy, including example questions and explanations.
Conduction
Fluffy animals
Wool, thick fur, and feathers are nature’s clothing for animals, keeping them warm even during cold days. These fibres need to be fluffy as the trapped air acts as an insulator, reducing heat loss from the body.
Materials of cooking utensils
Cooking pots and pans usually have bodies made of metal (e.g. aluminum, iron, steel), and handles made of plastic or silicone. The base material needs to be a good conductor so that it can heat up quickly to cook the food. The handles are covered with an insulator of heat, so that the user will not be burnt by handling the cookware.
Think: When we want to boil a beaker of water, why do we place a metal gauze over the Bunsen burner before placing the beaker over it?
Convection
Placement of air-conditioning units
Notice how air-cons tend to be placed near the ceiling? This makes sense because dense, cool air sinks, circulating cool air around the room, while warmer air will rise and be cooled by the air conditioner. If the air-con is placed low in the room, the top of the room will remain warm as the cool air does not rise up. This principle is also the reason why heaters (in other countries) are usually placed near the ground.
Coastal breeze
Those who have spent a lot of time near the coast will realise that the direction of the breeze changes according to the time of the day.
In the day, when the surface temperature of the land is warmer than that of the sea, the warm air above land will rise. Cool air from above the sea will flow in to fill the space over land, forming a breeze blowing towards the land (“sea breeze”).
At night, the reverse occurs as the sea is warmer than land. As such, warm air over the sea rises, and cool air from the land flows towards the sea, resulting in “land breeze”.
Convection rainfall
Imagine a day that starts off hot and sweltering. But soon, a huge thunderstorm seems to come out of nowhere!
This is actually a common phenomenon known as convection rainfall. On a hot and humid day, the warm, moist air near the ground rises up to the upper atmosphere, which is cooler. There, the water vapour in the air condenses and finally precipitates as rain.
Radiation
Greenhouse
A greenhouse is used to create a warm environment for plants to grow in an otherwise cool climate.
Usually, the roof is made of glass, which allows shorter wavelengths of radiation from sunlight to enter, but prevents longer wavelengths of radiation from the plants to escape. Thus, heat is trapped within the greenhouse, keeping the temperature at an equilibrium which is warm.
Vacuum flask
Hot drinks can remain hot in a vacuum flask for many hours – this is achieved by a combination of design factors that reduce heat loss from the liquid to its surroundings.
The inner wall of the vacuum flask is shiny, reducing the rate of heat loss by radiation to the walls of the flask.
The vacuumed area (no air) between the double walls of the flask prevents heat loss by conduction and convection.
Wrapping food with aluminium foil
Did you know that you are supposed to wrap food with aluminium foil with the shiny side inside?
For cooking, the dull side should be facing outwards as the dull surface absorbs radiation at a faster rate, cooking the food at a quicker rate. The shiny surface inside reduces loss of heat by radiation and keeps food warm for longer.
Think: Why are astronaut suits usually white in colour?
Factors affecting rate of heat transfer
We hope the above examples will familiarise you with some of the type of application questions you would encounter in your exams! While you can memorise some of them, you also need to know how to recognise and tackle novel situations.
For example, you should look out for keywords like mention of colour (is it about radiation?), temperature difference (is it about convection?), or material (is it about conduction?). Knowing how the rate of heat transfer is affected by these factors will help you tremendously!
Below is a quick review for your convenience!
Heat conductor:
- Metals are good conductors (heat up quickly, and cool down quickly)
- Non-metals are usually poor conductors (heat up slowly, but retains heat for a longer time)
Convection:
- Hot air (less dense) rises, cool air (more dense) sinks
Factors affecting rate of heat transfer by radiation:
- Colour
- Black surfaces are good absorbers and emitters
- White surfaces are poor absorbers and emitters
- Texture
- Rough or matte surfaces are good absorbers
- Smooth, polished surfaces are poor absorbers and emitters (good reflectors)
- Surface temperature – the higher the temperature of the surface, the more energy it radiates
- Surface area – the higher the surface area, the more energy it radiates
Conclusion
Thermal physics is interesting and varied because of the diverse types of applications there are! Many of them are also commonplace in our daily lives, so you can easily relate them to what you know and see daily.
Hopefully, these examples will build up your bank of examples and applications of thermal energy transfer, so you won’t be stumped when you see them in your exam!
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