P1 Conservation of Energy and Power (page 170) Flashcards
What fact must you remember about Energy?
Energy is NEVER destroyed.
What is the conservation of energy principle?
Energy is always conserved. Energy can be transferred usefully, stored or dissipated, but can NEVER be created or destroyed.
(dissipated means energy is wasted).
When energy is transferred between stores, not all energy is transferred usefully into the store you want it to go into, why?
Some energy is always dissipated when an energy transfer takes place.
What is Dissipated Energy?
It is sometimes called ‘Wasted Energy’ because the energy is being stored in a way that is not useful (usually energy has been transferred into thermal energy stores).
A mobile phone is a system, explain how the phones energy is usually transferred?
Energy is usually transferred from a chemical energy store, of the battery in the phone, but some of the energy is dissipated in this transfer to the thermal energy store of the phone. (you may have noticed your phone feels warm if you’ve been using it for a while).
What is a closed system energy transfer?
A closed system is where NO energy is added and NONE can escape.
Give an example of a ‘closed system’ when a cold spood is dropped into an insulated flask of hot soup which is then sealed?
You can assume the flask is a perfect thermal insulator, so the spoon and the soup form a ‘closed system’.
Energy is transerred from the termal energy store of the soup to the useless thermal energy store of the spoon (causing the soup to cool down slightly).
Energy transfers have occured within the system, but no energy has left the system, so the net hange is zero (see page 167).
Power is the ‘Rate of Doing Work’ means what?
How much per second.
What is Power?
power is the rate of energy transfer, or the rate of doing work.
How is power measured?
in Watts
What is one Watt equal to?
one watt = 1 Joule of energy transferred per second.
How can you calculate power?
E - Energy transferred
Power (W) - P = ______
t - Time (s)
W - Work done (J)
Power (W) - P = \_\_\_\_\_\_\_\_
t - time (s)
see diagram on page 170
What can a powerful machine transfer?
A lot of energy in a short space of time.
A powerful machine is not necessarily one which can exert a strong force (although it usually ends up that way)
If you take two 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, what car will reach the finish line faster?
The car with the more powerful engine will finish the line faster than the other car, as it will transfer the same amount of energy but over less time.
It takes 8000J of work to lift a stund performer to the top of a building. Motor ‘A’ can lift the stun 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 poser of this motor?
1) Both motors transfer the same amount of energy, but motor A would do it quicker than motor B, so motor A is more powerful motor B, so motor A is more powerful motor
2) Plug the time taken and work done for motor A the equation
P = W ÷ t and find the power.
P = W ÷ t = 8000 ÷ 50 = 160 W
