What is the difference between 'wasted' energy and 'dissipated' energy?

'Wasted' energy is a functional description of any energy not used for the intended purpose. 'Dissipated' energy describes the physical state of that energy being spread out into the surroundings, usually as heat.

How does a closed system differ from an open system regarding energy dissipation?

In a closed system, energy can be dissipated to different stores within the system, but the total energy remains constant. In an open system, dissipated energy can leave the system boundaries and enter the wider environment.

Why is it incorrect to say that energy is 'lost' in a friction-heavy process?

According to the Law of Conservation of Energy, energy cannot be destroyed. It is not lost but rather transferred from a useful store (like kinetic) to a non-useful store (the thermal store of the surroundings).

What is a common mistake when identifying the 'surroundings' in energy dissipation?

Students often forget that the 'surroundings' include the air, the surface the object is on, and the components of the machine itself. All these parts absorb dissipated thermal energy.

Define 'Thermal Store' in the context of energy dissipation.

The thermal store is the energy held by an object or the environment due to the kinetic energy of its constituent particles. Dissipation increases this store by raising the particles' vibration or movement.

What does it mean for energy to be 'spread out'?

It means the energy has moved from a concentrated, high-quality source (like a battery or a moving weight) to a vast number of particles in the environment, making it impossible to recapture for work.

How does lubrication reduce energy dissipation in a gearbox?

Lubrication creates a thin layer between moving metal parts, preventing them from directly rubbing. This reduces the work done against friction, meaning less energy is transferred to the thermal store.

Why does a lightbulb feel hot if its purpose is to provide light?

The heat is dissipated energy. Electrical resistance in the filament or circuitry transfers some of the input electrical energy to the thermal store instead of the light (radiation) store.

In a perfect vacuum, would a moving object still dissipate energy via air resistance?

No, because air resistance requires a fluid medium to transfer energy to. However, the object might still dissipate energy through internal friction or radiation if it has moving parts.

What is the relationship between dissipation and efficiency?

Efficiency is inversely related to dissipation. As the amount of energy dissipated to non-useful stores increases, the percentage of useful energy output decreases, lowering the overall efficiency.

Dissipation of Energy | Pearson Edexcel GCSE Physics

8. Energy – Forces Doing Work

Dissipation of Energy

Summary

Dissipation is the process by which energy is transferred into non-useful stores, typically spreading out into the surroundings as thermal energy. While the total energy in a system remains constant according to the Law of Conservation of Energy, dissipated energy becomes effectively 'wasted' because it is too spread out to be used for further work.

1. Definition & Core Concepts

Dissipation refers to the process where energy 'spreads out' into the surroundings, making it less concentrated and harder to use for a specific purpose. This usually occurs when energy is transferred to the thermal store of the environment.
Wasted Energy is any energy transfer that does not contribute to the intended function of a system. For example, in a moving vehicle, energy transferred to the air as heat due to friction is considered wasted.
Useful Energy is the energy transferred to the specific store required for the system's purpose, such as kinetic energy in a motor or light energy in a lamp.

Sankey diagram showing energy input splitting into useful energy and dissipated energy.

2. Underlying Principles

The Law of Conservation of Energy dictates that energy cannot be created or destroyed, only transferred between stores. Therefore, 'lost' energy is never truly gone; it has simply moved to a store that is not useful for the task at hand.
Thermal Equilibrium is the state toward which dissipated energy moves. As energy spreads into the surroundings, it increases the temperature of the air and nearby objects until they reach a uniform temperature.
Entropy is the underlying thermodynamic reason for dissipation. Systems naturally tend toward a state of higher disorder, meaning energy naturally spreads out from concentrated sources to the wider environment.

3. Methods of Dissipation

Friction: When two surfaces rub together, work is done against the frictional force. This mechanical work is transferred into the thermal stores of the surfaces and the surrounding air.
Air Resistance: As an object moves through a fluid (like air), it must push particles out of the way. This transfers kinetic energy from the object to the thermal store of the fluid particles.
Electrical Resistance: When current flows through a wire, electrons collide with the ions in the lattice of the conductor. These collisions transfer energy to the thermal store of the wire, causing it to heat up.

4. Reduction Techniques

5. Key Distinctions

6. Exam Strategy & Tips