handout07 - 3 The First Law of Thermodynamics Introduction...

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3– The First Law of Thermodynamics Introduction REMINDER: There are two central laws in Thermodynamics: ’The energy of the universe is constant. The entropy of the universe tends to a maximum.’ Rudolph Clausius (1865) In this chapter, we will discuss energy, heat and work in more detail. NOTE: The concept of energy and that it is conserved evolved in the late 19th century. Energy can be inter-converted from/to many different forms (Kondepudi) Transformation Chemical Mechanical Work Heat Magnetism Electricity 3–1
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EXAMPLE: The Thermoelectric Effect: Electricity-Heat interconversion Named after the independent discovers Seebeck, Peltier and Thomson. NOTE: This is not the same as the Joule effect: an electric current flowing through a resistor will heat up. This is used in electric heaters of all sorts (e.g. hair driers). The thermoelectric effect is reversible (can generate electricity by a temperature difference and create a temperature difference by applying a voltage difference), the Joule effect is not. Seebeck discovered that a voltage is created between two different metals (or semiconductors) by a temperature difference: T2 T1 V This effect is very small: 10’s of microvolts per Kelvin difference. Applications include: Temperature sensors (thermocouples) Thermoelectric cooling by Peltier elements: reversible heat pumps without moving parts (e.g. camping cool boxes that cool and heat) 3–2
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fi Thermoelectric power generation: convert heat directly into electricity. For example, in a radioisotope thermoelectric generator (RTG): use radioactive decay to produce the required heat. Inspection of the Cassini spacecraft RTG before launch (NASA ) NOTE: We will see that heat, energy and work all have the same units (Joule), but that they are fundamentally different : internal energy is a state function . heat and work are not functions of state; they are path functions . 3–3
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QUESTION: What does this mean in practise? ANSWER: We can determine the energy of a particular system without knowing any of its history: we say that energy is path-independent Heat, Q , and work, W , are involved in processes (heating, cooling, gas expansion, etc.), we need to know the way in which the process evolves: we say that heat and work are path-dependent 3.1 Internal Energy, Heat and Work We have mentioned inner or internal or intrinsic energy in this class before without defining it further. . . QUESTION: So what is this internal energy U ? ANSWER: intuitively: Any energy that can be defined within a gas container is internal.
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