Civil Engineering (CE) Exam > Civil Engineering (CE) Videos > Internal Energy
Internal Energy Video Lecture - Civil Engineering (CE)
FAQs on Internal Energy
| 1. What is internal energy? |  |
Ans. Internal energy refers to the total energy contained within a system, including both the kinetic and potential energies of its particles. It is a measure of the system's microscopic energy, including the energy associated with the motion and interaction of its atoms and molecules.
| 2. How is internal energy related to temperature? | |
Ans. Internal energy is directly related to the temperature of a system. As the temperature increases, the internal energy of the system also increases. This is because temperature is a measure of the average kinetic energy of the particles in a system, and internal energy includes the kinetic energy of these particles.
| 3. What factors can change the internal energy of a system? | |
Ans. Several factors can change the internal energy of a system. These include changes in temperature, pressure, and the addition or removal of heat. Additionally, work done on or by the system can also alter its internal energy.
| 4. How is internal energy different from heat and work? | |
Ans. Internal energy, heat, and work are all forms of energy, but they differ in their definitions and how they transfer energy. Internal energy is the total energy of a system, while heat is the transfer of energy due to a temperature difference, and work is the transfer of energy due to a force acting through a distance.
| 5. Can the internal energy of a system be negative? | |
Ans. No, the internal energy of a system cannot be negative. By definition, internal energy is a measure of the total energy contained within a system. While the energy of individual particles within the system can be negative, the overall internal energy will always be positive or zero.
Text Transcript from Video
In the last video we defined
internal energy as literally
all the energy that's
in a system.
That's kind of the most
inclusive version,
at least in my head.
So that's my system, it's
some type of container.
And I have a bunch of
particles in here.
It's literally the sum of
the kinetic energies
of all these particles.
If they had potential
energies, we'd
throw that in there.
If they have electrical
potential, we'd
throw that in there.
If they have bonds with each
other, the energies in that
bond, we would throw
it in there.
It's everything.
It's all inclusive.
And, I told you in the last
video it's unintuitive that U
stands for internal energy.
But I kind of think that, you
know, U, it contains the
universe of energy.
You know, that's just for
me to memorize it.
Don't think too deeply into
what I just said.
But this is internal energy.
If you show me a system, it
has some internal energy.
It's a function of its state.
I don't care how it got to that
state, but if you told me
a system at a certain state,
maybe with a certain pressure,
or a certain volume, or at a
certain temperature-- although
if you give me pressure and
volume that should be enough--
I can tell you what its
internal energy is.
Especially if you tell me the
type of molecule I have, and
things like that.
Now we also said in the last
video that because internal
energy is all the energy in
the system, it can't be
randomly created or destroyed.
It can just be transformed
from one form to another.
So if I have a change in
internal energy, it can only
be due to-- well, it can be
due to more than what I'm
describing.
But in our simple world where
all of the energy in a
system-- and maybe we're dealing
with gases, because
that's normally what you deal
with in a first-year chemistry
course-- it's going to be the
heat that could be added to
the system, plus the work
done on the system.
And like I said in the last
video, sometimes they'll say,
instead of plus the work done
on the system, sometimes
they'll say, minus the work
that the system does.
Either way.
And here I just want to make
another side discussion here,
because I decided to write
it without the
little deltas here.
And the reason why I did that
is if you were to write this
equation, which you will see,
you'll see it in textbooks,
teachers will do it-- nothing
inherently criminal about
that-- but I just do that
because it clears in my mind
what heat and what work are
relative to internal energy.
If I were to write delta U is
equal to heat delta Q plus
delta W to me this implies that
at some point I had some
amount of heat in my system,
and then I have a different
amount of heat in my system,
and I took the difference
between the two and I got
a change in heat.
So this implies that heat is
somehow an inherent macrostate
of the system, and that's
not the case.
I can tell you what the
internal energy
of this system is.
I can tell you its pressure.
I can tell you its volume.
I can tell you its
temperature.
These are all macrostates
of the system.
I don't know how it got to this
situation, but I can tell
you about it.
I cannot tell you what the
heat is of this system.
And that might be a little
unintuitive, because if I ask
you, hey I have a cup of coffee,
what's the heat of it?
You might say, oh, it's, you
know, you might give me the
temperature.
Because in our everyday language
we use things like
heat and temperature
interchangeably.
But in thermodynamics, heat
is a transfer of energy.
A way to think about it
is, if internal energy
is your bank account.
So you could say change
in bank account.
And it really is like
the energy bank
account of a system.
If you have a change in a bank
account, you had some deposits
or withdrawals.
And heat and work are really
just deposits or withdrawals
into our energy bank account.
Heat is one kind.
Maybe heat is like
a wire transfer.
So you could say it's wire
transfers to your account.
Transfers to your account
plus check deposits.
Now it makes a lot of sense to
say, you could say, what is my
value of my bank account?
Or you could say what is my
change in my bank account?
You take two snapshots of
your bank account at
two different times.
But would it make sense to say--
you know, I could say I
wire transferred $10, right?
So I could say, this statement
would be plus $10.
And I could say that I
wrote $20 in checks,
minus $20 in checks.
In which case, my change
in my bank account
would be minus $10.
Now would it make sense
for me to say
change in wire transfer?
That implies that when I started
off maybe my bank
account had $100 in it
and now it has $90.
When my bank account had $100
in it, did it have any wire
transfer amount?
No, wire transfer was how money
was deposited or taken
away from my account.
Likewise, it didn't have a
checking deposit account, so I
can't really-- it just seems
weird to me to say change in
wire transfer is $10, or change
in check deposits is
$20 or minus $20.
Would you say, I made a
$10 wire transfer and
I paid $20 in checks.
So I had a net change in
my bank account of $10.
Likewise, I say how much work
was done to me or I did, which
is essentially a deposit or
withdrawal of energy.
Or I could say much heat was
given to me or how much heat
was released, which is another
way of depositing or
withdrawing energy from my
energy bank account.
So that's why I like
to stick to this.
And I like to stay
away from this.
And just like I said, you can't
say how much heat is in
the system.
So someone will say, oh, how
much heat is in this?
You cannot say that.
There's no heat state variable
for that system.
You have internal energy.
The closest thing to heat,
we'll talk about it in a
future video, is enthalpy.
Enthalpy is essentially a way of
measuring how much heat is
in a system, but we won't talk
about that just now.
And you can't say how much
work is in a system.
You can't say, oh I have x
amount of work in a system.
The system can do work or have
work done to it, but there's
not a certain amount of work,
because that energy in the
system could be all used for
work, it could all be used for
heat, it could all be used for
a ton of different things.
So you can't say those things.
And that's why I don't like
treating them like state
variables, or state functions.
So with that said, this
is our definition.
Let's do a couple of
simple problems,
just to give you intuition.
And I really want to make
you comfortable.
My real goal is to make you
comfortable with, when to know
to use plus or minus
on the work.
And the best way to do it is not
to memorize a formula, but
just to kind of think about
what's happening.
So let's say that I have some
system here and, I don't know,
it's a balloon.
And let's say that I have no
change in internal energy.
Internal energy is 0.
And for our purposes we can kind
of view it as the kinetic
energy of the particles
haven't changed.
And let's say by expanding a
bit, by my balloon expanding a
bit-- I did some work.
I'll do this in more detail
in the next video.
So the system does 10
joules of work.
My question is, how much heat
was added or taken away from
the system?
So the way I can think about
it-- you don't even have to
write the formula down, or you
can write it-- you could say,
look, the internal energy, the
amount of energy in the system
hasn't changed.
The system did 10
joules of work.
So that's energy going
out of the system.
It did 10 joules.
So 10 joules went out
in the form of work.
If the internal energy did not
change, then essentially 10
joules of energy had to
go into the system.
10 joules had to go
into the system.
If it didn't, the internal
energy would have gone down by
the amount of work we had.
And the only way, if is this is
the net work, the only way
that we're talking about, that
we can add energy outside of
work, is through heat.
So 10 joules of heat must
have been added.
So we can write 10 joules
of heat added to system.
Now let's look at that from
the point of view of the
actual formula.
If we have delta U is equal to
heat added, plus work done to
the system, then we would
say, OK, this was 0.
It's equal to the heat
added to the system.
Remember, in this way we're
saying this is the heat done,
or the work done,
to the system.
W is work done.
Now, the system did work
to something else.
It didn't have work
done to it.
So if this is work done to the
system, and it did work, then
this is going to
be a minus 10.
Minus 10 joules.
And then you solve both
sides of this.
You add 10 to both sides and
you get 10 is equal to Q,
which is exactly what
we got up here.
But this can get confusing
sometimes, because you're
like, oh, is this heat
that the system did?
Is this heat that was added to
the system or taken away?
The convention tends to be
that this is heat added.
But then sometimes
it's confusing.
Is this the work done to the
system or work done by?
And that's why I like just
doing it this way.
If the system does work
it loses energy.
If the system has work done
to it, it gains energy.
So let's do another problem.
I mean, I could have done this
exact same thing using the
other formula that you'll
sometimes see.
Delta U is equal to Q minus the
work that the system does
by the system-- Work
done by the system.
And in this case, once
again, change in
internal energy was 0.
That is equal to heat
added to the system,
minus the work done.
So minus-- I told you at the
beginning of the problem that
the system did 10 joules of
work-- so minus the work done.
Minus 10 joules.
We get the same situation
up here from
two different formulas.
And we got 10 is equal to Q.
Either way, the heat added to
the system is 10 joules.
Let's do one more.
Let's say that, I don't know,
5 joules of heat taken away
from system.
And let's say that 1 joule
of work done on system.
So maybe we're compressing the
balloon on the system.
What is our change in
internal energy?
Let's just figure
out our change.
So the way I think of it is 5
joules of heat taken away from
the system, that's going
to reduce our internal
energy by minus 5.
And if 1 joule work is done onto
the system, we're putting
energy into it so that'll
be plus 1.
So minus 5, plus 1, is
going to be minus 4.
Or enter our change in internal
energy is minus 4 joules.
Now we could have done that a
little bit more formally with
the formula, change in internal
energy is equal to
heat added to the system, plus
work done on the system.
So it's equal to heat
added to the system.
We had 5 joules heat taken away,
so this is minus 5, plus
work done on the system.
We have 1 joule of work done
on the system, and there we
get, once again, minus 4.
Now, I could have written that
same formula the other way.
I could have said change in
internal energy is equal to
heat added to the system minus
the work that the system does.
I want to do this both ways,
because I don't want you to
get confused, either way you
see it on problems or in
school, or maybe you take one
class that does it one way and
one class that does
it the other.
If you use it this way,
what was the heat
added to the system?
We had heat taken away,
so it's minus 5.
So minus the work that the
system does, how much work
does the system do?
Well, the system had 1 joule
of work done to it.
So it did itself minus
1 joules of work.
So it's minus minus 1.
I want to be clear, when I use
this formula, this is work
done by system, done by.
This is work done to.
The system had work
done to it.
So it had minus 1 joules of work
done by the system, so
these become pluses, and you
get back to a minus 4.
Do whatever's intuitive
for you.
For me, this tends to be the
most intuitive, where you
don't even use a formula.
We just think, if I'm doing
work, I'm using up energy.
If I get work done to me, I'm
being handed over energy.
If I have heat taken away from
me, I'm giving away energy.
If I have heat added to me,
I'm getting energy.
See you in the next video.
internal energy as literally
all the energy that's
in a system.
That's kind of the most
inclusive version,
at least in my head.
So that's my system, it's
some type of container.
And I have a bunch of
particles in here.
It's literally the sum of
the kinetic energies
of all these particles.
If they had potential
energies, we'd
throw that in there.
If they have electrical
potential, we'd
throw that in there.
If they have bonds with each
other, the energies in that
bond, we would throw
it in there.
It's everything.
It's all inclusive.
And, I told you in the last
video it's unintuitive that U
stands for internal energy.
But I kind of think that, you
know, U, it contains the
universe of energy.
You know, that's just for
me to memorize it.
Don't think too deeply into
what I just said.
But this is internal energy.
If you show me a system, it
has some internal energy.
It's a function of its state.
I don't care how it got to that
state, but if you told me
a system at a certain state,
maybe with a certain pressure,
or a certain volume, or at a
certain temperature-- although
if you give me pressure and
volume that should be enough--
I can tell you what its
internal energy is.
Especially if you tell me the
type of molecule I have, and
things like that.
Now we also said in the last
video that because internal
energy is all the energy in
the system, it can't be
randomly created or destroyed.
It can just be transformed
from one form to another.
So if I have a change in
internal energy, it can only
be due to-- well, it can be
due to more than what I'm
describing.
But in our simple world where
all of the energy in a
system-- and maybe we're dealing
with gases, because
that's normally what you deal
with in a first-year chemistry
course-- it's going to be the
heat that could be added to
the system, plus the work
done on the system.
And like I said in the last
video, sometimes they'll say,
instead of plus the work done
on the system, sometimes
they'll say, minus the work
that the system does.
Either way.
And here I just want to make
another side discussion here,
because I decided to write
it without the
little deltas here.
And the reason why I did that
is if you were to write this
equation, which you will see,
you'll see it in textbooks,
teachers will do it-- nothing
inherently criminal about
that-- but I just do that
because it clears in my mind
what heat and what work are
relative to internal energy.
If I were to write delta U is
equal to heat delta Q plus
delta W to me this implies that
at some point I had some
amount of heat in my system,
and then I have a different
amount of heat in my system,
and I took the difference
between the two and I got
a change in heat.
So this implies that heat is
somehow an inherent macrostate
of the system, and that's
not the case.
I can tell you what the
internal energy
of this system is.
I can tell you its pressure.
I can tell you its volume.
I can tell you its
temperature.
These are all macrostates
of the system.
I don't know how it got to this
situation, but I can tell
you about it.
I cannot tell you what the
heat is of this system.
And that might be a little
unintuitive, because if I ask
you, hey I have a cup of coffee,
what's the heat of it?
You might say, oh, it's, you
know, you might give me the
temperature.
Because in our everyday language
we use things like
heat and temperature
interchangeably.
But in thermodynamics, heat
is a transfer of energy.
A way to think about it
is, if internal energy
is your bank account.
So you could say change
in bank account.
And it really is like
the energy bank
account of a system.
If you have a change in a bank
account, you had some deposits
or withdrawals.
And heat and work are really
just deposits or withdrawals
into our energy bank account.
Heat is one kind.
Maybe heat is like
a wire transfer.
So you could say it's wire
transfers to your account.
Transfers to your account
plus check deposits.
Now it makes a lot of sense to
say, you could say, what is my
value of my bank account?
Or you could say what is my
change in my bank account?
You take two snapshots of
your bank account at
two different times.
But would it make sense to say--
you know, I could say I
wire transferred $10, right?
So I could say, this statement
would be plus $10.
And I could say that I
wrote $20 in checks,
minus $20 in checks.
In which case, my change
in my bank account
would be minus $10.
Now would it make sense
for me to say
change in wire transfer?
That implies that when I started
off maybe my bank
account had $100 in it
and now it has $90.
When my bank account had $100
in it, did it have any wire
transfer amount?
No, wire transfer was how money
was deposited or taken
away from my account.
Likewise, it didn't have a
checking deposit account, so I
can't really-- it just seems
weird to me to say change in
wire transfer is $10, or change
in check deposits is
$20 or minus $20.
Would you say, I made a
$10 wire transfer and
I paid $20 in checks.
So I had a net change in
my bank account of $10.
Likewise, I say how much work
was done to me or I did, which
is essentially a deposit or
withdrawal of energy.
Or I could say much heat was
given to me or how much heat
was released, which is another
way of depositing or
withdrawing energy from my
energy bank account.
So that's why I like
to stick to this.
And I like to stay
away from this.
And just like I said, you can't
say how much heat is in
the system.
So someone will say, oh, how
much heat is in this?
You cannot say that.
There's no heat state variable
for that system.
You have internal energy.
The closest thing to heat,
we'll talk about it in a
future video, is enthalpy.
Enthalpy is essentially a way of
measuring how much heat is
in a system, but we won't talk
about that just now.
And you can't say how much
work is in a system.
You can't say, oh I have x
amount of work in a system.
The system can do work or have
work done to it, but there's
not a certain amount of work,
because that energy in the
system could be all used for
work, it could all be used for
heat, it could all be used for
a ton of different things.
So you can't say those things.
And that's why I don't like
treating them like state
variables, or state functions.
So with that said, this
is our definition.
Let's do a couple of
simple problems,
just to give you intuition.
And I really want to make
you comfortable.
My real goal is to make you
comfortable with, when to know
to use plus or minus
on the work.
And the best way to do it is not
to memorize a formula, but
just to kind of think about
what's happening.
So let's say that I have some
system here and, I don't know,
it's a balloon.
And let's say that I have no
change in internal energy.
Internal energy is 0.
And for our purposes we can kind
of view it as the kinetic
energy of the particles
haven't changed.
And let's say by expanding a
bit, by my balloon expanding a
bit-- I did some work.
I'll do this in more detail
in the next video.
So the system does 10
joules of work.
My question is, how much heat
was added or taken away from
the system?
So the way I can think about
it-- you don't even have to
write the formula down, or you
can write it-- you could say,
look, the internal energy, the
amount of energy in the system
hasn't changed.
The system did 10
joules of work.
So that's energy going
out of the system.
It did 10 joules.
So 10 joules went out
in the form of work.
If the internal energy did not
change, then essentially 10
joules of energy had to
go into the system.
10 joules had to go
into the system.
If it didn't, the internal
energy would have gone down by
the amount of work we had.
And the only way, if is this is
the net work, the only way
that we're talking about, that
we can add energy outside of
work, is through heat.
So 10 joules of heat must
have been added.
So we can write 10 joules
of heat added to system.
Now let's look at that from
the point of view of the
actual formula.
If we have delta U is equal to
heat added, plus work done to
the system, then we would
say, OK, this was 0.
It's equal to the heat
added to the system.
Remember, in this way we're
saying this is the heat done,
or the work done,
to the system.
W is work done.
Now, the system did work
to something else.
It didn't have work
done to it.
So if this is work done to the
system, and it did work, then
this is going to
be a minus 10.
Minus 10 joules.
And then you solve both
sides of this.
You add 10 to both sides and
you get 10 is equal to Q,
which is exactly what
we got up here.
But this can get confusing
sometimes, because you're
like, oh, is this heat
that the system did?
Is this heat that was added to
the system or taken away?
The convention tends to be
that this is heat added.
But then sometimes
it's confusing.
Is this the work done to the
system or work done by?
And that's why I like just
doing it this way.
If the system does work
it loses energy.
If the system has work done
to it, it gains energy.
So let's do another problem.
I mean, I could have done this
exact same thing using the
other formula that you'll
sometimes see.
Delta U is equal to Q minus the
work that the system does
by the system-- Work
done by the system.
And in this case, once
again, change in
internal energy was 0.
That is equal to heat
added to the system,
minus the work done.
So minus-- I told you at the
beginning of the problem that
the system did 10 joules of
work-- so minus the work done.
Minus 10 joules.
We get the same situation
up here from
two different formulas.
And we got 10 is equal to Q.
Either way, the heat added to
the system is 10 joules.
Let's do one more.
Let's say that, I don't know,
5 joules of heat taken away
from system.
And let's say that 1 joule
of work done on system.
So maybe we're compressing the
balloon on the system.
What is our change in
internal energy?
Let's just figure
out our change.
So the way I think of it is 5
joules of heat taken away from
the system, that's going
to reduce our internal
energy by minus 5.
And if 1 joule work is done onto
the system, we're putting
energy into it so that'll
be plus 1.
So minus 5, plus 1, is
going to be minus 4.
Or enter our change in internal
energy is minus 4 joules.
Now we could have done that a
little bit more formally with
the formula, change in internal
energy is equal to
heat added to the system, plus
work done on the system.
So it's equal to heat
added to the system.
We had 5 joules heat taken away,
so this is minus 5, plus
work done on the system.
We have 1 joule of work done
on the system, and there we
get, once again, minus 4.
Now, I could have written that
same formula the other way.
I could have said change in
internal energy is equal to
heat added to the system minus
the work that the system does.
I want to do this both ways,
because I don't want you to
get confused, either way you
see it on problems or in
school, or maybe you take one
class that does it one way and
one class that does
it the other.
If you use it this way,
what was the heat
added to the system?
We had heat taken away,
so it's minus 5.
So minus the work that the
system does, how much work
does the system do?
Well, the system had 1 joule
of work done to it.
So it did itself minus
1 joules of work.
So it's minus minus 1.
I want to be clear, when I use
this formula, this is work
done by system, done by.
This is work done to.
The system had work
done to it.
So it had minus 1 joules of work
done by the system, so
these become pluses, and you
get back to a minus 4.
Do whatever's intuitive
for you.
For me, this tends to be the
most intuitive, where you
don't even use a formula.
We just think, if I'm doing
work, I'm using up energy.
If I get work done to me, I'm
being handed over energy.
If I have heat taken away from
me, I'm giving away energy.
If I have heat added to me,
I'm getting energy.
See you in the next video.
About this Video
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Apr 19, 2026 Last updated
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Internal Energy for Civil Engineering (CE)2026 is part of Civil Engineering (CE) preparation. The notes and questions for Internal Energy have been prepared according to the Civil Engineering (CE) exam syllabus. Information about Internal Energy covers all important topics for Civil Engineering (CE)2026 Exam. Find important definitions, questions, notes, meanings, examples, exercises and tests below for Internal Energy.
Introduction of Internal Energy in English is available as part of our Civil Engineering (CE) preparation & Internal Energy in Hindi for Civil Engineering (CE) courses. Download more important topics, notes, lectures and mock test series for Civil Engineering (CE) Exam by signing up for free.
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Internal Energy of covers all the important topics, helping you prepare for the Civil Engineering (CE) exam on EduRev. Start for free!
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Here you can find the Internal Energy defined & explained in the simplest way possible. Besides explaining types of Internal Energy theory, EduRev gives you an ample number of questions to practice Internal Energy tests, examples and also practice Civil Engineering (CE) tests.
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