Chapter 11: Energy Flow and Power

CPO Science
Foundations of Physics
Unit 4, Chapter 11
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
Chapter 11 Objectives
1. Give an example of a process and the efficiency of a
2. Calculate the efficiency of a mechanical system from
energy and work.
3. Give examples applying the concept of efficiency to
technological, natural and biological systems.
4. Calculate power in technological, natural, and
biological systems.
5. Evaluate power requirements from considerations of
force, mass, speed, and energy.
6. Sketch an energy flow diagram of a technological,
natural, or biological system.
Chapter 11 Vocabulary Terms
 efficiency
 horsepower
 herbivore
 process
 producer
 carnivore
 input
 energy
 decomposer
 output
 flow
 food web
 food calorie
 watt
 reversible
 cycle
 energy
 irreversible
 food chain
 power
 power
 steady state
 ecosystem
11.1 Efficiency
Key Question:
How efficient is the straight track?
*Students read Section 11.1 AFTER Investigation 11.1
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
 The efficiency of a
process is the ratio of
output to input.
11.1 Efficiency
Efficiency can also mean the ratio of energy
output divided by energy input.
e = Eo
Energy input (J)
Energy output (J)
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.
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
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
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.
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.
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.
11.2 Energy and Power
Key Question:
How powerful are you?
*Students read Section 11.2 AFTER Investigation 11.2
11.2. Energy and Power
 It makes a difference how fast you do work.
11.2 Power
 A unit of power is called
a watt.
 Another unit more
familiar to you is
 One horsepower (the
avg power output of a
horse) is equal to 746
11.2 Power
Power is equal to the amount of work done
divided by the time it takes to do the work.
Power (W)
Change in time (sec)
Change in work
or energy (J)
11.2 Calculate power
 A 70 kg person goes up stairs 5 m high in 30
— a) How much power does the person need to use?
— b) Compare the power used with a 100-watt light
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)
Force (N)
P=F v
Velocity (m/sec)
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.
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 %.
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 × 1026 W.
 The power received from the sun is what drives
the weather on Earth.
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.
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.
11.3 Energy flow in systems
Energy flows almost always involve energy
To understanding an energy flow:
1. Write down the forms that the energy takes.
2. Diagram the flow of energy from start to finish for
all the important processes that take place in the
3. Try to estimate how much energy is involved and
what are the efficiencies of each energy conversion.
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.
11.3 Energy flow in human technology
The energy flow in technology can usually be
broken down into four types of processes:
1. Storage ex. batteries, springs, height, pressure
2. Conversion ex. a pump converting mechanical
energy to fluid energy
3. Transmission ex. through wires,
tubes, gears, levers
4. Output ex. heat, light, electricity
11.3 Energy flow
 The energy flow diagram
for a rechargeable electric
drill shows losses to heat
or friction at each step.
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
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.
11.3 Energy flow in natural systems
 The energy pyramid
is a good way to
show how energy
moves through an
11.3 Energy Flow in Systems
Key Question:
Where did the energy go?
*Students read Section 11.3 BEFORE Investigation 11.3
Application: Energy from Ocean Tides