Readings

Read Chapter 4 of Physical Science, 13^th edition by Bill Tillery McGraw Hill Education

  Machines

In this module, machines are devices that operate in a cycle and that accomplish a task by taking in heat and/or work and producing an output of heat and/or work. Common examples of these machines are heat engines and refrigerators. 

Lecture Video

Time: 10:03

Topics: In this lecture we will cover machines such as heat engines and refrigerators, and the Second Law of Thermodynamics.

ASL version of Machines

Direct Link to the non-ASL Version

Machine Diagrams

Follow these steps when creating a machine diagram.

Metric of Goodness

Once all of the heat and work inputs and outputs of a machine have been identified, we want some way of accessing how effective it is at doing its job. In general, the way to compute this is to take the ratio between the output that is desired and the input that must be paid for

In the case of heat engines, heat is supplied as an input and the operator pays for this heat in the cost of the fossil fuels that are needed to run the engine, and the engine has two outputs, one is desired and one is not. The desired output is the work that can be transferred to the transmission. This ratio is called the engine's efficiency, and because the output work can never be more than the input this ratio is less than one, and is normally represented as a percentage.

Where $0<\text{efficiency}<1$

In the case of refrigerators, electrical work is done on a gas (the refrigerant) to compress it. When the gas expands, heat is pulled from the surrounding environment thereby cooling it, but as the compressor works it gets hot and radiates heat away. The energy transfer that is desired is the heat transfer from a cold environment, and the energy transfer that the operator must pay for is the electrical work being done. This ratio is called the coefficient of performance, COP

Where $\text{COP}>1$

Joules and Watts

Joules are the SI unit for energy, work, and heat whereas watts are the SI units for power and rates of energy or heat transfer, and one watt equals one joule per second. The machines in this module operate in a cycle; that means that the state of the machine is exactly the same at the end of the cycle as it was at the beginning. With this being the case, the device operates the same each cycle. Because of this, it is common to talk about the input power and output power instead of input work and output work per cycle; and instead of heat per cycle we talk about the rate of heat transfer. This won't change the result when computing the metric of goodness described above.

  Practice and Apply

Equation Sheets

Print off these equation sheets to help you complete the mastery assessments.

Equation Sheet - Module 4.pdf

PROMT How much energy does a refrigerator remove from 100.0 g of water at 20.0℃ to make ice at -10.0℃?

PROMT A heat engine operates with 65.0 Kcal of heat supplied and exhausts 40.0 kcals of heat. How much work did the engine do?