熱是有形還是無形？..

熱（heat）是無形 - not a material
It can flow and being transferred.
e.g. When it is hot you sweat, then heat energy is transferred to air through vaporization. That is a kind of heat conduction (傳導). How? Heat is conducted through sweat (water) from your body to air.

Heat can be transferred through radiation and convection too.

Radiation (輻射)
e.g. Solar energy (sun)
Heat transfer to earth and you through air and space no conductor (導體) in between.

Convection (對流)
e.g. Heater (電暖爐)
You will feel the heat energy when a heater is turned on in a room.

Heat is energy. It is basically the kinetic energy of particles.
so it is 無形.

heat can comes from heat from somewhere else (eg. you heat up your food with fire)
or when you try to "move" the particles with other type of energy
(eg. light bulb is hot and microwave can vibrate the particles of food)

In technical terms or in books, we generally describe heat transfer by three different ways
1) conduction
2) convection
3) radiation

1) Conduction: is how heat flows from hot to cold place. Because when particles move, they push the next particles. In the end, all the particles move. It is how the heat transfer from one end of the metal to the other end.

2) convection: conduction cannot be done in air because particles are separated apart. So they can't push the next one. However, you do feel hot air. It is because when air particles are heated, they move faster as they are free to move. The density of the hot air is therefore lower, so this air rise. This is how why hot air rises and how hot air balloon works

3) radiation: This is how the sun emmits heat to the earth. it is like how light is transferred, the radiation of heat is travelling as fast as light to us.

熱是能量的一種。
熱由溫度較的一處轉移至溫度較低的地方。
除此之外，我們亦可以透過作功(例如：以鎚仔打一塊鐵片)來產生熱。