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### Course: AP®︎/College Physics 2>Unit 2

Lesson 2: Thermal conduction and heat transfer

# Thermal conduction

Intuition behind how heat gets transferred through thermal conduction.

## Want to join the conversation?

• I still don't get the intuition behind heat. How is it different from thermal energy or kinetic energy of molecules and what exactly do we mean when we say that kinetic energy is lost because of friction to heat. And some places on the internet tell me that heat is a form of energy while other places say it is not so. What exactly is it?
• The trick is that, at the heart of it, heat and energy (and work) are the same thing, they are all measured in joules. Kinetic energy is the work added to a particle (truck, bullet or gas molecule) when it is set in motion. Thermal energy is basically the kinetic energy bound up in individual molecules of a gas, liquid or solid, and temperature is how we describe the average thermal energy of those molecules. Heat, in one sense, is the total amount of work done to a group of molecules to get them from no thermal energy at all (at 0 degrees Kelvin) to their current temperature, Usually, though, we use heat to mean the energy or work (same thing, remember?) added to or taken from a system when we do something with it.
Now remember that, other than in atom bombs, energy/work/heat is conserved, which means that we can change it from one form to another but when we add up all the forms it can take in a system, we have to end up with exactly the amount we started with. When we say we lose kinetic energy to heat because of friction, the friction is a process that transforms some of the kinetic energy to heat. Sometimes we mean to like when we apply the brakes on a car or bicycle, and sometimes we just can't avoid it like the tiny amount of heating that goes on in a good set of wheel bearings.
• Thermal conduction is ascribed to the molecule collision between two different gases. What is the thermal convection? Could you please explain the difference between thermal conduction and thermal convection? Thank you very much.
• Thermal conduction happens between molecules when the more energetic one transfers some of its energy to the less energetic one. In a solid, that is pretty much the only way to move heat. However, in liquids and gasses, as soon as a tiny pocket of molecules gains some heat by conduction, it becomes less dense and starts to rise, which moves the heat energy to different, higher part of the container, where it will eventually lose the heat it gained to other, cooler pockets of molecules it comes in contact with.
Note that we need gravity to make convection work, they've done experiments, and there is no real convection on the International Space Station, although fans still work to cool their computers.
• In some sites that i research, i see that heat transfer is also referred to the amount of free electrons. How does that relate to the particles bumping into one another and transferring heat?
• Yes that's true, but it only occurs in metal. Metal has a giant metalic structure, so it possesses positive metal ions and a 'sea' of delocalized (free-moving) electrons. When conduction takes place in metal, it is assisted by free electron diffusion, where the higher energy and free moving electrons collide with the particles in the solid, causing them to vibrate more vigorously, and speeding up the process of transfer of kinetic energy and achieving thermal equilibrium.
• how is momentum different from kinetic energy qualitatively?
• - Momentum is 𝑚𝑣 and kinetic energy is 1/2𝑚𝑣^2

- Momentum has a direction, kinetic energy not

- Momentum is conserved, kinetic energy not (but energy is)

- Momentum depends linear on velocity, kinetic energy depends quadratically on velocity
• If temperature is the average K.E, then why doesn't it have the same unit (Joule) as K.E?
• Temperature is not equal to the average kinetic energy.
Average kinetic energy of an ideal gas molecule is proportional to the temperature of the gas. Which means that increasing the temperature of the gas will make the molecules of the gas move at higher speed and therefore the average kinetic energy per mole of the gas will increase as well.
• the gas molecules in the pot can also be heated by convection other than conduction as explained in the previous video? So what is the primary reason for transfer of heat here? Conduction or convection?
• When Sal started talking about the pot, he mentioned that it was empty, so we are just thinking about the heat moving through the metal by conduction. In real life, of course, we would also be heating the air in the room by convection from both the inside and outside of the empty pot.
• How does a thermometer measure the average kinetic energy in a system?
• Depends on which type of thermometer. A couple of examples are thermometers based on the volume change of a known substance (like quicksilver) and thermometer based on the electromagnetic wave emission pattern of a dark body.
For volume change thermometers, you just need to put it in contact with the object you're interested at until equilibrium is reached. By then, you should be able to observe a volume change in mercury due to temperature change, a known and studied effect. The manufacturer surely marked the relation between volume and temperature on the scale, so you can quickly read it.
The other type I mentioned deals with the fact that dark bodies emit a known spectrum of electromagnetic waves which corresponds with its temperature. By building the measuring conditions in such a way that the energy absorbed is similar to that of a dark body (usually putting the thermometer in a cavity), one can read the absolute temperature with a good precision.
• A cup of tea at 90°c has less heat than a bathtub full of water at 70°c. Please explain ..me in 7th std