Work Done by a Gas Work=(Force)x(distance) Work=(Force)x(distance) =F  y Force=(Presssure)x(Area) Force=(Presssure)x(Area) W=P(A  y) W=P(A  y) =P 

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Presentation transcript:

Work Done by a Gas Work=(Force)x(distance) Work=(Force)x(distance) =F  y Force=(Presssure)x(Area) Force=(Presssure)x(Area) W=P(A  y) W=P(A  y) =P  V

First Law of Thermodynamics Conservation of energy When heat is added into a system it can either 1) change the internal energy of the system (i.e. make it hotter) or 2) go into doing work. When heat is added into a system it can either 1) change the internal energy of the system (i.e. make it hotter) or 2) go into doing work. Q=W +  U. Note: For our purposes, Internal Energy is the part of the energy that depends on Temperature.

Heat Engines If we can create an “engine” that operates in a cycle, we return to our starting point each time and therefore have the same internal energy. Thus, for a complete cycle If we can create an “engine” that operates in a cycle, we return to our starting point each time and therefore have the same internal energy. Thus, for a complete cycleQ=W

Model Heat Engine Q hot = W+Q cold Q hot = W+Q coldor Q hot -Q cold =W Q hot -Q cold =W (what goes in must come out)

Efficiency We want to write an expression that describes how well our heat engine works. We want to write an expression that describes how well our heat engine works. Q hot =energy that you pay for. Q hot =energy that you pay for. W=work done (what you want.) W=work done (what you want.) Q cold = Waste energy (money). Q cold = Waste energy (money). Efficiency = e = W/Q hot

If we had a perfect engine, all of the input heat would be converted into work and the efficiency would be 1. If we had a perfect engine, all of the input heat would be converted into work and the efficiency would be 1. The worst possible engine is one that does no work and the efficiency would be zero. The worst possible engine is one that does no work and the efficiency would be zero. Real engines are between 0 and 1 Real engines are between 0 and 1

Newcomen Engine (First real steam engine) e=0.005

Example Calculation In every cycle, a heat engine absorbs 1000 J from a hot reservoir at 600K, does 400 J of work and expels 600 J into a cold reservoir at 300K. Calculate the efficiency of the engine. In every cycle, a heat engine absorbs 1000 J from a hot reservoir at 600K, does 400 J of work and expels 600 J into a cold reservoir at 300K. Calculate the efficiency of the engine. e= 400J/1000J=0.4 e= 400J/1000J=0.4 This is actually a pretty good engine. This is actually a pretty good engine.

Second Law of Thermodynamics (What can actually happen) Heat does not voluntarily flow from cold to hot. Heat does not voluntarily flow from cold to hot.OR All heat engines have e<1. (Not all heat can be converted into work.) All heat engines have e<1. (Not all heat can be converted into work.)

Carnot Engine The very best theoretically possible heat engine is the Carnot engine. The very best theoretically possible heat engine is the Carnot engine. The efficiency of a Carnot engine depends on the temperature of the hot and cold reservoirs. The efficiency of a Carnot engine depends on the temperature of the hot and cold reservoirs.

Example Calculation Part II In every cycle, a heat engine absorbs 1000 J from a hot reservoir at 600K, does 400 J of work and expels 600 J into a cold reservoir at 300 K. Calculate the efficiency of the best possible engine. In every cycle, a heat engine absorbs 1000 J from a hot reservoir at 600K, does 400 J of work and expels 600 J into a cold reservoir at 300 K. Calculate the efficiency of the best possible engine. e= 1-300/600 =0.5 e= 1-300/600 =0.5 Recall that the actual engine has e=0.4. Recall that the actual engine has e=0.4.