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How a Microwave Oven Heats Food

7th grade nonfiction passage about microwave ovens: 275 words, six questions, an answer key and vocabulary.

7th grade nonfiction microwave ovens 275 words

How a Microwave Oven Heats Food

A microwave oven can warm a bowl of soup in two minutes, yet the inside of the oven never gets hot. The secret is that it does not heat the air around the food. It sends energy straight into the food itself.

The energy travels as microwaves, a kind of electromagnetic wave, in the same family as radio waves and visible light. A part inside the oven called a magnetron produces them. The waves bounce off the metal walls and move through the space where your food sits.

Food contains water, and a water molecule has a slight positive charge on one side and a slight negative charge on the other. The microwaves create an electric field that flips back and forth billions of times each second. Water molecules try to line up with the field, so they twist back and forth at the same rapid rate. All this jostling makes the molecules rub against their neighbors, and the rubbing produces heat.

Microwaves only reach about an inch into most foods. The outer layer heats first, and the center warms as the heat spreads inward. Dry foods heat less than moist ones because they contain less water. Because the waves form patterns with hot and cool spots, many ovens have a turntable that rotates the food for even heating.

The glass window in the door has a metal screen with tiny holes. The holes are small enough to keep the microwaves inside, but large enough to let you see your food. The idea for the oven came in the 1940s, when an engineer noticed that radar equipment had melted a candy bar in his pocket.

Questions

  1. What kind of wave does a microwave oven use to heat food? in the text

  2. What part of the oven produces the microwaves? in the text

  3. What happens to water molecules in the microwave field? in the text

  4. How far do microwaves reach into most foods? in the text

  5. Why does a turntable help the food heat evenly? think about it

  6. Why do dry foods heat less than moist ones? think about it

Words to know

microwaves
a kind of wave that carries energy and can heat food
electromagnetic
having to do with electricity and magnetism together
molecule
the smallest piece of a substance, made of atoms
jostling
pushing and bumping against others
radar
a system that uses radio waves to find objects
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One passage, three levels

The same topic and facts written three ways, so a mixed class can read and talk about one text. Each group’s sheet shows only a shape; the key says which is which.

● Easier · 266 words · Flesch-Kincaid grade 4.4

A microwave oven can warm a bowl of soup in two minutes. Yet the inside of the oven never gets hot. The secret is that it does not heat the air. It sends energy right into the food.

The energy travels as microwaves. These are a kind of electromagnetic wave, like radio waves and visible light. A part inside the oven called a magnetron makes them. The waves bounce off the metal walls and move through the space where your food sits.

Food has water in it. A water molecule has a slight positive charge on one side and a slight negative charge on the other. The microwaves make an electric field that flips back and forth billions of times each second. Water molecules try to line up with the field. So they twist back and forth very fast. All this jostling makes the molecules rub against their neighbors. The rubbing makes heat.

Microwaves reach only about an inch into most foods. The outer layer heats first. Then the heat spreads to the center. Dry foods heat less than moist ones because they have less water. The waves make patterns with hot and cool spots. So many ovens have a turntable that turns the food and heats it evenly.

The glass window in the door has a metal screen with tiny holes. The holes are small enough to keep the microwaves in. But they are big enough to let you see your food. The idea for the oven came in the 1940s. An engineer noticed that radar equipment had melted a candy bar in his pocket.

▲ This passage · 275 words · Flesch-Kincaid grade 7.2

A microwave oven can warm a bowl of soup in two minutes, yet the inside of the oven never gets hot. The secret is that it does not heat the air around the food. It sends energy straight into the food itself.

The energy travels as microwaves, a kind of electromagnetic wave, in the same family as radio waves and visible light. A part inside the oven called a magnetron produces them. The waves bounce off the metal walls and move through the space where your food sits.

Food contains water, and a water molecule has a slight positive charge on one side and a slight negative charge on the other. The microwaves create an electric field that flips back and forth billions of times each second. Water molecules try to line up with the field, so they twist back and forth at the same rapid rate. All this jostling makes the molecules rub against their neighbors, and the rubbing produces heat.

Microwaves only reach about an inch into most foods. The outer layer heats first, and the center warms as the heat spreads inward. Dry foods heat less than moist ones because they contain less water. Because the waves form patterns with hot and cool spots, many ovens have a turntable that rotates the food for even heating.

The glass window in the door has a metal screen with tiny holes. The holes are small enough to keep the microwaves inside, but large enough to let you see your food. The idea for the oven came in the 1940s, when an engineer noticed that radar equipment had melted a candy bar in his pocket.

■ Harder · 288 words · Flesch-Kincaid grade 9.1

A microwave oven can warm a bowl of soup in a mere two minutes, yet the interior of the oven never becomes hot. The secret is that it does not heat the surrounding air at all. Instead, it transmits energy directly into the food itself.

That energy travels in the form of microwaves, a type of electromagnetic wave belonging to the same family as radio waves and visible light. A component inside the oven called a magnetron generates them. The waves rebound off the metal walls and sweep through the chamber where your food is sitting.

Food contains water, and every water molecule carries a slight positive charge on one side and a slight negative charge on the other. The microwaves produce an electric field that reverses direction billions of times each second. Water molecules attempt to align themselves with the field, so they twist back and forth at the same astonishing rate. All of this jostling causes the molecules to rub against their neighbors, and the friction generates heat.

Microwaves penetrate only about an inch into most foods. The outer layer heats first, and the center warms as the heat gradually spreads inward. Dry foods heat less than moist ones because they contain less water. Since the waves form patterns with hot and cool spots, many ovens include a turntable that rotates the food so that it heats evenly.

The glass window in the door contains a metal screen perforated with tiny holes. The holes are small enough to keep the microwaves safely inside, yet large enough to let you watch your food. The idea for the oven originated in the 1940s, when an engineer noticed that radar equipment had melted a candy bar in his pocket.

Download as one PDF

How to use it

  1. Press Read it big (or B) to put the text on the projector, or Full screen (F) to fill the whole screen.
  2. Ask the questions one at a time and use Show answer when the class has decided. A shows or hides every answer.
  3. Print student copy for the class, or Print with answers for yourself.

Ideas for class

  • Before reading, put microwaves, electromagnetic, molecule, jostling and radar on the board with their meanings from the Words to know section and have students predict what the text will be about.
  • Read it aloud once while students follow along, then have them reread it alone or with a partner. At a calm 100 words a minute it takes about 3 minutes to read aloud.
  • Use the Read it big button (or B) to put the text on the projector, and Show answer on one question at a time so the class commits to an answer first.
  • Question 5 asks the reader to infer. Ask students to point to the words in the passage that led them to their answer.
  • Ask students to write one question the passage did not answer, then look for the answer in a book or on a classroom screen.
  • Print the student copy for independent work and keep the answer-key copy for yourself.

Related

Questions

What reading level is "How a Microwave Oven Heats Food"?

It was written for 7th grade. It has 275 words and works out to a Flesch-Kincaid grade level of about 7.2. That is a formula's estimate from sentence length and syllables, and it is not a Lexile or a guided reading level, so check it against your students.

What do the questions ask?

There are six. 4 ask for something the text says directly and 2 ask the reader to work something out from clues. Each has an answer in the key, and the answers are written as a teacher would accept them, not as the only possible wording.

How long does it take to read?

About 3 minutes read aloud at 100 words a minute. For a one-minute fluency check, paste it into the fluency timer and use the word count.

Can I print it?

Yes. Print student copy gives the passage, the questions with lines to write on, and the word meanings. Print with answers gives the same page with the answers filled in. Both are made in your browser.

Is this passage from a published book?

No. It was written for Peabody Teach, so there is no book to buy and no login to read it.

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