What is the direction of energy transfer, and in what form is the energy transferred (work or heat)?
1. A person goes swimming in cold water.
2. A truck pulls a trailer.
3. The explosion of gasoline in an engine cylinder pushes the piston and increases the temperature of the engine.
2 Answers | Add Yours
The best way to remember this is that energy in thermodynamics really only gets transferred in either of those two forms: heat and work. If something moves (higher than only the molecular level),work has been done. If something gets hotter (moving faster at the molecular level only), then heat has been transferred. Two things you need to remember for this problem, specifically, work requires that an object be moved a certain distance (no movement, no work), and heat can only flow fromhot to cold.
Now, you can solve each portion of the question. If a person swims in cold water, based on our hot to coldheat flow, energy will be transferred from the person to the water in the form of heat. Because water is being moved, work is being done; however, because this point is not brought up in the question, I would not worry about it.
In the second situation, just remember that pulling a trailer isn't going to cause it to heat up in any significant way. One object is simply causing another to move: the truck is exerting work on the trailer.
Finally, in an engine, the temperature is increasing and things are moving! The heat energy here is actually flowing from the hot gas generated by combustion to the engine block, causing it to get warmer. Also, the expanding gas does work on the piston, causing it to move. In fact, because of all of these thermodynamic properties in engines, you will see most of these terms used for standardized engine examples!
For situation 1, we should always remember that energy as heat flows from high temperature region to low temperature region. Thus, the direction of energy trasfer is from the person to the cold water. (Answer: Energy transfer is from the person to the water; Heat)
For situation 2, there is a work exerted to the trailer and the trailer moved because of the pulling of the truck. (Answer: Work)
For situation 3, this is a combination of heat and work. Heat flows from the engine cylinder to the piston causing an increased in temperature. Thus, there is energy transfer. Furthermore, the piston moved, thus a work is created. (Answer: Energy transfer is from the engine cylinder to the piston; Heat and Work)
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