Unit 04 The First Law.pdf

CEE 230 CEE 230 Energy and Environment Unit 4: The First Law source material: C&B Chapter 2 CEE 230 Goals for today?s lecture ?Describe methods of calculating different forms of work. ?Consider definitions of device efficiency. ?Introduce the First Law of Thermodynamics & discuss the principle of energy conservation. CEE 230 Heat and Work Work (W): energy transferred from the system to the surroundings (or vice versa) that deforms the system boundary, and hence changes the volume of the system Heat (Q): energy transferred from the system to the surroundings (or vice versa) that does not change the shape of the system boundary CEE 230 Heat and Work By IUPAC convention, heat and work are both positive if the transfer of either occurs from the surroundings and into the system. Your textbook (C&B 6 th ed.) uses a different sign convention: heat transfer into a system is positive; work done on a system is negative. Some textbooks use a different convention: heat and work are both positive when added to the system. CEE 230 Heat Transfer Mechanisms Heat transfer from a system to its surroundings occurs by conduction, convection and radiation. CEE 230 Forms of Work ?Shaft (mixing) work ?Boundary (mechanical) work ?Electrical work (of a current of electrons) ?Other forms (e.g. elastic deformation) W = W s + W b + W e + W other CEE 230 Mechanical (Boundary) Work work required to deform a system boundary W = ??F ds = ??P dV CEE 230 Electrical Work work required to move electrical charge through a potential difference W = ??VI dt CEE 230 Mechanical Forms of Work Problem 2-31 Acceleration of a car Problem 2-32 Lifting of a beam by a construction crane CEE 230 Device efficiency General definition: useful heat or work obtained from device heat or work put into the device example: water heaters CEE 230 Energy efficiency is important ? but cost and aesthetics matter too. CEE 230 The First Law of Thermodynamics ?Equation of energy conservation: ?E system + ?E surroundings = 0 system surroundings system + surroundings = universe boundary enclosing system CEE 230 First Law for Closed Systems ?E = ?U + ?KE + ?PE = Q - W ? = difference of final & initial states of system system ?KE ?PE ?U t U = internal energy of system due to translation, rotation & vibration of its molecular contents CEE 230 First Law for Closed Systems Q and W represent the net heat input and the net work output respectively Q = 12 kJ W = -6 kJ CEE 230 First Law for Closed Systems For systems that undergo large charges in elevation or velocity, the potential or kinetic energy change may be significant. Otherwise, for a closed stationary system ?U = Q - W CEE 230 The First Law: Air Conditioning of a Classroom Problem 2-50 Christian Lastoskie Microsoft PowerPoint - Unit 04 The First Law