Conservation of Energy & Power

• Energy is never destroyed
• Conservation of energy principle = Energy can be transferred usefully, stored or dissipated, but can never be created or destroyed
• Dissipation = 'wasted' energy because it isn't useful
• When energy is transferred between stores, not all of the energy is transferred usefully into the store.
• Some energy is always dissipated when an energy transfer takes place
• 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 of this energy is dissipated in this transfer to the thermal energy store of the phone
• A cold spoon is dropped into an insulated flask of hot soup, which is then sealed. You can assume that the flask is a perfect thermal insulator so the spoon and the soup form a closed system
• A cold spoon is dropped into an insulated flask of hot soup, which is then sealed. Energy is transferred from the thermal energy store of hte soup to the useless thermal energy store of the spoon (causing the spoon to cool down slightly). No energy has left the system - the net change in energy is still 0
• Power is the rate of doing work / energy transfer
• Power is measured in watts. One watt = 1 joule of energy transferred per second
• P(W) = E(J) or W(J) / t(seconds)
• Power(J) = energy or watts(J) / time (s)
• A poweful machine is not necessarily one which can exert a strong force (usually ends up being so). A powerful machine is one which transfers a lot of energy in a short space of time
• Take 2 cars that are identical in every way apart from the power of their engines. Both cars race the same distance along a straight race track to a finish line. The car with the more powerful engine will reach the line faster than the other car. (It will transfer the same amount of energy but over less time
• It takes 8000J of work to lift a stunt performer to the top of the building. Motor A can lift the stunt performer to the correct height in 50s. Motor B would take 300s to lift the performer to the same height. Which motor is most powerful? Calculate the power of this motor

  P = W/t so 8000/50 = 160W