Joules

Definition

Joules are the standard unit of energy in the International System of Units (SI). They measure the amount of work done or energy transferred in various physical and chemical processes, including those related to phase changes and thermodynamics.

5 Must Know Facts For Your Next Test

1. One joule is defined as the amount of work done when a force of one newton is applied over a distance of one meter.
2. Joules are used to measure the energy required for phase changes, such as the latent heat needed to melt ice or boil water.
3. The Second Law of Thermodynamics states that the entropy of an isolated system not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium, which is measured in joules per kelvin.
4. The relationship between energy, work, and heat is described by the first law of thermodynamics, which states that the change in the internal energy of a system is equal to the sum of the work done on the system and the heat added to the system, all measured in joules.
5. Joules are also used to measure the energy content of food, with one calorie being equivalent to 4.184 joules.

Review Questions

• Explain how joules are used to measure the energy required for phase changes, such as melting ice or boiling water.
  • The energy required to change a substance from one phase to another, known as the latent heat of the phase change, is measured in joules. For example, the latent heat of fusion for ice is 334 joules per gram, which is the amount of energy needed to melt one gram of ice at 0°C and atmospheric pressure. Similarly, the latent heat of vaporization for water is 2,260 joules per gram, which is the energy required to convert one gram of liquid water into water vapor at the boiling point. These phase change energies, measured in joules, are crucial for understanding the energy transformations that occur during physical changes of state.
• Describe how joules are used to quantify the relationship between energy, work, and heat as described by the first law of thermodynamics.
  • The first law of thermodynamics states that the change in the internal energy of a system is equal to the sum of the work done on the system and the heat added to the system, all measured in joules. This means that energy can be transformed between different forms, such as work and heat, but the total energy of the system remains constant. For example, if a system does 50 joules of work and receives 75 joules of heat, the change in its internal energy would be 75 joules - 50 joules = 25 joules. The use of joules as the unit of measurement allows for the precise quantification of these energy transformations and the conservation of energy principle.
• Explain how joules are used to measure the entropy changes described by the Second Law of Thermodynamics, and how this relates to the concept of energy degradation.
  • The Second Law of Thermodynamics states that the entropy of an isolated system not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium. Entropy is measured in joules per kelvin, which quantifies the degree of disorder or randomness in a system. As a system undergoes processes that increase its entropy, the energy available for useful work decreases, a phenomenon known as energy degradation. This is because the energy becomes more dispersed and less concentrated, making it less useful for performing work. The use of joules per kelvin to measure entropy changes allows for the understanding of how the inevitable increase in disorder affects the ability to extract useful work from a system, a fundamental principle in the study of thermodynamics.

Related terms

Kinetic Energy: The energy of motion, which is directly proportional to the mass of an object and the square of its velocity.

Potential Energy: The stored energy of an object due to its position or state, such as the energy of a raised object in a gravitational field.

Work: The transfer of energy by a force acting on an object, measured as the product of the force and the distance moved in the direction of the force.