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