Fundamentals of Energy, Work, and Thermodynamics

Understanding Energy and Mechanical Work

Energy is the physical quantity representing the ability of bodies to perform changes on themselves or others. It is measured in joules (J).

Properties of Energy

The properties of energy are fundamental to our civilization: energy can be transformed from one form to another, transferred between systems, and remains constant. However, with each transformation, energy becomes less capable of performing further work; it degrades.

Mechanical Work

In physics, a force performs work when its point of application moves in a direction not perpendicular to it. The mechanical work (W) of a constant force on a body is expressed as the scalar product of force and displacement, measured in Joules (N·m).

Types of Work

• Motor Work: If the angle is between 0° and 90°, work is positive. It promotes movement and increases the body's energy.
• Zero Work: If the force is perpendicular to the displacement, no work is performed.
• Resistant Work: If the angle is between 90° and 180°, work is negative. It opposes motion and consumes the body's energy (e.g., frictional forces).

When several forces act on a body, the sum of all work done equals the work performed by the resultant force.

Power

Power is the work done by a system per unit of time, measuring the speed of energy transfer. Its unit is the watt (W), equivalent to 1 J/s. Note that one horsepower equals 735.5 W (1 HP = 735.5 W).

Conservative Forces and Energy Theorems

A force is conservative when the work done depends only on the initial and final positions of its point of application.

Kinetic and Potential Energy

• Kinetic Energy: The ability of a body to perform work due to its motion.
• Work-Energy Theorem: The total work done on a body moving from position 1 to 2 is equal to the change in its kinetic energy.
• Gravitational Potential Energy: The ability of a body to perform work due to its position relative to the Earth's surface.

Conservation of Mechanical Energy

The Principle of Conservation of Mechanical Energy states: "If the only forces acting on a body are conservative, the mechanical energy of the body remains constant."

Thermodynamics and Internal Energy

Body temperature is a measure of the average kinetic energy of the particles forming the body. Heat is energy transferred between two bodies due to a temperature difference.

The Laws of Thermodynamics

Thermodynamics is the branch of physics that studies energy exchanges between a system and its environment. The internal energy of a system is the total energy of its constituent particles, including translational, vibrational, and rotational kinetic energies.

First Law of Thermodynamics: The internal energy change of a system is equal to the sum of the heat exchanged with the environment and the work done by or on the system.