Definition
Dissipation
Dissipation is the spreading of transferred energy into the thermal energy stores of the surroundings, which makes the energy less useful for further transfers.
- Every real change in a system transfers some energy into stores that were not wanted, and that energy is said to be dissipated.
- Dissipated energy almost always ends up in the thermal energy stores of the object and its surroundings.
- Conservation of energy still holds, so all of the joules are still there and none have been destroyed.
- Dissipated energy is described as less useful because it has been shared out between huge numbers of particles in the surroundings.
- Spread that thinly, the energy raises the temperature of the surroundings by only a tiny amount and cannot be gathered back to drive the change again.
- Whether a transfer counts as useful depends on the job the device is meant to do, so heating is the useful transfer in a kettle and an unwanted one in a drill.
- Sound waves also carry energy away from a system, and those joules finish in the thermal energy store of the surroundings as well.
Why mechanical processes waste energy
- A mechanical process is one in which forces do work, such as a motor turning a shaft or a wheel rolling along a road.
- Whenever two surfaces slide or rub across each other, friction acts between them and work is done against it.
- That work transfers energy into the thermal energy stores of both surfaces, so their temperature rises.
- A mechanical process becomes wasteful when it causes this rise in temperature, because the energy is then dissipated into the surroundings by heating.
- Moving through air or water means doing work against air resistance or drag, which warms both the fluid and the moving object.
- The rise in temperature is the signature of the waste, so a machine that runs hot is dissipating a large share of the energy supplied to it.
- The unwanted transfer also depends on how long the process runs, because a longer run dissipates more joules.
Tracking dissipation in real systems
- A bouncing ball rebounds to a lower height each time, because energy is dissipated at every bounce.
- During a bounce the ball squashes and then recovers, and the work done inside the rubber warms both the ball and the ground.
- The bounce also makes a sound, so a further share of the energy leaves the ball as sound waves.
- Less energy is therefore returned to the gravitational potential energy store each time, so each bounce is lower than the one before.
- A car left in neutral rolls to a stop because friction in the bearings and air resistance move its kinetic energy store into thermal energy stores.
- A filament lamp transfers most of the energy supplied to it by heating the surroundings rather than by light, so most of that transfer is unwanted.
- Brakes are the case that proves the rule, because they are designed to raise a thermal energy store, yet once those joules are spread through the brakes and air they can no longer move the car.
Example
Bounce heights of a dropped ball
- A tennis ball is dropped from 1.0 m1.0\ \text{m}1.0 m onto a hard playground surface and rebounds to about 0.55 m0.55\ \text{m}0.55 m.
- Falling empties the ball's gravitational potential energy store and fills its kinetic energy store.
- At the moment of impact the ball squashes, so work is done inside the rubber and against the ground.
- That work transfers energy into the thermal energy stores of the ball, the ground and the air, and some energy leaves as sound waves.
- Only the energy still in the ball's kinetic store after the bounce can refill its gravitational store, so the ball rises to a lower height.
- Every later bounce dissipates more energy, so the heights keep falling until the ball stays on the ground.
Example
Dissipation in an electric drill
- A drill is used to make a hole in a wall, and its casing feels warm afterwards.
- Energy is transferred electrically from the supply and increases the kinetic energy store of the rotating bit, which is the useful transfer.
- Friction acts in the bearings, in the gears and between the bit and the wall, so work is done against friction.
- That work transfers energy by heating into the thermal energy stores of the drill, the wall and the surrounding air.
- The warm casing shows where the wasted joules have gone, and they cannot be recovered to turn the bit.
Exam technique
Explaining dissipation for full marks
- Name the store that holds the energy at the start, such as the kinetic energy store of the car.
- Name the cause of the unwanted transfer, which is usually friction, air resistance or the squashing of a material.
- State that work is done against that force, so energy is transferred by heating.
- Name the thermal energy stores that increase, and include the surroundings as well as the object itself.
- Finish by saying that the energy has become spread out and is now less useful, which is the mark most answers miss.
Common Mistake
- Do not write that energy is lost or destroyed, and write that it is dissipated or transferred to less useful stores instead.
- Do not write heat energy, because energy is transferred by heating into a thermal energy store.
- Do not claim the total energy falls during dissipation, because the total is unchanged and only its spread has altered.
- Do not treat sound as a store, because sound waves are a way of carrying energy away from the system.
Self review
- Define dissipation and name the store dissipated energy usually ends in.
- Explain what less useful means for dissipated energy.
- State what makes a mechanical process wasteful.
- Explain why a bouncing ball does not return to its starting height.
- Name two forces that cause unwanted energy transfers in a moving vehicle.
