Power and Efficiency Physics 11

Power (copy) The rate at which work is done. OR The rate at which energy is transferred. 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.

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

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

Answer:

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?

Answer A) She has gone 400.m. Work here is change in potential energy (gravitational) as she is climbing the hill. W = ΔE = mghf –mghi W = (60)(9.81)(400) - 0= 2.35 x 105 J B) P = W/t = (2.35 x 105)/(60.0) P = 3.92 x 103 W

Efficiency (copy) Not all systems are closed. Think back to types of systems… Energy can be lost through friction and other ways (as seen in the Q for Conservation of Total energy)

Efficiency (copy) It is a scalar. We can also look at efficiency in terms of work:

UNITS??? (copy) There is no unit for efficiency.

Efficiency Example 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 0.500 kg rocket travels 1.00x102 m? Where does the “lost” energy go???

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

Practice Problems Power, Efficiency Energy Review Page 274: 2, 4, 6 13, 15, 16, 17, 18, 19, 20 Page 276: 22, 23 Page 266 Questions 41, 42, 43 (Ignore the horsepower stuff!) Page 270 44-50 Page 271 – Section Review 1,3