10.2 PROCESSES 10.3 THE SECOND LAW OF THERMO/ENTROPY Student Notes

Transcription

1 10.2 PROCESSES 10.3 THE SECOND LAW OF THERMO/ENTROPY Student Notes I. THE FIRST LAW OF THERMODYNAMICS A. SYSTEMS AND SURROUNDING B. PV DIAGRAMS AND WORK DONE V -1 Source: Physics for the IB Diploma Study Guide, Kirk 1

2 + W U + Q EXAMPLE 1 A gas in a container with a piston expands isothermally (at constant T). If 10 5 J of thermal energy is given to the gas, what is the work done on/by the gas? [10 5 J done by the gas] II. TYPES OF PROCESSES A. ISOCHORIC 2

3 B. ISOBARIC C. ISOTHERMAL 3

4 D. ADAIBATIC EXAMPLE 2 Did you know.. that 1.0 g of water (1.0 cm 3 ) becomes 1671 cm 3 of steam when boiled at a constant pressure of 1.0 atmosphere (1.013 x 10 5 Pa)? The latent heat of vaporization of steam at this pressure is Lv = x 10 6 Jkg -1. Calculate: a) The work done by the water when it vaporizes. [+169 J] b) Its increase in internal energy. [2087 J] 4

5 Pressure E. INTERPRETING PV DIAGRAMS P 2 A B P 1 D C V 1 volume V 2 III. THE SECOND LAW OF THERMODYNAMICS A. HEAT TRANSFER AND THE WAY THINGS NATURALLY HAPPEN DEFINE: Spontaneous = something that occurs without the action of another agent It will never happen that thermal energy will spontaneously transfer from a cold to a hot object. Although does the first law forbid this? NO. Think: - Will air in a room suddenly occupy one half of the room and leave the other half completely empty? - Will a glass of water at room temperature suddenly freeze, thereby causing the temperature of the room to rise? B. ENTROPY AND DISORDER Thermodynamic processes that occur in nature all occur spontaneously in one direction but not in the other. The direction of the process is always to increase the disorder of the system. These processes are irreversible because they lead to a greater state of disorder. Therefore, thermodynamics has been related to the arrow of time, the direction in which all natural processes take place. 5

6 Irreversibility can be quantified by entropy. Entropy is a measure of the randomness or disorder os a system. The greater the entropy, the greater the disorder. The Second Law of Thermodynamics states that the entropy of a system can never decrease. Implying that: heat flows spontaneously from a substance at a higher T to a substance at a lower T and does not flow spontaneously in the reverse direction. For example: Imagine a book sliding across a table and coming to rest. We have the kinetic energy of the book being transferred to heat energy by friction. We never get the opposite process occurring (i.e. the book cooling down and transferring this heat energy to kinetic energy). This is because kinetic energy is associated with organized, coordinated motions of many molecules, but heat transfer involves changes in energy of random, disordered molecular motion. Therefore conversion of mechanical energy involves an increase in the randomness or disorder. Another example: The melting of ice. Left in a room at 20 0 C, heat will flow into ice and melt it. We would never expect a beaker of water to suddenly melt in the room. This is because if it were to do so, the entropy of the system (water and room) would have decreased (that is entropy increase of the room would not have been as great as the entropy loss of the water). A NOTE ON THE ENERGY CRISIS In the news we are always hearing about the impending energy crisis. But the problem facing our world is not an energy crisis. We have the same amount of energy as ever, because energy cannot be destroyed. The problem is that the available energy is slowly being degraded. This means that the energy is slowly becoming dispersed because as we burn our fuels, the entropy of the universe increases. Although the first law tells us that the total quantity of energy in any process is constant, the second law tells us that as soon as any physical process involves the transfer of energy to thermal energy that energy begins to disperse. Once dispersed, the energy cannot then be used to do as much work as it could before. We are therefore suffering an entropy crisis rather than an energy crisis! So how do we solve the problem? 6

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