What are the two general ways to increase the internal energy of a substance? Describe an example of each.

When we talk about increasing the internal energy of a substance, one effective way to achieve this is through the process of transferring energy as heat. Heat can be thought of as the flow of thermal energy from one object to another because of a difference in temperature. This process is fundamental in thermodynamics, the science concerned with heat and its relation to other forms of energy and work.

Let's consider the act of boiling water as an illustrative example: as you heat the water on the stove, the thermal energy provided by the heat source (the stove) is transferred to the water. This transfer causes the water molecules to move more energetically, which manifests as an increase in the temperature of the water. Once the temperature reaches a certain threshold, the water begins to boil, with the molecules moving so vigorously that they transition from the liquid to the gaseous phase. Hence, the internal energy of the water is increased through this introduction of heat.

Another gateway to escalating the internal energy of a substance is by doing work on it. In the realm of physics, ‘work’ is defined as the process of applying a force over a distance. When work is done on a substance, it typically involves a change in its physical state or shape that requires energy—energy that becomes part of the substance's internal makeup.

Compression and Internal Energy

Consider compressing a piston filled with gas—a classic example. When you apply force to compress the gas, you effectively decrease its volume. The gas molecules, which previously had more space to move around, are now packed closer together, and therefore they collide with each other and the walls of the container more frequently and with increased energy. This elevated rate of collision translates to a rise in internal energy for the gas. The work done on the gas doesn't disappear; instead, it's stored within as increased internal energy.

Thermodynamics is a branch of physics that deals with the relationships and conversions between heat and other forms of energy. It is essential for understanding how to increase the internal energy of a substance, as it provides us with the laws and principles that govern energy transfer.

There are four main laws of thermodynamics that describe how energy behaves. The first law, often referred to as the law of energy conservation, states that energy can neither be created nor destroyed, only transformed from one form to another or transferred from one substance to another. This law underpins both methods of increasing internal energy—whether it's by heating (transferring thermal energy) or doing work (transferring mechanical energy). This law assures us that the internal energy of a substance increases when heat is added or when work is done upon it because energy is being conserved and merely changing forms.

Delving down to the microscopic view of substances, molecular motion is at the heart of how internal energy is ramped up. Substances consist of atoms and molecules that are constantly in motion; this motion comprises vibrations, rotations, and translations (movements through space). The kinetic energy associated with this motion is a significant portion of a substance's internal energy.

The temperature of a substance is a measure of the average kinetic energy of its molecules. When a substance absorbs heat, this kinetic energy increases, causing faster molecular motion—if we're talking about a gas, its molecules move more quickly and spread out, leading to an increase in temperature and sometimes a change in state from liquid to gas. On the other hand, doing work on a substance, like compressing it, forces the molecules into a smaller volume which affects their motion as well, contributing to an increase in internal energy through enhanced molecular interactions.