15.1 The First Law of Thermodynamics

The energy in this tea kettle is moving.

The water in the kettle is turning to water because of the heat being transferred from the stove to the kettle.
The work is done when the system gets hotter.

The conserve of energy principle is important if we are interested in how heat transfer is converted into work.

The first law of thermodynamics states that heat transfer and doing work are the methods of transferring energy into and out of the system.

The internal energy of the system has changed.

If there is a net heat transfer into the system, then there is net work done by the system.

Positive takes energy from the system and negative takes energy from it.
The difference between heat transfer into the system and work done is stored as internal energy.
The transfer of heat into a heat engine is a good example of this.

The first law of thermodynamics states that heat and work can transfer energy to a system in thermal equilibrium.

The change in the internal energy of the system is related to heat and work.

The law of conserve of energy is the first law of thermodynamics.

The relationship between heat transfer, work done, and the change in internal energy of a system is given in the first law.

Taking energy out of a system is one of the ways in which heat transfer is used.

The processes are not the same.
The process of heat transfer is driven by temperature differences.
Work involves a force through a distance.
Both heat and work can cause a temperature increase.
When the Sun warms the air in a bicycle tire, it can increase its temperature, and so can work done on the system, as when the bicyclist pumps air into the tire.

It is not possible to tell if the temperature increase was caused by heat transfer or by doing work.

This uncertainty is important.
Neither heat transfer nor work is stored as such in a system.

Both can change the internal energy of a system.

Internal energy is not the same as heat or work.

The internal energy of a system can be thought of in two different ways.

The atomic and molecular view looks at the system on the atomic scale.
It is called mechanical energy.
The sum of mechanical and atomic energy is called internal energy.
We have to deal with averages and distributions because it is impossible to keep track of all individual atoms.
The internal energy of a system can be viewed in terms of its macroscopic characteristics, which are very similar to atomic and average values.

Many detailed experiments have verified that where is the change in the total energy of the system.

The internal energy of a system depends on the state of the system and not how it gets to that state.
The function of pressure, volume, and temperature is independent of past history such as whether there has been heat transfer or work done.
If we know the state of a system, we can calculate changes in its internal energy from a few variables.

When making calculations of how a system behaves, we often use the macroscopic picture, while the atomic and molecular picture gives underlying explanations.

This will be seen in later sections of this chapter.
calculations will be made using the atomic and molecular view

To get a better idea of how to think about the internal energy of a system, we should look at a system going from State 1 to State 2.

No matter how it got to either state, the system has internal energy in State 1 and State 2.

The change in internal energy is not related to what happened.
Is independent of path.
The method of getting from the beginning to the end is called path.

Both depend on path.

Internal energy is easier to consider than heat transfer or work done.

There is a heat transfer of 25.00 J out of the system and 4.00 J of work done on the system.

We need to find the net heat transfer and net work done from the given information.

The first law of thermodynamics can be used to find a change in internal energy.
The equation can be used directly if the net heat transfer and work done are given.

We can see the change in internal energy for each step.

The change in internal energy is the same regardless of whether you look at the overall process or break it into steps.

A very different process produces the same change in internal energy.

The change in the system is related to the system as a whole.
The system ends up being the same in both places.
There are two different paths for the system to follow between the same starting and ending points, and the change in internal energy for each is the same.

There are two processes that produce the same change.

The internal energy is changing.
The final state of the system is related to internal energy, not how it was acquired.

The first law of thermodynamics is called metabolism.

The first law of thermodynamics gives us another look at these topics.

The first law can be used to examine heat transfer, doing work, and internal energy in activities ranging from sleep to heavy exercise.

Body temperature is kept constant by heat transfer.
The body works on the outside world.
The body loses internal energy when it is negative.

Think about the effects of eating.

Eating increases the internal energy of the body by adding chemical potential energy.
All the food we eat is absorbed by the body.
metabolism is an oxidation process in which the chemical energy of food is released Food input is in the form of work.

The units are determined by burning food in a calorimeter.

The energy required to raise the temperature of one gram of pure water by one degree Celsius is defined in chemistry and biochemistry.

The Calorie is often used by weight watchers and Nutritionists, and is spelled with a capital C. The amount of energy needed to raise the temperature of one kilogram of water by one degree Celsius is called one food Calorie.
One kilo calories is equal to one kilo calories for the chemist, so one must be careful to avoid confusion.

The body has lost internal energy.

This internal energy can be used to heat transfer, to do work, and to store fat.
The internal energy of the body is taken out by heat transfer and doing work.

In the long run, whatever you lose to heat transfer and doing work is replaced by food.

If you eat too much, your body stores the extra energy as fat.
If you eat too little, the reverse is true.
If it is negative for a few days, the body will use its own fat to maintain its temperature and work that takes energy from the body.
Dieticians use this process to produce weight loss.

Any dieter knows that life is not always easy.

If energy intake changes for a period of several days, the body stores fat.
Your body alters the way it responds to low energy intake if you have been on a major diet.
The BMR is the rate at which food is converted into heat and work is done while the body is at rest.
The body adjusts its metabolism to make up for undereating.
The body will not eliminate its own fat in order to replace lost food intake.
As a result of the lower metabolism, you will not lose weight as fast and you will feel less energetic.
Exercise raises your metabolism even when you are not working and it helps to lose weight because of the heat transfer from your body and work.
Weight loss is aided by the low efficiency of the body in converting internal energy to work, so that the loss of internal energy resulting from doing work is much greater than the work done.

The body shows us that many processes are irreversible.

Under certain conditions, an irreversible process can go in one direction but not the other.
Although body fat can be converted to do work and produce heat transfer, work done on the body and heat transfer into it cannot be converted to body fat.
We could skip lunch by walking down stairs.
Another example of an irreversible process is photosynthesis.
The intake of one form of energy--light--by plants and its conversion to chemical potential energy is the process.
One advantage of the first law of thermodynamics is that it accurately describes the beginning and end points of complex processes, such as metabolism and photosynthesis.
There is a summary of terms relevant to the first law of thermodynamics.

Work done by the body removes internal energy while food intake replaces it.

There are many subcategories, such as thermal and chemical energy.

Depends on the state of a system, not how the energy enters it.
It is path independent when it comes to internal energy change.