Question

we use thermo flask to keep hot liquids . explain its work

Answer 1

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. Heatconduction happens only when things touch.



Artwork: How convection pumps heat through a saucepan of soup. Warming soup (red arrows) swirls upward, while cooling soup (blue arrows) sinks down to take its place. The cooler soup is then warmed by the hot plate beneath and rises up in its turn. Gradually, the soup gets hotter and hotter.

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 byglass with a vacuum inside.