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# The First Law of Thermodynamics & Energy Transfer

This lesson aligns with Next Generation Science Standards (NGSS) PS3.B

Introduction
To understand the first law of thermodynamics, we need to understand the concept of internal energy. Internal energy is a thermodynamic property of the system known as energy associated with the molecules of the system which includes potential energy and kinetic energy. The first law of thermodynamics, also known as the law of energy conservation, is a fundamental principle that governs the behavior of energy. In this article, we will explore the first law of thermodynamics and its significant role in energy transfer, providing concrete examples to aid in understanding this fundamental concept.

The First Law of Thermodynamics:
Law of Energy ConservationWhenever any change occurs in a system due to the interaction of work, heat, and internal energy, it undergoes various energy transfers and conversions. However, during these conversions and transfers, there is no net change in the total energy.

Similarly, if we talk about the first law of thermodynamics, it affirms that heat is a form of energy. The first law of thermodynamics states that “energy cannot be created or destroyed but can only be transferred from one form to another”. This law is based on the principle of energy conservation, which means that the total energy of a closed system remains constant over time. In simpler terms, the energy input must equal the energy output.

A thermodynamic system in an equilibrium state keeps the state variable known as internal energy (E). The change in the internal energy between two systems is equal to the heat transfer into the system minus the work done by the system, as shown in Figure 1.

Role of the First Law in Energy Transfer
The first law of thermodynamics plays a vital role in energy transfer. Let's look at a few examples to understand its practical application.

Electrical Energy Conversion
When you plug in an electrical device, such as a smartphone charger, electrical energy from the power source is transferred into the device. The first law of thermodynamics ensures that the electrical energy input is equal to the energy output, which can be in the form of heat, light, or mechanical work. The law allows us to calculate the efficiency of this energy conversion process. It also helps us determine how much of the input energy is converted into useful output energy.

Burning Fossil Fuels
The first law is also crucial in understanding the energy transfer that occurs when we burn fossil fuels, such as gasoline. When fuel is burned in a car's engine, the chemical energy stored in the fuel is transformed into heat energy. As a result, it is converted into mechanical work to move the vehicle. The first law ensures that the total energy input from the fuel combustion is equivalent to the energy output in the form of work and heat.

Solar Energy
Solar panels convert sunlight into usable electrical energy. When sunlight falls on the solar cells, the energy from photons is absorbed, causing electrons to move and generate an electric current. The first law of thermodynamics guarantees that the total energy input from the sun is equal to the electrical energy output, accounting for losses due to inefficiencies in the conversion process.

Nuclear Power
In nuclear power plants, the first law is essential in understanding the energy transfer that takes place during nuclear fission. When the nucleus of an atom splits, a tremendous amount of energy is released in the form of heat. This heat is used to produce steam, which drives a turbine to generate electricity. The first law ensures that the input energy from the nuclear reaction is equivalent to the energy output in the form of electricity.

Summary
• The first law of thermodynamics is the law of energy conservation.
• This law states that energy cannot be created or destroyed but can only be transferred or transformed from one form to another.
• The change in internal energy (∆U) of a system equals the heat added to the system minus the work done.
• Electrical energy conversion, burning fossil fuels, solar energy, and nuclear power are examples that illustrate the application of the first law in practical scenarios.

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