Agenda. Chapter 10, Problem 26. All matter is made of atoms. Atomic Structure 4/8/14. What is the structure of matter? Atomic Terminology

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1 Agenda Today: HW Quiz, Thermal physics (i.e., heat) Thursday: Finish thermal physics, atomic structure (lots of review from chemistry!) Chapter 10, Problem 26 A boy reaches out of a window and tosses a ball straight up with a speed of 10 m/s. The ball is 20 m above the ground as he releases it. Use conservation of energy to find: a. The ball s maximum height above the ground. b. The ball s speed as it passes the window on its way down. c. The speed of impact on the ground. What is the structure of matter? All matter is made of atoms Atom Electron Cloud Nucleus Characteristics of Atoms: Very tiny Very numerous Always in motion Substances made of only 1 type of atom are called elements Atomic Structure Atomic Terminology An atom is composed of Nucleus Contains most of the atomic mass made up of protons (+ electric charge) and neutrons (no electric charge) Very, very small (<10-14 m) Electrons even smaller than nucleus 2000 times less massive than nucleus negative electric charge (same amount as proton) may be thought of as orbiting nucleus Number of electrons in a neutral atom is equal to number of protons in nucleus Atomic Number = # of protons in nucleus Atomic Mass Number = # of protons + neutrons Molecules: consist of two or more atoms (H 2 O, CO 2 ) 1

2 Elements & The Periodic Table Atomic Terminology Isotope: same # of protons but different # of neutrons. ( 4 He, 3 He) Transforming Energy Recall: energy is always conserved, just changes from one form to another Thermal energy is not very useful (incredibly difficult to transform into another form of energy!), we mostly consider thermal energy to be lost Other forms (spring energy, kinetic energy, gravitational potential energy, work done by conservative forces) are not considered lost in this way. Efficiency Real-life processes almost always include some friction, which means we ll lose some energy to heat. How much we lose determines the efficiency of the process: e = Energy out/ Energy in (Efficiency is a percentage, so no units) Temperature Temperature (T) is a measure of how hot or cold something is Temperature measures the random KE of each particle in an object. The greater the motion/vibration the greater the T The smaller the motion/vibration the lower the T SI Unit: kelvin (K) Room temperature is about 295K Kelvin Temp. Scale The Kelvin scale has the same step size (size of one degree) as the Celsius scale, but the Kelvin scale has its zero at absolute zero. Conversion between a Celsius temperature and a Kelvin temperature: 2

3 Thermometers Thermometers are instruments designed to measure temperature. In order to do this, they take advantage of some property of matter that changes with temperature. Length of a solid or liquid column Volume of a solid, liquid, or gas Electromagnetic waves (infrared light) given off by hot objects Thermal Expansion When you heat something up, it expands! (usually ) The effect is less dramatic in solids than in liquids or gases Common Thermometers Liquid-in-tube Bimetallic Strip If you have a glass jar with a metal lid that s stuck, which of these might help you loosen the lid? A. Running the lid under hot water B. Running the lid under cold water C. Running the lid under lukewarm water Thermal Energy Careful! Temperature and thermal energy are NOT the same thing, even though thermal energy depends on temperature. Temperature: KE avg Thermal Energy: KE tot E th = (3/2) Nk B T N = number of atoms k B = 1.38 x J/K (Boltzmann s constant) T = temperature in Kelvins 3

Two gases have the same thermal energy. One gas has twice as many atoms as the other. Which gas is at a higher temperature? A. The gas with fewer atoms. B. The gas with more atoms. C.

4 Two gases have the same thermal energy. One gas has twice as many atoms as the other. Which gas is at a higher temperature? A. The gas with fewer atoms. B. The gas with more atoms. C. Neither; they are at the same temperature. Two gases have the same thermal energy. One gas has double the temperature of the other. Which gas contains a larger number of atoms? A. The gas with the lower temperature. B. The gas with the higher temperature. C. Neither; they contain the same number of atoms. Heat (Q) Definition of heat: Heat is the energy transferred between objects because of a temperature difference, or through work. Objects are in thermal contact if heat can flow between them. Connection from momentum: we treat collisions between atoms as perfectly elastic collisions Thermal Equilibrium When the transfer of heat between objects in thermal contact stops, they are in thermal equilibrium. The objects will then be at the same temperature (they won t necessarily have the same thermal energy!) Units of Heat Since heat is just a flow of energy, the SI unit is the energy unit, the joule (J). Other heat units calorie (cal): Heat needed to raise temperature of 1 gram of water by 1 C (or 1 K) Calorie (Cal or kcal): Heat needed to raise temperature of 1 kg of water by 1 C (or 1 K) Calorie also used to measure energy content of food Conversions: 1 cal = J 1 kcal = 1 Cal (food Cal.) = kj Specific Heat Capacity Specific heat capacity is the amount of heat energy required to raise the temperature of one unit mass of a material by one degree. SI Unit: J/(kg K) 4

Heat Transfer Methods Conduction: Thermal kinetic energy passed from particle-to-particle

Radiation: Thermal energy carried by electromagnetic waves (light) Conduction Heat conduction

Thermal kinetic energy is passed along as hotter particles collide with colder ones.

Some materials are good thermal conductors (like the tile), others are insulators (like the

Conduction Convection Convection is flow of fluid due to difference in temperatures, such as

Natural convection: Warm fluid will rise because it is less dense then cold fluid.

5 Heat Transfer Methods Conduction: Thermal kinetic energy passed from particle-to-particle along a length of material. Convection: Thermal energy carried by moving fluid. Radiation: Thermal energy carried by electromagnetic waves (light) Conduction Heat conduction can be visualized as occurring through molecular collisions. Thermal kinetic energy is passed along as hotter particles collide with colder ones. Conduction is heat flow by direct contact. Some materials are good thermal conductors (like the tile), others are insulators (like the wood). Conduction Convection Convection is flow of fluid due to difference in temperatures, such as warm air rising. Fluid carries heat with it as it moves. Natural convection: Warm fluid will rise because it is less dense then cold fluid. Convection Heat transfer in a fluid often occurs mostly by convection. Buoyancy causes warm air to rise, which carries thermal energy directly by its motion. Fiberglass Insulation Air is a poor thermal conductor but easily transfers heat by convection. Fiberglass insulation is mostly air, with the fibers disrupting the convection flow. 5

6 If you were trying to warm your hands with a bonfire, where would you place your hands to warm them up as quickly as possible? A. To the left of the bonfire B. To the right of the bonfire C. Above the bonfire D. Anywhere; the exact location doesn t matter Radiation How does energy get from the Sun to Earth? No atmosphere out in space, so it s can t be convection or conduction The energy is transferred through radiation; specifically, electromagnetic radiation Conceptual Check: Spend a few minutes discussing with a neighbor how you could cool a hot cup of coffee. Try to explain how the cooling would happen using the ideas we ve learned about heat transfer. 0 th Law of Thermodynamics Imagine three systems: A, B, and C If A and B are each in thermal equilibrium with C, then A and B must also be in thermal equilibrium with each other. A B C 1 st Law of Thermodynamics When heat flows to (or from) a system, the system gains (or loses) an amount of energy equal to the amount of heat transferred. Caution: Remember that numerically, ΔQ ΔTemp! 1 st Law of Thermodynamics ΔE th = ΔQ + work This is the thermal version of conservation of energy! You can never get more energy out of a system than you originally put in In an adiabatic process, ΔQ = 0 so the ΔE th is the same as the work done by the system 6

Imagine you do work on an object (such as sanding a piece of wood) but there is very little heat flowing out of the object. What is happening to the energy?

7 Imagine you do work on an object (such as sanding a piece of wood) but there is very little heat flowing out of the object. What is happening to the energy? Work and Q can be positive or negative numbers A. The energy is staying in the wood, making the molecules in the wood move faster. B. The energy is dissipating into the surrounding air. C. The energy is staying in the wood, heating it up. D. Both A and C. 2 nd Law of Thermodynamics Heat of itself never flows from a cold object to a hot object. Efficiency Revisited e = Energy out/ Energy in e = W out / Q H And, you can harness some but not all of that heat for useful work: W out = Q H Q C e max = 1- (T H / T C ) T H, Q H ΔQ X T C, Q C This is not a technological limit, but a real physical limit to the efficiency of a heat engine. Usually, the real efficiency isn t even as high as the theoretical efficiency! Heat Engines vs. Heat Pumps We can harness heat flowing from a hightemperature object to a low-temperature object to do work We can also do the reverse: do work on a system to force heat to flow the wrong direction. Entropy: Order vs. Disorder In natural processes, high-quality energy tends to transform into lower-quality energy order tends toward disorder. So what in the world do high-quality and low-quality energy mean?? COP max = T C / (T H T C ) 7

8 Quality of Energy: Order & Usefulness Can you re-use the potential energy you give to a box when you lift it? Can you re-use the energy you give to your hands when you rub them together quickly? In which case above is the energy more useful? Entropy: Amount of Disorder Will a box lift itself onto a shelf spontaneously? Will the energy in your warm hands dissipate into the air spontaneously? In which case above is there more entropy? The Heat Death of the Universe Stars are producing energy (via fusion more on that later), and it radiates out in space. Stars have finite (although long) lifetimes; eventually they ll stop shining! What happens when no more stars can form? Thermodynamics Summary: You can t win, you can t break even, and you can t get out of the game. 8

Conduction. Heat Transfer Methods. Conduction. Conduction

Conduction. Heat Transfer Methods. Conduction. Conduction Heat Transfer Methods Conduction: Thermal kinetic energy passed from particle-to-particle along a length of material. Convection: Thermal energy carried by moving fluid. Radiation: Thermal energy carried