Curl Metals with Heat

A hot-tempered science project from Science Buddies

Key ConceptsPhysicsTemperatureThermal expansionMetals

IntroductionDo 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!

BackgroundYou 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

Preparation

Procedure

Observations and ResultsDid 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.

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

More to ExploreHow Thermometers Work, from HowStuffWorksA Temperature Measurement Metallic Spiral Strip Metal Thermometer, from Abdelrahman Hamdy and YouTubeThermal Expansion, from The Physics HypertextbookSecret of Ribbon Curling Revealed, from Scientific AmericanSTEM Activities for Kids, from Science Buddies

This activity brought to you in partnership with Science Buddies

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