Cool Hot Rod
Objects change size when heated or cooled
Changes in temperature cause objects to expand or contract. You may have noticed the effects of this kind of change around your house. If you run cool water on a hot glass, it may break, as some parts of the glass contract more rapidly than others. You can loosen a tight jar lid by running hot water over it, causing the metal lid to expand more than the glass. The expansion and contraction of materials when they are heated or cooled is commonly used to make thermometers and thermostats. This Snack allows you to directly observe the expansion and contraction of a metal tube.

(one hour or less)

Insert one end of the copper tubing into the plastic tubing. Slip the plastic tubing over the end of the funnel. Make sure that the tubes all fit snugly together.

Place the ring stand near the edge of a table. Position the ring so that it supports the funnel a few inches above the tabletop. Position the copper tubing so that the end farthest from the funnel sticks out beyond the edge of the table by a few inches. Place a small block under the copper tubing at the end near the funnel. Clamp the tubing and block to the table so that they can't move.

Place the second block under the other end of the tubing. Put the needle between the copper tubing and the block, positioned perpendicular to the tubing. Make sure that the eye of the needle extends past the block. Stick the toothpick through the eye of the needle. As the tubing expands and contracts, the needle will rotate, rolled along by the movement of the tubing. The toothpick will shift from an upright position to a slanted position as the needle rotates, making the rotation more evident.

Put the bucket under the end of the copper tubing that sticks out beyond the edge of the table. The bucket will catch the water that you will pour through the copper tubing.

(15 minutes or more)

Pour hot water into the funnel to heat the tubing. For best results, heat the water to near boiling. When you do this, remember to keep your hands away from the copper tubing: It will become very hot!

When you pour the hot water into the funnel, notice the direction in which the needle rotates. Immediately pour cold water through the funnel, and watch the needle again. Notice the direction in which it rotates.

The copper tubing, like everything else in the world, is made of atoms that are constantly vibrating. The higher the temperature, the faster the atoms vibrate. When you pour hot water into the tubing, heat flows from the water to the copper, giving energy to the copper atoms, which vibrate faster. This increase in vibration causes the atoms to collide with each other more often and more violently, so the space between the atoms increases. As a result, the whole tube gets longer and thicker. The needle turns as the tube expands.

When you pour cold water into the tube, the copper atoms give up some of their heat energy to the water, vibrate less violently, and move closer together. The tube shrinks and the needle turns in the opposite direction as the tube contracts.

The copper tube expands by 1.7 x 10-5 of its length for every 1.8 degrees Fahrenheit (1 degree Celsius) of temperature increase. So a copper tube that is 3.3 feet (1 meter) long will expand by 5.6 x 10-3 feet (1.7 x 10-3 m) over a 180°F (100°C) temperature change, lengthening by almost 0.06 inch (1.7 mm). As the copper tube expands, it will make the needle roll over this 0.06 inch (1.7 mm) distance. When an average-sized needle rolls 0.06 inch (1.7 mm), it makes more than two complete revolutions. The toothpick in the eye of the needle dramatically amplifies the motion of the expanding or contracting rod.

If the needle slips instead of rotating, try placing a microscope slide between the wood and the needle. You can also increase the friction by wrapping a rubber band around the wood and the tubing to hold them together more tightly.