Simple explanation of a Vacuum Flask
Explain it like I'm Five. Vacuum flasks (or Thermos) keep your drink hot or cold. Heat struggles to leave or enter. How? The vacuum flask holds the liquid in an inner bottle which is surrounded by a vacuum. Heat doesn't get through that vacuum.
What is a vacuum?
A vacuum is a space without air in it, which is a great insulator.
Why Vacuum Flasks?!
Vacuum flasks pop up in Physics exams so often because a science concept is applied to a tool in daily life that we experience outside a lab.
Insulation
Vacuum Flasks work by insulation. So, heat cannot enter or leave through conduction.
- The air trapped in the sponge/foam is a bad conductor of heat. This is good because it means that heat is not lost or gained.
- The plastic case of the flask is not a good conductor of heat, thus reducing heat loss. It provides additional insulation.
The 3 ways thermal energy gets transferred
Thermal energy can be transferred through Conduction, Convection or Radiation.
Conduction - Dipped in boiling tea, metal spoons get hot QUICK. Why? Transferred through a substance with the substance moving itself. The substance is heated and its particles gain energy and vibrate more. The vibrating particles then bump into nearby particles and make them vibrate more. This passes thermal energy through a substance from the hot end to the cold end.
Convection - Transferred through fluids (liquids or gases) by the upward movement of warmer, less dense regions of fluid. The particles in liquids and gases can move from place to place. Convection happens when particles with a lot of thermal energy in a liquid or gas move, and take the place of particles that have less thermal energy. The thermal energy is transferred from hot places to cold places. (E.g. Radiators)
Radiation - Transferred by Infra-Red (IR) Waves. All objects transfer thermal energy by infrared radiation. The hotter an object, the more IR Radiation it gives off. Unlike conduction and convection, no particles are involved. This means that thermal energy transfer by radiation can even work in places with no air (e.g. Space). This is how we can feel the Sun's heat even though it is extremely far away. Infrared cameras, which are used by police helicopters to find a suspect who is trying to hide or flee in the dark, work in the dark because they detect heat (Infra-Red), not the visible light spectrum.
Conduction - Dipped in boiling tea, metal spoons get hot QUICK. Why? Transferred through a substance with the substance moving itself. The substance is heated and its particles gain energy and vibrate more. The vibrating particles then bump into nearby particles and make them vibrate more. This passes thermal energy through a substance from the hot end to the cold end.
Convection - Transferred through fluids (liquids or gases) by the upward movement of warmer, less dense regions of fluid. The particles in liquids and gases can move from place to place. Convection happens when particles with a lot of thermal energy in a liquid or gas move, and take the place of particles that have less thermal energy. The thermal energy is transferred from hot places to cold places. (E.g. Radiators)
Radiation - Transferred by Infra-Red (IR) Waves. All objects transfer thermal energy by infrared radiation. The hotter an object, the more IR Radiation it gives off. Unlike conduction and convection, no particles are involved. This means that thermal energy transfer by radiation can even work in places with no air (e.g. Space). This is how we can feel the Sun's heat even though it is extremely far away. Infrared cameras, which are used by police helicopters to find a suspect who is trying to hide or flee in the dark, work in the dark because they detect heat (Infra-Red), not the visible light spectrum.
How come... Vacuum Flasks will eventually get cold?
This could come up in exams disguised as 'the drawbacks of Vacuum Flasks'. Convection can occur by the hot liquid/solid transferring heat with the trapped air inside the flask. When hot water transfers heat with the cold air inside of the flask, the hot air transfers heat with the cold lid. If the lid is hotter than the surroundings (which is likely), the lid transfers heat with the air molecules around it. However, this only affects the temperature by a small amount as most of the liquid will condense again.
Vacuum Flasks minimise thermal energy transfer
Conduction:
- The pockets of air around the middle of the flask which contains the liquid (the container) are poor conductors, because air is a poor conductor.
- The vacuum prevents conduction from occurring because there is no air (and therefore no atoms for these heat transfers to take place).
- The thinness of the walls stops heat entering or leaving the flask by conduction.
- Air is trapped in areas around the liquid container. Convection is prevented by stopping the air from circulating.
- The vacuum between the container and the flask prevents heat moving by convection.
- The lid prevents convection from taking place.
- The cap stops convection.
- The inside of the liquid container is shiny and not black, preventing heat transfer by radiation. Shiny mirrored surfaces on the inside and outside of the liquid container reflect heat and prevents heat from being lost (good for hot liquids) and reflects heat radiation (good for cold liquids).
- The lid reduces evaporation because the hot liquid isn't exposed to the air.
- Vacuum prevents evaporation.
Positives of Vacuum Flasks
- Radiation can only escape if the lid is taken off.
- The air trapped in the sponge/foam is a bad conductor of heat. This is good because it means that heat is not lost or gained.