1. Notes 10.2: Power and efficiency

2. Are the powers different ?
Same work but different power to do it … How ?
SAME DISTANCE
Are the powers different ?
POWER
500 W
100 W
WORK
500 J
TIME
1 s
5 s
The work to move the chair is the same in the two cases. The only thing that is different is the time taken to move them.

3. Scottish inventor and engineer
Power
Power takes its unit (W) from James Watt who contributed to the steam engine development
James WATT
1736 – 1819
Scottish inventor and engineer

4. Power is the rate of doing work
You do the same amount of work whether you move the chair quickly or slowly.
But your power is greater if you do the work in a smaller amount of time
Power =
Work or Energy
Time
P = unit (Watts) W t

5. Ex: a cat who weighs 40 N climbs a tree that is 15 meters tall in 10 seconds. Calculate the work done by the cat and the cat’s power
Relationships: W = Fd P = W / t
Work d = 15m W = Fd
F = 40 N W = 40 x 15
W = ? W = 600J
Power W = 600 J P = W / t
t = 10 s P = 600 / 10
P = ? P = 60 W

6. Efficiency
Question: If I put one liter of gasoline into my car, do you think that all of the energy inside that liter of gasoline will be turned into energy to move my car ?

7. Input or initial energy ( J )
efficiency describes how well energy is converted from one form into another
A process is 100 % efficiency if no energy is “lost” due to friction, sound, … In reality no process is 100 % efficient
Output or final energy ( J )
Input or initial energy ( J )
Efficiency =

8. A bobsleigh drops 50 meters in height of a hill
A bobsleigh drops 50 meters in height of a hill. The potential energy of the bobsleigh at the top of the hill is 70,000 J. The kinetic energy at the bottom of the hill is 28,000 J. What is the efficiency of the ride?
Relationships: (Efficiency) e = final / initial = Ke / Pe
Ke = 28,000 J
Pe = 70,000 J
e = ?
e = Ke / Pe
e = 28,000 J / 70,000 J
e = 0.4
e = 40%
No Units !!!