A car's engine would obviously be considered a machine. However, it operates a bit differently than the machines we have been discussing. In a later chapter we will be going over this in more detail. But for now, it is important to understand that heat can be equated with energy and if something moves (even just molecules), then it produces heat. 

Heat always flows from hot to cold. If a hot object comes in contact with a cold object, heat will be exchanged. The heat from the hot object will flow into the cold. In a closed system, the amount of heat lost (Qlost) from the hot object will equal the amount of heat gained (Qgained) by the cold. Using the letter Q to represent heat, then:


Qlost = Qgained



Let’s use a hot cup of coffee as an example. If it is really hot, one might put an ice cube into the coffee to “cool it off.” But what is really happening? Because the coffee has greater internal (heat) energy, this energy in the form of heat will move into the ice cube. As the ice cubes internal energy increases, it begins to melt. The higher internal energy of the coffee continues to flow to the cooler melted “ice cube” until they reach thermal equilibrium. The ice cube does not cool off the coffee, but the coffee loses heat energy to the ice cube. Once this law of heat exchange is understood, it becomes clearer how virtually all systems work on the principle of heat exchange.

As a simple example, this principle is used when we blow on a hot piece of food. The heat energy from the hot food is transferred to the cooler air. The lower internal energy of the food is now at a more reasonable temperature for us to eat.



REVIEW QUESTION

1. With the idea of heat loss and heat gain in mind, explain what happens when you put an ice cube in a hot cup of coffee. 

Record your answer in the logbook!


A more complex example is that of a running car engine. The heat energy of the exploding gas inside the engine is transferred to the metal engine block. As we learned about thermal equilibrium, if this heat exchange continues, the engine block could potentially reach the temperature of exploding gas. Of course if this happened the engine block would warp and “seize” (not able to work any longer). So to prevent this we add another heat exchange system. 


Water is circulated into the engine gaining some of the internal energy of the engine block. The water then goes through a radiator where air is blowing over it, and the water loses heat energy to the air and becomes cooler. This cycle is then repeated, keeping the car engine at a normal operating temperature.


REVIEW QUESTION

2. Why do car engines circulate water within the engine block?

Record your answer in the logbook!


Normally when we think of an engine we picture a car or motorcycle engine. Although these are heat engines, they are only one type. So what then is a heat engine? Simply put, a heat engine is a machine that converts heat energy into mechanical energy. It takes heat that is in or added to a system and turns it into motion. Indeed, a car engine accomplishes this by taking the tremendous heat energy of exploding gas and turning it into motion. An important consideration in any heat engine is a difference in temperature. Remember, heat flows from hot to cold. Let’s use a hurricane, a natural heat engine, as an example.


The hurricane obtains its heat from the warm, humid air over the tropical ocean. It releases this heat as the water changes phase from condensation (gas) to rain (liquid). The released heat warms the air which causes it to rise up into the cooler atmosphere. This heat exchange is continually repeated and as the air rises it causes an effect similar to water going down a drain, swirling round and round, getting larger and faster during the heat exchange process. The animation below shows this process.

 


REVIEW QUESTION

3. Name two different types of heat engines.

Record your answer in the logbook!


The steamboat you built is a basic heat engine that converts heat energy (from the candle) to mechanical energy (movement forward). As with any heat engine there is a difference in temperature. There is the hot steam inside the boiler and the cold water outside the boat (in the bowl). There is also a heat exchange process. The heat from the flame is transferred to the water in the boiler, turning it to steam. The steam travels into the colder pipes and exchanges its heat with the pipe and the colder water in the pipe.

 

Click through the animation below to learn the mechanics of how your steam engine works and why it moves forward.

 


REVIEW QUESTION

4. In your own words, explain the basic heat exchange process of the steam boat.

Record your answer in the logbook!


 

 

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