By explaining the structure and principles of a thermos, specifically how heat is transferred by conduction, convection, and radiation, this course explains the science behind why a thermos keeps drinks hot longer, and in doing so, explores the scientific principles behind everyday objects.
A thermos bottle keeps cold drinks cold and warm drinks warm. These ubiquitous objects first appeared in 1881, over a hundred years ago. The thermos was first designed by Weinhold, then improved by James Dewar in 1892, and is still in use today. It was originally called the Dewar bottle after its inventor, James Dewar.
Thermoses seem like they could hold a lot of beverages, but when you fill them, they don’t hold as much as you think. Why do they hold so little? Let’s take a look at how a thermos protects your beverage, its structure, and the three ways heat is transferred.
First, heat is the flow of energy from a higher-temperature substance to a lower-temperature substance when two objects of different temperatures come into contact. There are three ways this heat can be transferred. Conduction, convection, and radiation, with conduction and convection occurring when there is a medium through which heat can be transferred, and radiation occurring without a medium.
For example, when extinguishing a fire, there are three ways to do it. The first is for firefighters to run over with buckets of water and put out the fire by hand. The second way, if you have enough firefighters, is to line up in a line and pass the buckets around to put out the fire. The final method is to directly extinguish the fire by shooting water from a hose from a fire truck. Think of firefighters as a heat transfer medium and water as the heat. The first and second methods correspond to heat transfer via convection and conduction, respectively. The last method mentioned can be thought of as radiation, where heat is transferred without a medium.
Let’s take a closer look at each of these methods in more detail, and how they relate to the structure of a thermos.
Conduction is the transfer of heat from a higher temperature to a lower temperature without involving the movement of a substance (medium). The measure of how well a material can transfer heat is called its “thermal conductivity” – the smaller the value, the less well the object transfers heat. This is unique to each material and is generally smaller for solids, liquids, and gases, in that order. Solids have more molecules per unit volume than liquids or gases because they are better able to transfer heat through their molecules. Conduction does not occur in a vacuum.
Also, among solids, metals are the best conductors of heat because they have more free electrons, which allows for additional heat conduction. For example, when boiling a pot of stew, a metal ladle gets hot quickly, while a wooden ladle heats up slowly. This happens because the thermal conductivity of metal is much greater than that of wood.
Thermoses make the bottle a double walled structure so that the beverage is not affected by the outside temperature due to conduction. By leaving an empty space between the double structure, it creates a medium-free vacuum to minimize heat loss.
Heat transfer by convection is a phenomenon that usually occurs within fluids, where molecules in a fluid transfer heat through diffusion. When you heat a fluid, the difference in density causes the hotter parts to rise to the top, and the cooler parts fill in their place, creating a circulation and heating the entire fluid. An example of convection is when you turn on a stove and warm air rises to the top and cooler air falls to the bottom, warming the entire room.
A thermos uses a vacuum to prevent heat conduction, which also prevents heat loss due to convection because there is no medium for it to occur.
Finally, radiation is the release of heat from a higher temperature to a lower temperature via electromagnetic waves, without a medium to carry the heat. Because it doesn’t require a medium, radiation is the fastest of the three heat transfer methods. The closest example is solar energy. The sun transfers heat to Earth by emitting radiant energy in the form of infrared, ultraviolet, and visible light. When radiant energy hits an object, some of it is reflected and some is absorbed, causing a change in the object’s temperature. A black surface absorbs energy better, while a bright or shiny surface reflects energy better.
The thermos prevents convection and conduction in a vacuum, but radiation still occurs. To prevent radiation from affecting the temperature of the beverage, the inside of the thermos is plated with polished silver to reflect the radiation. This method is also applied to satellites, which are sensitive to temperature, so their surfaces are plated with silver or gold to block solar radiation.
It’s not just the beverage inside the thermos that allows us to enjoy hot or cold drinks for longer, but also the scientific idea of preventing heat transfer, such as conduction, convection, and radiation. In recent years, with the movement to reduce the use of disposable products, many people are using thermoses to help the environment. The vacuum structure of the thermos is also applied to vacuum insulated windows in homes.
This blog post started with the question, “Why does a thermos only hold a little bit of drink?” It led us to explore the scientific laws related to heat transfer. Wouldn’t it be fun to find out what other scientific principles are hidden in everyday objects?
