1. Power, and Efficiency
Physics 11

2. Power The rate at which work is done. OR
The rate at which energy is transferred.

3. Power Formula: Symbol: P Units: Watts (W)
NOTE: 1 Watt = 1 Joule/second

4. Power
Any machine that does mechanical work or any device that transfers energy via heat can be described using a power rating.
Your appliances at home have a power rating.

5. Example:
A crane is capable of doing 1.50x105J of work in 10.0s. What is the power of the crane?

6. Answer: x 104 W

7. Example 2:
A cyclist and her bike have a mass of 60.0kg. She is able to cycle up a hill that changes her altitude by 400. m in 1.00 min. Assume no friction.
A) How much work does she do against gravity in climbing the hill?
B) How much power is she able to generate?

8. Answer
A) She has gone 400m. Work here is change in potential energy (gravitational) as she is climbing the hill.
W = ΔE = mgh2 –mgh1
W = (60)(9.81)(400) = 2.35 x 105 J
B) P = W/t = 2.35 x 105 J/60.0 s
P = 3.92 x 103 W

9. Practice Problems Page 266 Questions 41, 42, 43
Ignore the horsepower stuff!

10. Efficiency Not all systems are closed.
Energy can be lost through friction and other ways
Efficiency of energy conversion in open systems.

11. Efficiency There is no unit for efficiency. It is a scalar.
We can also look at efficiency in terms of work:

12. UNITS???

13. Efficiency Consider the following:
A rocket has 3.50x103 J of chemical potential energy. The stored chemical energy is transformed into gravitational potential energy when it is launched. What is the efficiency of the rocket’s transformation of energy if the kg rocket travels 1.00x102 m? Where does the “lost” energy go???

14. Answer
Most of the energy is “lost” meaning it transforms into thermal (heat) energy and light energy instead of Eg.

15. Practice Problems Page 270 Page 271 – Section Review Power, Efficiency
Chapter 6 Review
Page 274:
2, 4, 6
Page 275:
13, 15, 16, 17, 18, 19, 20
Page 276:
22, 23,
Page 270
44-50
Page 271 – Section Review
1,3