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Course: UP Class 11 Physics>Unit 6

Lesson 4: Heat transfer

There are three forms of thermal energy transfer: conduction, convection, and radiation. Conduction involves molecules transferring kinetic energy to one another through collisions. Convection occurs when hot air rises, allowing cooler air to come in and be heated. Thermal radiation happens when accelerated charged particles release electromagnetic radiation, which can be felt as heat.

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• let's say that all the planets have no atmosphere (i can somehow stand on the gas giants) , so if i stand on mercury i feel that the sun is hot as it releasing heat as radiation and if stand on jupiter I feel that the sun is less hot. so my question is this how is the heat being dimnished the farther away I go from the sun because space is a vacuum?
• The radiation doesn't "go away," it's just more spread out. Think of a balloon expanding, covered in dots. The dots get farther away from each other. So the radiation from the sun becomes more spread out, and therefore less intense, the father you are away from it.
• If you're standing far away from a fire, and you can still feel heat, isn't that an example of conduction? The molecules from the fire are warming up the air particles, which then reach your skin and warm you up. How is it an example of radiation but not conduction?
• Sometimes it can be difficult to know whether the heat you feel is due to conduction, convection, or radiation. Did the fire just heat up all the air in the room (conduction), did it heat some air that flowed over to you due to drafts or currents (convection), or is the heat reaching you in the form of infrared waves (radiation)? One way to know that it is almost surely radiation is if the side of you facing the fire feels much warmer than the side of you facing away from the fire. Another way to be almost certain that it is radiation is if you suddenly block the direct path from the fire to your face with something (a piece of paper or even your hands), and you immediately feel a big difference in how warm your face feels, it is radiation.

Finally, if you are sitting near a fire OUTDOORS, then the warmth you feel has certainly got to be radiation. Convection will take the heat straight up, and there is no way the fire's heat can reach you by conduction outdoors since the volume of air is so enormous and any air that does get warm will rise. (Well, unless you are directly ABOVE the fire, but then you are in big trouble. :) )
• At , I have 2 related doubts:
1) How does acceleration of charged particles cause radiation?
2) It is said that radiation contains energy to produce light. Is light a form of energy?
I don't get around the concept of Charged particles acceleration-radiation-light
• 1) This is a non-classical question and difficult to understand.
Essentially EM radiation is a moving electromagnetic field. (watch kurzgesagt's video 'what is light' on Youtube).
A charged particle has an electric field. If you accelerate or move it, you also change the shape of the electromagnetic field. Because everything travels at the speed of light, this 'change' also travels at the speed of light. you can interpret this as light.

The faster (more violent) the change, the smaller the region of space this propagating change exists over. This can be interpreted as the wavelength of the light - helping explain why short wavelength light has more energy.

Not sure how much this helped but its a complex question and a proper explanation would delve deep into forces and the standard model
• I still don't really understand why the colder, more dense molecules float down, below the hotter, less dense molecules. If someone could point me to another khan video, or explain themselves, that would be great. Thank you!
• We know that when matter is heated, it usually expands and hence its volume increases. When matter cools, it contracts (exception: water freezing).
Since density equals mass divided by volume:
If the volume is increased, and mass stays constant, density must decrease (i.e. density is inversely proportional to volume).
Decreasing density of a substance means that less buoyant force (as Rodrigo referred to) is required to push the substance up (in this case, hot air molecules), as compared to colder, denser air. Since the buoyant force from surrounding atmospheric air acts with the same force on both cold and hotter air molecules, the hotter, less dense air will tend to rise. Conversely, the cold air molecules will sink lower down.
Cool fact: Convection currents keep birds such as hawks in the air for so long. They glide on the rising masses of warm air heated by the ground.
• can you identify and give the definition of thermal radiation, conduction,covection for a simple memorable way.
(1 vote)
• Conduction is the transfer of thermal energy through direct contact. Convection is the transfer of thermal energy through the movement of a liquid or gas. Radiation is the transfer of thermal energy through thermal emission. Hope this helps!
• How can some camera can see infrared energy
• Instead of programming a camera to detect visible light, like the colors we can see, the camera is equipped to detect waves on the infrared spectrum, so things giving off more heat will show up on the camera, not normal light.
• Can one energy form affect a different energy form relating to temperature, amount of charged particles, etc.?
• where do these particles-electromagnetic particles- go after we stop feeling their heat?