Unit 4, Chapter 11
CPO ScienceFoundations of Physics
Unit 4: Energy and Momentum
11.1 Efficiency
11.2 Energy and Power
11.3 Energy Flow in Systems
Chapter 11 Energy Flow and Power
Chapter 11 Objectives1. Give an example of a process and the efficiency
of a process.
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
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
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 output.
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.
= Eo
Ei
Energy output (J)
Energy input (J)
Efficiency
11.1 Efficiency
The work output is reduced by the work that is converted to heat, resulting in lower efficiency.
According to the law of conservation of energy, energy cannot ever be lost, so the total efficiency of any process is 100%.
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.
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.
11.1 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 horsepower.
One horsepower (the avg power output of a horse) is equal to 746 watts.
11.2 PowerPower is equal to the amount of work done divided
by the time it takes to do the work.
P = E t
Change in workor energy (J)
Change in time (sec)
Power (W)
11.2 Calculate power
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.
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.
Velocity (m/sec)Force (N)
Power (W) P = F . v
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 conversions.
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 system.
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 example.
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 ecosystem.
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