Conservation of Energy and Power

The Conservation of energy principle:

  1. Energy can be transferred usefully, stored or dissipated but can never be created or destroyed.
  2. When energy is transferred between stores, not all the energy is transferred usefully(where you want it to go). Some energy is always dissipated when an energy transfer takes place.
  3. Dissipated is sometimes called ‘wasted energy’, because the energy is being stored a way that is not useful(usually energy has been transferred into the thermal energy store).

Example:

A mobile phone is a system. When you use the phone, energy is usefully transferred from the chemical energy store of the battery in the phone. But some energy is dissipated in this transfer to the thermal energy store of the phone(that’s why it feels warm after a while).

Understand to describe energy transfers of a closed system(does not allow transfer of matter in and out the system):

A cold spoon is dropped into an insulated flask of hot soup, which is sealed. If you assume the flask is a perfect thermal insulator so the spoon and soup form a closed system. Energy is transferred from the thermal energy store of the soup to the useless thermal energy store of the spoon(causing the soup to cool down slightly).

Energy transfers have occurred within the system, but no energy has left the system- so the net energy is zero.

Power is the ‘rate of doing work’-i.e. how much per second

  1. Power is the rate of energy transfer, or the rate of doing work.
  2. Power is measured in watts. One watt = 1 joule of energy transferred per second.

Equation P=E/t or P=W/t

Power(watts) = Energy transferred(J) / time(s) or Power(watts)=Work done/time(s)

  1. A powerful machine is one which transfers a lot of energy in a short space of time. I.e In a car the faster one is the one that transfers the same amount of energy in less time.

Example:

It takes 8,000J of work to lift a stunt performer to the top of a building. Motor A can lift the performer in 50s. Motor B would take 300s to lift the performer. Which motor is more powerful?