 # VII. The second low of Thermodynamics

## Presentation on theme: "VII. The second low of Thermodynamics"— Presentation transcript:

VII. The second low of Thermodynamics
1. Reversible and irreversible processes Mechanics Thermodynamics Heat can flow spontaneously from a hot object to a cold object; heat will not flow spontaneously from a cold object to a hot object. 2. Heat engines H.E. transforms heat partly into work or mechanical energy Working substance Cycle process: ΔU = 0 ΔU = Q - W = 0 W = Q = QH + QC = |QH| - |QC| TH efficiency: |QH| W engine 3. Refrigerators |QC| coefficient of performance: TC

4. The second law of thermodynamics and heat engines
It is impossible to build a heat engine with 100% efficiency 5. The Carnot Cycle (reversible) Maximum efficiency: P a b TH d c TL V 6. The Carnot Cycle and the second law of thermodynamics No device is possible whose sole effect is to transform a given amount of heat completely into work No device is possible whose sole effect is to transform heat from one system at temperature TL into a second system at higher temperature TH

Example 1: A nuclear power plant generate steam at 6300 C.
The steam is sent through a series of turbines and exits at 1000 C. What is the maximum possible efficiency of the power station? TH = 630ºC TC = 100ºC εC - ?

Example 2: An ice-making machine operates in a Carnot cycle.
It takes heat from water at 0.0C and rejects heat to a room at 24.0C. Suppose that 50.0 kg of water at 0.0C are converted to ice at 0.0C. How much energy must be supplied to the device? Heat of fusion of water is 334*103 J/kg. TH = 24.0C TC = 0.0C m = 50.0 kg L = 334*103 J/kg W - ?

The thermal efficiency of the heat engine is ___ %.
Question In one cycle, a heat engine does 10 J of work and exhausts 20 J of waste heat. The thermal efficiency of the heat engine is ___ %. 20 33 50

The temperature of the hot reservoir is ___ K.
Question In a Carnot heat engine, the cold temperature reservoir is at 300 K (= 27 ˚C) and the thermal efficiency is 50%. 400 600 800 1000 The temperature of the hot reservoir is ___ K.

Question A refrigerator has a coefficient of performance K = Work is done on the refrigerator at a rate Pwork = dW/dt = 100 Watts. 50 100 200 400 The rate at which heat is removed from inside the refrigerator is d|QC|/dt = ___ Watts.

The internal-combustion engine
A fuel vapor can be compressed, then detonated to rebound the cylinder, doing useful work.

r - compression ratio γ – CP/CV 6. Otto cycle
Example: An automobile engine has a compression ratio of 10.0. The maximum theoretical thermal efficiency of the engine is ___ %. ( = 1.40 for air)