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Temperature/Heat/States of matter

ENERGY - The ability to cause change.

Units - SI uses joules (J) and USCS uses calories

4184 J = 1 food Calorie = 1000 calories

You are well aware of the fact that when the temperature reaches 32oF (0oC) water freezes and turns into ice. When water reaches 100oC (212oF) it boils and turns into steam. The goals of this unit is to better understand why and how these events, and other related occurrences, take place as well as our perceptions of hot and cold.

We know that temperature is a measurement we use to tell us how hot or cold something is with respect to some standard. When we say our hands get warmer we mean the temperature is rising. But what is temperature really measuring?

Our everyday use of temperature simply gives us a reference frame for how hot or cold things are. We are familiar with the Fahrenheit scale here in the U.S., while most people in other countries use the Celsius/centigrade scale. These are both scales of relative hotness to chosen standards. Celsius is based on 0 degrees = freezing point of water and 100 degrees = boiling point of water. Then 100 equal divisions were marked off between these points, thus determining the range of 1 degree centigrade. The Fahrenheit scale was similarly determined with one degree F being smaller than one degree Celsius.

What temperature is actually measuring is a bit more complicated, but the basic idea is based on the actual particles that compose a material. Each particle has some potential (stored) and some kinetic (motion) energy. The total energy of a material is the total potential and kinetic energy of all particles in that material. This is often referred to as thermal/internal energy. The temperature is directly related to the average kinetic energy of all of the particles in a material. The temperature scale used most in scientific research is the Kelvin (K) scale. This is an absolute temperature scale. An absolute temperature scale starts at zero degrees since the average kinetic energy of no moving particles would be zero. 0oC = 273 K

Letís look at the states of matter: solid, liquid, gas, and plasma.

A picture of a solid has all the particles packed tightly with motion due mostly to vibrations of the particles themselves.

A liquid shows loosely packed particles in which the vibrations have increased to the point where the particles can flow around each other.

A gas has particles moving at fairly high speeds and numerous brief collisions. So in terms of the average kinetic energy for each state the solid should have the least and the gas the most.

The plasma state contains particles that have started to break apart. A high temperature is not always necessary to produce a plasma, just a high amount of energy.

In order to move from solid to plasma a change must take place, this requires energy to be added to the particles.

If you consider the relative temperatures of theses states you find a general increase from solid to gas. This means that a gas is hot and a solid is cold, relative to each other for the same material.

Letís take a more detailed look at how materials change from on state to another: melting/freezing, vaporizing(boiling)/condensing, subliming, and evaporating are the changes we are most familiar with. Most of us would say that in order to melt a piece of ice we need to make it warmer or heat it up.

The basic principle here is energy transfer. When we say, "heat" something we are describing the addition of thermal energy to a material in order to increase its temperature. To "cool" a material would require the opposite. Both of these scenarios are defined as heat, the flow or movement of thermal energy due to differences in temperature. Thermal energy naturally flows from regions of high temperature to regions of low temperature.Materials do not contain ďheatĒ.

A simple example would be a cup of hot coffee sitting on your kitchen counter. Eventually the coffee will be the same temperature as the kitchen. If the coffee is 180 oF (82 oC) and the kitchen air is about 72 oF (22 oC) there will be a flow of thermal energy from the coffee to the air. As a result the coffee cools while the air warms and when they reach the same temperature there is no more flow of thermal energy. This is called thermal equilibrium. The opposite happens with a glass of iced tea in your kitchen.

When you put some ice in a pot you have a solid state. You can "heat" the pot on a stove which will warm to a higher temperature when on. The temperature difference between the burner and the pot results in heat, the temperature difference between the pot and the ice also results in heat, and so the thermal energy flows form the burner to the ice. This thermal energy causes the particles to vibrate faster, which increases the temperature measured. When the temperature reaches 0oC the thermal energy provided causes the bonds between the particles to break. The temperature does not change but the ice melts. When you have all liquid the added thermal energy will cause the temperature to increase, indicating an increase in the average kinetic energy of the particles. The water will reach 100 oC and begin to vaporize. It will remain at 100 oC until all of the bonds holding the particles together are broken. Now you have water vapor/steam. At this point it will be hard to heat the steam since you cannot keep it in the pot. But if you could, the temperature would continue to increase.

During the rising of temperature the particles are gaining kinetic energy. Different materials need more energy to increase their temperature than others due to their composition. This property of matter is known as specific heat capacity. Specific heat capacity is the amount of energy required to change the temperature of a unit of mass of a material by 1 degree Celsius. Itís kind of like thermal inertia, a materialís tendency to stay at its current temperature. Materials with high specific heat capacities heat up and cool down slowly, or tend to resist change. Materials with low specific heat capacities heat up and cool down quickly, very little resistance to change. Water has a high specific heat capacity (4.184 J/goC or 1 cal/goC). Explore - sandy beach and hot water bottles

The amount of thermal energy required to simply change the temperature can be found with the following equation:

Q = (mass) (specific heat) (change in temperature)

During changes of state the amount of thermal energy required is known as the heat of transformation. More specifically the heat of fusion (freeze/melt) and the heat of vaporization (boiling/condensing).

The amount of thermal energy required to change the state of matter can be found with the following equation:

Q = (mass) (heat of transformation)

If you want to reverse the changes in state discussed above simply remove the appropriate amounts of thermal energy.

Recall the law of conservation of energy - you cannot create or destroy energy. If we are to add energy to one material another must be losing that energy as it provides.

There are three basic paths that energy can take from one location to another: conduction (particle contact), convection (fluid motion), and radiation (electromagnetic waves). The example of melting the ice in a pot was heat conduction.

Heat conduction is the transfer of thermal energy through the collisions of particles in the material(s) due to a difference in temperature. Solids are generally better conductors since the particles are closer together and metals are usually good conductors because of the "freedom" of the electrons in a metal. It is not wise to hold a metal rod over a heat source for long, the heat will reach you fairly quickly. Poor conductors such as wood, asbestos, glass, plastic, air, and water are called insulators. Insulators are materials that do not allow energy to be transferred easily. Something that reduces the formation of convection currents would also be considered an insulator.

Convection is a method of energy transfer due to the movement of large amounts of a material, this flow is called fluid motion and is most often seen in liquids and gases. As air is heated it will expand (thermal expansion) as it expands its density decreases and rises through the cooler, more dense air which falls to where the heated air was before heating. This air is then heated resulting in the same process. This makes boiling water much easier, adds to our weather, heats our homes, and allows marine life to survive during the winter among other things.

The third path for energy is the most prevalent but often the least noticed and understood, radiation. Do not confuse this with radioactivity of a material. Irradiated foods have not been exposed to radioactive materials in the uranium, plutonium sense. As a matter of fact, if you microwave your dinner you have irradiated your dinner. Radiation is simply the transfer of energy through electromagnetic waves. The transfer of thermal energy in this manner is known as infrared radiation. Radiation does not require a material to travel through and can travel through the vacuum of empty space. However, infrared radiation has a tough time getting through a cloud cover or glass.