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### Course: High school chemistry > Unit 6

Lesson 1: Thermal energy and equilibrium# Thermal energy, temperature, and heat

Thermal energy refers to the kinetic energy of randomly moving particles in a substance. Particles can have translational, rotational, and/or vibrational kinetic energy, depending on the state of matter. Temperature is a measure of the average kinetic energy of the particles in a substance. Heat is thermal energy that transfers into or out of a system. Heat can transfer by conduction, convection, or radiation. Created by Mahesh Shenoy.

## Want to join the conversation?

- What would the thermal energy be measured in when Mahesh computes it at4:29?(2 votes)
- The thermal energy is the total sum of the kinetic energy in the particles and there is 300 particles with each containing on average, 2 kinetic energy. So I think you do 300*2 = 600 kinetic energy which is the thermal energy.(1 vote)

- Hi there, I had a question about when they were explaining at the start what is thermal energy, isn't thermal energy also the process where heat is transferred from a hotter substance to a cooler substance resulting in the two substances reaching equilibrium?(1 vote)
- Thermal energy refers to the kinetic energy of the particles in a substance. Objects in thermal contact of differing temperatures will transfer energy (heat to the cooler substance) and reach thermal equilibrium. This process is called heat transfer. Conduction, convection and radiation are methods of heat transfer.

I hope this explanation helps! =D(2 votes)

- What about potential energy, isn't also the sum of P.E(1 vote)
- At4:45Wouldn't the thermal energy not necessarily be 200 because some particles could be greater or less than two?(1 vote)
- I know that temperature is measured in a variety of ways (e.g. Kelvins, degrees Fahrenheit, degrees Celsius, etc.). At4:19, when he is explaining temperature as average kinetic energy, he implies that the temperatures of the water samples are "2".

I looked this up and found that the Kelvin and Rankine scales are proportional to average kinetic energy. How would one convert average kinetic energy to Kelvin or Rankine?(1 vote)

## Video transcript

- [Instructor] I have
two vessels of water. I start heating them with, pretty much, the same amount of heat. They're similar stoves. What do we find? We find that the one which has less water starts boiling fast. That's not very surprising. This means that the one
which has less water, its temperature rises quicker, and it reaches the boiling point, that is the 100 Celsius, much quicker than the other one. But the question is, why does this happen? Why is it that if you
have less amount of water, its temperature shoots faster? Well, to answer this question, we need to understand the
difference between temperature, heat energy, and thermal energy. And that's exactly what
we'll do in this video. So let's begin. So let's start by asking
ourselves, what is thermal energy? Well, thermal energy is basically
the sum of kinetic energy of all particles. What does that mean? Well, for that, let's zoom into our water. We'll find water molecules, and these molecules are held together by intermolecular forces. Now, the forces are weak enough that the particles can actually move. They're all moving randomly. As a result of that,
they have kinetic energy. Now, there are different
kinds of motion over here. So there is translatory motion where particles move from
one place to another. Now have not drawn the arrow
mark for all the particles because I don't want
this to be too cluttered, but all particles can move. Some particles will move very slowly. Other particles will may move very fast, but there is kinetic
energy because of that. But that's not it.
Particles can also spin. So you have rotational kinetic energy. And particles can also
vibrate. They can jiggle. So you get jiggle,
jiggling kinetic energy. Now you add all that kinetic
energy of all the molecules, all the particles over here, that total energy is what
we call the thermal energy of that water. This is true for not just liquids, this is true for solids and gases as well. The only difference is
if you consider a solid, let's consider a solid like, say, ice. The big difference is
that, over here, particles, because of very strong
intermolecular forces, particles are locked in space. So they can't move from
one look place to another, but they can still vibrate. As a result of that, they
do have kinetic energy. And so when it comes to solids, the thermal energy comes
from the vibrational, the jiggling kinetic
energy that they have. What about gases? Well, in gases, they hardly
have any intermolecular forces. So the particles are free to move about, and therefore, when it comes to gases, the thermal energy comes from the translational kinetic
energy of the particles. But you can see in all the cases, thermal energy eventually
comes from where? The kinetic energy of all the particles. Okay, so that's thermal energy. What is temperature now? Isn't it the same as
thermal energy? It's not. Temperature is a measure
of average kinetic energy of the particles. Which means if you take the total energy of all the particles, which is basically the thermal energy, divided by the total number
of particles you have, you now get average
energy of each particle. That is a measure of what temperature is. And to see how different
these two things are, let's take some numbers here. So let's say we have about 100
molecules of water over here. Now, of course, we both
know that we are dealing with trillions and trillions of molecules, but let's just keep simple numbers. So if here, if you have 100, let's say here you have,
since you have more, you have about 300
molecules of water, okay? Now, if the average kinetic energy is two, two units, what does that mean? It means there'll be some
particles, some molecules, which will be moving with, which will be having more
than two kinetic energy. Some particles will be
having less than two kinetic, two units of kinetic energy. But if you average it out, you'll get two, and that represents the temperature. If this number is bigger,
temperature will be higher. If this number is smaller,
temperature would be lower. Okay, now this water
is at room temperature. I haven't started heating them yet. That means they should also have the same temperature as this one, which means they should also have the same average kinetic energy. Because if it had any
different kinetic energy, this water would be at
a different temperature compared to this one. But that's not true. We right now have the same
temperature, room temperature. Okay, so now comes the question, what is the thermal energy here and here? Well, the thermal energy here
would be, we have on average, each molecule has two units of energy, but there are total 100 molecules. So the total energy would be 200. That represents the thermal energy here. What's the thermal energy here? Well, again, each molecule, on average, has two units of energy, but there's total 300 molecules now, which means total would
be 600 units of energy. So the thermal energy over
here would be 600 units. Right in front of your eyes, you can see they have
different thermal energy. This water has more thermal
energy compared to this one, mainly because it has
a lot more particles. But look, they have the same temperature. So you can see they're not the same thing. This now brings us to the
third and the final piece, heat energy. What is heat energy? Is it the
same as thermal energy? No. Heat energy represents the
amount of thermal energy that is transferred. So whenever you are adding
or removing thermal energy from an object, that's when we say, that's when we use the word heat. So in science, it
doesn't make sense to say that this water has 600
units of heat energy. No. Whatever water has is thermal energy. But if you add some thermal energy to it or you remove from some
thermal energy to it, that's when we use the word heat. We say heat energy was added
or heat energy was removed. So you don't have heat energy,
you only have thermal energy. And again, we'll take some numbers. It'll make a lot more sense. But before we do that, one
question we could have is how do you transfer energy? How do you transfer thermal energy? Well, it turns out there are
actually three ways to do that. The first one is conduction. This is where you transfer thermal energy without the particles themselves moving. For example, if we
consider how heat energy is transferred in this vessel, let's look at the atoms of that vessel. The bottom of that vessel,
the atoms at the bottom, will have high thermal energy because it is directly in
contact with the flame. But how does that thermal energy
get transferred over here? Well, since the particles are
mostly jiggling over here, remember it's a solid, so the thermal energy is mostly jiggling, because the particles are
jiggling, they're vibrating, they will come in contact
and make the particles, you know, close to them
jiggle, next to them jiggle, and then these particles
will make the particles next to them jiggle
and so on and so forth. And that's how, look, thermal
energy is transferred, but the matter itself did not move. The particles did not move
from one place to another. So without any matter motion, you have thermal energy transferred. That's what we call conduction. But that's not it. There's a second kind of, there's a second way in which you can transfer thermal energy, which we call convection. This is kind of the opposite. Here, matter moves from
one place to another, and that's how thermal
energy is transferred. And this cannot happen in solids because in solids, remember, matter cannot move from
one place to another, which means it can only happen
in fluids, liquids and gases. So for this, consider the
water inside our vessel. And again, if you consider
that the bottom of this is, let's say, hot, it has
high thermal energy, then this means the particles
over here are moving with very high speeds, and
so they are farther apart, and therefore they'll have less density. And so this part of water
will now start rising up, allowing the rest of the
cooler water to come down. And then that heats up,
and then that rises up, and then the rest comes down. This is how, look, by
making the matter move, the matter is moving,
and as a result of that, thermal energy is being transferred. This can only happen in liquids and gases. But there's a third
one. That is radiation. Now, this is where
transfer of thermal energy can happen without any matter at all. This can happen in the vacuum of space using electromagnetic radiation. The best example of this
is how we receive heat from the sun. Between the sun and the
earth, there's vacuum so you cannot have
conduction or convection. But what we do get is radiation, which can travel through space, and that's how we receive
heat from the sun. So going back to our example, let's say we switch on the
stove and wait for some time, we'll transfer mostly the
same amount of energy. It's the same stuff and everything. So let's say we transferred
about 300 units of energy. This is the heat energy
that we transferred, okay? As a result of that, what
happens to our thermal energy? Well, thermal energy will increase. This will go from 600 to
600 plus 300, 900 units. This will go from 200 to
200 plus 300, 500 units. So you can see again, at
any given moment of time, this has more thermal energy than this. But now comes the key moment.
What is the new temperature? Well, the temperatures will be higher, but which one will be
having more temperature? Will it be the same? Will it be different? Will you pause the video
and think about it? Okay, remember for temperature, I need to think about
average kinetic energy. Now, this has 900 units of energy, but that's divided among 300
molecules, 300 particles. So if you average it
out, 900 divided by 300, you get about three. So the temperature has increased because the average kinetic
energy has increased from two to three, okay? Now let's look at over here. Although we have less
thermal energy, 500 units, that's divided among only
100 molecules, 100 particles, which means if I divide
it by 100, I get five. The new average kinetic
energy will be five units. So you can see the average
kinetic energy over here is higher, therefore the temperature
over here is higher. This makes sense, right? Because you have less molecules,
less particles over here, when you distribute
that energy, on average, each one ends up having more. That's why this one's temperature rises much quicker than this one. And that's the reason why that one eventually hits the boiling
point earlier than this one. So if you had to see the
difference between the two in one picture, this picture kind of sums it up. If you add up all the kinetic energy of all the particles over here, it'll be more here compared to over here because you have more particles
over here to begin with. That's why this still
has more thermal energy compared to this one. But think about the average. On an average, the particles are moving
pretty slowly over here. So it has less average kinetic
energy, less temperature. On average, the particles are
moving very quickly over here. So on average, you have a much higher
kinetic energy per particle, and therefore you have
a higher temperature. Now, here's something to think about. If you compare this vessel
of water with the ocean, you'll find that the ocean
has way higher thermal energy compared to this one, same idea. But this water in my kitchen
is at a higher temperature. It's still hotter than the ocean because the average
kinetic energy is higher than that in the ocean. That's incredible, isn't it?