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Physics Lisa Payne
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Draw a neat diagram of a thermos flask and explain how it is possible to keep hot things hot and cold things cold in it for a long time.​

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Answer:Humans can be pretty contrary at the best of times. When it's cold, we want to warm up; when it's hot, we want to cool down. That's because we're warm-blooded creatures who need to keep our body temperatures more or less constant, at around 37°C (98.6°F), just to survive. Vacuum flasks are a bit like people in this respect: they like to keep things at steady temperatures. If you put hot drinks in them, they keep them hot; if you put cold drinks in them, they keep them cool. They're simple, neat, and effective—but how exactly do they work?

Photo: A typical Thermos® vacuum flask. Vacuum flasks are widely known as "Thermos" flasks for the German company, founded by Reinhold Burger, that commercialized the technology in 1904.


How heat travels

Why your coffee goes cold

How vacuum flasks work

Inside a vacuum flask

Find out more

How heat travels

A scientist wearing blue protective gloves pours liquid nitrogen from a gray Dewar flask

Photo: Scientists also use vacuum flasks, but they tend to call them Dewar flasks or Dewar bottles. That's because the idea was originally conceived in the early 1890s by a Scottish scientist named Sir James Dewar (1842–1923). According to John Rowlinson's biography (see references below), Dewar used his flasks only for keeping laboratory chemicals cold, didn't anticipate the huge commercial market for keeping drinks hot, and consequently never patented the idea! In this photo, a NASA scientist is pouring very cold liquid nitrogen from a Dewar. Photo by Tom Tschida courtesy of NASA.

Before we can understand why flasks are so fantastic, we need to understand a bit more about how heat travels.

Heat is a kind of energy that moves around our world in three different ways called conduction, convection, and radiation. If you touch something hot, heat flows straight into your body because there's a direct connection between you and the hot object. Heat conduction happens only when things touch.

Convection, on the other hand, can happen without the need for direct contact. If you switch on a fan heater, it blows hot air through a grille into your room. Hot air is less dense (lighter, effectively) than cold air so it rises upwards. As hot air starts to climb up from a fan heater, it has to push colder air out of its way. So the cooler air near the ceiling of your room moves back toward the floor to get out of the way. Pretty soon, there's a kind of invisible conveyor belt of warming, rising air and cooling falling air and this gradually warms up the room. When heat moves in this way, using a moving liquid or gas to travel from one place to another, we call it convection. Heating soup in a saucepan is another way of using convection.

Radiation is slightly different again from conduction and convection. When objects are hot, they give off light. That's why camp fires glow red, orange, and yellow. This happens because the atoms in hot objects become "excited" and unstable when they gain extra heat energy from the fire. Since they're unstable, the atoms quickly return to their normal state—and give off the energy they had as light. (Read more about how and why this happens in our longer article about light.) Sometimes we can see the light that atoms produce and sometimes not. If the light they produce is just a bit too red for our eyes to see, it's called infrared radiation and, rather than seeing it, we feel it as heat. You can feel the infrared given off by hot objects even if you're not touching them (so there's no conduction) and there's no air or liquid present to carry heat either (so there's no convection). Radiation explains why we can feel heat coming from old-style, incandescent lamps even though they're surrounded by glass with a vacuum inside.

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