QOD All objects sitting in a room should reach the same temperature. Yet if you pick up a cup made of glass, it feels cooler than a cup made of plastic. How is this possible? How can two objects be the same temperature and yet one feels cooler? Heat (Thermal Energy) and Atoms, Molecule and the Meaning of Heat Heat is kinetic energy, the kinetic energy of molecules When you rub your hands, they feel warmer because, after rubbing, the molecules are shaking back and forth faster than before. Heat = the shaking of atoms and molecules, random in direction, rapid in speed, but microscopic in distance. Atoms, Molecule and the Meaning of Heat All substances are made up of approx 100 different types of atoms These atoms can join together to make molecules In all materials, the molecules are constantly shaking. The more vigorously they shake, the hotter the material is. Atoms, Molecule and the Meaning of Heat How fast do they shake? About 700 miles per hour or 330 meters per second - the speed of sound. Is this a coincidence? Atoms, Molecule and the Meaning of Heat Sound travels through air by molecules bumping into each other. So the speed of sound is actually determined by the speed of molecular motion. Sound traveling through a gas cannot move faster than the velocity of the gas molecules. 1
Atoms, Molecule and the Meaning of Heat What about in a solid? The sound can travel faster than the molecules because the molecules are essentially touching (solid). They don't have to move in order to transmit a force to the next molecule. The Speed of Sound The average speed of molecules that compose any solid is the speed of sound. They are moving in random directions, thus the solid stays still. However, if you could get them to all move at the same direction the entire solid would move at the speed of sound! In either instance the total energy of the solid is exactly the same!!! This example illustrates the ENORMOUS energy contained in the heat of ordinary objects. Energy in Heat Unfortunately, it's often not possible to extract the energy from the last example to do useful work. There is no good way to change the directions of he shaking so that all the molecules move together. Yet we can do the opposite. When an asteroid hit the earth 65 million years ago, killing the dinosaurs, all the molecules were initially moving at 30 km/s in the same direction. After impact, the directions were all different - kinetic energy was turned to heat energy. The molecules changed from being neatly ordered to disordered. Thermodynamics The concept of disorder is popular in physics and the amount of disorder can be quantified. Disorder has a special name - entropy. When an object is heated, its entropy (the randomness of molecular motion) increases. A Property of Matter 2
is closely related to heat - it's what you read with a thermometer. What exactly is being measured when you take the temperature of something? is a measure of the hidden kinetic energy of the molecules. (The usually unobserved energy of fast but microscopic shaking.) increases when the average shaking of energy of an objects molecules is greater. (The word average is used because not all molecules move at the exact same velocity. There are some moving faster than others (like dancers on a dance floor) If two objects have the same temperature, then their molecules have the same kinetic energy of vibration. Suppose that two bars, one made of iron and the other copper, have the same temperature. Will the Fe and Cu molecules have the same average speed? NO! KE = 1/2 mv2 Cu and Fe have different molecular masses (m). So, the heavier Cu must have a smaller velocity (v) in order to have the same kinetic energy (KE) See why temperature was once even more of a mystery than heat? Remember: At the same temperature, lighter molecules move faster (on average) than heavier ones. The reason temperature is such a useful idea in physics is the simple fact that two things that touch each other tend to reach the same temperature. This is why a thermometer gives you the temperature of the air - it's in contact with the air, so it gets to the same temperature. The fact that objects in contact with each other eventually reach the same temperature is the zeroth law of thermodynamics. 3
What happens when you touch a hot object to a cooler one? Say hot iron to cold copper. The fast moving molecules in the hotter iron bang into the slower moving copper molecules transferring energy (heat) Eventually the temperature is the same and the energy transfer has stopped. The flow of heat is actually the sharing of kinetic energy. If you put a bunch of objects in the same room and wait, eventually they will all reach the same temperature. (This isn't true if one of the objects is the source of energy e.g., wood burning.) Scales On the Fahrenheit scale,, there are 180 F between the freezing and boiling points and on the Celsius scale, there are 100 C. 180 F = 9 F = 1.8 F 100 C 5 C 1 C In the formula for the Fahrenheit temperature, adding 32 adjusts the zero point of water from 0 C to 32 F. or T F T F Fahrenheit Formula = 9/5 T C + 32 = 1.8 T C + 32 Kelvin Scale The Kelvin temperature scale has 100 units between the freezing and boiling points of water. 100 K = 100 C or 1 K = 1 C is obtained by adding 273 to the Celsius temperature. = T C + 273 T K contains the lowest possible temperature, absolute zero (0 K). 0 K = 273 C s Scale Conversions To convert from Celsius to Fahrenheit: T f = 9/5 T c + 32 (T C is obtained by rearranging the equation for T F. To convert from Celsius to Kelvin: T k = T c + 273 F. ) 4
Conversion Practice Page 278 1-4 Thermometers Definitions Heating is the transfer of energy from an object with more random internal energy to an object with less (always hot to cold!) is a measure of the average energy of the randomly moving molecules in a substance Same temperature=thermal equilibrium History Daniel Gabriel Fahrenheit Chose two reference points Coldest (mixture of salt, water, ice) 0F Body temperature 96 (slightly in error = 98.6) First to utilize mercury Anders Celius His two reference points: boiling point of water and the freezing point of water Lord Kelvin Zero point is absolute zero (-273.15 o C) and 0 o C is 273.15 K Fixed Reference Points and Thermometer Calibration Systems whose temperatures are determined by some physical process that is universal and repeatable Phase transitions are commonly used as fixed reference points Transducers (sensors) used to measure temperature Liquid expansion devices Bimetallic expansion devices Change-of-state indicators Metallic resistance devices Thermistors Thermocouplers 5
Liquid Expansion Thermometers Contain liquid that expands or contracts with temperature changes Different objects expand and contract at different rates. Liquids and gases expand and contract much more dramatically than solid objects, like metals or glass. Bimetallic Expansion Thermometer Constructed by fusing together two metal strips, typically one of brass and one of iron, which have different coefficients of expansion Change-of-State Indicators Varied products, all of which change color or form when exposed to heat Resistance Thermometry Metallic resistance thermometers and thermistors are two types of thermometers based on the principle that the electrical resistance of materials changes as their temperature changes Resistance temperature detectors use metallic wires (length of fine coiled wire wrapped around a ceramic or glass core.) Thermistors use semiconductor materials Thermocouples Consists of two wires made of different metals that are joined together Metal wire 2 Metal wire 1 Current flows when the Two junctions are at Different temperatures Infrared The design essentially consists of a lens to focus the infrared thermal radiation on to a detector, which converts the radiant power to an electrical signal that can be displayed in units of temperature. This permits temperature measurement from a distance without contact with the object to be measured. 6
Thermal Expansion Thermal Expansion When atoms heat up they push neighbor atoms further away. Because of this, most solids expand when heated. Typically this expansion is between 1 part in 1000 to one part in 100,000. Though this sounds small here s an example Thermal Expansion If you re a bridge builder how might thermal expansion come into play when designing the bridge? The span of the Verrazano-Narrows bridge in New York is 4260 ft. When temps change from 20 F to 92 F, the bridge expands in length by about two feet. This is a factor for the height of the bridge too. The colder, shorter cables in the winter raise the middle of the bridge 12 ft. Thermal Expansion SR-71 Spy Plane Thermal expansion is the reason glass shatters when you place hot glass into cool water. The outside cools more rapidly than the inside, making it a different size. It starts to bend, and since glass is brittle it breaks. Pyrex is a special glass used for cooking. It doesn t expand as much as regular glass and therefore won t break as easily when cooled quickly. 7
SR-71 Spy Plane These planes fly so fast that friction heats the outer surface to 1000 F. Thermal expansion is so great that if made the normal way the wings would crack. Designers had to make fittings on the plane loose. They don t fit until plane s metal expands at high speeds. A tricky consequence of this was the fact that the planes leaked fuel until the outside heated up sufficiently. What about contraction? Does everything contract when cooled? No! Water between 39 F and 32 F expands as its cooled. It expands even more as it s frozen. Without this peculiar behavior of water, life on earth would have a hard time enduring. Since ice is less dense than water it floats. Thus, lakes freeze from the top down. Some people speculate that the entire ocean would eventually reach the freezing point, turn to ice, and all life die, if it wasn t for this property. Heat Transfer Heat Transfer Heat energy transfer from one object to another occurs in three different ways: 1. Conduction 2. Convection 3. Radiation Heat Transfer: Conduction Heat transfer through conduction occurs via molecular collisions. (Ex.: Sticking a spoon into a hot bowl of soup.) With the spoon ex., it s the collisions of free e- within the metal that are responsible for conduction. There must be a difference in temperature between the two points to occur. Heat Transfer: Conduction Thermal conductivity (k): a numerical value of a material s ability to transfer heat. Substances with a large k conduct heat rapidly and are said to be good conductors Substances with a small k conduct heat poorly and are said to be good insulators. 8
Thermal Conductivity (k) Substance Thermal Conductivity (k) Silver 420 Copper 380 Aluminum 200 Steel 40 Ice 2 Glass 0.84 Brick 0.84 Concrete 0.84 Water 0.56 Human Tissue 0.2 Wood 0.1 Fiberglass 0.048 Cork 0.042 Wool 0.040 Goose down 0.025 Air 0.023 Heat Transfer: Convection Convection: The process of heat flowing through the mass movement of molecules from one place to another Examples: forced air heat in your house, hot air rising, ocean currents, wind Blood in the human body acts as a convective fluid to transfer heat from inside the body to just beneath the surface of the skin, where it is then conducted over a very short distance to the skin. Hiking in the summer, picking a tent location Heat Transfer: Radiation Radiation: The transfer of heat over empty (or nearly empty) space with no medium at all. The sun, a fireplace (air rises up by convection through chimney and does not reach us), radiator, heat lamp. Consists essentially of electromagnetic waves. Heat Transfer: Radiation Best way to enjoy a warm house is through radiant heat (walls, floors, etc.) It is estimated that radiation accounts for about 50% of the heat loss from a sedentary person in a normal room. Believe it or not, rooms are most comfortable when the floors and walls are hot and the air is not. Even the Romans knew this 2000 years ago. Their houses in the remote province of Great Britain made use of hot-water and steam conduits in the floor to heat their houses. Applying these principles How does a thermos work? 1. Glass envelope holding a vacuum (a perfect vacuum has zero atoms), which eliminates heat loss through conduction and convection. 2. The glass is silvered (like a mirror) to reduce infrared radiation. The combination of a vacuum and the silvering greatly reduces heat transfer by convection, conduction and radiation. Internal Thermal Energy Internal Thermal Energy: The sum total of all the kinetic energy of all the molecules in an object. If three beakers (25 ml, 250 ml, and 500 ml) all have the same temperature, does any one have a greater thermal energy? 9
Thermos Challenge Using the principles of heat transfer, create a thermos using 5 sheets of paper, 1 sq ft aluminum foil, 1 wool sock, 1 piece of black plastic bag, a thermometer, and a water bottle. 15 minutes to design Add water Record temperature every 1 minute for 15 min. Calculate percent loss and record. [(T i -T f )/T i ]*100 = percent loss Create a table and graph displaying T vs time record percent loss on graph paper. The winning team will have the lowest percent loss. Gas and Pressure The Ideal Gas Law What is a gas and how does it act? To understand, let s compare a solid to a gas. In a solid, the atoms move back and forth but never leave their relative positions. As the solid gets hotter the expanded bouncing causes it to expand (thermal expansion). When the energy of the molecules becomes great enough, the atoms push their way out, escaping. The molecules no longer stay in the same place but move much more freely. The molecules now bounce quickly into other molecules and the walls of the container they re in. The bouncing causes the walls to push outwards and a force must be applied in return The Ideal Gas Law The pressure of a gas is defined as the force it exerts on one square meter of area. There is a relationship between the amount of pressure a gas exerts and its temp. This relationship is quantified by the following equation: P = RT (kelvin kelvin) Where P = pressure, R = Avogadro's constant, and T = temp in K This is a simplified version of the Ideal Gas Law: PV = nrt The Ideal Gas Law The importance of this law is as follows: If you double the temperature, you double the pressure of the gas. If you raise the temperature by a factor of 167 (as when TNT explodes), then the pressure increases by a factor of 167. This is why hot gases exert so much pressure (force) and can do a lot of work! Automobile Airbags and Airplane Escape Chutes A big balloon that inflates very rapidly (1/1000 of a second) How do you fill a balloon that rapidly? An explosion! P=RT Airbags contain about 50 to 200 grams of an explosive called sodium azide. When triggered by an electric pulse, it explodes into sodium metal and nitrogen gas, which inflates the airbag. 10
How does a fridge work? If you pour a little rubbing alcohol on your skin, it'll feel cold -- really cold. It isn't refrigerated, so how does this happen? Alcohol evaporates at room temperature the way water evaporates at a low temperature in an oven. As it evaporates, it absorbs the heat on the surface of your skin, making your skin cooler. Fridges use a special coolant, called a refrigerant, that functions the same way. The coolant is trapped inside a series of coils. As it makes a circuit through them, it changes back and forth from a liquid to a gas. A refrigerator uses five major components: Compressor Heat-exchanging exchanging pipes (serpentine or coiled set of pipes outside the unit) Expansion valve Heat-exchanging exchanging pipes (serpentine or coiled set of pipes inside the unit) Refrigerant (liquid that evaporates inside the refrigerator to create the cold temperatures) The Refrigeration Cycle The compressor compresses the ammonia gas. The compressed gas heats up as it is pressurized (orange). The coils on the back of the refrigerator let the hot ammonia gas dissipate its heat. The ammonia gas condenses into ammonia liquid (dark blue) at high pressure. The high-pressure ammonia liquid flows through the expansion valve.. You can think of the expansion valve as a small hole. On one side of the hole is high-pressure ammonia liquid. On the other side of the hole is a low-pressure area (because the compressor is sucking gas out of that side). The liquid ammonia immediately boils and vaporizes (light blue), its temperature dropping to -27 F. This makes the inside of the refrigerator cold. The cold ammonia gas is sucked up by the compressor,, and the cycle repeats. A little more info Pure ammonia gas is highly toxic to people and would pose a threat if the refrigerator were to leak, so all home refrigerators don't use pure ammonia. You may have heard of refrigerants know as CFCs (chlorofluorocarbons), originally developed by Du Pont in the 1930s as a non-toxic replacement for ammonia. CFC- 12 (dichlorodifluoromethane) has about the same boiling point as ammonia. However, CFC-12 is not toxic to humans, so it is safe to use in your kitchen. Many large industrial refrigerators still use ammonia. In the 1970s, it was discovered that the CFCs then in use are harmful to the ozone layer, so as of the 1990s, all new refrigerators and air conditioners use refrigerants that are less harmful to the ozone layer. 11