Conservation of energy Revision Notes

Non-Renewable Energy Sources

Non-renewable energy sources are finite and take millions of years to form, meaning they cannot be replenished as quickly as they are consumed. Fossil fuels (coal, oil, natural gas) are formed from ancient organic matter under heat and pressure. They are burned in power stations to heat water, produce steam, and drive turbines, generating electricity. A major drawback is the release of greenhouse gases, contributing to climate change. Nuclear fuel (uranium) is used in nuclear power plants, where controlled nuclear fission releases heat energy to produce steam. While not producing greenhouse gases, it generates highly radioactive waste that must be stored safely for thousands of years.

Renewable Energy Sources

Renewable energy sources are naturally replenished, making them sustainable over the long term. Solar power directly converts sunlight into electricity using photovoltaic cells or concentrates it to heat water for steam turbines. Wind power harnesses the kinetic energy of moving air with wind turbines. Hydro-electricity uses the potential energy of water stored at height, released to turn turbines. Tidal power captures the energy from the movement of ocean tides using barrages or turbines. Bio-fuels are derived from recently living plant or animal matter and can be burned or processed into liquid fuels. While generally producing fewer greenhouse gases than fossil fuels, some renewable sources have large land footprints or depend on variable natural conditions.

Energy Transfers and Stores

Energy is always conserved. When a system undergoes a change, energy moves between different stores or is transferred to the surroundings. Common energy stores include kinetic (due to motion), gravitational potential (due to height), elastic potential (due to stretching/compressing), thermal (due to temperature), chemical (due to chemical bonds), and nuclear. Common transfers include mechanical work, electrical work, heating, and radiation.

Examples of Energy Changes

When an object is thrown upwards, its initial kinetic energy is converted into gravitational potential energy as it gains height. A car braking converts kinetic energy into thermal energy in the brakes due to friction. An electric kettle converts electrical energy into thermal energy to heat water. Impact events convert kinetic energy into various forms, including kinetic energy of fragments, thermal energy, and sound energy.

Analysing Energy Changes

To analyse energy changes, identify the initial and final energy stores, and the mechanisms of energy transfer. For example, a constant force accelerating an object does work on it, increasing its kinetic energy. A vehicle slowing down means its kinetic energy decreases, often converted to thermal energy (in brakes and tyres) and sound energy.

What is a Closed System?

A closed system is a defined region where matter cannot enter or leave. However, energy can typically still be transferred into or out of the system, contrary to a common misconception. For the purpose of energy conservation, a 'closed system' often implies a system where we are focusing on internal energy transfers without considering external energy inputs or outputs to simplify the concept. In reality, truly isolated systems where neither matter nor energy can cross the boundaries are very rare, but the concept is useful for understanding energy transformations.

The Principle of Energy Conservation

The fundamental principle of energy conservation states that energy cannot be created or destroyed. It can only be transferred from one store to another or converted from one form to another. This means that for any process occurring within a closed system, the total sum of all energy forms and stores before the process must equal the total sum after the process. There is no net gain or loss of energy; it simply shifts its state or location within the system boundaries.

Illustrative Example: Bouncing Ball

Consider a ball dropped from a height inside a closed room. As it falls, its gravitational potential energy is converted into kinetic energy. When it hits the ground, some kinetic energy is converted into elastic potential energy (deforming the ball), sound energy, and thermal energy in the ball and ground. The ball then bounces up, converting elastic potential energy back into kinetic and then gravitational potential energy. Throughout this sequence, the total energy (gravitational potential + kinetic + elastic potential + sound + thermal) of the ball-earth-air system remains constant, assuming no energy leaves the room.

Core Principle

The conservation of energy is a fundamental principle in physics stating that energy can never be created or destroyed. It can only be transferred from one form to another, or from one object to another. The total energy within a closed system remains constant.

Energy Transfers and Transformations

While the total energy is conserved, it frequently changes form. For example, chemical potential energy in fuel can be transformed into kinetic energy of a vehicle and thermal energy. In many real-world scenarios, some energy is dissipated, often as heat or sound, into the surroundings, reducing the useful energy available within the system but not destroying the energy itself.

Closed vs. Open Systems

The principle holds true for a closed system, which is one where no matter or energy can enter or leave. In an open system, energy can be exchanged with the surroundings, making it seem like energy is 'lost' or 'gained' from the perspective of the system, but the total energy of the universe remains constant.