Heat Transfer

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Now that we know how pressure makes a popcorn kernel pop, we need to look at how exactly the popcorn kernels are heated in order to raise their temperature. According to the Merriam-Webster online dictionary the definition of heat is, “Energy transferred from one body to another as the result of a difference in temperature.” (“Heat”). By looking at all three of our different methods of popping it can be seen that energy is transferred from the air popper, stove top and fire to the popcorn kernels. Now we need to look at the different methods of energy transfer; there are three main ways energy is transferred which are conduction, convection, and radiation. A good visualization of these three types is shown in the image on the left. Conduction occurs when two things are in contact with each other, like the person’s hand and the pot handle. Convection, one of the quickest methods of energy transfer, occurs when heat is transferred through a fluid. This is shown taking place inside the pot in the water. Finally, radiation is the process in which electromagnetic waves travel and when they hit things convert to heat. Radiation is shown coming from the fire under the pot. One last thing before we continue this discussion is defining the properties of a fluid. Two main properties of a fluid is that it does not resist deformation (or only slightly) and it has the ability to flow. This means that water, oil, and even different gasses are fluids. Looking back at popcorn, we can look at each method of popping and determine which methods of energy transfer are present. As stated in the methods of popping section, one method of popping corn is a hot air popper. With our new insight to heat transfer and fluids we can come to the conclusion that a hot air popper takes advantage of heat transfer from convection (mainly) and uses air as the medium (fluid) to transfer energy to the kernels. Looking at some sort of kettle either in a popcorn machine or on a stove top you can see that the electric heating element transfers energy conductively through the kettle and into the oil. The oil transfers energy directly to the kernels through convection. The oil is the fluid. The last two methods, microwave popcorn and cooking popcorn over a fire, rely on the transfer of energy through radiation; however they differ slightly in the fact that the microwave transfers microwaves (wavelength of 1 mm to 1 meter) and the fire does so by infrared waves (wavelength of 750 nm to 1 mm). This concludes our brief discussion of the physics of popcorn.