AGUS HARYANTO

 Introduce concept of energy and its various forms.  Discuss the nature of internal energy.  Define concept of heat and terminology associated  Define concept of work and forms of mechanical work.  Define energy conversion efficiencies.  Discuss relation of energy conversion and environment.

 BENTUK ENERGI: 1. Energi Kinetik (KE)  2. Energi Potensial (PE)  PE = mgh 3. Energi dakhil atau Internal Energy (U)  ENERGI TOTAL: E = U + KE + PE e = u + ke + pe(per satuan massa)

 Thermodynamics deals only with the change of the total energy (  E). Thus E of a system can be assigned to zero (E = 0) at some reference point.  The change in total energy (  E) of a system is independent of the reference point selected.  For stationary systems, the  E is identical to the change of internal energy  U.

 The macroscopic forms of energy are those a system possesses as a whole with respect to some outside reference frame, such as kinetic and potential energies.  The microscopic forms of energy are those related to the molecular structure of a system and the degree of the molecular activity, and they are independent of outside reference frames. The sum of all the microscopic forms of energy is called the internal energy of a system and is denoted by U.

The internal energy of a system is the sum of all forms of the microscopic energies. SENSIBLE and LATENT energy CHEMICAL energy NUCLEAR energy

The portion of the internal energy of a system associated with the kinetic energies of the molecules is called the sensible energy The various forms of microscopic energies that make up sensible energy

 The internal energy associated with the phase of a system is called the latent energy.  The amount of energy absorbed or released during a phase-change process is called the latent heat coefficient.  At 1 atm, the latent heat coefficient of water vaporization is kJ/kg.

 The internal energy associated with the atomic bonds in a molecule is called chemical energy.  The tremendous amount of energy associated with the strong bonds within the nucleus of the atom itself is called nuclear energy.

Energy crosses the boundaries in the form of:  Heat  Work  Mass flow* *) 1,2 for Clossed System; 1,2,3 for Open System

Heat : the form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference

 Mechanics: work is the energy transfer associated with a force acting through a distance (W = F.s).  Thermodynamics: work is an energy interaction that is not caused by a temperature difference between a system and its surroundings.

 Q in = + (positive)  Q out = - (negative)  W in = - (negative)  W out = + (positive)

1. Both are recognized at the boundaries of a system as they cross the boundaries. That is, both heat and work are boundary phenomena. 2. Systems possess energy, but not heat or work. 3. Both are associated with a process, not a state. Unlike properties, heat or work has no meaning at a state. 4. Both are path functions (i.e., their magnitudes depend on the path followed during a process as well as the end states), and not point functions.

 Path functions have inexact differentials designated by  (  Q or  W) NOT dQ or dW.  Properties are point functions (i.e., they depend on the state only, and not on how a system reaches that state), and they have exact differentials designated by d. A small change in volume, for example, is represented by dV.

 Properties are point functions  Heat and Work are path functions

A candle is burning in a well-insulated room. Taking the room (the air plus the candle) as the system, determine (a) if there is any heat transfer during this burning process and (b) if there is any change in the internal energy of the system.

(a) The interior surfaces of the room form the system boundary. As pointed out earlier, heat is recognized as it crosses the boundaries. Since the room is well insulated, we have an adiabatic system and no heat will pass through the boundaries. Therefore, Q = 0 for this process. (b) The internal energy involves energies that exist in various forms. During the process just described, part of the chemical energy is converted to sensible energy. Since there is no increase or decrease in the total internal energy of the system,  U = 0 for this process.

A potato initially at room temperature (25°C) is being baked in an oven that is maintained at 200°C, as shown in Fig. 2–21. Is there any heat transfer during this baking process?

This is not a well-defined problem since the system is not specified. Let us assume that we are observing the potato, which will be our system. Then the skin of the potato can be viewed as the system boundary. Part of the energy in the oven will pass through the skin to the potato. Since the driving force for this energy transfer is a temperature difference, this is a heat transfer process. Note: if the system is the oven, then Q = 0

 A well-insulated electric oven is being heated through its heating element. If the entire oven, including the heating element, is taken to be the system, determine whether this is a heat or work interaction.  How if the system is taken as only the air in the oven without the heating element. Electrical Work: W el = V.I.t = I.R.I.t

The energy content of the oven obviously increases during this process, as evidenced by a rise in temperature. This energy transfer to the oven is not caused by a temperature difference between the oven and the surrounding air. Instead, it is caused by electrons crossing the system boundary and thus doing work. Therefore, this is a work interaction.

This time, no electrons will be crossing the system boundary at any point. Instead, the energy generated in the interior of the heating element will be transferred to the air around it as a result of the temperature difference between the heating element and the air in the oven. Therefore, this is a heat transfer process.

1. Kinetical Work W k = F.s W b = P.A.ds = P.dV

Sebuah tangki kokoh berisi udara pada 500 kPa dan 150 o C. Akibat pertukaran panas dengan lingkungannya, suhu dan tekanan di dalam tangki berturut-turut turun menjadi 65 o C dan 400 kPa. Tentukan kerja lapisan batas selama proses ini. W b = 0 karena dV = 0

2. Shaft Work W sh = 2. .n.   = torsi = F.r Daya Poros:

Determine the power transmitted through the shaft of a car when the torque applied is 200 N.m and the shaft rotates at a rate of 4000 revolutions per minute (rpm).

The shaft power is determined directly from: = = 83.8 kW (112 HP)

3. Spring Work W sp = 0.5 k (x 1 2 – x 2 2 ) k = spring constant (kN/m) F = kx

 n = normal stress  n = F/A

W g = m.g.  z = m.g.  h W a = 0.5 m.(V 2 2 -V 1 2 )

Consider a 1200-kg car cruising steadily on a level road at 90 km/h. Now the car starts climbing a hill that is sloped 30° from the horizontal (Fig. 2–35). If the velocity of the car is to remain constant during climbing, determine the additional power that must be delivered by the engine.

The additional power required is simply the work that needs to be done per unit time to raise the elevation of the car, which is equal to the change in the potential energy of the car per unit time:

Determine the power required to accelerate a 900-kg car shown in Fig. 2–36 from rest to a velocity of 80 km/h in 20 s on a level road.