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

cycle

decomposer

ecosystem

efficiency

energy conversions

energy flow

food calorie

food chain

food web

herbivore

horsepower

irreversible

power

power transmission

producer

reversible

steady state

watt

Inv 11.1 EfficiencyInvestigation 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 EfficiencyEfficiency 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%.

1. You are asked for efficiency.

2. You are given input force and distance, output mass and speed.

Calculating efficiencyA 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.

3. Input work = Output energy, so W = f x d, Ek = ½ mv2 and = Eo÷ Ei

4. Solve: = (.5) (0.012 kg) (6.5 m/s)2 = 0.26 = 26%(10 N) (0.15 m)

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.

P = E t

Change in workor energy (J)

Change in time (sec)

Power (W)

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 wattsa. 114 wattsb. This is a little more than a100 watt light bulb.

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.

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.

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.

1. 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

Estimating 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.

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

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.

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 systemsEnergy 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 technologyThe energy flow in technology can usually be

broken down into four types of processes:1. Storage ex. batteries, springs, height, pressure2. 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.

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.

Energy from Ocean Tides