Energy Transfers, Conservation & Dissipation of EnergyOxford AQA IGCSE Physics: Flashcards

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Name three energy stores between which energy can be transferred.

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Name three energy stores between which energy can be transferred.: Kinetic, gravitational potential, and elastic potential (any three).
Describe the energy transfers in a swinging pendulum.: Energy transfers back and forth between the kinetic store (greatest at the bottom of the swing) and the gravitational potential store (greatest at the top of the swing).
State the law of conservation of energy.: Energy cannot be created or destroyed — it can only be transferred from one store to another, or dissipated.
What does it mean for energy to be dissipated?: It has spread out into the surroundings, usually as heat, and is no longer useful.
Name two processes that commonly dissipate energy as heat.: Friction and air resistance.
Give the equation for efficiency in terms of energy.: efficiency = useful energy output / total energy input (can be x100 for a percentage).
Give the equation for efficiency in terms of power.: efficiency = useful power output / total power input (can be x100 for a percentage).
Can any real device be 100% efficient? Why/why not?: No — some energy is always dissipated (usually as heat) rather than usefully transferred.
What is a Sankey diagram used for?: To show how energy is redistributed in a system, with arrow width proportional to the amount of energy represented.
In a Sankey diagram, what does a wide arrow branching off to the side usually represent?: Wasted/dissipated energy, e.g. heat lost to the surroundings.
A device has a total energy input of 200 J and a useful output of 150 J. Calculate its efficiency as a percentage.: 150/200 x 100% = 75%.
Why can efficiency never be expressed as greater than 1 or 100%?: Because useful energy output can never exceed total energy input — the law of conservation of energy forbids creating energy.