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Electromotive Force

The potential difference within a circuit that is needed to maintain current flow.

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Credit: Chris Kelly
Source: https://www.flickr.com/photos/ckelly/4846047005/
License: CC BY-NC 3.0

Since their invention in 1945, microwaves have revolutionized the industrialized world. Today, the appliance is a standard fixture in modern kitchens. Microwaves are able to heat food using electromagnetic radiation. By bombarding its contents with microwave radiation, the appliance excites molecules in the food, increasing their kinetic energy and therefore the food's temperature.

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  • Microwave ovens function by using electromagnetic waves that have a frequency of 2.45 gigahertz. These 2.45 GHz waves are created through the use of a vaccuum tube called a magnetron. A magnetron generates microwaves by passing a stream of electrons through a magnetic field. The microwaves emitted from the magnetron are directed into the main chamber of the microwave, in which food is placed. Since one of the properties of microwaves is that they reflect off of metal, the walls of the main chamber are made out of metal.
  • The microwaves that are directed into the chamber are optimally absorbed by water molecules in the food placed inside. Because microwaves are electromagnetic waves, they produce constantly changing electric fields. Water molecules are polar. In other words, they have an electrical charge distribution that is not symmetrical. In the presence of a constantly changing electric field, the water molecules will rotate, trying to align themselves with the field. As these water molecules rotate, they collide with other molecules, resulting in a transfer of kinetic energy. Since the average kinetic energy of the food placed inside is increased, the food becomes hotter.
  • Learn more about the engineering of microwaves by watching the video below: http://www.youtube.com/watch?v=M2qzCQ58_7Q

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Using the information provided above, answer the following questions.

  1. What is the method of heat transfer between a microwave and the food inside: conduction, convection, or radiation?
  2. Describe how the charge is distributed on a water molecule.

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Image Attributions

  1. [1]^ Credit: Chris Kelly; Source: https://www.flickr.com/photos/ckelly/4846047005/; License: CC BY-NC 3.0

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