UNIT 2:WAVE AND PARTICLE NATURE OF LIGHT►

UNIT 1:APPLICATIONS OF THERMODYNAMICS LAWS

  • UNIT 1:APPLICATIONS OF THERMODYNAMICS LAWS

    Mutesi is a parent of two children at a certain school. Before she takes them 
    to school, she first makes sure that she prepares food and drinks for them 
    and packs some in flasks so that her children can eat and drink during lunch 
    time. 
    She then drives them to school before she reports to her working place and 
    then from the school she then diverts to her working place which is about 5 
    km away from the school.
    The parking yard at her work place is a plain place without any shade but she 
    makes sure that her car is parked near a tree that is near the parking yard 
    to prevent it from different damages among which is destruction of tyres of 
    the car.
    a) Explain why Mutesi makes use of flasks not normal utensils like metallic 

    bowels while parking foods and drinks for her children.

    b) Is there heat exchange inside the flasks? Explain your reasoning.
    c) Imagine on a certain day these two children only eat food and leaves, 
    the drink in the flask and by mistake they forget flask in the store and 
    the mother come to pick it the next day. Do you think the contents in the 
    flask will be at the same temperatures? Explain all scientific phenomena 
    that may lead to either loss or gain in energy of the contents in the flask.
    d) Explain why in most cases the outer covering of a flask is always made 
    of a poor conductor? Explain how quality and efficiency of these flasks 
    can be improved by manufactures.
    e) Based on statements above, Mutesi normally parks her car under a 
    shade to prevent her car from being exposed to sunshine. Explain how 
    during hot days the tyres of a car may burst.
    f) Her Car uses petrol in operation. During operation of her car, the engine 
    draws fuel (Petrol) air mixture from the tank into the engine, explain 

    all the processes that take place in the engine.

    Thermodynamics refers to the study of heat and its transformation into 
    mechanical energy.
    In thermodynamics, the internal energy is one of the two extremely important 
    state functions of the variables of a thermodynamic system. It refers to total 
    energy contained within the system excluding the kinetic energy of motion of 
    the system and the potential energy of the system due to external forces. It 

    keeps account of the gains and losses of energy of the system.

    The internal energy of a system may be changed by 
    i) heating the system
    ii) doing work on it, 
    iii) adding or taking away matter.
    The thermal energy is the portion of internal energy that changes when the 
    temperature of the system changes. Sometimes the term thermal energy is 
    used to mean internal energy. Heat is defined as the transfer of energy across 
    the boundary of a system due to a temperature difference between the system 
    and its surroundings. 
    When you heat a substance, you are transferring energy into it by placing it 
    in contact with surroundings that have a higher temperature. For example, 
    when you place a pan of cold water on a stove burner, the burner is at a higher 
    temperature than the water, and so the water gains energy. 
    In daily life, we recognize the difference between internal energy and heat. The 
    heat transfer is caused by a temperature difference between the system and its 
    surroundings. However, in some systems there are no temperature and pressure 

    gradients, such systems are said to be in thermodynamic equilibrium.



    Consider a gas contained in a cylinder fitted with a movable piston. At 
    equilibrium, the gas occupies a volume V and exerts a uniform pressure P on 
    the cylinder’s walls and on the piston. If the piston has a cross-sectional area 
    A, the force exerted by the gas on the piston is F = PA. Now let us assume that 
    we push the piston inward and compress slowly to allow the system to remain 

    essentially in thermal equilibrium.


    With work done by the force due to pressure, we find the same relation but 
    having a negative sign. The force is exerted in opposite direction and the final 

    volume is less than the initial one.

    The total work done on the gas as its volume changes from initial volume (Vi ) 
    to final volume (Vf ) is given by the above equation.
    If the gas is compressed, ΔV is negative and the work done on the gas is positive 
    (Work done by the gas is positive) and if the gas expands, ΔV is positive and 
    the work done on the gas is negative (Work done on the gas is negative). If 
    the volume remains constant, the work done on the gas is zero. Thus, no work 
    done. To evaluate this relation, one must know how the pressure varies with 
    volume during the process.
    The work done on a gas in a quasi-static process that takes the gas from an 
    initial state to a final state is the negative of the area under the curve on a PV 
    diagram, evaluated between the initial and final states.
    Based on the processes of compressing a gas in the cylinder indicated in figure 
    1.1, the work done depends on the path taken between the initial and final 

    states

    1.3.1. First law of Thermodynamics
    It states that the change in internal energy of a system is equal to the heat added 
    to the system minus the work done by the system. Therefore, the law stated 
    gives mathematical treatment of internal energy of a system shown below.

    Hence the first law of thermodynamics.

    Note: 
    - The first law of thermodynamics is a special case of the law of conservation 
    of energy that encompasses changes in internal energy and energy 
    transfer by heat and work. 
    - It is a law that can be applied to many processes. It is noticed that energy 
    can be transferred between a system and its surroundings. 
    - One is work done on the system, which requires that there be a macroscopic 
    displacement of the point of application of a force. 
    - The other is heat, which occurs on a molecular level whenever a 
    temperature difference exists across the boundary of the system.
    - Both mechanisms result in a change in the internal energy of the system 
    and therefore usually result in measurable changes in the macroscopic 
    variables of the system, such as the pressure, temperature, and volume 
    of a gas.
    - The increase in internal energy of a system is the sum of the work done on 
    the system and the heat supplied to the system.
    - One of the important consequences of the first law of thermodynamics 
    is that there exists a quantity known as internal energy whose value is 
    determined by the state of the system. The internal energy is therefore a 
    state variable like pressure, volume, and temperature.
    - The first law of thermodynamics is an energy conservation equation 
    specifying that the only type of energy that changes in the system is the 

    internal energy ΔU.


    1.3.3. Applications of first law of Thermodynamics
    The first law of thermodynamics that we discussed relates the changes in 
    internal energy of a system to transfers of energy by work or heat. In this case 
    , we consider applications of the first law in processes through which a gas is 

    taken as a model.

    ISOBARIC PROCESS
    A process that occurs at constant pressure is called an isobaric process. In such 
    processes, the values of the heat and the work are both usually nonzero. The 

    work done during isobaric process is simply

     

    Since the first law of thermodynamics states that energy is conserved. There 
    are, however, many processes we can imagine that conserve energy but are not 
    observed to occur in nature. Lets consider an example below of the first law to 
    introduce the second law.
    For example, when a hot object is placed in contact with a cold object, heat 
    flows from the hotter one to the colder one, never spontaneously the reverse. 
    If heat were to leave the colder object and pass to the hotter one, energy could 
    still be conserved. Yet it doesn’t happen spontaneously the reverse.
    There are many other examples of processes that occur in nature but whose 
    reverse does not. To explain this lack of reversibility, scientists in the latter half 
    of the nineteenth century formulated a new principle known as the second 

    law of thermodynamics. 

    The second law of thermodynamics is a statement about which processes occur 
    in nature and which do not. It can be stated in a variety of ways, all of which are 
    equivalent. One statement is that: “Heat can flow spontaneously from a hot 
    object to cold object; heat will not flow spontaneously from a cold object 
    to a hot object”. 
    The development of a general statement of the second law of thermodynamics 
    was based partly on the study of heat engines. A heat engine is any device 
    that changes thermal energy into mechanical work, such as steam engines and 

    automobile engines.

    1.4.2.3. Impact of heat engines on climate
    Most of air pollution is caused by the burning of fuels such as oil, natural gas 
    etc. The air pollution has an adverse effect on the climate. Climate change is the 
    greatest environmental threat of our time endangering our health. When a heat 
    engine is running, several different types of gases and particles are emitted that 
    can have detrimental effects on the environment.
    Of concern to the environment are carbon dioxide, a greenhouse gas; and 
    hydrocarbons. Engines emit greenhouse gases, such as carbon dioxide, which 
    contribute to global warming. Fuels used in heat engines contain carbon. The 
    carbon burns in air to form carbon dioxide. 
    The Carbon dioxide and other global warming pollutants collect in the 
    atmosphere and act like a thickening blanket and destroy the ozone layer. 
    Therefore, the sun’s heat from the sun is received direct on the earth surface 
    and causes the planet to warm up.
    As a result of global warming, the vegetation is destroyed, ice melts and water 
    tables are reduced. Heat engines especially diesel engines produce Soot which 
    contributes to global warming and its influence on climate. 
    The findings show that soot, also called black carbon, has a warming effect. 
    It contains black carbon particles which affect atmospheric temperatures in a 
    variety of ways. The dark particles absorb incoming and scattered heat from the 
    sun; they can promote the formation of clouds that can have either cooling or 
    warming impact.Therefore soot emissions have significant impact on climate 
    change.
     Similarly, some engines leak, for example, old car engines and oil spills all over. 
    When it rains, this oil is transported by rain water to lakes and rivers. The oils 
    then create a layer on top of the water and prevent free evaporation of the water.
    1.4.3. Carnot cycle and Carnot engine
    In 1824 a French engineer named Sadi Carnot described a theoretical engine, 
    now called a Carnot engine, which is of great importance from both practical 
    and theoretical viewpoints. He showed that a heat engine operating in an ideal, 
    reversible cycle—called a Carnot cycle—between two energy reservoirs is the 
    most efficient engine possible.
    An ideal engine establishes an upper limit on the efficiencies of all other engines. 
    That is, the net work done by a working substance taken through the Carnot 
    cycle is the greatest amount of work possible for a given amount of energy 
    supplied to the substance at the higher temperature. 
    Carnot’s theorem can be stated that no real heat engine operating between 
    two energy reservoirs can be more efficient than a Carnot engine operating 
    between the same two reservoirs.
    Note: No Carnot engine actually exists, but as a theoretical idea it played an 
    important role in the development of thermodynamics.
    The idealized Carnot engine consisted of four processes done in a cycle, two of 
    which are adiabatic (Q = 0) and two are isothermal (ΔT = 0). This idealized cycle 
    is shown in figure 1.8.

    From P-V diagram for the Ideal Diesel cycle, the cycle follows the numbers 1-4 
    in clockwise direction. The image on the top shows a P-V diagram for the ideal 
    Diesel cycle; where P is pressure and V is specific volume. The ideal Diesel cycle 
    follows the following four distinct processes (the color references refers to the 
    color of the line on the diagram.
    • Process 1-2 is isentropic (adiabatic) compression of the fluid (blue 
    color). 
    • Process 2-3 is reversible (isobaric constant pressure heating (red). 
    • Process 3-4 is isentropic (adiabatic) expansion (yellow). 
    • Process 4-1 is reversible constant volume cooling (green). 
    The Diesel is a heat engine; it converts heat into work. The isentropic processes 
    are impermeable to heat; heat flows into the loop through the left expanding 
    isobaric process and some of it flows back out through the right depressurizing 
    process, and the heat that remains does the work. 













    UNIT 2:WAVE AND PARTICLE NATURE OF LIGHT►