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The laws of thermodynamics
 

The laws of thermodynamics

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    The laws of thermodynamics The laws of thermodynamics Presentation Transcript

    • Connections between heat and work
    • Connections Between Heat and Work
    • In studying thermodynamics… Surroundings SYSTEM
    • Zeroth Law of Thermodynamics  If objects A, B and C are in contact with each other, and A is in thermal equilibrium with B and B is in thermal equilibrium with C, then A is in thermal equilibrium with C. A B C
    • Zeroth Law at Work
    • First Law of Thermodynamics Heat (Q) Internal Energy (U) Work (W)
    • First Law of Thermodynamics  The change in internal energy (ΔU) is equal to the difference of the heat (Q) added/removed to/from the system and the work (W) done on/by the system. ΔU = Q – W
    • Sign Conventions for the First Law For Heat (Q) + if the heat is added to the system if the heat is removed from the system For Work (W) + if work is done by the system if work is done on the system
    • Sample Word Problems on 1st Law  Suppose 2500 J of heat is added to a system and 1800 J of work is done on the system. What is the change in internal energy of the system?  You have a motor that absorbs 3000 J of heat while doing 2000 J of work. What is the change in the motor’s internal energy?  Say that a motor does 1000 J of work on its surroundings while releasing 3000 J of heat. By how much does its internal energy change?
    • The First Law at Work
    • Thermodynamic Processes  Adiabatic (constant heat)  Isothermal (constant temperature)  Isochoric (constant volume)  Isobaric (constant pressure)
    • Second Law of Thermodynamics  Review: How does heat flow?  Would it be possible for heat to flow from an area of lower to higher temperature?
    • Second Law of Thermodynamics  Clausius Statement Heat can flow spontaneously from a hot object; heat will not flow spontaneously from a cold object to a hot object.
    • Second Law and Heat Engines  It is a machine that turns energy into mechanical energy or motion, especially one that gets its energy from a source of heat, such as burning of a fuel.  Can be classified as external combustion and internal combustion engines.
    • Water-Tube Type  It is common with stationary engines and turbines.  Water is allowed to pass through tubes while the flames and hot gaseous products of combustion follow a path over around tubes.
    • Fire-Tube Type  Used in steam locomotives  Flames and heat are made to enter the tubes which are horizontally arranged in the boiler and are surrounded by water.
    • Other Classification of Steam Engines  Condensing type  Non-condensing type
    • Gasoline Engines  These are engines whose working substance is gasoline. It is internally burned unlike steam engines.
    • Parts of a Gasoline Engine
    • Intake Stroke
    • Compression Stroke
    • Power Stroke
    • Exhaust Stroke
    • Diesel Engines  These are engines whose working substance is diesel. It is internally burned unlike steam engines.
    • Diesel Engine Structure
    • How efficient are heat engines?
    • Thermal Efficiency of an Engine  It is defined as the ratio of the net work (W) done by the engine during one cycle to the energy absorbed at the higher temperature (QH) during the cycle.
    • Sample Problem on Thermal Efficiency  Find the efficiency of a heat engine that absorbs 2000 J of energy from a hot reservoir and exhausts 1500 J to the cold reservoir.  Your car is powered by a heat engine and does 3.0 x 107 J of work getting you up a small hill. If the heat engine is 80 percent efficient, how much heat did it use and how much did it exhaust?
    • Sadi Carnot  A French engineer who established the concept of an ideal engine known as the Carnot engine.  He developed the Carnot’s theorem which states that No real engine operating between two energy reservoirs can be more efficient than a Carnot engine operating between the same two reservoirs.
    • Basic Concept of the Carnot Engine  The ideal efficiency of an engine depends on the difference of the hot and cold reservoirs.  You can’t have it all.
    • Sample Word Problems on Carnot Efficiency  If an engine extracts heat from a 2730 K reservoir and expels heat at 1730 K reservoir, what is its efficiency?  How about if the engine extracts heat from a 10 730 K reservoir instead?  What if the engine reservoirs are working at the same temperatures?
    • Wait a minute…  It is possible to produce work from heat – that is heat transferred from a hot reservoir to a cold reservoir. Would it be possible to do the reverse?
    • Second Law of Thermodynamics  Kelvin-Planck Statement It is impossible to construct a heat engine that, operating in a cycle, produces no effect other than the absorption of energy from a reservoir and the performance of an equal amount of work.
    • The Impossible Heat Pump
    • Heat Pumps  These are heat engines running in reverse.  Heat is transferred from a cold reservoir to a hot reservoir by performing work.  This is done through the aid of phase change.  Examples are refrigerators and air conditioning units.
    • The Structure of a Heat Pump
    • Refrigerators
    • Questions to Ponder  What do you notice with heat engines and heat pumps?  What do they have in common?  What is its impact to the environment?
    • Entropy  Natural processes tend to undergo increased state of disorder.  A measure of the amount of energy in a physical system not available to do work.
    • Second Law of Thermodynamics  On entropy The total entropy of an isolated system that undergoes a change can never decrease.
    • Entropy increases…
    • Third Law of Thermodynamics  It is impossible to reach absolute zero.
    • Let’s wrap up…