Create a Marshmallow Map of Your Microwave Oven!

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!

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

Preparation

Procedure

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,

More to Explore

• Measuring the Speed of 'Light' with a Microwave Oven, from Science Buddies
• Tasty Arrangements: How to Position Food in Microwave Cooking, from Science Buddies
• Science Activities for All Ages!, from Science Buddies

Credits