Learning standards covered by this activity:
Major Understanding
- 4.3k All frequencies of electromagnetic radiation travel at the same speed in a vacuum.
- 4.3m When waves of a similar nature meet, the resulting interference may be explained using the principle of superposition. Standing waves are a special case of interference.
Process Skill
- 4.3 vi Predict the superposition of two waves interfering constructively and destructively (indicating nodes, antinodes, and standing waves)
The above learning standards were taken from the Core Curriculum Physical Setting/Physics, The University of the State of New York, The State Education Department.
Materials
- Glass baking dish (or other microwavable baking dish)
- 8 oz bag of marshmallows
- Metric ruler
- Microwave oven
- Pot holders
- Butter (optional)
- Toasted rice cereal (optional)
- 13 x 9 x 2 in pan greased on the bottom (optional)
Procedure
- Coat the bottom of the glass pan/microwaveable dish with butter. (This will allow a much easier clean up and will give you the option of creating a tasty snack afterwards.
- Evenly spread the marshmallows on the bottom of the glass dish.
- Remove the turntable from the microwave so the pan will not rotate.
- Place the pan of marshmallows in the microwave and microwave on high until about five hot spots develop in the marshmallows. The hot spots will be where the marshmallows begin to puff up, eventually the hot spots will sink and turn a light brown color.
- Remove the glass dish carefully using the pot holders.
- Use the ruler to measure the distance between the antinodes of the microwaves in the marshmallows. Record your data.
- Look at the tag on the back of the microwave for the frequency of the microwave and record this number as well. This should be about 2500 MHz or 2,500,000,000 Hz.
Optional Procedure (for fun only)
- Have the students mix in 3 cups of toasted rice cereal to the melted marshmallows and spread into a baking sheet for a tasty treat!
Data Analysis
Average the difference between the antinodes. The student should get about 6 cm for most microwaves.
The distance between two antinodes is only ½ a wavelength, so to get the total wavelength the student must multiply by 2.
Calculate the speed of light using the formula:
velocity = frequency x wavelength
velocity = (2,500,000,000 Hz)(.12 m) = 300,000,000 m/s
Explanation
Microwaves are a type of electromagnetic wave with a wavelength of about 0.01 cm to 1 meter. Microwave ovens work by setting up standing waves inside the oven that twist the water molecules in the food back and forth until they heat up. Most microwave ovens have a wavelength of about 12 cm. In the picture of the standing wave below you can see the nodes and antinodes of the string which is a visual of what the wave inside the microwave would look like. The highest energy of the wave occurs at the nodes of the wave. This is where the hot spots in a microwave oven occur. Most microwaves have a turn table because the food needs to be rotated to place most of the food in the path of the hot spots. A hot spot occurs ever ½ a wavelength or approximately 6 cm.
Reinforcement Activities
The standing wave demonstrator from Cenco Physics is shown in the diagram above. Since we can not see microwaves, this provides an excellent visual representation of a standing wave and can be purchased through the following the link below.
Have students create their own standing waves and using 2.0 meter long spiral springs. These springs easily produce standings waves when stretched between two students and give hands on learning experiences with wave characteristics.
