Thermal energy is an interaction between two objects or systems of objects, which involves transferring energy from one object to another due to temperature differences. Regarding thermoses, the different modes of heat energy are restricted due to their structure and shape. Still, some of the heat does shift from the thermos after several hours.

Different Modes of Heat Transfer In a Thermos 

 There are four different heat transfer modes: radiation, convection, conduction, and natural convection.


Conduction is the main mode of heat transfer in a thermos. It occurs between two solid bodies that are heated, one solid body being much hotter than the other. In a thermos, this occurs between the container walls and the hot liquid inside. With this heat transfer process, heat is transferred through solid objects to other liquids or slurries. 


Convection is the process of transporting fluids from one area to another. In a nutshell, it is the transfer of heat from one area to another through fluids. In a thermos, convection primarily takes place through the movement of the gases inside the thermos – usually air – which allows for heat insulation and less heat loss from the thermos. 


Radiation is a form of thermal energy emitted from one object to another and spreads out in all directions. Radiation occurs between the hot side of the thermos walls and the colder outside air through convection. Though convection will occur next to the wall, radiation will occur everywhere else.

How does a Thermos Insulate? 

A thermos will insulate its contents from direct heat transfer by being covered in a vacuum or another material. Such materials are usually insulators, which have the ability to stop or slow down heat transfer. These materials are embedded in a thermos flask so that heat cannot transfer well to induce temperature changes quickly.

The double-walled insulation comprises of two insulating layers, which are separated by a vacuum. The vacuum is only present on the inner side, which is used to keep the hot contents. The outer wall is made of metal and the thermos is generally lined with rubber to make it fit snugly in your hands or pockets.

The silver lining of the thermos is made of silver or aluminum. It is usually used for conducting heat away from the inner walls of the thermos. Also, it keeps away electromagnetic waves from passing through. On the other hand, aluminum thermoses are generally lined with rubber to make the thermos last longer.

The internal surfaces of a thermos are lined with plastic to maintain an airtight seal. The lining may also help create insulation since plastics are made with low thermal conductivity.

Lastly, the thermos lid is usually made of plastic which provides insulation by controlling the flow of heat.

For how long can a liquid sustain its temperature in a thermos? 

If the liquid is kept at a constant temperature, then the time taken for the liquid to cool down is proportional to how much heat is transferred to it. A thermos can sustain its internal temperature for many hours with various liquids.

For example, if the thermos is kept at room temperature for the whole day, then the contents of the thermos will be kept at the same temperature in which they were poured in. As a result, thermoses allow for maintaining a fixed temperature for extended periods of time. This is the basic functioning of a thermos. 

Why is Vacuum created in a thermos flask? 

Due to the thermal conductivity of solids, the temperature inside the thermos will rise quickly after adding coffee or any other hot liquid into it. To reduce this temperature change, a vacuum is produced inside the thermos. The vacuum prevents heat transfer through conduction between solid surfaces, thus reducing temperature fluctuation inside the thermos. This is called insulation or insulating effect. Also, it prevents excessive heat loss through the outer walls of the thermos.

Hey! Do You Wanna Buy Best Thermos Then Click Me.


Thermal energy transfer that occurs in a thermos is called conduction, which results from heat flowing from one object to another due to differences in temperatures. Additionally, the thermos insulates the contents from the surroundings by creating an airtight seal with the help of plastic and rubber. The vacuum present inside also helps reduce temperature fluctuation by preventing heat transfer through convection. Radiation is restricted somehow by the silver lining that the thermos entails.

Frequently Asked Questions

Q. How long do the drinks stay hot in a thermos?

A. It depends on several factors such as the liquid’s temperature at the time of sealing, the insulation of the thermos and how often it is opened. If the liquid is in the range of 70-80 °C, it can remain hot for several hours in a completely insulated environment. 

Q. How much does a thermos cost?

A. Thermoses range from $35 to upwards of $100. Plastic bottles can be bought in packs or from grocery stores for much less. If you want a thermos that will last for a long time, you should consider buying the stainless steel ones that cost more but keep the liquid at its desired temperature for a complete day. 

Q. How does Convection occur in a thermos flask? 

A. Convection transfers hot gases from one area to another, which allows for insulation and less heat loss from the thermos. The gas or air inside the thermos moves around while the hot liquid stays stationary in the flask, thus transferring heat to the nearby cooler air. 

Q. What is the function of the silver lining?

A. The silver lining provides conductivity to spread heat away from the inner walls of the thermos. It also prevents radiation, which is a form of thermal energy transfer that occurs through electromagnetic waves.