You either have matter or no matter, in which case you have a vacuum. And by matter I mean all the forms of matter: solid, liquid and gas. Liquid and gas together are in a category we call ‘fluid’. So, we have solids and fluids. And where none of these are present, we have a vacuum. Simple enough.
Now, think of how you can send a message to someone. You can shout or you can write down the message on a piece of paper and hand it over to that person. In both cases, you need a medium through which the message can be sent. However you look at it, you need a material medium to send that message: whether it’s through air as your voice travels in the form of sound waves and reaches the other person’s ears or it’s on the sheet of paper. There’s at least another way of sending the message where you don’t have to rely on a material medium to transmit that message: a phone call. Once you’ve dialled the other person’s number, the signal is sent via electromagnetic waves (radio waves, to be precise) from the mobile phone transmitter to the satellite and eventually to the antenna on the other person’s mobile phone. This mode of transmitting a message doesn’t depend on a material medium because the electromagnetic waves can travel in vacuum. Unlike sound, for example, light can travel through empty space. Think of how the light from the Sun reaches us via vast distances of empty space. Light is an electromagnetic wave, just like radio waves. See my blog Spectrum on electromagnetic waves for a broader description of what they are. So, in short, sending a message across can be done both via material and non-material media.
There’s another physical quantity which can be transmitted in a similar fashion: thermal energy. We sometimes refer to that as ‘heat’ but the point here is that this quantity which we often relate to a body’s temperature can be transmitted both via material and non-material media.
If we take the case of material medium then we have two means by which heat can move from a system to another. Either via solids or via fluids as these are everything that make up matter. If we consider the non-material medium, or vacuum, then there’s a third way by which this transfer of thermal energy takes place. In all, therefore, there are three ways by which one system can transfer heat. Let’s look at the first one.
When the heat transfer involves solids then we call it conduction. Now, how does that work? For conduction to work there needs to be physical contact between the two systems in question. Clasping a warm mug in your hands, for example, involves the physical contact between your hands and the mug. This contact then allows heat from the mug to transfer to your hands. It helps to understand what thermal energy is in order to understand how the transfer takes place.
I’ve already described this in my blog on heat but, in short, matter is composed of molecules and atoms. Those atoms vibrate and jiggle about whether they are atoms of solids, liquids or gases. The kinetic energy of these atoms is the energy they possess as a result of them moving about and vibrating and jiggling. Their kinetic energies put together is what we observe as the temperature of that system. So, if our system in question is a mug and the coffee inside it, then the kinetic energy of the molecules of coffee put together is what determines the temperature of the coffee. The greater the kinetic energy of the molecules the higher the temperature. That is, the faster those molecules move about, the hotter the coffee becomes. As these coffee molecules move about and vibrate, they hit against the molecules of the mug. In so doing, they pass on some of their kinetic energy to the mug molecules. Think of it as the sound which emanates from you clapping. Your hands move closer together until they hit each other. Your moving hands possess kinetic energy due to their motion and, as they hit each other, some of this kinetic energy gets converted into sound energy which you hear as the clap. This is an example of how energy gets transferred from one system to another, from one form to another. Likewise, when the coffee molecules hit the mug molecules, kinetic energy is transferred. As the mug molecules accumulate energy they move faster. This increase in their kinetic is what makes the mug’s temperature increase. Now, the mug being in our hands, the mug molecules hit against our molecules on the skin of our palms. Same process of molecules bashing against each happens whereby kinetic energy gets transferred. As the palm molecules gain more kinetic energy, we interpret that as our palms (and hands) warming up. This is how our hands warm up when we hold a mug of hot coffee. It’s all done through continuous and successive collisions among the different molecules. This is how conduction works: molecules collide to transfer kinetic energy which we then observe as heat being transferred from one system to another.
Now you might have noticed that not all materials conduct heat the same way. Some allow heat to transfer much more easily than others. Metals are generally good heat conductors. Plastic or wood, not so much. A cooking pan could be made of aluminium or steel, which are both good heat conductors, whereas the handle of the pan is made of plastic. If you imagine of how thermal energy is transferred by molecules hitting against on each other then you can think of electrons in those molecules or atoms as well. The electrons also contribute to the heat transfer by jiggling about and passing on the kinetic energy to their comrades. In general, metals have many more free and vibrating electrons compared to non-metals. This is also a reason why they are good electrical conductors not just heat conductors. So by having a whole army of molecules and electrons shaking and vibrating, there is a much easier way to pass on the kinetic energy and, therefore, a more pronounced tendency to conduct heat well.
Another interesting observation is that heat generally moves from a system where the temperature is high to a system with low temperature. To put it simply, heat moves from hot to cold. Why should that be the case? If you think of how energetic and restless the molecules of a hot substance is, then they are most likely to be the ones to hit against the molecules of the colder system than the “cold” molecules are to hit on the “hot” ones. In that sense, the direction of heat transfer is most likely to be from hot to cold.
So much for heat transfer by conduction via solids. How about when fluids are involved. How is heat transferred in liquids and gases? We call this process of heat transfer in fluids, convection. It fluids, regions with low temperature are more dense than regions of high temperature. This has to do with the fact that, wherever the temperature is higher than the surroundings, the molecules in that regions are moving faster than average. Not only that, they move further apart from their average position. As a result, they occupy slightly more space than the molecules from colder regions. By occupying a greater space, a greater volume, the density of that region is less than the surrounding, colder regions. And because of buoyancy, the region with smaller density tends to rise and float in a surrounding of higher density. Think of an air bubble in water. That little pocket of air has a smaller density than the surrounding water. And because of buoyancy, the air bubble rises to the surface of the water. Similarly, pockets of hot fluid will rise if surrounded by cooler fluid. By moving to cooler regions, the heat is then distributed across the whole system. As the pocket of hot fluid rises, its temperature decreases the more is distributes its thermal energy to its surroundings. And so, once it’s reached the top, it’s cooled down, its density has increased to be larger than the warmer pockets of fluid beneath it and it therefore sinks back to the bottom. This cycle of upwards and downwards movement of hot pockets of fluid is called convection. It goes on until the whole system is at a uniform temperature.
A hot air balloon rises because the pocket (albeit a large one) of air inside the balloon is kept warmer than the surrounding air so that its density is smaller. This is what makes the hot air balloon rise. This principle is no different from what drives the process of convection in fluids.
The mug containing hot coffee, therefore, has at least two ways in which heat is being transferred. Within the coffee, convection is what is distributing the heat throughout the coffee. Between the coffee and the inner wall of the coffee mug, heat is transferred by conduction. And finally, between the outer wall of the mug and your palms, again conduction is responsible for heat transfer.
Now, you might have noticed that you probably don’t even have to touch the mug to know that its content is hot. Simply by placing your hand close enough to it you can sense the temperature inside the mug. That gap between your hand and the mug is also involved in some heat transfer between the mug and your hand. There is some convection going on there as well as the air around the mug warms up due to the warm mug. This convection relays some of the warmth to your hand. But there is also a different means by which heat is being transferred from the mug to your hand.
The third and final way by which heat can be transferred is called radiation. The warmth from the mug to your hand also reaches you via radiation even if there is no direct contact between your hand and the mug. Convection is also involved, like I mentioned above. What type of radiation is it exactly that transfers heat from one place to another? Well, it’s the same type as the radiation which relays colour from one place to another. In other words, heat (via radiation) and light are from the same type of radiation called the electromagnetic spectrum. Light is the only visible part of the spectrum and is more commonly known as the colours of the rainbow. Heat is the part which comes just before the red portion of the rainbow. As such, we’ve called it the infrared part of the spectrum. Heat, therefore, can be transferred by infrared. We don’t see it but we can detect it nevertheless.
Every body radiates heat, regardless of its temperature. The higher the temperature of the body, the higher the intensity of the radiation emitted from the body. It is by analysing how a body radiates heat that Max Planck came up with the concept of quantised energy. This, eventually, developed into the revolutionary theory of quantum physics. Please see my blog here on quantum physics.
The energy from the sun reaches us in the form of electromagnetic radiation. Two important components of the radiation are the visible part and the infrared part. The first one we relate to as light and the second as heat. This method of heat transfer can be done even in the absence of any material medium. However, it doesn’t mean that a material medium doesn’t have any effect on the radiation. Just like light can bounce off a mirror, heat can also bounce off a suitably reflective surface. Sometimes, the heat that reaches the earth from the sun, bounces off the ground but then gets trapped by the clouds. This trapped heat is a result of what we call the greenhouse effect. Carbon dioxide is a greenhouse gas in that its high concentration in the atmosphere acts as this barrier preventing heat from reflecting off the ground. It is fine to trap some heat because otherwise our planet would be always too cold to live on. The planet Mercury doesn’t have an atmosphere so the side which faces away from the sun gets extremely cold (the average temperature is minus 180 degrees Celsius!) whereas the side which faces the sun can get as hot as 400 degrees Celsius! This wide variation in surface temperature is due to the lack of atmosphere on the planet and that Mercury is the planet closest to the sun.
It is as equally important to understand how heat gets transferred from one system to another as it is to understand how to prevent or minimise heat transfer. So, just as we have good heat conductors (for example metals) we can also investigate bad conductors or insulators. Air is usually a bad conductor of heat but it can allow heat to pass in the form of radiation. In some woollen sweaters you will notice that the knitting pattern is such that there are small gaps between the crochets. You might think that such a sweater would not be a good insulator because it has ‘holes’ in it.
The small gaps in fact work in our favour and are good insulators against body heat loss. It works because those small gaps trap small pockets of air that cannot move about. Hence this minimises any convection that might otherwise arise and cause heat to be transferred from your body to the surroundings. In addition to that, air is not a good heat conductor. As a result, the sweater is minimising heat loss by not allowing convection to happen and by being a bad conductor.
Thus, knowing the different modes of heat transfer and knowing how different materials react to heat, we can use this knowledge to make good heat conductors or good heat insulators. A vacuum flask is another good example of how heat loss is minimised by having poor heat conductors, by preventing convection and by reducing the amount of heat radiating off the content in the flask. Kettles are generally coated with a shiny or glossy layer. Ideally, heat radiators in houses should be black to increase the amount of heat radiation but black radiators do not look as nice as white ones. Air cooling units should be placed near the ceiling, unlike heat radiators, because the warmer air at the top needs to be cooled down. Due to convection currents, hot air rises and cooler air sinks. So it would not be wise or efficient to place the air cooling unit near the floor.
The follow-up to this is figuring out what happens when the different types of materials absorb heat. That is, if something is a good heat conductor then what would happen to it as it conducts the heat. Does its temperature rise? Does it start to melt? Does it change colour? Sometimes the conduction of heat is also related to the conduction of electricity. Some materials are therefore used in that respect. So as you can imagine, the world of thermal energy is pretty vast. Understanding heat and how it affects materials is a whole field of study in physics. For now, though, it is sufficient to appreciate that heat gets around in 3 ways and by understanding those three modes of heat transfer, we can use that knowledge to our advantage while being practical with our designs.