Energy Transfers, Conservation & Dissipation of EnergyOxford AQA IGCSE Physics: Revision notes
Section 1
What happens to energy when a system changes?
When a system changes, energy is transferred between energy stores — for example between kinetic (movement), gravitational potential (height) and elastic potential (stretched/squashed) stores.
A simple pendulum is a classic example of oscillating motion: as it swings, energy is continually transferred back and forth between the kinetic energy store (fastest at the bottom of the swing) and the gravitational potential energy store (greatest at the top of each swing, where it is briefly at rest).
Section 2
What is the law of conservation of energy?
The law of conservation of energy states that energy cannot be created or destroyed — it can only be transferred from one store to another, or dissipated. The total energy of a closed system is always the same before and after any change.
Section 3
Why is not all energy usefully transferred?
When energy is transferred in a system, only part of it is usefully transferred to where it is wanted — the rest is dissipated (wasted), often as heat spreading into the surroundings.
- Friction and air resistance dissipate energy by heating the surfaces/air involved and the surroundings
- Once energy has dissipated into the surroundings, it becomes increasingly spread out and harder to use for further useful work
Dissipated energy is not destroyed — it still exists, but has spread out and become far less useful. Never write that energy is 'lost' or 'used up' without qualifying that it has been transferred/dissipated, not destroyed.
Section 4
How do we calculate and represent efficiency?
Efficiency compares the useful energy (or power) output to the total energy (or power) input:
efficiency = useful energy output / total energy input
or
efficiency = useful power output / total power input
Efficiency can be expressed as a decimal (0 to 1) or as a percentage (multiply by 100%). No real device is 100% efficient, because some energy is always dissipated.
A Sankey diagram is a way to show how energy is redistributed in a system: the width of each arrow is proportional to the amount of energy it represents, showing the useful output(s) and the wasted output(s) branching from the total input.
A device with 100 J total energy input and 80 J useful energy output has an efficiency of 80/100 = 0.8, or 80%.
Must Know
- Energy is transferred between stores (kinetic, gravitational potential, elastic potential) when a system changes
- A pendulum transfers energy between kinetic and gravitational potential stores as it swings
- Law of conservation of energy: energy cannot be created or destroyed, only transferred or dissipated
- Only part of transferred energy is usefully transferred; the rest is dissipated, often as heat, by friction and air resistance
- efficiency = useful energy (or power) out / total energy (or power) in, as a decimal or percentage
- A Sankey diagram shows energy redistribution with arrow width proportional to energy amount
That's the notes covered.
Carry on to the next subtopic.