handout02 - 2 Properties of Matter AIM In this chapter we...

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2– Properties of Matter AIM: In this chapter, we will introduce the fundamental concepts that we will use in the rest of the course. 2.1 Thermodynamic Systems We divide the world into a ’system’ of interest and its ’exterior’; The two are separated by a ’boundary’. Depending on the kind of boundary, we have the following kind of systems: isolated : the system does not exchange energy or matter with the exterior. closed : the system exchanges energy (heat & mechanical energy), but not matter with the exterior. open : the system exchanges energy and matter with the exterior. insulation 2–1
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2.2 Equilibrium & Non-equilibrium BACKGROUND: We know from experience that an isolated system will relax to a particular state and then no longer change: it has reached thermodynamic equilibrium . EXAMPLE: we pour cold milk into an (isolated) cup of hot coffee. as we pour , the system is not at equilibrium (e.g. temperature is dropping) once the milk is well mixed into the coffee, the system is at equilibrium: the temperature will no longer change. NOTE: Again, the evolution towards equilibrium is an irreversible process. We will see that the entropy of a system is maximised at equilibrium. Another way of looking at this: there are thermodynamic forces driving the system towards equilibrium. The forces vanish when equilibrium is reached. QUESTION: What is a non-equilibrium state? ANSWER: A non-equilibrium state is one in which irreversible processes are taking place which drive the system towards the equilibrium state. 2–2
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2.3 State Variables & Functions AIM: We want to describe a thermodynamic system such as a gas in a chamber at a particular time t . For now, lets assume the system is at equilibrium. REMINDER: There are two levels of describing the gas: Microscopic: positions and velocities of all atoms Macroscopic: things we can measure directly In Thermodynamics, we are mainly concerned with macroscopic state variables such as N: number of particles (or n : moles of a substance) V: volume of the chamber T: absolute temperature p: pressure of the system . . . NOTE: There are two classes of state variables: intensive : independent of system size extensive : dependent of system size 2–3
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QUESTION: How do we determine whether a state variable X is intensive or extensive? ANSWER: (Scotty’s easy recipe) double N,V Imagine a system with a certain fixed N and V ’Measure how much X there is’ fi Double the system size with equal density; we now have 2 N particles in 2 V volume fl Decide whether we now have • ’double as much X ’ ( extensive); or • ’the same amount of X ’ ( intensive). EXAMPLE: T is intensive; inner energy U is extensive. 2.4 State Functions AIM: We can calculate further interesting properties about our gas in a chamber by defining functions of state variables. These are called state functions . Examples include: U = U ( T , N , V ) : total internal energy of the system S = S ( T , N , V ) : entropy of the system 2–4
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NOTE:
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