Conservation of energy and power Flashcards
What is the conservation of energy principle
Energy can be used usefully, stored or dissipated but can never be created or destroyed.
Why is all energy not used usefully ?
when energy is transferred between stores, not all energy is transferred usefully into the store that you want it to go in. Some energy is always dissipated when an energy transfer takes place.
Why is dissipated energy often called wasted energy ?
because energy stored in a way that’s not useful.
Example of system with dissipated energy
A mobile phone is a system. When you use your phone, energy is transferred usefully from chemical energy store of battery in phone. Some energy is dissipated in this transfer to TES of phone = warmer
Example of closed system with dissipated energy
a cold spoon is dropped in insulating flask of hot soup, which is then sealed. Then you assume the flask is perfect thermal insulator so spoon and soup form a closed system. Energy transferred from TES of soup to useful TES of spoon. Energy transfer occurs within system and no energy left system - net change in energy =0
What is power and what’s it measured in
Rate of energy transfer
Measured in watts, One watt = 1 joule of energy transferred per second
2 power equations
P (W) = E(J) / T (S) P (W) = work done (J) / T (S)
Example
identical cars race same distance. Car with more powerful engine reach finish line faster.
