11.1 Efficiency 1
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. 2
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 Efficiency Investigation Key Question: How efficient is the smart track? 4
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.
Calculating 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. You are asked for efficiency. You are given input force and distance, output mass and speed. Input work = Output energy, so W = f x d, Ek = ½ mv2 and e = Eo÷ Ei Solve: e = (.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.