Energy conservation and energy transfers explained
Principles of energy • Conservation and dissipation of energy
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Key concepts
What you'll likely be quizzed about
Conservation of energy - clear definition
Energy cannot be created or destroyed. The total energy in a closed system remains the same even when energy moves between different stores. Transfers change which store holds the energy (kinetic, chemical, thermal, gravitational potential, elastic, magnetic, electrostatic, nuclear), but not the total quantity.
Energy stores and methods of transfer
Energy is stored in specific ways: kinetic for motion, gravitational potential for height, chemical in fuels or foods, internal (thermal) in particle motion, elastic in stretched or compressed materials, and others. Energy transfers take place by mechanical work (forces moving objects), by heating (thermal energy flow), by electrical work (charges moving in circuits) and by radiation (light and sound transfer energy without storing it). Light, sound and electricity act as transfer mechanisms rather than long-term stores.
Closed systems and no net change
A closed system has no net energy exchange with the surroundings or no external forces acting on the objects inside the system. Within a closed system energy can move between stores while the total remains constant. Examples include two colliding trolleys on an isolated track where kinetic energy transfers between trolleys and possibly into deformation or heat, but the total energy in the system remains the same. Momentum and energy conservation arguments apply when external influences are absent.
Energy dissipation and less useful stores
Every real process dissipates some energy to less useful stores, typically as thermal energy spread into the surroundings. Dissipation reduces the amount of energy available for useful work because the energy becomes more widely spread and harder to harness. Practical examples include car engines where chemical energy mainly becomes thermal energy in the engine and surroundings, brakes converting kinetic energy into heat, and power station generators producing waste heat through friction and vibrations. Minimising dissipation increases efficiency.
Key notes
Important points to keep in mind