HEAT

1. HEAT BONUS: Why does Popcorn pop? Chapter 9 HOLT Physics

2. Temperature • Determining an object’s temperature with precision requires a standard definition of temperature and a procedure for making measurements that establish how “hot” or “cold” objects are.

3. Quick Lab – Sensing Temperature • Materials Needed: • 3 identical bowls/cups • hot and cold tap water • ice Fill one bowl/cup with hot tap water. Fill another with cold tap water, and add ice until about 1/3 of mixture is ice. Fill the third bowl/cup with an equal mixture of hot and cold tap water. Place left hand in hot water and right hand in cold ice water for 15 seconds. Then place both hands in the lukewarm water for 15 seconds and describe whether the water feels hot or cold to either of your hands.

4. CH9 – Section 1Temperature & Thermal Equilibrium • Temperature is proportional to the KE of atoms and molecules. Temperature is actually the measure of the average KE of particles in a substance. • Adding or removing energy usually changes temperature (or could change phase). • Matter typically expands as its temperature increases– Thermal expansion • Temperature units depend on the scale used (°F, °C, K)

5. Internal Energy • Atoms and Molecules move in different ways: • Translational Motion • Rotational Motion • Vibrational Motion -The energies associated with atomic motion are referred to as internal energy, which is proportional to the substance’s temperature (assuming no phase change). -Internal energy is the energy of a substance due to both the random motions of its particles and to the potential energy that results from the distances and alignments between the particles. Internal Energy (U) - also called Thermal Energy Change in Internal Energy (ΔU) Units of Internal Energy are Joules

6. Conceptual Challenge • If two cups of hot chocolate, one at 50°C and the other at 60°C, are poured together in a large container, will the final temperature of the double batch be… • Less than 50°C? • Between 50°C and 60°C? • Greater than 60°C?

7. Thermal Equilibrium • Thermal equilibrium is the state in which two bodies in physical contact with each other have identical temperatures (Zeroth Law of Thermodynamics). • By placing a thermometer in contact with an object and waiting until the column of liquid in the thermometer stops rising or falling, you can find the temperature of the object. • The reason is that the thermometer is in thermal equilibrium with the object. The temperature of any two objects in thermal equilibrium always lies between their initial temperatures. Page 301 – Did you know?

8. Thermal Expansion • In general, if the temperature of a substance increases, so does its volume. This phenomenon is known as thermal expansion. • Different substances undergo different amounts of expansion for a given temperature change. • The thermal expansion characteristics of a material are indicated by a quantity called the coefficient of volume expansion. • Gases have the largest values for this coefficient. Liquids have much smaller values, and solids typically have the smallest values for this coefficient

9. What about Water? Question I am learning that the particles in matter expand when heated and contract when cooled. But, when you put water in the freezer to make ice cubes, the ice expands…why? Shouldn't the water have contracted because its being cooled? Water is really strange stuff, because of the very strong hydrogen-oxygen bonds. One odd thing is that water is most dense at 4°C -- that's why lakes freeze from the top down; the warmer water sinks. Every substance expands when you heat it -- because the average space between the atom's nuclei increases when the atoms have more energy (vibrating faster, more heat). So water DOES expand when you heat it, like anything else -- it's only when it freezes and crystallizes that the “weirdness” (expansion of ice) happens. The reason water expands when frozen is the crystalline structure that forms, and those strong O-H bonds. The hydrogen atoms have a very strong attraction for the ‘unbonded’ electrons in the nearby molecules. In an ice crystal, each oxygen atom has it's own 2 electrons, and grabs hold of 2 more electrons from the water molecule next door. And so they crystallize into a big hexagon shape, which takes up more space than the same molecules do when the water is in liquid form. An ice crystal's form what is called a 'network structure.' Same molecules, but takes up more space. http://en.allexperts.com/q/Science-Kids-3250/water-expand-freezer.htm

10. Measuring Temperature • The most common thermometers use a glass tube containing a thin column of mercury, colored alcohol, or colored mineral spirits. • When the thermometer is heated, the volume of the liquid expands. • The change in length of the liquid column is proportional to the temperature.

11. Measuring Temperature • When a thermometer is in thermal equilibrium with a mixture of water and ice at one atmosphere of pressure, the temperature is called the ice pointor melting pointof water. This is defined as zero degrees Celsius, or 0°C. • When the thermometer is in thermal equilibrium with a mixture of steam and water at one atmosphere of pressure, the temperature is called the steam pointor boiling pointof water. This is defined as 100°C.

12. Temperature Scales There are three temperature scales widely used today: • Fahrenheit • Celsius • Kelvin

13. Measuring Temperature • Temperature values in the Celsius and Fahrenheit scales can have positive, negative, or zero values. • Because the kinetic energy of the atoms in a substance must be positive, the absolute temperature that is proportional to that energy should be positive also. • A temperature scale with only positive values is called the Kelvin scale.

14. Measuring Temperature • A temperature difference of one degree is the same on the Celsius and Kelvin scales. The two scales differ only in the choice of zero point. • Thus, the ice point (0.00°C) equals 273.15 K, and the steam point (100.00°C) equals 373.15 K. • The Celsius temperature can therefore be converted to the Kelvin temperature by adding 273.15:

15. Sample Problem • What are the equivalent Celsius and Kelvin Temperatures of 50°F?

16. Review CH9 Section 1 • Temperature can be changed by transferring energy to or from a substance. • Thermal equilibrium is the condition in which the temperature of two objects in physical contact with each other is the same. This is The Zeroth Law of Thermodynamics • The most common temperature scales are the Fahrenheit, Celsius, and Kelvin (or absolute) scales. • What is the difference between Heat, Temperature and Internal Energy?

17. CH9 Section 2Heat and Energy • Heat(Q) is the energy transferred between objects because of a difference in their temperatures. • From a macroscopic viewpoint, energy transferred as heat tends to move from an object at higher temperature to an object at lower temperature. The directionin which energy travels as heat can be explained at the atomic level, as shown on the next slide.

18. Transfer of particles’ kinetic energy as heat Energy is transferred as heat from the higher-energy particles to the lower-energy particles, as shown on the left. The net energy transferred is zero when thermal equilibrium is reached, as shown on the right.

19. Heat and Energy • The atoms of all objects are in continuous motion, so all objects have some internal energy. Because temperature is a measure of that energy, all objects have some temperature. • The greater the temperature difference between 2 objects, the greater the rate of energy transfer. • Heat, on the other hand, is the energy transferred from one object to another because of the temperature difference between them. When there is no temperature difference between a substance and its surroundings, no net energy is transferred as heat.

20. Heat and Energy • Just as other forms of energy have a symbol that identifies them (PE for potential energy, KE for kinetic energy, U for internal energy, W for work), heat is indicated by the symbol Q. Q=ΔU • Because heat, like work, is energy in transit, all heat units can be converted to joules, the SI unit for energy.

21. Thermal Units and Their Values in Joules

22. Thermal Conduction • The type of energy transfer that is due to atoms transferring vibrations to neighboring atoms is called thermal conduction. • The rate of thermal conduction depends on the substance. • Substances that rapidly transfer energy as heat are called thermal conductors. • Substances that slowly transfer energy as heat are called thermal insulators. When this burner is turned on, the skillet’s handle heats up because of conduction.

23. Conductors and Insulators • Metals are typically good conductors of heat energy. • Some good insulators are: Asbestos Cork Ceramic Cardboard Fiberglass

24. Conduction, Convection and Radiation • Conductive heat flow involves the transfer of heat from one location to another in the absence of any material flow. Conduction is only one mechanism for transferring energy. • Two other mechanisms for transferring energy as heat are convection and electromagnetic radiation. • Convectionis the process of heat transfer from one location to the next by the movement of fluids. (“heat rises” = heated fluid rises) • Natural Convection • Forced Convection • Radiationis the transfer of heat by means of electromagnetic waves and does not involve the movement or the interaction of matter.

25. Conceptual Challenge • Why does a high quality thermos bottle have a vacuum lining as a major component of its insulating ability? Conduction and convection are heat transfer methods which depend upon the presence of materials to transfer heat. By lining a thermos bottle with a vacuum lining, energy cannot escape the contents of the bottle by two of the three forms of heat transfer.

26. Heat and Work As internal energy increases, temperature also increases. Internal energy can be used to do Work. • Some of the energy required to overcome friction is transformed into internal energy. • For solids, bending/deforming a material can also change the internal energy and raise temperature. (examples: bending metal or stretching a rubber band) • For liquids, mixing rapidly can actually increase internal energy and raise temperature. (example: electric mixer and water) • For gases, compressing a gas will increase internal energy and raise temperature.

27. Conservation of Energy • If changes in internal energy are taken into account along with changes in mechanical energy, the total energy is a universally conserved property. • In other words, for a closed system, the sum of the changes in potential, kinetic, and internal energy is equal to zero. CONSERVATION OF ENERGY DPE + DKE + DU = 0 the change in potential energy + the change in kinetic energy + the change in internal energy = 0

28. Sample Problem At Niagra Falls, if 505 kg of water fall a distance of 50m, what is the increase in the internal energy for the water at the bottom of the falls? Assume that all of the initial PE goes into increasing the water’s internal energy and that the final KE is zero.

29. Review CH9 Section 2 • Heat is energy that is transferred from objects at higher temperatures to objects at lower temperatures. • Heat Energy is transferred by thermal Conduction through particle collisions. Convection and Radiation are other mechanisms for transferring Heat Energy. • Energy is conserved when mechanical energy and internal energy are taken into account. Thus, for a closed system, the sum of the changes in kinetic energy, potential energy, and internal energy must equal zero.

30. CH9 Section 3Changes in Temperature and Phase • The specific heat capacity of a substance is defined as the energy required to change the temperature of 1 kg of that substance by 1°C. • Every substance has a unique specific heat capacity. • This value tells you how much the temperature of a given mass of that substance will increase or decrease, based on how much energy is added or removed as heat.

31. Specific Heat Capacity cont. • Specific Heat Capacity is expressed mathematically as follows: • The subscript p indicates that the specific heat capacity is measured at constant pressure. • In this equation, DT can be in °C or in Kelvin.

32. Calorimetry • Calorimetryis used to determine specific heat capacity. • Calorimetry is an experimental procedure used to measure the energy transferred from one substance to another as heat. A simple calorimeter allows the specific heat capacity of a substance to be determined.

33. Calorimetry cont. Because the specific heat capacity of water is well known (cp,w= 4.186 kJ/kg•°C), the energy transferred as heat between an object of unknown specific heat capacity and a known quantity of water can be measured. energy absorbed by water = energy released by substance Qw= –Qx cp,wmw∆Tw = –cp,xmx∆Tx

34. Latent Heat • When substances melt, freeze, boil, condense, or sublime, the energy added or removed changes the internal energy of the substance without changing the substance’s temperature. • These changes in matter are called phase changes. • The energy per unit mass that is added or removed during a phase change is called latent heat, abbreviated as L. Q = mL energy transferred as heat during phase change = mass  latent heat

35. Latent Heat cont. • During melting, the energy that is added to a substance equals the difference between the total potential energies for particles in the solid and the liquid phases. This type of latent heat is called the heat of fusion, abbreviated as Lf. • During vaporization, the energy that is added to a substance equals the difference in the potential energy of attraction between the liquid particles and between the gas particles. In this case, the latent heat is called the heat of vaporization, abbreviated as Lv.

36. Heating Curve

37. Sample Problem Because of the pressure inside a popcorn kernel, water does not vaporize at 100°C. Instead, it stays liquid until its temperature is about 175°C, at which point the kernel ruptures and the superheated water turns into steam. How much energy is needed to pop 95g of corn if 14% of a kernel’s mass consists of water? Assume that the latent heat of vaporization for water at 175°C is 0.9 times its value at 100°C and that the kernels have an initial temperature of 175°C.