Color Temperature

A very hot object-like the filament of a lightbulb-glows with a color that depends on its temperature. In fact, color was one of the first ways temperature was judged. The traditional instructions for making a samurai sword, for instance, called for quenching the sword once it was heated to the color of the morning sun. Here are examples of what happens to metals at different temperatures:

Yellow 1500 C Iron melts

Orange 1300 C Stainless Steel melts

1100 C Gold melts

Reddish 900 C Silver melts

800 C Coals of a wood fire

650 C Aluminum melts

500 C Metals glow faintly

Red Hot

Did you know that by blowing hard on the heating elements of you toaster, you can stop them from toasting? Pressing the button on this exhibit sends an electric current through the toaster wire. The internal resistance of the wire transforms most of the electrical energy to heat. Heating causes the metal wire to expand and sag. As it cools it shrinks back to its room-temperature length. Heating also causes metals to emit visible light as they reach high temperatures. The color of glowing metal is actually a good indication of its temperature. You can dim a white incandescent light bulb and watch the metal filament cool from glowing white to a red glow like this wire. Despite this wire reaching almost 1500 degrees F, it is easy to cool off when you blow because the wire is so thin. It would take more than a puff of breath to get thick wire to stop glowing.

Hot Rod

All forms of matter generally expand when heated and contract when cooled. This change in size may be subtle but on a large scale can be pretty dramatic—the temperature swings between winter and summer cause long power transmission lines to sag more than 50 feet. Thermal expansion is why mercury thermometers rise as the temperature increases. Heating causes the molecules that make up a substance to jiggle faster and collide together with more energy. This forces the molecules further apart, expanding the substance. The aluminum rod inside the tube is clamped on its left end and as it heats up, its entire length increases and expands towards its right end. In contact with the right end is a dial indicator gauge so sensitive that a full revolution of the dial hand represents the rod lengthening by just ten thousandths of an inch. The gauge is even sensitive to the slight bending of the table when you touch it.

Thermoelectrics

This exhibit has two demonstrations of thermoelectrics:

1) The thermoelectric device in Demo 1 heats or cools depending on which direction the electric current is flowing.

2) The reverse happens in Demo 2 where the heat from your finger generates an electric current whose direction of flow depends on which side of the thermoelectric module you touch.

Although the thermoelectric module has no moving parts, it is able to move heat towards the white plate or away depending on the direction of the electrical current flowing through it. The rate of heat flow depends upon the amount of electrical current. These modules are designed for cooling purposes where reliability is important and the amount of cooling small. Uses include cooling electronic equipment and small travel refrigerators. The module consists of 142 blocks of semiconductor material forming 71 junctions that will either absorb or generate heat as a result of current flow. This is called the Peltier effect. As shown in Demonstration #2, this effect is reversible—heat flow through the unit can produce electricity. This is called the Seebeck effect.

Bimetal Thermostat

The coiled strip is made of two layers of different metals bonded together. The metals expand at different rates when heated. The metal layer on the inside of the coil expands more than the outside layer, causing the coil to unwind to the left. As the coil cools, the inside layer contracts more than the outside layer and the coil winds tighter. Temperature control devices like house and oven thermostats take advantage of the different rates of expansion and contraction of dissimilar metals to regulate temperatures. The bimetallic strip’s response to temperature repeatedly turns the heating or cooling device on and off to maintain the required temperature.

Come on in to see these new exhibits, and many more!