Fundamental Principles of Work, Power, and Energy

Work and Power in Physics

Work acts on a body when a force moves it through a distance; we say that force has done a job. Therefore, the work will be calculated as: W = F · s. The work unit in the SI is the Joule (J), which is defined as the work performed by applying a force of 1 Newton over 1 meter (1J = 1N · 1m).

Conditions for No Work

No work is done when:

• 1. There is no displacement (space is 0); the work is zero.
• 2. The force and displacement are perpendicular (90°).

Power and Efficiency

Power: The quantity that relates work over time; therefore, P = W / t. The SI unit is the Watt (W), defined as work performed at 1 Joule per second (1W = 1J / 1s). It can also be expressed as: P = F · v.

Efficiency: The performance of a machine is never 100% because a portion of the energy consumed is lost as heat. It is expressed as a percentage (%); it has no units because it is the quotient between two energies.

Energy and Its Forms

Energy: The ability of a body to perform work; therefore, the concepts of energy and work are related and measured in the same units (J). Depending on the ability to produce different classes of work, energy is converted into others: mechanical, chemical, heat, light, and nuclear.

Kinetic and Potential Energy

Energy is presented in two aspects: Kinetic Energy and Potential Energy. Potential energy is the energy a body has by virtue of its position with respect to the surface of the Earth (i.e., placed at a given height). It depends on body mass and height: E.pot = mgh.

Conservation of Mechanical Energy

Principles of Conservation of Mechanical Energy: The total mechanical energy of a system that is considered isolated (meaning no energy exchange with the outside world) remains constant.

Thermal Energy and Equilibrium

Thermal Energy: Heat energy is the energy transmitted from hot bodies to cold bodies when they come into contact. Thermal energy depends on three factors: body mass, temperature difference, and the nature of the body. The formula is Q = m · Ce · (T1 - T2).

Specific Heat and Temperature

The Specific Heat (Ce) is defined as the amount of heat to be provided to a body so that, at a pressure of one atmosphere, its temperature increases by 1°C. In the SI, it is measured in Joules (J). There is another unit for measuring heat, and temperature is defined as the measure of the energy level of bodies.

Thermal Equilibrium

Thermal Equilibrium: When two bodies come into contact at different temperatures, the hot body cools and the cold body heats up until they reach the same temperature, called the balance temperature.

• Hot body (Heat lost): Qc = m1 · Ce1 · (T1 - Tx)
• Cold body (Heat gained): Qg = m2 · Ce2 · (Tx - T2)
• Equilibrium Equation: Qc = Qg