Heat without touching
Radiation travels straight from a hot thing to us.
By the fireplace
Pema and Palden sit near the fireplace and feel warm. They are not touching the fire. The air between them is not what brings them the heat.
How does the heat reach them?
What this lesson covers
The idea
Radiation is the transfer of heat directly from a hot object to us without any medium; the heat of the Sun reaches us by radiation.
By the fireplace
Pema and Palden sit near the fireplace and feel warm. They are not touching the fire. The air between them is not what brings them the heat.
How does the heat reach them?
- Straight from the fire to them
- Only through the floor
- Only by the smoke
Send the heat
Send heat in both pictures: the fireplace, and the Sun and Earth.
No go-between needed
In both pictures, heat went from the hot object to us directly. Between the Sun and the Earth there is empty space, and the heat still arrives. So no medium is needed.
Radiation is the transfer of heat directly from a hot object to us without any medium. The heat of the Sun reaches us by radiation.
Notes
Radiation is the transfer of heat directly from a hot object to us without any medium. The heat of the Sun reaches us by radiation.
Check yourself
How does the heat of the Sun reach us?
Radiation needs …
Pema and Palden feel warm near a fireplace because heat reaches them by …
Between the Sun and the Earth there is empty space. Can heat still cross it?
- By conduction through the air. The space between the Sun and the Earth is empty. The heat comes by radiation.
- By convection of water. Convection needs liquid or gas particles to carry the heat. Sun's heat comes by radiation.
- By radiation — correct. Yes!
- no medium — correct. Yes!
- a solid medium. Radiation does not need any medium, not even a solid one.
- a liquid medium. Radiation does not need a liquid or any other medium.
- conduction through the floor. The heat reaches them from the fire directly. That is radiation.
- radiation — correct. Yes!
- convection in water. There is no water between them and the fire. They feel the heat by radiation.
- No, heat needs a medium for every journey. Conduction and convection need a medium. Radiation does not, so the Sun's heat reaches us.
- Yes, but only by conduction. Conduction needs a material medium. The heat crosses by radiation.
- Yes, by radiation — correct. Yes!