跟读练习: First Law of Thermodynamics, Basic Introduction - Internal Energy, Heat and Work - Chemistry - 通过视频学习英语口语

In this video, we're going to talk about the first law of thermodynamics.

and how it relates to internal energy.

heat and work.

So what is the basic idea behind the first law of thermodynamics?

The gist of it is this: Energy cannot be created or destroyed.

It can simply be transferred from one place to another. So let's say...

if we have a system and everything outside of it.

is the surroundings.

Energy can flow into or out of the system.

And there's two ways in which energy can do so.

and that is through heat. and work.

So if heat flows into the system, The system gains energy.

And that energy is known as the internal energy of the system.

represented by The symbol capital U.

Now, the surroundings can do work on a system.

So those are the two ways in which a system...

can increase It's internal energy.

is by the transfer of heat energy into the system, or if the surroundings perform work on a system.

So let's say if the surroundings perform 300 Joules on his system. That means...

the system's internal energy goes up by 300.

So the change is positive.

so delta u increases The energy of the surroundings, however, has to decrease by 300.

And so, energy is not created or destroyed.

is simply transferred from one place to another.

The system Didn't just get the 300 jewels from nowhere.

that 300 joules of energy came from somewhere.

It came from the surroundings.

A good way to illustrate this is to use money.

So let's say if...

you decide to sell a laptop for $500.

Someone.

We'll buy the laptop for $500.

Let's see if someone does buy it.

your bank account will increase by 500.

but that person's bank account will decrease by 500.

And so the money just doesn't come from nowhere, just doesn't, just magically appears.

It comes from somewhere.

So in order for you to and gain 500, someone else has to lose 500.

Now, granted...

The government could print more money if they wanted to. But...

in a practical sense, in everyday uh, life.

When someone gains money, someone has to lose money.

And so in that sense within that transaction Money is not created or destroyed.

It simply transferred from one place to another.

energy is follows that same principle.

unless you're God Energy cannot be created or destroyed.

as far as we know. But...

In today's world, Under practical conditions, energy is simply transferred from one place to another.

So in our example, If the system gains 300 joules of energy, That means the surroundings loses 300 Joules of energy.

If the system loses 500 joules of energy, the surroundings have to gain 500 joules of energy.

And in that sense, within that practical area, Energy is not created or destroyed.

Thus we have the first law of thermodynamics.

Now there's three types of systems that You want to be familiar with.

The first one is an open system.

The second... is a closed system.

And the third, is an isolated system.

So in the open system, Matter.

can enter into an open system.

So oxygen from the air can Go inside of it.

And also.. heat energy can flow into an open system.

So matter and energy can be transferred into and out of an open system.

Now in a closed system, Matter cannot flow into it.

So oxygen just It can't go inside of a closed system.

However, energy...

can still flow into a closed system So in a closed system, only energy can go into and out of a closed system but matter cannot.

In an isolated system, Energy or matter cannot enter or leave an isolated system.

So the mass within an isolated system is fixed.

It doesn't change and the total energy of an isolated system also doesn't change.

Because no energy can flow into it or out of it.

The equation for the change in the internal energy of a system is is q plus w.

And this is the equation that you'll see in a typical chemistry textbook.

Q represents the heat energy flows into or out of the system and W represents the work.

Now in physics...

The equation is a bit different.

In the physics textbook, you'll see Delta you is equal to q minus w.

Now you might be wondering, Why is it different?

Why is it not the same?

and a reason for that is the point of view taken by the scientists.

and chemistry, we take the system's point of view. In physics, Engineers take...

the viewpoint of the surroundings.

So in chemistry, W is negative when work is done.

by the system.

Anytime work is done by the system, The system has to expend energy to do work and so the internal energy of the system decreases.

Imagine if you're going to the gym to work out.

and As you lift weights, Your body is burning energy.

the internal energy of your body decreases. as you burn calories.

And so work is being done by the system.

That is, you're the system.

You're doing the work.

and the surroundings might be the waist that you're lifting.

So as you burn calories to lift up a weight from, let's say, to ground, to an elevated position.

the potential energy of that weight increases.

Bye.

The internal energy of your body decreases because you're burning calories.

You're losing weight.

In this case, Whenever w is negative, work is done by the system in the case of chemistry.

and When w is positive, work is done.

on this system.

Now in physics, It's different.

100

we take the surroundings point of view in physics.

101

So W is negative when work... is done. on the system.

102

and W is positive, when work is done.

103

by the system.

104

So let's analyze these two cases and work is done by the system.

105

So in chemistry, when work is done by the system, as we said before.

106

the internal energy decreases.

107

and because the energy of the system is decreasing, work is negative.

108

Remember, we're taking the viewpoint of the system.

109

In physics, we want to take the viewpoint of the surroundings.

110

So when work is done by the system, the internal energy of the system still decreases.

111

However, We say W is positive because the surroundings is gaining energy we're taking the viewpoint of the surroundings.

112

And so since the surroundings gain energy, w is positive.

113

In the case of Chemistry.

114

When work is done by the system, the system loses energy, so we say W is negative.

115

And so it's the viewpoint.

116

And that's why the equations are a bit different.

117

In chemistry, we take the viewpoint of the system, But in physics, We take the viewpoint.

118

on the surroundings.

119

So when work is done by the system, the system loses energy the surroundings gain energy.

120

So if you're focused on the surroundings, then W is positive because the energy of the surroundings go up.

121

If you focus on a system, the system loses energy when work is done by the system, and so W is negative.

122

But in this video, I'm gonna take the system's perspective.

123

So going back to this equation, delta u is q plus w.

124

You need to know that Q is positive.

125

whenever the system meaning the reactants instead of products, absorb heat energy.

126

So anytime he...

127

flows into the system heat energy is absorbed by the system, so Q is positive.

128

This is an endothermic process.

129

whenever heat energy is absorbed.

130

Now Q is negative.

131

whenever the system releases...

132

heat energy.

133

And so heat energy flows out of the system.

134

into the surroundings.

135

And so this is an exothermic process.

136

So anytime heat energies release...

137

it's exothermic.

138

and when heat energy is absorbed by the system, It's endothermic.

139

So, For an exothermic process, Q is negative, and for an endothermic process, Q is positive.

140

And as was mentioned before, W is positive when work is done on the system.

141

This is in Chemistry.

142

and W is negative whenever work is negative.

143

done by the system.

144

So you need to be familiar with the sign conventions.

145

if you're gonna use this equation.

146

So in another video, I'm going to give you some practice problems.

147

on calculating the change in internal energy.

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using heat and work.

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