1. Foundations of Physics
CPO Science
Foundations of Physics
Unit 4, Chapter 11

2. Unit 4: Energy and Momentum
Chapter 11 Energy Flow and Power
11.1 Efficiency
11.2 Energy and Power
11.3 Energy Flow in Systems

3. Chapter 11 Objectives
Give an example of a process and the efficiency of a process.
Calculate the efficiency of a mechanical system from energy and work.
Give examples applying the concept of efficiency to technological, natural and biological systems.
Calculate power in technological, natural, and biological systems.
Evaluate power requirements from considerations of force, mass, speed, and energy.
Sketch an energy flow diagram of a technological, natural, or biological system.

4. Chapter 11 Vocabulary Terms
efficiency
process
input
output
food calorie
reversible
irreversible
power
horsepower
producer
energy
flow
watt
cycle
food chain
power transmission
herbivore
carnivore
decomposer
food web
energy conversion
steady state
ecosystem

5. 11.1 Efficiency Key Question: How efficient is the straight track?
*Students read Section AFTER Investigation 11.1

6. 11.1 Efficiency Efficiency is defined for a process.
A process is any activity that changes things and can be described in terms of input and output.
The efficiency of a process is the ratio of output to input.

7. 11.1 Efficiency
Efficiency can also mean the ratio of energy output divided by energy input.
Energy output (J)
e = Eo Ei
Efficiency
Energy input (J)

8. 11.1 Efficiency
According to the law of conservation of energy, energy cannot ever be lost, so the total efficiency of any process is 100%.
The work output is reduced by the work that is converted to heat, resulting in lower efficiency.

9. 11.1 Calculate efficiency
A 12-gram paper airplane is launched at a speed of 6.5 m/sec with a rubber band.
The rubber band is stretched with a force of 10 N for a distance of 15 cm.
Calculate the efficiency of the process of launching the plane.
1) You are asked for the efficiency.
2) You are given the input force and distance and the output mass and speed.
3) Efficiency is output energy divided by input energy.
The input energy is work = F x d.
The output energy Ek = 1/2 mv2.
4) Solve:
ε = (0.5)(0.012 kg)(6.5 m/sec)2 / (10 N)(0.15 m)
= 0.26 or 26%

10. 11.1 Efficiency in natural systems
Energy drives all the processes in nature, from winds in the atmosphere to nuclear reactions occurring in the cores of stars.
In the environment, efficiency is interpreted as the fraction of energy that goes into a particular process.

11. 11.1 Efficiency in biological systems
In terms of output work, the energy efficiency of living things is typically very low.
Almost all of the energy in the food you eat becomes heat and waste products; very little becomes physical work.

13. 11.1 Efficiency in biological systems
Think of time as an arrow pointing from the past into the future.
All processes move in the direction of the arrow, and never go backward.

14. 11.1 Efficiency in biological systems
Since processes in the universe almost always lose a little energy to friction, time cannot run backward.
If you study physics further, this idea connecting energy and time has many other implications.

15. 11.2 Energy and Power Key Question: How powerful are you?
*Students read Section AFTER Investigation 11.2

16. 11.2. Energy and Power It makes a difference how fast you do work.
Suppose you drag a box with a force
of 100 newtons for 10 meters in 10 seconds. You do 1,000 joules of work. Your
friend drags a similar box and takes 60 seconds. You both do the same amount of
work because the force and distance are the same. But something is different. You
did the work in 10 seconds and your friend took six times longer.

17. 11.2 Power A unit of power is called a watt.
Another unit more familiar to you is horsepower.
One horsepower (the avg power output of a horse) is equal to 746 watts.

18. 11.2 Power
Power is equal to the amount of work done divided by the time it takes to do the work.
Change in work
or energy (J)
Power (W)
P = E t
Change in time (sec)

19. 11.2 Calculate power A 70 kg person goes up stairs 5 m high in 30 sec.
(1) You are asked for power.
(2) You are given mass, distance, and time.
(3) Relationships that apply:
Ep = mgh P = E/t
(4) Solve:
Ep = (70 kg)(9.8 N/kg)(5 m) = 3430 J
P = (3,430 J)/(30 sec) = 114 watts
(a) 114 W
(b) This is a little more power than a 100-watt light bulb.
Many human activities use power comparable to a light bulb.
A 70 kg person goes up stairs 5 m high in 30 sec.
a) How much power does the person need to use?
b) Compare the power used with a 100-watt light bulb.

20. 11.2 Power
Another way to express power is as a multiple of force and it's velocity, if the velocity and force are both vectors in the same direction.
Power (W)
P = F . v
Velocity (m/sec)
Force (N)

21. 11.2 Power in human technology
You probably use technology with a wide range of power every day.
Machines are designed to use the appropriate amount of power to create enough force to do work they are designed to do.

22. 11.2 Estimate power A fan uses a rotating blade to move air.
How much power is used by a fan that moves 2 m3 of air each second at a speed of 3 m/sec? Assume air is initially at rest and has a density of 1 kg/m3.
Fans are inefficient; assume an efficiency of 10 %.
(1) You are asked for power.
(2) You are given volume, density, speed, and time.
(3) Relationships that apply:
ρ = m/V, Ek = 1/2 mv2, P = E/t
(4) Solve:
m = ρV = (1 kg/m3)(2 m3) = 2 kg
Ek = (0.5)(2 kg)(3 m/sec)2 = 9 J
At an efficiency of 10 percent, it takes 90 joules of input power to make 9J of output energy.
P = (90 J)/(1 sec) = 90 watts

23. 11.2 Power in natural systems
Natural systems exhibit a much greater range of power than human technology
The sun has a total power output of 3.8 × W.
The power received from the sun is what drives the weather on Earth.

24. 11.2 Power in biological systems
200 years ago, a person’s own muscles and those of their horses were all anyone had for power.
Today, the average lawn mower has a power of 2,500 watts—the equivalent power of three horses plus three people.
Most of the power output of animals takes the form of heat.
The output power from plants is input power for animals.

25. 11.2 Estimate power An average diet includes 2,500 food calories/day.
Calculate the average power this represents in watts over a 24-hour period.
One food calorie equals 4,187 joules.
(1) You are asked for power.
(2) You are given the energy input in food calories and the time.
(3) Relationships that apply:
1 food calorie = 4,187 J, P = E/t
(4) Solve:
E = (2,500 cal)(4,187 J/cal)
= 10,467,500 J
There are 60 x 60 x 24 =
86,400 seconds in a day.
P = (10,467,500 J) / (86,400 sec) = 121 watts
This is a bit more than the power used by a 100-watt light bulb.

26. 11.3 Energy flow in systems
Energy flows almost always involve energy conversions.
To understanding an energy flow:
Write down the forms that the energy takes.
Diagram the flow of energy from start to finish for all the important processes that take place in the system.
Try to estimate how much energy is involved and what are the efficiencies of each energy conversion.

27. 11.3 Energy flow in systems
A pendulum is a system in which a mass swings back and forth on a string.
There are 3 chief forms of energy: potential energy, kinetic energy, and heat loss from friction.

28. 11.3 Energy flow in human technology
The energy flow in technology can usually be broken down into four types of processes:
Storage ex. batteries, springs, height, pressure
Conversion ex. a pump converting mechanical energy to fluid energy
Transmission ex. through wires, tubes, gears, levers
Output ex. heat, light, electricity

29. 11.3 Energy flow
The energy flow diagram for a rechargeable electric drill shows losses to heat or friction at each step.

30. 11.3 Energy flow in natural systems
The energy flows in technology tend to start and stop.
Many of the energy flows in nature occur in cycles.
Water is a good example.

32. 11.3 Energy flow in natural systems
A food chain is a series of processes through which energy and nutrients are transferred between living things.
A food chain is like one strand in a food web.
A food web connects all the producers and consumers of energy in an ecosystem.

33. 11.3 Energy flow in natural systems
The energy pyramid is a good way to show how energy moves through an ecosystem.

34. 11.3 Energy Flow in Systems Key Question: Where did the energy go?
*Students read Section BEFORE Investigation 11.3

35. Application: Energy from Ocean Tides