Lesson 5

Temperature and Density

Concepts

Matter expands when heated and contracts when cooled

Expansion and contraction can be used to measure temperature

Density changes with temperature

Objectives

Use a laboratory thermometer and discuss how it works

Construct and calibrate a thermometer

Use the constructed thermometer to measure temperature

Know that movement in the thermometer is due to thermal expansion

Compare the expansion of liquids and gases

Observe thermal expansion in solids

Discuss change of density with pressure

Notes

Galileo is credited with the invention of the thermometer in 1596

His invention was called a thermoscope

Some thermometers are made with bimetal strips

Digital thermometers measure electrical force between two unlike forces

Heat is a measure of energy and moves from a hot place to a cooler place

Temperature is a measure of kinetic energy in particles of matter

Heat moves from hot places to cooler places

When a thermometer is placed in hot water the heat in the water moves to the thermometer. When the kinetic energy of the thermometer matches the kinetic energy of the water the temperature will be the same. This is a state of equilibrium.

How Does the Thermometer Work?

A thermometer is a device that measures the temperature of things. The name is made up of two smaller words: "Thermo" means heat and "meter" means to measure. You can use a thermometer to tell the temperature outside or inside your house, inside your oven, even the temperature of your body if you're sick.

One of the earliest inventors of a thermometer was probably Galileo. We know him more for his studies about the solar system and his "revolutionary" theory (back then) that the earth and planets rotated around the sun. Galileo is said to have used a device called a "thermoscope" around 1600 - that's 400 years ago!!

The thermometers we use today are different than the ones Galileo may have used. There is usually a bulb at the base of the thermometer with a long glass tube stretching out the top. Early thermometers used water, but because water freezes there was no way to measure temperatures less than the freezing point of water. So, alcohol, which freezes at temperature below the point where water freezes, was used.

The red colored or silver line in the middle of the thermometer moves up and down depending on the temperature. The thermometer measures temperatures in Fahrenheit, Celsius and another scale called Kelvin. Fahrenheit is used mostly in the United States, and most of the rest of the world uses Celsius. Kelvin is used by scientists.

Fahrenheit is named after the German physicist Gabriel D. Fahrenheit who developed his scale in 1724. Ice freezes at 32

degrees Fahrenheit (F for short), and water boils at 212 degrees F. He arbitrarily decided that the difference between

the freezing point and boiling point of water should be 180 degrees.

The Celsius scale is named after Anders Celsius. The Celsius scale used to be called the "centigrade" scale. Centigrade

means "divided into 100 degrees." Anders Celsius developed his scale in 1742. He started with the freezing point of water

and said that was 0 degrees Celsius (C for short). At the point where water boils, he marked that at 100 degrees C. This scale

is much more scientific because the measurement is broken down into an even 100 degrees. This is similar to the scientific

system of measuring distance and weight called the metric system.

Kelvin is named after Lord Kelvin, whose full name is Sir William Thomson, Baron Kelvin of Largs, Lord Kelvin of Scotland. His scale

starts at 0 degrees Kelvin, which is called absolute temperature. Lord Kelvin took the idea of temperature one step further with his

invention of the Kelvin Scale in 1848. The Kelvin Scale measures the coldest temperature there can be. He said there was no upper limit of how hot things can get, but he said there was a limit as to how cold

things can get. Kelvin developed the idea of Absolute Zero. This is at minus 273.15 degrees Celsius (or -523.67 F)! At this temperature,

absolute zero is the lowest possible temperature, occurring when no heat energy remains in a substance. Absolute zero is the point at which molecules do not move (relative to the rest of the body).

As far as scientists know, nothing in the universe can get that cold!

How A Thermometer Works

When you look at a regular outside bulb thermometer, you'll see a thin red or silver line that

grows longer when it is hotter. The line goes down in cold weather.

This liquid is sometimes colored alcohol but can also be a metallic liquid called mercury. Both mercury and alcohol grow bigger when heated and smaller when cooled. Inside the glass tube of a thermometer, the

liquid has no place to go but up when the temperature is hot and down when the temperature is cold.

Numbers are placed alongside the glass tube that mark the temperature when the line is at that point.

The other type of common thermometer is a "spring" thermometer. A coiled piece of metal that is sensitive to heat is used. One end of the spring is attached to the pointer. As the air heats, the metal expands and the pointer moves higher. As the air cools, the metal contracts and the pointer moves lower. Typically, these type of thermometers are less accurate than bulb or digital thermometers.

How does a Galileo thermometer work?

Based on a thermoscope invented by Galileo Galilei in the early 1600s, the thermometer on your co-worker's desk is called a

Galileo thermometer. A simple, fairly accurate thermometer, today it is mostly used as decoration. The Galileo thermometer consists of a sealed glass tube that is filled with water and several floating bubbles. The bubbles are glass spheres filled with a colored liquid mixture. This liquid mixture may contain alcohol, or it might simply

be water with food coloring. Attached to each bubble is a little metal tag that indicates a

temperature. A number and degree symbol are engraved in the tag. These metal tags are actually calibrated counterweights. The

weight of each tag is slightly different from the others. Since the bubbles are all hand-blown glass, they aren't exactly the same size and shape. The bubbles are calibrated by adding a certain amount of fluid to them so that they have the exact same density. So, after

the weighted tags are attached to the bubbles, each differs very slightly in density (the ratio of mass to volume) from the other

bubbles, and the density of all of them is very close to the density of the surrounding water

If you have read this question, then you know that an object immersed in a fluid experiences two major forces: the

downward pull of gravity and the upward push of buoyancy. It is the downward force of gravity that makes this thermometer

work. The basic idea is that as the temperature of the air outside the thermometer changes, so does the temperature of the water surrounding the bubbles. As the temperature of the

water changes, it either expands or contracts, thereby changing its density. So, at any given density, some of the

bubbles will float and others will sink. The bubble that sinks the most indicates the approximate current

temperature.

Consider this example: Let's say there are five bubbles in the

thermometer: A blue bubble that represents 60

degrees A yellow bubble that represents 65

degrees A green bubble that represents 70

degrees A purple bubble that represents 75

degrees A red bubble that represents 80

degrees

The blue bubble (60 degrees) is the heaviest (densest) bubble, and each bubble thereafter is slightly lighter, with the red bubble being

the lightest. Now, let's say the temperature in the room is 70 degrees. Since the surrounding air is 70 degrees, we know the water inside the

thermometer is also about 70 degrees. The blue and yellow bubbles (60 and 65 degrees, respectively) are calibrated so that they have

higher densities than the water at this temperature, so they sink. The purple and red bubbles each have a density that is lower than the surrounding water, so they float at the very

top of the thermometer. Since the green bubble is calibrated to represent 70 degrees, the same temperature as the water, it sinks slightly so that it is floating just below the

purple and red bubbles -- thereby indicating the room's temperature!