Essential Science for Teachers: Physical Science
Heat and Temperature A Closer Look
A Closer Look
Look for the following topics in the video, indicated by the onscreen icon, and click below to learn more.
Why do We Need Three Scales of Temperature?
As mentioned in the video, the two scales we are most familiar with, the Fahrenheit and Celsius (also known as centigrade) scales, are defined in terms of events that are universal: a temperature of zero on the Fahrenheit scale is the temperature of a mixture of equal parts ice, water, and salt, and the freezing point of water is what sets the zero point on the Celsius scale. A difference of one degree Celsius is larger than a difference of one degree Fahrenheit.
So why do we need yet another temperature scale?
If we examine the definition of temperature as it relates to our particle model, we can see that there is something more fundamental on which to base our temperature scale. Temperature is related to the average energy of the motion of the particles of the object. Therefore, a natural point for a temperature scale is the point at which all particle motion stops. This point is defined as zero on the Kelvin scale. The unit of the Kelvin scale is referred to as a “Kelvin,” and the magnitude of one Kelvin is the same as the magnitude of one degree Celsius. The zero temperature on the Kelvin scale is called absolute zero.
Is it possible to reach absolute zero?
In a word, no. As we’ve seen in previous sessions, successful scientific models often break down when they are applied to circumstances with extreme conditions: for example, mass conservation breaks down when we deal with the high temperatures and pressures inside a star. Very low temperatures are another extreme. As it turns out, the best model for understanding the world of the very small, quantum mechanics, dictates that we cannot completely stop the motion of any particles, no matter how hard we try. However, with certain special techniques (including using the light from lasers to slow down particles), scientists have been able to lower the temperature of matter to just a fraction of a Kelvin above absolute zero.
At these very low temperatures, matter behaves very differently than it does near room temperature. As a result, scientists had to develop further refinements to their particle model. However, since everyday life is experienced between the extremes of high and low temperatures, we do not observe or experience any of these unusual effects.
Why do We Need Heated Towel Racks?
In this video, weather forecaster Bill Babcock tells us that, when we step out of the shower, it takes energy from our skin to turn liquid water into water vapor via evaporation. To understand this process better, let’s take a closer look at what happens at the microscopic level.
Recall that earlier in the session we looked at what happens when energy is transferred as heat from a hot mug of tea to your hand:
Essentially, the process we are about to describe is the reverse of that process: heat flows from your skin to the water.
In both your skin and the water on your skin, the particles are moving with a distribution of speeds (i.e., some move faster than others) but the average energy of their motion is related to the temperature of your skin and the temperature of the water. However, the particles in the water that are moving faster than average may be moving fast enough to overcome the pull they feel from their neighbors and break away from the surface of the liquid water into the air. By doing this, the remaining water is at a lower temperature because the average energy of motion of all the particles has gone down. See the side illustration for clarification.
Since there is now a temperature difference between your skin and the water on it, heat flows to the water. On a microscopic level, your skin particles collide with the water molecules, transferring some energy of motion. As a result, the water molecules move faster and your skin particles move slower. This leaves the temperature of your skin lower and you feel colder, while the temperature of the water goes up and the faster-moving molecules escape. The process keeps repeating until all the water is gone. This is why it doesn’t feel as cold if you towel off quickly.
This same process happens at the surface of any liquid that is evaporating. Try observing what happens in the microscopic world when heat is transferred in the Session 3 Virtual Particle Lab.
Session 1 What Is Matter?: Properties and Classification of Matter
What is matter? This question at first seems deceptively simple — matter is all around us. Yet how do we define it? What does a block of cheese have in common with the Moon? What are the characteristics of matter that set it apart from something that is definitely not matter? Matter is one of the big ideas in science. Most areas in physical science can be discussed and explained in terms of matter or energy, and matter is a subject that naturally bridges to the other sciences (chemistry, life, earth science, etc.). In this session, we’ll build a working definition of matter, learn to distinguish between its “accidental” and “essential” properties, and explore it through classification, an activity with a rich history in science.
Session 2 The Particle Nature of Matter: Solids, Liquids, and Gases
What simple idea links together all of chemistry and physics? How can a close study of the macroscopic differences among solids, liquids, and gases support a microscopic model of tiny, discrete, and constantly moving particles? In this session, participants learn how the "particle model" can be turned into a powerful tool for generating predictions about the behavior of matter under a wide range of conditions.
Session 3 Physical Changes and Conservation of Matter
What happens when sugar is dissolved in a glass of water or when a pot of water on the stove boils away? Do things ever really "disappear?" In everyday life, observations that things "disappear" or "appear" seem to contradict one of the fundamental laws of nature: matter can be neither created nor destroyed. In this session, participants learn how the principles of the particle model are consistent with conservation of matter.
Session 4 Chemical Changes and Conservation of Matter
How can the particle model account for what happens when two clear liquids are mixed together and they produce a milky-white solid? What happens when iron rusts? Where do the elements come from? In this session, participants extend the particle model by looking inside the particles, learn about some early chemical pioneers, and in the process discover how the law of conservation of matter applies even at the scale of atoms and molecules.
Session 5 Density and Pressure
What makes a block of wood rise to the surface of a bucket of water? Why do your ears pop when you swim deep underwater? In this session, participants examine density, an essential property of matter. They also look at how particles of matter are in constant motion, which leads to a deeper understanding of fluid pressure. Lastly, the concepts of pressure and density are investigated to explain the macroscopic phenomenon of rising and sinking.
Session 6 Rising and Sinking
Why does a hot air balloon rise into the sky? Why does ice rise in water, when a lump of solid wax will sink in a jar full of molten wax? In this session, participants generalize the model that has been developed about what rises and what sinks, using the idea of balance of forces.
Session 7 Heat and Temperature
What makes the liquid in a thermometer rise or fall in response to temperature? Which contains more heat — a boiling teakettle on the stove or a swimming pool of lukewarm water? In this session, participants focus on the difference between heat and temperature, and examine how both are defined in terms of particles. The particle model is then used to explain a number of everyday phenomena, from why things expand when they are heated to the role that temperature plays in changes of state.
Sessions 8 Extending the Particle Model of Matter
In this session, participants extend their understanding of the particle model to explain additional macroscopic phenomena, including the electrical properties of matter. Participants review the progression of ideas covered in the course and anticipate future developments in the understanding of matter.