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##### Unit 4, Chapter 11

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**Unit 4, Chapter 11**CPO Science Foundations of Physics**Unit 4: Energy and Momentum**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 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**Key Question:**How efficient is the straight track? 11.1 Efficiency *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. Energy output (J) e = Eo Ei Efficiency 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.**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.**Key Question:**How powerful are you? 11.2 Energy and Power *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 Power**Power is equal to the amount of work done divided by the time it takes to do the work. Change in work or energy (J) P = E t Power (W) Change in time (sec)**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.**P = F . v**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) Velocity (m/sec) Force (N)**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: • Write down the forms that the energy takes. • Diagram the flow of energy from start to finish for all the important processes that take place in the system. • 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: • Storage ex. batteries, springs, height, pressure • Conversion ex. a pump converting mechanical energy to fluid energy • Transmission ex. through wires, tubes, gears, levers • 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.**Key Question:**Where did the energy go? 11.3 Energy Flow in Systems *Students read Section 11.3 BEFORE Investigation 11.3