Key Concepts
Wavelength, frequency, energy

Introduction

Have you ever bitten into a microwave burrito, and the first bite is scalding hot, whereas the second bite is still frozen? If you’ve experienced this, then you know that microwaves have hot and cold spots, which is why they usually have a rotating tray to ensure that your food is evening cooked. In this activity, you’ll use marshmallows to map out where the hot and cold spots are in your microwave. Get ready to heat things up!

This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Background

You probably use your kitchen microwave oven so often, you don’t even think about how it works, or why it can heat your food up so quickly.  Despite being an everyday kitchen appliance, microwave ovens are fairly sophisticated pieces of scientific equipment!

Although they’re easy to take for granted, for this activity it’s important to understand the basics of how microwave ovens heat your food.

Microwave ovens convert electrical energy (coming from the wall outlet) into very small radio waves. Because these radio waves are so small, we call them microwaves! Microwaves travel at the speed of light (186,000 miles/second), and carry a huge amount of energy, enough to damage living tissue. This is why you can’t (and should never try to) operate a microwave oven with the door open – the door prevents the microwaves from escaping.

When you turn on your microwave oven, you’re actually turning on a microwave generator called a magnetron. The magnetron takes the electrical energy from your wall socket and turns it into small, powerful radio waves (microwaves). These microwaves enter the food compartment of the microwave oven, where they bounce back and forth off the walls. However, the microwaves stop bouncing when they hit food. Instead of being reflected, the microwaves penetrate the food, causing the food molecules to vibrate. The vibrating food molecules collide with each other, causing more vibrations. With these vibrations, the temperature of the food increases, and the more the molecules vibrate, the hotter the food becomes.

However, you have probably noticed that microwave ovens don’t always heat your food evenly. This is because microwaves are waves, and just like light, sound and even ocean waves, they have ‘peaks’ and ‘valleys’. Microwaves carry the most energy at their peaks and valleys, and very little energy at the transition point. As these waves travel across your microwave oven, they hit the food at different points in their wave cycle. Therefore, after cooking a burrito in your microwave oven, your first bite might be boiling hot and the next bite still frozen.

In this activity we’re going to map the ‘peaks’ and ‘valleys’ of the microwaves traveling through your microwave oven, and observe how your oven’s rotating tray helps your food cook more evenly.

Materials

  • A microwave with a removable rotating tray
  • 2 large, microwave safe casserole dishes
  • 2 bags of marshmallows (chocolate chips will work too)
  • Butter (at least 4-6 tablespoons)
  • Oven mitts
  • A ruler (optional)
  • A fork
  • A piece of paper
  • A pen or pencil
  • An adult helper
  • A large bowl

Preparation

  1. Butter both the casserole dishes (this will make it easier to clean once you’ve finished experimenting!)
  2. To avoid wasting the marshmallows used in this activity, collect the ingredients to make marshmallow treats!

Procedure

*This activity requires handling hot materials, please ask an adult to help you with these steps!

  1. Arrange your marshmallows in rows across your first casserole dish, so that the marshmallows are packed together closely.
  2. Place the casserole dish on the microwave’s rotating tray.
    Set the microwave for low power, for 1 minute. Hit start. Note: Microwave power varies. If your marshmallows aren’t affected by this temperature and time, try increasing the time that your marshmallows are in the microwave.
  3. When the time is up, carefully remove the dish and observe the marshmallows. Caution: the dish may be hot! Use your oven mitts and ask an adult to help you!
  4. Use your fork to gently poke the marshmallows. Note the texture and appearance.  How did the marshmallow’s appearance change? How did the marshmallow’s texture change – are they softer or stickier than before? Do all the marshmallows look and feel the same, or did some change more than others? What do you think caused the marshmallows to change?
  5. Set the dish aside.
  6. Arrange the second bag of marshmallows in your second casserole dish, the same way that you did the first.
  7. Remove the rotating tray from your microwave. Ask an adult for help with this step!
  8. Place the casserole dish in the microwave.
  9. Set the microwave for low power, for 1 minute. Hit start.
  10. When the time is up, carefully remove the dish and observe the marshmallows. Caution: the dish may be hot! Use your oven mitts and ask an adult to help you!
  11. Again, use your fork to gently poke the marshmallows. Note the texture and appearance.  How did the marshmallow’s appearance change? How did the marshmallow’s texture change – are they softer or stickier than before? Do all the marshmallows look and feel the same, or did some change more than others? What do you think caused the marshmallows to change?
  12. See if you can find a pattern in which marshmallows melted, and which ones didn’t. Try to identify spots in the dish where the marshmallows did not melt, and other places where the marshmallows did melt. If you would like, use your ruler to measure the distance between two adjacent ‘melted’ spots. Is the distance similar between melted spots?
  13. Compare the marshmallows in the two dishes. Are more marshmallows melted in one dish compared to the other? Which dish seems to have melted the marshmallows most evenly?

Observations and Results

In this activity we used marshmallows to map the hot and cold spots in your microwave oven that are caused by the wavelength of the microwaves that heat your food. In this first part of the activity, your marshmallow dish was placed on the rotating tray inside your microwave oven. In this case, you may have found that some marshmallows were slightly more melted than others.

In contrast, during the second part of the activity you removed the rotating tray, so that the marshmallow dish didn’t move as it was heated. In this case, you should have observed a more distinct contrast between melted and un-melted sections of marshmallows. Because the dish wasn’t spinning on the tray,

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Credits

Megan Arnett, PhD, Science Buddies

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Key Concepts
Wavelength, frequency, energy
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