MÁQUINAS TÉRMICAS - TERMOLOGIA - Aula 18 Prof. Boaro

Summary

This physics lesson on thermodynamics explores the principles behind thermal machines like engines and refrigerators. It explains how thermal motors convert heat into mechanical work, demonstrating the second law of thermodynamics and the inevitability of energy loss. The efficiency of these systems is calculated by comparing the work done to the heat absorbed. Examples include internal combustion engines and refrigerators, illustrating how heat is transferred from cold to hot sources. The lesson also covers real-life applications, providing a clear understanding of how energy is transformed in everyday machines.

Takeaways

• 😀 Thermal machines convert heat energy into work. Key types include heat engines and refrigerators.
• 😀 Heat engines, like steam engines, absorb heat from a hot source to do work, and then expel some of that heat to a cold source.
• 😀 According to the second law of thermodynamics, it’s impossible for a heat engine to convert all heat energy into work without losing some to a cold source.
• 😀 In a cyclic thermodynamic process, the change in internal energy is zero because the system returns to its initial state.
• 😀 The efficiency of a heat engine is the ratio of work done to the heat absorbed from the hot source.
• 😀 A simple formula for efficiency is: Efficiency = (Work Output) / (Heat Input) = (Q_H - Q_C) / Q_H.
• 😀 Efficiency is limited because not all heat energy can be converted into work. Some is always lost to the cold source.
• 😀 A refrigerator works by transferring heat from a cold source to a hot source, using work done by a motor (e.g., a compressor).
• 😀 The efficiency of a refrigerator is calculated as the ratio of heat extracted from the cold source to the work required to transfer that heat.
• 😀 In a real-world example, a thermal engine with an 80 kcal heat intake and 60 kcal heat rejection has an efficiency of 25%.
• 😀 Thermal machines are critical in industrial history, particularly since the 19th century during the industrial revolution, where their development was key to advancing technology.

Q & A

What is a thermal engine, and how does it work?

A thermal engine uses heat from a hot source to generate work. For example, in a steam engine, heat from burning fuel (like coal) is used to convert water into steam, which then moves a piston to perform mechanical work. However, not all energy from the hot source is converted into work; some energy is lost to the cold source.

What is the Second Law of Thermodynamics, and how does it apply to thermal engines?

The Second Law of Thermodynamics states that it is impossible to create a machine that converts all energy from a hot source into work without losing some energy to the cold reservoir. This means thermal engines always have some energy that is wasted and cannot be used to perform work.

How is the efficiency of a thermal engine calculated?

The efficiency of a thermal engine is calculated by dividing the work done by the engine by the heat absorbed from the hot source. The formula is: Efficiency = Work / Heat received from the hot source. This ratio indicates how much of the energy from the hot source is converted into useful work.

Can you give an example of how to calculate the efficiency of a thermal engine?

For example, if a machine absorbs 1000 joules of energy from a hot source and rejects 700 joules to the cold source, the remaining 300 joules are used to perform work. The efficiency is calculated as 300 / 1000 = 30%, meaning the engine is 30% efficient.

What is the difference between a thermal engine and a refrigerator?

A thermal engine converts heat from a hot source into work, while a refrigerator works in reverse, transferring heat from a cold space to a warmer one. Refrigerators require work to be done on a refrigerant gas to move heat from the inside of the fridge (cold source) to the outside (warm source).

How does a refrigerator work, and what role does work play in the process?

A refrigerator removes heat from a cold space (like the inside of the fridge) and transfers it to a warmer space (outside of the fridge). This requires a motor to do work, typically by compressing a refrigerant gas, which helps to move the heat from inside to outside.

What is the efficiency of a refrigerator, and how is it calculated?

The efficiency of a refrigerator is the ratio of the heat removed from the cold space to the work required to do so. The formula is: Efficiency = Heat removed from cold space / Work required. This efficiency tells us how effective the refrigerator is at cooling relative to the amount of energy it consumes.

Why is the term 'efficiency' used for refrigerators instead of 'rendimento'?

The term 'efficiency' is used for refrigerators instead of 'rendimento' because the focus is on how much heat is moved from a cold space to a warm one, relative to the work required. This measure compares the amount of energy transferred (heat) to the energy input (work), offering a clear understanding of the refrigerator's performance.

What is an example of calculating the efficiency of a refrigerator?

If a refrigerator removes 400 joules of heat from a cold space and uses 100 joules of work to do so, the efficiency would be 400 / 100 = 4. This means the refrigerator is 4 times more effective at moving heat than the energy it consumes for work.

How does the example of a car engine relate to thermal efficiency?

In a car engine, if 80 kcal of energy is absorbed from the fuel and 60 kcal is rejected as waste heat, the remaining 20 kcal is used to perform work. The efficiency of the engine is calculated as 20 / 80 = 0.25, or 25%. This illustrates how much energy is effectively used for work compared to how much is lost.