Systems and Environments Basic concept: The Universe can be divided into two parts: the system and its environment. A system is the physical one that we are interested in. Its environment is the rest of the Universe. The system is much smaller than its evironment. 1

2 Nonisolated System A nonisolated system is one that interacts with or is influenced by its environment A new analysis model An isolated system would not interact with its environment The Work-Kinetic Energy Theorem can be applied to nonisolated systems

3 Energy Transfer Work is a method of energy transfer Work has the effect of transferring energy between the system and its environment If positive work is done on the system, energy is transferred to the system from its environment. Negative work indicates that energy is transferred from the system to its environment

4 Internal Energy The energy associated with an object’s temperature is called its internal energy, E int The work done by the book on the surface increases the internal energy of the surface, rather than the kinetic energy. Fig 6.14

5 6.7 Work-Kinetic Energy Theorem with Fiction Forces When kinetic friction is involved in a problem, you must use a modification of the work-kinetic energy theorem  W other forces – ƒ k d =  K The term ƒ k d is the work associated with the frictional force Also,  e int = ƒ k d when friction is the only force acting in the system

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10 Fig 6.17(b)

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18 A friction force transforms the kinetic energy in a system to internal energy For a system in which the frictional force alone acts, the increase in the internal energy of the system is equal to its decrease in kinetic energy

19 Ways to Transfer Energy Into or Out of A System Work – transfers by applying a force and causing a displacement of the point of application of the force Mechanical Waves – allow a disturbance to propagate through a medium Heat – is driven by a temperature difference between two regions in space

20 More Ways to Transfer Energy Into or Out of A System Matter Transfer – matter physically crosses the boundary of the system, carrying energy with it Electrical Transmission – transfer is by electric current Electromagnetic Radiation – energy is transferred by electromagnetic waves

21 Conservation of Energy Energy is conserved This means that energy cannot be created or destroyed If the total amount of energy in a system changes, it can only be due to the fact that energy has crossed the boundary of the system and its environment by some method of energy transfer

22 Conservation of Energy, cont. Mathematically,  E system =  E system is the total energy of the system T is the energy transferred across the system boundary Established symbols: T work = W and T heat = Q Others do not have standard symbols The Work-Kinetic Energy theorem is a special case of Conservation of Energy

23 Continuity Equation The conservation of energy equation is an example of an continuity equation Specifically, it is the continuity equation for energy A continuity equation arises in any situation in which the change in a quantity in a system occurs solely because of transfers across the boundary

24 Conservation of Energy, Completed The primary mathematical representation of the energy analysis of a nonisolated system is  K +  E int = W + T MT + T ET + T ER If any of the terms on the right are zero, the system is an isolated system The Work-Kinetic Energy Theorem is a special case of the more general equation above

Power The time rate of energy transfer is called power The average power is given by when the method of energy transfer is work

26 Instantaneous Power The instantaneous power is the limiting value of the average power as  t approaches zero This can also be written as

27 Power Generalized Power can be related to any type of energy transfer In general, power can be expressed as dE/dt is the rate rate at which energy is crossing the boundary of the system for a given transfer mechanism

28 Units of Power The SI unit of power is called the watt 1 watt = 1 joule / second = 1 kg. m 2 / s 3 A unit of power in the US Customary system is horsepower 1 hp =550 ft. lb/s = 746 W Units of power can also be used to express units of work or energy 1 kWh = (1000 W)(3600 s) = 3.6 x10 6 J

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36 Exercises of Chapter 6 3, 10, 18, 21, 23, 30, 35, 45, 46, 53, 59, 64