Why are microwaves quicker than ovens

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Sensemaking is actively trying to figure out how the world works science or how to design solutions to problems engineering. Students do science and engineering through the science and engineering practices. Engaging in these practices necessitates students be part of a learning community to be able to share ideas, evaluate competing ideas, give and receive critique, and reach consensus. Whether this community of learners is made up of classmates or family members, students and adults build and refine science and engineering knowledge together.

Popping popcorn or heating up other foods in a microwave oven is a common experience for most middle school students, and they may have noticed that some microwave ovens take longer than others to pop those last few kernels of popcorn.

Ask students if they have ever popped a bag of popcorn in the microwave and whether they have used different microwave ovens to do so. Optionally, you could use a microwave oven to pop some popcorn to share with your students during or after this discussion.

Ask whether they have noticed that different microwaves take different amounts of time to pop the same amount of popcorn. Ask students why they think this is true and have them share their ideas with a partner. Next, ask students how they think microwave ovens cook food.

Give students time to think independently and record ideas. Then ask students to share their ideas in a small group. Students may have many different ideas, but some students are likely to share the idea that microwave ovens somehow use waves or radiation to cook food.

Students might use terms like power or energy to support their ideas. Tell students every microwave oven has a sticker on it that lists important information specific to that particular model. Share the image below, or use the specifications sticker from your microwave oven. Tell students you have some data that might help them start to make sense of the information on this sticker. Tell them to record observations in the notice column and questions in the wonder column.

Share the data table below. Students are likely to notice that the frequency is the same for all four ovens but that the rated output varies. They are also likely to notice the pattern that cooking time decreases as the rated output increases. Students may have questions about the value and units for frequency and output. After students have a chance to review the data, ask them to share their observations with a partner. Next, ask students to share observations and questions publicly.

Record them on a display board projection screen, white board, dry erase board, chalkboard, etc. Say to students, "Many of us have questions about how the 'rated output' can be different for different ovens when the frequency stays the same. Does it make sense to answer this question first? Say to students, "Let's see if we can use this rope to model waves to help us explain why the microwaves have different cook times for popcorn when the number representing frequency is the same.

Give each pair of students a length of rope about 6 ft. Say to students, "Use your observations of the different rope waves you modeled and the patterns you noticed to make a claim about what is DIFFERENT between the microwave ovens explains why some microwave ovens have faster cooking time than others. Then, ask students to share their ideas in small groups. You might use partner conversational supports to facilitate the small group discussion.

What evidence is that based on? Bring the class back together. Consider holding an building ideas discussion using prompts such as:. Students may disagree on what causes the microwave ovens to have different cooking times for popcorn. Some students may claim waves that are closer together cause faster cooking times in microwave ovens while others may claim higher waves cause faster cooking time.

Some other may say both closer waves and higher waves cause the faster cooking for popping popcorn. Students can use some or all of the online resources below to obtain information about waves and their characteristics and behaviors. The three videos are embedded in the interactive lesson, but they could be used on their own to target more directly the science ideas addressed in this Daily Do.

The microwaves also penetrate into the food, effectively heating it from within. Of course, you don't want things outside the oven getting heated up, which is why they are made of metal. This metal cage traps the microwaves inside the oven: they bounce around from wall to wall until they hit something that can absorb them. That is made of glass that I can see through! Does that mean every time I look through this my eyeballs are being boiled?

Fortunately, the answer is no. If you look closely, you'll see a grid of wires inside the clear glass door. These are connected to the metal walls of the oven, and block the microwaves from getting out. It is one of the odd quirks of radiation that if a hole is smaller than the wavelength of the radiation, most of it won't pass through the hole.

So, to the microwaves, this grid of wires looks like a solid wall, and most of it keeps bouncing around inside the oven until it hits an excitable water molecule. It's the same reason why broadcast TV antennas made of grids of wire work: the grid looks like a solid surface to the radiation.

Likewise, with all the tricks you see on YouTube about putting CDs or metal objects in microwaves. This current has got to go somewhere, and the easiest path between bits of metal is sometimes through the air such as between the tines of a fork. This creates a plasma of ionized gas which can start a fire. The reason you get the oddly beautiful effect when you microwave a CD is that the radiation induces a current in the metal, and the small holes that the data is stored on are just wide enough to create a strong electric current which arcs across the gap, melting the metal and the plastic coating.

As the metal film that the data is stored on in the CD melts, the arc moves around the data track of the CD. The microwaves themselves are created inside the magnetron. You can usually locate this in your microwave by looking for a plastic panel inside the cooking cavity.

This panel covers the magnetron output, where the microwaves come from. The magnetron generates microwave radiation by bouncing electrons around inside a vacuum filled cavity that is exposed to a strong magnetic field. This magnetic field forces these electrons to circle around inside the cavity, absorbing energy.

Eventually, this energy is released as a microwave. This microwave radiation is then gathered and directed into the cooking space of the microwave by a device called a wave guide. Some ovens also feature a rotating metal fan called the stirrer which spreads the microwave beam to make it more random. This process of generating microwaves requires a very high voltage, usually in excess of a thousand volts 1 kilovolt. In a microwave, the containers we use are not heated directly by the microwaves.

The microwaves heat the food, which can in turn heat the container it sits in — but never make it hotter than the food itself. So once my pizza leaves the microwave, it doesn't benefit from the same ongoing source of heat as one that was in the oven. To avoid this, transfer food from the microwave directly to a heated surface.

For thick foods like, say, leftover chili or take-out pad Thai, the issue here is uneven heating. Microwaves heat food unevenly due in part to differing amounts of energy in different parts of the appliance. This often leaves the center of the food cold while the edges are piping hot. So once my microwaved food has been left to sit for a few minutes, the heat from the edges will have migrated to warm up the center, for an overall cooler temperature. Avoid this issue by stirring your food midway through the heating process, or if it's something un-stirrable like lasagna , move it to another part of the microwave partway through cooking.

When it comes to coffee and other liquids, there's enough fluid circulation that uneven heating doesn't occur. But my half-finished cup of coffee has a much smaller volume and only slightly smaller surface area than the full cup I started with.

Even if it's heated to the same temperature as a full cup, the half-finished cup will always cool faster due to a proportionately larger surface area through which heat can escape.