First Law and Internal Energy

In physics, thermodynamics is a branch of science that deals with heat, work, and forms of energy. The first law of thermodynamics and the concept of internal energy are fundamental topics in this field. For students learning about these concepts for the first time, it is important to understand them in a simple and straightforward manner.

First law of thermodynamics

The first law of thermodynamics, also called the law of conservation of energy, states that energy cannot be created or destroyed, it can only be converted from one form to another. This principle is fundamental because it establishes that the total energy in an isolated system remains constant.

ΔU = Q - W

In this formula, ΔU represents the change in the internal energy of the system, Q is the heat added to the system, and W is the work done by the system on its surroundings. Let's break each of these down:

• ΔU (change in internal energy): It is the change in energy of the system due to heat and work.
• Q (heat): This is the energy transferred into the system. If heat is added, it is considered positive. If heat is taken out, it is considered negative.
• W (Work): This is the energy used by the system to do work on its surroundings. If the system does work on its surroundings, it is considered positive. If work is done on the system, it is considered negative.

The first law emphasizes the conservation of energy, and states that the energy input into a system will always equal the energy output.

Understanding internal energy

Internal energy refers to the total energy present in a system. It includes all the microscopic energies, such as the kinetic and potential energies of the molecules in a substance. Internal energy can be changed by heat transfer or by work being done on or by the system.

Internal energy can be viewed through three different scenarios:

• Heating a gas: When a gas is heated, its internal energy increases because the kinetic energy of the molecules increases. This means the molecules are moving faster.
• Compression of a gas: When a gas is compressed, work is done on the gas, which increases its internal energy. The molecules are pressed closer to each other, which increases their potential energy.
• Expansion of gas: When a gas expands, it does work on the surroundings. This requires energy, which usually reduces the internal energy and results in slowing down the motion of the molecules.

First law and examples of internal energy

Example 1: Heating water in a vessel

Suppose water is being heated in a vessel on the stove. Heat from the stove is transferred to the water, raising its temperature. The heat added is Q If the water is allowed to expand while being heated, it can do work on the air above it, represented by W

In this scenario, according to the first rule:

ΔU = Q - W

The increase in internal energy of water will be equal to the heat added to it minus the work done for expansion.

Example 2: Compressing air in a bike pump

Consider using a bike pump to put air into a tire. When you push the pump down, you are doing work on the air inside, represented by W This work increases the internal energy of the air, causing it to heat up. No heat Q is added from the outside, so:

ΔU = 0 - (-W) = W

Here, the increase in internal energy is equal to the work done on the gas, since there is no heat transfer in or out.

Example 3: Cooling the engine

In an engine, the internal energy may decrease when it loses heat Q to its surroundings. For example, a car engine loses heat while running, and the work it does may be in the form of physical motion W In this case, the first law can be expressed as:

ΔU = Q - W

The change in internal energy is the energy obtained by subtracting the work done by the engine from the energy lost as heat emitted.

Visual representation with simple diagrams

Let's use some simple diagrams to understand these concepts:

To understand how energy flows into and out of a system, consider this simplified diagram of a gas inside a piston:

Here, heat Q enters the system (piston) while work W is done by the gas, causing the piston to move upward. The boundary of the system is marked by the gray rectangle.

Conclusion

The first law of thermodynamics is a cornerstone in understanding how energy is managed in a physical system. It underlies the idea of conservation, indicating that the forms of energy can change, but it cannot easily disappear or be created from nothing.

In a broader sense, this law applies not just to the examples of gases and engines, but to all systems where energy is transferred. By studying the first law along with the concept of internal energy, students can better understand fundamental concepts that are important in physics and engineering.

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