Question

A gas in a piston-cylinder device is compressed, and as a result its temperature rises. Is this a heat or work interaction?

Short answer

Answer: The temperature increase in the piston-cylinder device is caused by a work interaction. This is because the mechanical compression of the gas results in increased internal energy, leading to a rise in temperature.

Step-by-step solution

Understand the definitions of heat and work

Heat and work are two different forms of energy transfer. Heat is the transfer of energy due to a temperature difference across a boundary, while work is the transfer of energy due to mechanical or electrical means.

Identify the energy transfer in the problem

In this situation, a gas is being compressed in a piston-cylinder device. Since there is mechanical compression involved, this indicates that work is being done on the gas.

Analyze the effect of work on the gas

As work is being done on the gas, its particles are pushed closer to each other, resulting in an increase in the gas's internal energy. This increased internal energy causes an elevation in the gas's temperature.

Conclusion

Since the temperature increase is due to the mechanical compression of the gas, it is a work interaction, not a heat interaction.

Key concepts

Heat vs Work

Understanding the difference between heat and work is fundamental in thermodynamics, as both are methods of energy transfer. Heat, denoted as 'Q', is energy transferred between systems or objects with different temperatures. It occurs naturally and flows from a higher temperature to a lower one, driven by the temperature difference. In contrast, work, denoted as 'W', is the process of energy transfer that results from a force acting on an object over a distance. This can manifest as a variety of actions, such as mechanical compression, electrical power transfer, or any other instance where a force causes displacement.

For instance, when you rub your hands together, they get warmer through friction—that's work leading to a temperature rise. On the other hand, when you hold an ice cube, your hand gets cold as heat flows from your warmer hand to the cooler ice cube. In thermodynamics problems, determining whether energy transfer is occurring due to temperature differences (heat) or forces causing displacement (work) is crucial for understanding the system's behavior.

Piston-Cylinder Device

The piston-cylinder device is a classic example of a system encountered in thermodynamic studies, often used to illustrate the principles of energy transfer. This apparatus consists of a cylinder with a movable piston at one end that can compress or expand the gas contained within it. The movement of the piston changes the volume of the gas, and with it, the gas's pressure and temperature can also change.

The operation of a piston-cylinder device can be involved with both heat transfer and work done by or on the gas. When the piston is pushed inward, work is done on the gas as it compresses, while pulling the piston out does work by the gas as it expands. The nature of these processes and the resultant changes in energy are dependent on the specific conditions and constraints of the problem.

Temperature Change in Gases

When considering gases, temperature change is closely tied to changes in other properties, including volume and pressure, according to the gas laws. The mechanism of this change can depend on whether it's caused by heat transfer or work done on or by the gas.

For example, during mechanical compression in a piston-cylinder device, as the volume of the gas decreases, the molecules become more densely packed and their collisions become more frequent and energetic. This raises the internal energy and thus the temperature of the gas. Alternatively, heating a gas at constant volume will similarly increase the temperature by adding energy directly to the gas molecules, increasing their kinetic energy. The understanding of how temperature changes relative to heat and work leads to better comprehension of thermodynamic cycles and processes.

Mechanical Compression

Mechanical compression is an essential concept in thermodynamics and is particularly significant when describing the work done in piston-cylinder devices. It involves decreasing the volume of gas by applying an external force via the piston. During compression, the gas molecules are forced into a smaller space, which increases their speed due to an increase in collisions among themselves and with the walls of the container. These heightened collisions result in an increase in the internal energy and hence an increase in temperature.

It's important for students to recognize that mechanical compression is a work interaction. Unlike heat, which can be transferred due to a difference in temperature, mechanical compression does not directly involve thermal energy but rather converts mechanical energy into increased internal energy of the gas. This concept is central to understanding engines and refrigeration cycles, where the compression and expansion of gases play vital roles.