Sposoby zmiany energii wewnętrznej ciała

Introductory questions.

1. What is matter built of?

2. What types of energy do you know?

3. What kind of energy do the particles have? Can you change it? How?

4. How could we change the temperature of, for example, the aluminum rod?

5. Is there any relationship between the bar's temperature and the energy of molecules?

Conclusion

Temperature is the size determining the state of the body due to the kinetic energy of the moving particles of the body. The greater the kinetic energy of moving body molecules, the higher the body temperature.

The concept of internal energy is closely related to the potential and kinetic energy of all body molecules.

The kinetic energy of molecules is related to the speed of molecules and the potential energy with interaction between them.

The kinetic energy of molecules changes, for example, when the body temperature changes, and the potential energy of molecules changes when the state of body changes.

The unit of internal energy is joule [J].

It follows that the value of internal energyinternal energyinternal energy depends on three factors.

• The number of atoms and molecules which are part of this body – the more molecules, the greater the number of components of the sum.

• Body temperature - higher temperature means a higher value of the average kinetic energy of the particles, so the total energy of the system is greater.

• The type of substance and its state of matterstate of matterstate of matter - the amount of potential energy which comes from intermolecular interactions changes with the state of matter of the substance and is different for different substances. This will be explained comprehensively in the subsection devoted to the processes of changing the states of matter.

The following examples explain the above statements.

• At the same constant temperature, a bucket of water has more internal energyinternal energyinternal energy than a glass of water, because in the bucket there are many more particles.

• A glass of hot tea has more internal energyinternal energyinternal energy than the same amount of cold tea, because the particles of hot liquid have higher average kinetic energy. In both glasses there is the same number of particles.

• A glass of water at 0°C has more internal energyinternal energyinternal energy than the ice which was formed of the water at the same temperature.

The internal energyinternal energyinternal energy of the body can be changed in two ways.

Examples of changes in internal energy through work:

• putting your hands on you (internal energy grows),

• pump the bicycle wheel using a pump (internal energy increases, the pump feels hot),

• hammering a metal sheet with a hammer (internal energy is growing, the metal sheet is hot),

• sprays aerosol (internal energy decreases, the container feels cold).

Examples of changes in internal energy through heat exchange:

• cooling of dishes in the refrigerator (internal energy of the food decreases),

• water heating in the kettle (internal energy of water increases),

• freezing of water on the lake (internal energy of the water decreases and the surrounding area grows),

• water evaporation from the human skin after leaving the bath - we feel the coolness (the internal energy of the water decreases and the surroundings grow).

The mechanism of internal energyinternal energyinternal energy transfer due to the temperature difference is the thermal flow of energy.

Joule is a unit of heatheatheat and is used for all other kinds of energy.

Thermal flow of energy is possible only between bodies of different temperatures. It always takes place from the body with a higher temperature to the body with the lower temperature and stops after the temperatures have equalised.

The first law of thermodynamicsthermodynamicsthermodynamics

The change in the internal energyinternal energyinternal energy of the body/body system (the body system can be e.g. water in a glass, or water in a bucket, etc.) is always equal to the sum of work done on the body (or by the body) and the heatheatheat supplied to the body (given out by the body):

where:
ΔU - the change of the internal energyinternal energyinternal energy of the body,
W - the work done on the system (by system),
Q - the heatheatheat supplied to the system (rendered by the system).

In order to avoid misunderstandings, the following agreement was introduced to differentiate the work done on the body from the work done by the system.

• W – the work done on the system is always positive (W > 0).

• W – the work done by the system is always negative (W < 0).

There is also a similar agreement which is related to heat exchangeheat exchangeheat exchange with the environment.

• Q – the heatheatheat supplied to the system is always positive (Q > 0).

• Q – the heatheatheat emitted by the system is always negative (Q < 0).

If we apply the above findings, then ΔU < 0 means the decrease of the internal energy of the body, and thus the decrease of its temperature. By contrast, ΔU > 0 means an increase in the internal energy of the body, and thus an increase in body temperature.

If we skillfully use certain processes, we can build a thermal engine, which converts part of the heatheatheat taken from a heater warmed to high temperature into usable work.

Summary

1. The internal energyinternal energyinternal energy of the body is the sum of the kinetic and potential energies of all atoms or molecules that this body consists of.

2. The value of internal energyinternal energyinternal energy depends on:

• the number of atoms and molecules which are part of this body – the more molecules, the greater the number of components of the sum,

• body temperature - higher temperature means a higher value of the average kinetic energy of the particles,

• the type of substance and its state of matterstate of matterstate of matter - the amount of potential energy which comes from intermolecular interactions changes with the state of matterstate of matterstate of matter of the substance and is different for different substances.

3. The first law of thermodynamics determines the relationship between the change of the internal energy of the body and the work done by the body (or over the body) and heat (delivered or given away by the body).
The first law of thermodynamics states that the change in the internal energy of the system ΔU is equal to the sum of the heat supplied Q and the work done on the system W.

4. The work done by the body against the friction force can increase its internal energyinternal energyinternal energy. Rubbing hands or starting fire by rubbing two properly prepared dry woods are the examples of such situation.

Exercises

Glossary

Keywords

internal energyinternal energyinternal energy

heatheatheat

heat exchangeheat exchangeheat exchange

doing workdoing workdoing work

temperature increasetemperature increasetemperature increase