2.6, 2.7, 2.8

The First Law of Thermodynamics

- energy change = energy at final state - energy at initial state

- E(in) -  E(out) = [Q(in) - Q(out)] + [W(in) - W(out)] + [E(mass,in) - E(mass,out)] = energy change

- Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear, and their sum constitutes the total energy, E of a system Thermodynamics deals only with the change of the total  energy.

EFFICIENCY

Frequently used terms in thermodynamics to indicate how well an energy conversion or transfer process is accomplished.

OVERALL EFFICIENCY

Generator:

A device that converts mechanical energy to electrical energy.

Generator efficiency:

The ratio of the electrical power output to the mechanical power input. 

Thermal efficiency of a power plant:

The ratio of the net electrical power output to the rate of fuel energy input.

 ENERGY AND ENVIRONMENT

-Energy conversions affects the environment in many ways, and thus the study of energy is not complete without considering its impact on the environment.

-Pollutants emitted during the combustion of fossil fuels are responsible for smog, acid rain, and global warming.

1. Ozone and Smog

2. Acid Rain
3.The Greenhouse Effect: Global Warming and Climate Change.

Solutions:

1) Energy Efficient technology.

2) Renewable energy.

 

2.4 ENERGY TRANSFER BY HEAT

2.5 MECHANICAL FORMS OF WORK

#2 ENERGY, ENERGY TRANSFER, AND GENERAL ENERGY ANALYSIS

2.1 INTRODUCTION

Mechanisms of Energy Transfer

2.2 Forms of Energy

      

Mass flow rate is the amount of mass flowing through a cross section per unit time.

 m = pV    (kg/s)    where p is the fluid density and V is the volume of fluid.

Energy flow rate associated with a fluid flowing at a mass flow rate.

 E = me    (kJ/s or kW)             

2.3 ENERGY TRANSFER BY HEAT

Heat is defined as the form of energy that is transferred between two systems (or a system and its or surroundings) by virtue of a temperature difference. The rate of heat transfer is high when the temperature difference is large. 

A process during which there is no heat transfer is called an adiabatic process. The process is only adiabatic either when the system is well insulated so that only a negligible amount of heat can pass through the boundary, or both the system and the surroundings are at the same temperature and therefore there is no driving force (temperature difference) for heat transfer. 

1.7 PROCESS AND CYCLE

1- Thermodynamics Process is any change that a system undergoes from one equilibrium state to another.

2- Quasistatic or quasi-equilibrium process proceeds in such a manner that the system remains infinitesimally close to an equilibrium state at all times.

3- Process Path is the series of states through which a system passes during a process.

4- Cycle is one in which the process returns to its initial state.

5- Work and heat are not system properties because both are dependent on process.


1.8 TEMPERATURE AND THE ZEROTH LAW OF THERMODYNAMICS

The zeroth law of thermodynamics: If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other.





Two bodies reaching thermal equilibrium after being brought into contact in an isolated enclosure.

The way to indicate temperature
1. Absolute zero = the lowest temperature
2. Celcius: freezing and boiling point of water Kelvin: Absolute zero
3. Fahrenheit: freezing point of brine
4. Rankine: Absolute zero


1.9 PRESSURE



 Pressure: A normal force exerted per unit area (P=F/A)

    

1 atm is equal to,
1.013 bar (English unit)
101.325kPa (SI unit)
0.1MPa (SI that is accepted to describe 1atm)
14.7 psi (lbf/in2)
760 mmHg (milimeter of mercury)
760 Torr(not SI unit)

 

•Absolute pressure: measured relative to absolute vacuum (i.e., absolute zero pressure).
•Gage or Gauge pressure: measured relative to atmospheric pressure.
•Vacuum pressures: Pressures below atmospheric pressure. Degree of vacuum is also usually used.

1.10 MANOMETER

 

Measuring the pressure drop across a flow section or a flow device by a differential manometer

 

 

 

The basic manometer

 

1.11 THE BAROMETER AND ATMOSPHERIC PRESSURE

• Atmospheric pressure is measured by a device called a barometer; thus, the
  atmospheric pressure is often referred to as the barometric pressure.
• A frequently used pressure unit is the standard atmosphere, which is defined as
  the pressure produced by a column of mercury 760 mm in height at 0°C (Hg =
  13,595 kg/m3) under standard gravitational acceleration (g = 9.807 m/s2).