Key Concepts
Physics
Temperature
Thermal expansion
Metals

Introduction
Do you enjoy wrapping gifts for people? Perhaps you have even curled a ribbon with scissors (or watched someone else do it). Have you ever wondered why the ribbon curls when you run a scissor blade down one side of it? The answer is that when you apply pressure on the ribbon with the blade of the scissors, the outer layer of the ribbon stretches and expands. This makes the outside layer of the ribbon longer than the inside layer that is pressed against the blade. As a result the ribbon curls to make up for the different lengths of each of its layers. In this activity you will also make materials curl. For these materials, however, you won't need scissors: you will use heat instead!

Background
You probably know or have seen that materials are able to change their shape or volume when they are heated or cooled. This is true for solids, liquids and gases, which are all made up of atoms and molecules. Once exposed to heat these molecules begin to vibrate and move around faster. This makes the molecules spread out and take up more space. As a result a material expands slightly when it gets hot. On the other hand, when a material is cold, molecules move less, which makes them take up less space. Thus, materials shrink when they get cold. Although all materials expand when heated, they do not all expand to the same degree. How much a material expands when heated is described by its thermal expansion coefficient. For example, aluminum expands 21 to 24 micrometers per meter if you increase its temperature by 1 degree Celsius.

What happens when an object is made up of more than one material? They will both expand differently when heated up! In fact there are special materials called bimetals that make use of their different thermal expansion properties. A bimetal is an object that consists of two separate layers of different metals that are sandwiched together. When a bimetal is heated one of the metals will expand more than the other. This results in the bimetal curving (or curling) in one direction—just like the ribbon for the gift wrap. Because of this effect, bimetals are often used to indicate temperature changes, such as in pointer dial thermometers (such as those used in ovens or refrigerators). Inside these thermometers a bimetal coil is attached to a pointer. When the temperature changes the pointer moves depending on the amount of deformation of the bimetal coil.

In this activity you will see for yourself how different thermal expansion properties can make an object curl. Ready to bring on the heat?

Materials

• Aluminum foil
• Printer paper
• Scissors
• Glue
• Candle with a sturdy base
• Lighter or matches
• Large pitcher of water or fire extinguisher
• Other material sheets (plastic, copper, etc., are optional)

Preparation

• Cut four 0.5 by 9-inch strips of aluminum foil
• Cut four 0.5 by 9-inch strips of printer paper.
• Glue two of the aluminum strips together, so there are two layers of aluminum on top of each other.
• Glue two of the paper strips together, so there are two layers of paper on top of each other.
• Glue one strip of paper on top of one strip of aluminum.
• Repeat the previous step with the remaining aluminum and paper strip.
• Ensure the glue is set before beginning the activity.

Procedure

• With the help of an adult, light the candle. Ensure that you have a large pitcher of water or fire extinguisher nearby—just in case.
• Take the strip that is made up of two layers of aluminum. With the help of an adult carefully hold one end of the strip about 1.5 to 2 inches above the candle flame. Keep it there for about three seconds. What happens to the aluminum strip?
• Remove the aluminum strip from above the candle. Carefully touch the part of the strip that was above the flame. How does it feel compared to the rest of the strip?
• Take the strip that is made up of two layers of paper. Hold one end of the strip about 1.5 to 2 inches above the candle flame. Keep it there for three seconds, being careful to not accidentally burn the paper strip. What do you notice this time?
• Remove the paper strip from above the candle, and gently touch the part of the strip that was above the flame. How does it feel?
• Next take a strip that is made up of aluminum and paper. Hold one end of the strip above the candle flame as you did with the others. Have the aluminum layer face toward the candle and the paper layer toward the ceiling. Keep the strip 1.5 to 2 inches above the candle for three seconds. What do you observe? Are your results different than before?
• After removing the strip from above the candle carefully touch the strip on both sides where it was above the flame. Do both sides feel the same?
• Take the last strip made up of aluminum and paper. This time hold the strip above the flame with the paper layer facing the candle flame and the aluminum layer facing up. What happens this time? Can you explain your results?
• Remove the strip from above the candle and carefully touch it on both sides where it was above the flame. What do you notice?
• Extra: What happens when you glue the aluminum strip to other materials such as a plastic sheet or copper sheet? Do your results change?

Observations and Results
Did you see one of your strips curl above the candle flame? You should have! The all-aluminum strip and the all-paper strip should have not changed when you held them above the flame. This is because the two layers of the strip are made of the same material. If the strip gets hot, which you should have noticed when touching it, both layers expand the same amount. If the strip consists of two different materials, however, such as the one you made from paper and aluminum, something different will happen.

When holding the paper/aluminum strip above the flame you probably observed that it immediately started to bend or curl in one direction. It curls upward when the aluminum layer faces the candle and downwards when the aluminum layer faces the ceiling. This is because the aluminum and the paper layer heat up when above the flame. The aluminum expands more than the paper because of its higher thermal expansion coefficient. Because the paper and aluminum are glued together, the aluminum curls away from the paper strip to make up for the different lengths of each of its layers—like the curling gift wrap ribbon! You will see similar results with other strips that consist of two different materials that have different thermal expansion coefficients.

Cleanup
Make sure to blow out the candle. You can recycle any unused paper or aluminum.

More to Explore
How Thermometers Work, from HowStuffWorks
A Temperature Measurement Metallic Spiral Strip Metal Thermometer, from Abdelrahman Hamdy and YouTube
Thermal Expansion, from The Physics Hypertextbook
Secret of Ribbon Curling Revealed, from Scientific American
STEM Activities for Kids, from Science Buddies

This activity brought to you in partnership with Science Buddies