- In practice, most systems tend to be open systems
- Unlike other forms of energy, heat, light and sound have a tendency to spread out into the surroundings
- This is known as dissipation
- When they do so it becomes very difficult to “gather” the energy back together again
- As a result, the energy becomes less useful
- Because of this, whenever a process produces unwanted heat, light or sound, the energy that ends up in those forms is essentially wasted
- Useful energy can be defined as:
The energy that is transferred to the place where it is wanted and in the form that it is needed
- Wasted energy can be defined as:
The energy that is not useful for the intended purpose and is dissipated to the surroundings
A student uses an electric motor to lift a load.
In the motor, the electrical energy is transferred into other types of energy. Some of this energy is useful and the rest of the energy is wasted.
a) State the useful energy output from the electric motor.
b) State the main source of wasted energy and describe what eventually happens to it.
- The motor causes the load to move upwards
- Therefore, the useful energy output is kinetic energy and gravitational potential energy
- As the motor operates, it heats up
- In this case, heat energy is not useful, hence it is the main source of wasted energy
- This heat energy is eventually dissipated into the surroundings
Make sure you are able to identify different types of “useful” and “wasted” energy as this is commonly tested in exams!
When describing types of wasted energy, make sure to say the energy is lost to the surroundings, if you say the energy is simply “lost”, this will not gain you the mark as it implies energy is not conserved.
- Energy loss can be reduced by doing one of two things:
- By reducing the amount of unwanted (wasted) energy that is produced
- By preventing the energy from dissipating (if, for example, it has been produced deliberately)
- Friction is a major cause of wasted energy in machines
- For example, the gears on a bike can become hot if the rider has been cycling for a long time
- Energy is wasted as it is transferred from the kinetic energy store of the bike to the thermal energy store of the gears and the chain
- This friction makes them become hot and transfers energy by heating to the thermal energy store of the surrounding air
- This wasted energy can be reduced if the amount of friction can be reduced
- This can be achieved by lubricating the parts that rub together
Lubrication helps reduce friction in the parts of a cycle
- In many situations, the heat that is produced is wanted. For example:
- When heating a home
- When boiling a kettle
- If this heat can be prevented from dissipating, then less thermal energy will be needed to replace the heat that has been lost
- This can be achieved by surrounding the appliance with insulation
- The effectiveness of insulation depends upon:
- How well the insulation conducts heat
- How thick the insulation is