Peoples Physics Book Version-2

http://www.ck12.org Chapter 21. Heat Engines and The Laws of Thermodynamics

21.3 Examples

Example 1

Question:A heat engine operates at a temperature of 650K. The work output is used to drive a pile driver, which is
a machine that picks things up and drops them. Heat is then exhausted into the atmosphere, which has a temperature
of 300K.

a) What is the ideal efficiency of this engine?

b.) The engine drives a 1200kg weight by lifting it 50min 2. 5 sec. What is the engine’s power output?

c) If the engine is operating at 50% of ideal efficiency, how much power is being consumed?

d) The fuel the engine uses is rated at 2. 7 × 106 J/kg. How many kg of fuel are used in one hour?

Answer:

a) We will plug the known values into the formula to get the ideal efficiency.

η= 1 −

Tcold

T

hot=1-300K650K=54%

b) To find the power of the engine, we will use the power equation and plug in the known values.

P=

W

t

=

F d

t

=

mad

t

=

1200kg× 9 .8m/s^2 ×50m

2 .5sec

=240kW