魔法瓶の仕組みは？熱を通さない「からっぽ」のすき間があります

How Does a Thermos Keep Your Tea Hot?

On a cold day, you put hot tea in a thermos in the morning.

At lunch, the tea is still hot.

In a normal cup, it would be cold by then.

How does a thermos do this?

Heat moves out of hot things in three main ways.

First, it moves when things touch each other.

This is called conduction.

Second, warm air or water moves and carries heat with it.

This is called convection.

Third, heat travels as light that we cannot see.

This is called radiation.

A thermos has two walls, one inside the other.

Most of the air between the two walls is taken out.

This almost empty space is called a vacuum.

Without air, heat cannot easily move by conduction or convection.

The walls facing the vacuum are also shiny, like a mirror, so they send radiation back.

But there is one weak point: the top.

There is no vacuum at the neck and the opening, so most of the heat escapes there.

That is why you should close the lid tightly.

A thermos works for cold drinks, too.

The vacuum keeps the outside heat from coming in, so your cold water stays cold.

So the next time you drink from a thermos, think about the empty space inside its walls.