Transcript Document

Chapter 11: Energy Flow and Power
 11.1 Efficiency
 11.2 Energy and Power
 11.3 Energy Flow in Systems
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.
Chapter 11 Vocabulary
 carnivore
 herbivore
 cycle
 horsepower
 decomposer
 irreversible
 ecosystem
 power
 efficiency
 power transmission
 energy
conversions
 producer
 energy flow
 food calorie
 food chain
 food web
 reversible
 steady state
 watt
Inv 11.1 Efficiency
Investigation Key Question:
How efficient is the smart track?
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.
Efficiency
e = Eo
Ei
Energy output (J)
Energy input (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.
Calculating 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 efficiency.
2.
You are given input force and
distance, output mass and speed.
3.
Input work = Output energy, so W = f x d, Ek = ½ mv2
and e = Eo÷ Ei
Solve: e = (.5) (0.012 kg) (6.5 m/s)2 = 0.26 = 26%
(10 N) (0.15 m)
4.
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 Estimating efficiency of a human
 The overall energy efficiency
for a person is less than
eight percent.
 An average person uses 55–
75 kilocalories per hour
when just sitting still.
 The rate at which your body
uses energy while at rest is
called your baseline
metabolic rate (BMR).
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.
Chapter 11: Energy Flow and Power
 11.1 Efficiency
 11.2 Energy and Power
 11.3 Energy Flow in Systems
Inv. 11.2 Energy and Power
Investigation Key Question:
How powerful are you?
11.2. Energy and Power
 How fast you do work makes a difference.
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)
P=E
t
Change in work
or energy (J)
Calculating 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.
1.
You are asked for power.
2.
You are given mass, distance, and time.
3.
Use Ep = mgh, P= E ÷ t
4.
Solve Ep = (70 kg) (9.8 N/kg) (5 m) = 3,430 J
5.
Solve P = (3,430 J) ÷ (30 s) = 114 watts
a. 114 watts
b. This is a little more than a100 watt light bulb.
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 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.
Estimating power
1.
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 %.
You are asked for power.
2.
You are given volume, density, speed and time.
3.
Use density = m ÷ V, Ek = ½ mv2, P = E ÷ t
4.
Solve: m = (1 kg/m3) (2 m3)= 2 kg
5.
Solve Ek = (0.5) (2 kg)(3m/s)2 = 9 J
6.
With 10% efficiency, it takes 90 J input energy to make 9 J output,
solve: P = 90 J ÷ 1 s = 90 W
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.
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 energy input in food calories and time.
3.
1 day = 86,400 s, 1 food calorie = 4,187 J, use P = E ÷ t
4.
Solve: E = (2,500 cal) (4,187 J/cal) = 10,467,500 J
5.
P = (10,467,500 J) ÷ (86,400 s) = 121 watts
Chapter 11: Energy Flow and Power
 11.1 Efficiency
 11.2 Energy and Power
 11.3 Energy Flow in Systems
Inv. 11.3 Energy Flow in Systems
Investigation Key Question:
Where did the energy go?
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.
Energy from Ocean Tides
 The energy and power in tides is
enormous.
 The power that moves the oceans and
creates tides comes from the total potential
and kinetic energy of the Earth-Moon
system.
 Many experimental projects have been built
to harness the power of tides.
 Like hydroelectric power, energy from tides
creates no pollution, nor does it use up
fossil fuels such as petroleum or coal.