Chapter 11: Energy Flow and Power

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2 Chapter 11: Energy Flow and Power 11.1 Efficiency 11.2 Energy and Power 11.3 Energy Flow in Systems

3 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.

4 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

5 Inv 11.1 Efficiency Investigation Key Question: How efficient is the smart track?

6 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.

7 11.1 Efficiency Efficiency can also mean the ratio of energy output divided by energy input. Efficiency ε = E o E i Energy output (J) Energy input (J)

8 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.

9 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. Calculating efficiency 3. Input work = Output energy, so W = f x d, E k = ½ mv 2 and ε = E o E i 4. Solve: ε = (.5) (0.012 kg) (6.5 m/s) 2 = 0.26 = 26% (10 N) (0.15 m)

10 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.

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12 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.

13 11.1 Estimating efficiency of a human The overall energy efficiency for a person is less than eight percent. An average person uses 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).

14 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.

15 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.

16 Chapter 11: Energy Flow and Power 11.1 Efficiency 11.2 Energy and Power 11.3 Energy Flow in Systems

17 Inv Energy and Power Investigation Key Question: How powerful are you?

18 11.2. Energy and Power How fast you do work makes a difference.

19 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)

20 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 E p = mgh, P= E t 4. Solve E p = (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.

21 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.

22 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) P = F. v Force (N) Velocity (m/sec)

23 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.

24 A fan uses a rotating blade to move air. How much power is used by a fan that moves 2 m 3 of air each second at a speed of 3 m/sec? Assume air is initially at rest and has a density of 1 kg/m 3. Fans are inefficient; assume an efficiency of 10 %. 1. You are asked for power. 2. You are given volume, density, speed and time. 3. Use density = m V, E k = ½ mv 2, P = E t 4. Solve: m = (1 kg/m 3 ) (2 m 3 )= 2 kg 5. Solve E k = (0.5) (2 kg)(3m/s) 2 = 9 J Estimating power 6. With 10% efficiency, it takes 90 J input energy to make 9 J output, solve: P = 90 J 1 s = 90 W

25 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 W. The power received from the sun is what drives the weather on Earth.

26 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.

27 1. You are asked for power. 2. You are given energy input in food calories and time 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.

28 Chapter 11: Energy Flow and Power 11.1 Efficiency 11.2 Energy and Power 11.3 Energy Flow in Systems

29 Inv Energy Flow in Systems Investigation Key Question: Where did the energy go?

30 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.

31 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.

32 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

33 11.3 Energy flow The energy flow diagram for a rechargeable electric drill shows losses to heat or friction at each step.

34 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.

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36 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.

37 11.3 Energy flow in natural systems The energy pyramid is a good way to show how energy moves through an ecosystem.

38 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.