Energy and Systems

Unit 3 Energy and SystemsChapter 8 Energy Flow and

Systems8.1 Energy Flow

8.2 Power, Efficiency and Thermodynamics

8.3 Systems in Technology and Nature

8.1 Investigation: Energy Flow in a SystemKey Question:

How does energy move through a series of transformations?

Objectives: Create an energy flow diagram to document the energy

transformations that occur as the Energy Car moves along the SmartTrack.

Discuss the factors which affect the efficiency of a system. Suggest and test modifications to improve the efficiency

of a system.

Energy Flow Our universe is matter and energy organized into systems.

There are systems within systems ranging in scale from the solar system, to Earth, to a single animal, to a single cell in the animal, right down to the scale of a single atom.Earth is a system.

Energy and change

The energy available to a system determines how much the system can change.

By looking at how much energy there is in a system, and how much energy is used by the system, you can tell a lot about what kinds of changes are possible.

How fast will the green ball travel if the only source of energy is the falling blue ball?

Different forms of energyMechanical energy is the energy possessed

by an object due to its motion or its position.Radiant energy includes light, microwaves,

radio waves, x-rays, and other forms of electromagnetic waves.

The electrical energy we use is derived from other sources of energy.

Chemical energy is energy stored in the bonds that join atoms.

Nuclear energy is released when heavy atoms in matter are split up or light atoms are put together.

The workings of the universe can be viewed as energy flowing from one place to another and changing back and forth from one form to another.

Potential energy Systems or objects with potential energy

are able to exert forces (exchange energy) as they change.

Potential energy is energy due to position.

Kinetic energy Energy of motion is called kinetic energy.

A moving cart has kinetic energy because it can hit another object (like clay) and cause change.

Energy in a closed system

The conservation of energy is most useful when it is applied to a closed system.

Because of the conservation of energy, the total amount of matter and energy in your system stays the same forever.

Energy flow diagramsAn energy flow diagram is a good way to show what happens to the energy in a system that is changing.

Each place where energy changes form is called a conversion.

Unit 3: Energy and SystemsChapter 8: Energy Flow and

Systems8.1 Energy Flow

8.2 Power, Efficiency and Thermodynamics

8.3 Systems in Technology and Nature

8.2 Investigation: People Power

Key Question:

What’s your work and power as you climb a flight of stairs?

Objectives: Identify the factors that determine the work a person

does and his or her power output. Calculate work and power. Relate the work done in joules to the Calories used in

the process.

Power, Efficiency, and Thermodynamics In order to raise a 1,000-

kilogram car 1 meter, you need 9,800 joules of energy

Doing the lift in 10 seconds requires a power output of 980 watts.

This is more than a human can do.

PowerPower is the rate of converting energy, or

doing work.

How fast you do work makes a difference

Power in flowing energyPower is used to describe these three similar

situations. In each, you calculate the power by dividing the energy or work by the time it takes for the energy to change or the work to be done.1. Work is done by a force. Power is the rate at

which the work is done.2. Energy flows from one place to another;

power is the rate of energy flow.3. Energy is converted from one form to another.

Power is the rate at which energy is converted.

An example

The Colorado River flows at 700 m3/s.

Hoover Dam converts the potential energy of the Colorado River into electricity

A 2-kg owl gains 30 m of height in 10 s. How much power does the owl use?

Calculating power in a system

1. Looking for: … power.

2. Given: … owl’s mass (2 kg), height (30 m) and time (10 s).

3. Relationships: Power = energy ÷ time and Ep = mgh

4. Solution: Ep = (2 kg)(9.8 N/kg)(30 m) =588 joules

Power = 588 J ÷ 10 s = 58.8 watts

EfficiencyThe efficiency of a

process describes how well energy or power is converted from one form into another.

Efficiency is the ratio of useful output energy or power divided by input energy or power.

If you could add up the efficiencies for every single process in a system, that total would be 100 %.

A 12 g paper airplane is launched at a speed of 6.5 m/s 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.

Calculating efficiency of a process

1. Looking for: … efficiency.

2. Given: … plane’s mass (12 kg), and speed (6.5 m/s) and the rubber band’s force (10N) and distance (.15 m)

3. Relationships: Efficiency = Eo ÷ Ei Input energy is work = F × d; and Output energy: Eo = (½) mv2

4. Solution: e = [(0.5)(0.012 kg)(6.5 m/s)2] ÷ [(10 N)(0.15 m)]

= 0.17 or 17%

Energy in the U.S.

In the United States, about 89% of the energy sources used to generate electricity power are fossil fuels—coal, gas, oil—or nuclear energy.

ThermodynamicsThermodynamics is the physics and study of heat.

The law of conservation of energy is also called the first law of thermodynamics.

It states that energy cannot be created or destroyed, only converted from one form into another.

The second law of thermodynamics states that when work is done by heat flowing, the output work is always less than the amount of heat that flows.

Efficiency of an engineEntropy is a measure

of the energy in a system that is “lost” as waste heat and that cannot be used to do work.

Entropy helps explain why processes that are not 100 percent efficient are irreversible and why time only moves forward.

For example, 64% of the energy in gasoline flows out the car’s tailpipe, radiator, and other parts as waste heat!

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.

Estimating efficiency of a humanTo estimate the efficiency of

a person doing physical work, consider climbing a mountain 1,000 meters high.

A human body doing strenuous exercise uses about 660 kilocalories per hour.

If it takes three hours to climb the mountain, the body uses 1,980 Kcal (8,300,000 J).

The overall energy efficiency for a person is less than 8%.

Unit 3: Energy and SystemsChapter 8: Energy Flow and

Systems8.1 Energy Flow

8.2 Power, Efficiency and Thermodynamics

8.3 Systems in Technology and Nature

8.3 Investigation: Transportation EfficiencyKey Question:

Which transportation method is the most efficient?

Objectives: Discuss the factors that influence the efficiency of

transportation methods. Use the Internet to research the fuel consumption of

different types of vehicles. Compare the energy requirements for different

methods of vehicle and human-powered transportation.

Energy flowThe energy flow diagram

for a rechargeable electric drill shows losses to heat or friction at each step.

Power in human technologyYou probably use machines 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 requirements You can calculate the power

required if you know the force you need and the rate at which things have to move.

Suppose your job is to choose a motor for an elevator.

The elevator must lift 10 people, each with a mass of 70 kg.

Estimating power requirements

The smallest motor that would do the job is 19.6 hp.

The actual motor required would be about three times larger (60 hp) because our calculation did not include any friction and assumed an efficiency of 100 %.

Energy flow in natural systems

Steady state means there is a balance between energy in and energy out so that the total energy remains the same.

On Earth, radiant energy from the Sun is energy input.

The average energy of the Earth stays about the same because energy input is balanced by its energy output, energy that is radiated back into space.

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.

Power in natural systems

The power received from the Sun drives the weather on Earth.

A 10 m/s wind gust represents only 4% of the available solar power on a 1 km2 area.

Energy flow in natural systemsA 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.

Energy flow in natural systemsThe energy pyramid

is a good way to show how energy moves through an ecosystem.

Energy flows from producers to consumers.

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.