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Two Metals That Bend When They Warm Up

8th grade nonfiction passage about the bimetallic strip: 268 words, six questions, an answer key and vocabulary.

8th grade nonfiction the bimetallic strip 268 words

Two Metals That Bend When They Warm Up

Almost every material grows slightly when it is heated and shrinks slightly when it cools, and although the change is small, different materials change by different amounts. Brass, for example, expands more than steel for the same rise in temperature. Inventors learned to turn that difference into a useful tool called a bimetallic strip.

To make one, a manufacturer bonds a thin strip of brass to a thin strip of steel so tightly that they behave as a single piece. At room temperature the strip is straight. When it is warmed, the brass tries to grow longer than the steel. The two layers are stuck together, so the only way both can fit is for the strip to curve, with the brass on the outside of the bend. When the strip cools, it straightens again. If it becomes colder than its starting temperature, it bends the other way.

A traditional thermostat on a wall once relied on this trick. The strip was often shaped into a coil, because a coil magnifies a small bend into a larger movement. As the room warmed, the coil slowly turned. When the temperature reached the setting, the motion tipped a small switch or pulled a contact apart, and the furnace shut off. As the room cooled, the coil turned back, the switch closed, and the heat came on again.

The same strip appears in many other devices, such as some kitchen thermometers, toasters, and irons that cut off power when they become too hot. Its great advantage is simplicity, since the metal itself senses the temperature and moves, with no computer required.

Questions

  1. Which metal expands more for the same rise in temperature, brass or steel? in the text

  2. How is a bimetallic strip made? in the text

  3. Why was the strip in a thermostat often shaped into a coil? in the text

  4. What happened when the room reached the temperature setting? in the text

  5. Why does the strip curve instead of just growing longer? think about it

  6. Why is the strip described as simple? think about it

Words to know

expand
to grow bigger or take up more space
bimetallic
made of two different metals
bonded
joined together so firmly that the parts stay stuck
thermostat
a device that keeps a room or machine at a set temperature
magnifies
makes something seem or act bigger
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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 · 255 words · Flesch-Kincaid grade 3.5

Almost every material gets a little bigger when it is heated. It gets a little smaller when it cools. The change is small, but different materials change by different amounts. Brass grows more than steel for the same rise in heat. Inventors found a way to use this difference. They made a bimetallic strip.

To make one, a maker bonds a thin strip of brass to a thin strip of steel. They are stuck so tightly that they act as one piece. At room temperature the strip is straight. When it is warmed, the brass tries to grow longer than the steel. The two layers are stuck together, so the only way both can fit is to curve. The brass is on the outside of the bend. When the strip cools, it gets straight again. If it gets colder than where it began, it bends the other way.

An old wall thermostat used this trick. The strip was often a coil, because a coil turns a small bend into a bigger movement. As the room warmed, the coil slowly turned. At the set temperature, the motion tipped a small switch or pulled a contact apart, and the furnace shut off. When the room cooled, the coil turned back. The switch closed, and the heat came on.

The strip is used in other things too. Some kitchen thermometers, toasters and irons use one to cut power when they get too hot. It is simple. The metal itself feels the heat and moves. No computer is needed.

▲ This passage · 268 words · Flesch-Kincaid grade 7.6

Almost every material grows slightly when it is heated and shrinks slightly when it cools, and although the change is small, different materials change by different amounts. Brass, for example, expands more than steel for the same rise in temperature. Inventors learned to turn that difference into a useful tool called a bimetallic strip.

To make one, a manufacturer bonds a thin strip of brass to a thin strip of steel so tightly that they behave as a single piece. At room temperature the strip is straight. When it is warmed, the brass tries to grow longer than the steel. The two layers are stuck together, so the only way both can fit is for the strip to curve, with the brass on the outside of the bend. When the strip cools, it straightens again. If it becomes colder than its starting temperature, it bends the other way.

A traditional thermostat on a wall once relied on this trick. The strip was often shaped into a coil, because a coil magnifies a small bend into a larger movement. As the room warmed, the coil slowly turned. When the temperature reached the setting, the motion tipped a small switch or pulled a contact apart, and the furnace shut off. As the room cooled, the coil turned back, the switch closed, and the heat came on again.

The same strip appears in many other devices, such as some kitchen thermometers, toasters, and irons that cut off power when they become too hot. Its great advantage is simplicity, since the metal itself senses the temperature and moves, with no computer required.

■ Harder · 280 words · Flesch-Kincaid grade 8.8

Almost every material grows slightly when it is heated and shrinks slightly when it cools, and although the change is small, different materials respond by different amounts. Brass, for example, expands considerably more than steel for the same rise in temperature. Inventors eventually learned to turn that difference into a remarkably useful device called a bimetallic strip.

To make one, a manufacturer bonds a thin strip of brass to a thin strip of steel so securely that they behave as a single piece. At room temperature the strip is perfectly straight. When it is warmed, the brass tries to grow longer than the steel. The two layers are firmly attached to each other, so the only way both can fit is for the strip to curve, with the brass on the outside of the bend. When the strip cools, it straightens again. If it becomes colder than its starting temperature, it bends in the opposite direction.

A traditional thermostat on a wall once relied on this principle. The strip was often shaped into a coil, because a coil magnifies a small bend into a much larger movement. As the room warmed, the coil slowly turned. When the temperature reached the chosen setting, the motion tipped a small switch or pulled a contact apart, and the furnace shut off. As the room cooled, the coil turned back, the switch closed, and the heat came on once more.

The same strip appears in many other devices, such as some kitchen thermometers, toasters, and irons that cut off their power when they become too hot. Its greatest advantage is simplicity, since the metal itself senses the temperature and moves, with no computer required.

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 expand, bimetallic, bonded, thermostat and magnifies 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 "Two Metals That Bend When They Warm Up"?

It was written for 8th grade. It has 268 words and works out to a Flesch-Kincaid grade level of about 7.6. 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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