Energy Conservation (AQA A Level Physics): Revision Note

Conservation of energy in deformation

Loading and unloading a metal wire

• When a metal wire is loaded with a force and stretched beyond its limit of proportionality, it will undergo plastic deformation

• When the force is removed, the wire is unloaded, this causes the extension to decrease

• The unloading line is parallel to the loading line (since k does not change) however, it does not go through the origin

  • If the wire is permanently deformed, it will not be at zero extension when there is no force as it is now permanently extended

• The area between the loading and unloading lines represents the work done to permanently deform the wire

Loading and unloading a rubber band

• The force-extension graph for a material may not always be the same when loading (adding a force) and unloading (removing a force)

• The force-extension curve for stretching and contracting a rubber band is shown below

• Since the rubber band has no extension when the load is fully removed, it has no permanent extension

  • This means that the rubber band is elastic

• The graph shows the rubber band stores a greater amount of strain energy when it is loaded (stretched) than when it is being unloaded (contracted)

• The curve for contraction is always below the curve for stretching

• The key features of the area under the graph are:

  • Area X is the work done in heating the rubber (or the increase in thermal energy)

  • Area Y is the work done by the rubber when it is returned to its original shape

  • Area X + Y represents the work done in stretching the rubber band originally

• However, due to the conservation of energy, the difference in strain energy when loading and unloading must be accounted for

• A rubber band becomes warm when it is stretched and contracted hence some energy is transferred to heat energy

Energy conservation issues

• Vehicle suspension systems are made up of tyres, springs and shock absorbers which provide comfortable handling of a vehicle and improve the comfort of passengers

• Roads are often very bumpy filled with potholes and speed bumps

  • A bump in the road causes the wheel of a vehicle to move up and down perpendicular to the road surface

  • If a wheel loses contact with the road surface, it will slam back down again causing large vibrations within the car and potentially damage the vehicle

  • As well as this it would be very uncomfortable for the passengers and the driver could lose control of the vehicle

• This energy is absorbed by shock absorbers

  • These are elastic objects designed to absorb or dampen the compression and rebound of the springs above a vehicle’s tyres

  • They help keep the tyres on the road at all times

• When a vehicle hits a bump in a road, the shock absorbers dampen the movement of the springs in the suspension system

  • They do this by converting kinetic energy, from the movement of the car, into thermal energy which is dissipated

• The faster the springs in the suspension system move (say, if a vehicle hits a bump at a high velocity), the more resistance the shock absorber provides