Conservation of energy Revision Notes

Historical Dependence on Fossil Fuels

For decades, fossil fuels like coal, oil, and natural gas have been the primary energy sources. They are valued for their high energy output per unit mass and relatively inexpensive extraction and transportation infrastructure. This reliance has facilitated industrial growth and electrification but has also resulted in significant greenhouse gas emissions and air pollution, contributing to climate change and environmental degradation. The finite nature of these resources also raises concerns about long-term availability and energy security.

Shift Towards Renewable Energy

A prominent trend is the global transition to renewable energy sources, including solar, wind, hydroelectric, geothermal, and biomass. This shift is motivated by the need to reduce carbon emissions, combat climate change, enhance energy security by diversifying supply, and capitalize on the inexhaustible nature of these resources. While their initial setup costs can be higher and their output can be intermittent (weather-dependent), advancements in technology and decreasing costs are making them increasingly competitive and integrated into national grids.

Factors Influencing Energy Resource Trends

Key factors driving patterns in energy use include population growth, which directly increases energy demand; economic development, especially industrialization in emerging economies; technological progress in extraction, conversion, and storage; and global geopolitical stability influencing supply chains. Environmental policies and international agreements also play a crucial role in promoting cleaner energy and penalizing high-carbon emissions, further accelerating the move away from traditional fossil fuels towards sustainable alternatives.

Reducing Unwanted Energy Transfer through Lubrication

Lubrication involves applying a substance, such as oil or grease, between moving surfaces that are in contact. The primary purpose of lubricants is to reduce friction between these surfaces. Friction is a force that opposes motion and converts kinetic energy into thermal energy and sound energy, which are often unwanted. By reducing friction, lubrication minimises the amount of thermal energy generated, thereby increasing the efficiency of the system and preventing wear and tear on components. For example, engine oil reduces friction between the moving parts of a car engine, allowing them to move more freely and efficiently.

Reducing Unwanted Energy Transfer through Thermal Insulation

Thermal insulation is used to reduce the rate of thermal energy transfer between objects or regions at different temperatures. This is achieved by using materials that are poor conductors of thermal energy (insulators) and often by trapping air, which is an excellent insulator. In buildings, insulation materials like fibreglass or foam are installed in walls, roofs, and floors to prevent thermal energy from escaping in winter and entering in summer. This reduces the need for heating or air conditioning, saving energy. Similar principles apply to insulated containers like vacuum flasks, which minimise thermal energy transfer by conduction, convection, and radiation to keep contents hot or cold.

Sankey Diagrams

Sankey diagrams are flow diagrams that visually represent energy transfers and efficiency. The width of the arrows is proportional to the amount of energy involved. They typically show an input energy, useful output energy, and dissipated (wasted) energy. The useful output energy arrow continues straight, while wasted energy arrows branch off, often downwards, indicating dissipation to the surroundings, usually as heat or sound. The sum of the widths of the output arrows must equal the width of the input arrow, reflecting the law of conservation of energy.

Energy Transfer Chains

Energy transfer chains illustrate the sequence of energy transformations that occur in a system. They typically start with an initial energy store, followed by a series of transformations and transfers, ending with the final energy forms. For example, a battery powering a light bulb involves chemical energy (battery) -> electrical energy (wires) -> light energy + thermal energy (bulb). These diagrams help to identify the types of energy stores and pathways of transfer, making it clearer how energy moves and changes form within a system.

Common Energy Forms and Transfers

Energy exists in various forms, including kinetic (movement), gravitational potential (position in a gravitational field), elastic potential (stretched/compressed objects), chemical (stored in bonds), thermal (internal energy), electrical (moving charges), light (electromagnetic waves), and sound (vibrations). Transfers occur through mechanisms like heating (conduction, convection, radiation), electrical work (current flow), mechanical work (force over a distance), and sound waves. Understanding these forms and transfer methods is crucial for accurately drawing and interpreting energy diagrams.

Thermal Conductivity Defined

Thermal conductivity is a property of a material that indicates how well it conducts heat. Materials with high thermal conductivity (e.g., metals) allow heat to transfer quickly, while those with low thermal conductivity (e.g., insulation, trapped air) slow down heat transfer, acting as insulators. Buildings use materials with low thermal conductivity to reduce heat loss.

Effect of Wall Thickness

The thickness of a building's walls directly impacts heat transfer. Thicker walls mean a longer path for heat to travel from the inside to the outside. This increased path length slows down the rate of heat transfer, reducing cooling. Therefore, thicker walls provide better insulation, helping to keep the building warm.

Combined Effect on Cooling Rate

The rate of cooling of a building is inversely related to both the thickness of its walls and the insulation provided by its materials. Thicker walls (longer path for heat) and materials with lower thermal conductivity (poorer heat conductors) both contribute to a slower rate of heat loss from the building. This means the building cools more slowly, saving energy.