Physical Science Chapter 5 Cont2. Temperature & Heat

Physical Science Chapter 5 Cont2 Temperature & Heat

What are we going to study? Temperature Heat Specific Heat and Latent Heat Heat Transfer Phases of Matter The Kinetic Theory of Gases Thermodynamics

Chapter 5: Temperature Hot & Cold are relative terms. Temperature depends on the kinetic (motion) energy of the molecules of a substance. Temperature is a measure of the average kinetic energy of the molecules of a substance. Section 5.1

Thermometer Thermometer - an instrument that utilizes the physical properties of materials for the purpose of accurately determining temperature Section 5.1

Thermometer Thermal expansion is the physical property most commonly used to measure temperature. Expansion/contraction of metal Expansion/contraction of mercury or alcohol Section 5.1

Bimetallic Strip and Thermal Expansion Brass expands more than iron. The degree of deflection is proportional to the temperature. A/C thermostat and dial-type thermometers are based on bimetal coils. Section 5.1

Liquid-in-glass Thermometer Thermometers are calibrated to two reference points (ice point & steam point.) Ice point the temperature of a mixture of pure ice and water at normal atmospheric pressure Steam point the temperature at which pure water boils at normal atmospheric pressure Usually contains either mercury or red (colored) alcohol Section 5.1

Temperature Scales Celsius, Kelvin, Fahrenheit Section 5.1

Temperature Scales Celsius, Kelvin, Fahrenheit Scale Absolute Zero Ice Point Steam Point Diff. (Boil Ice) Fahrenheit - 460 o 32 o 212 o 180 Celsius -273 o 0 o 100 o 100 Kelvin 0 o 273 o 373 o 100 Section 5.1

Converting Temperatures is Easy! T K = T C + 273 (Celsius to Kelvin) T C = T K 273 (Kelvin to Celsius) T F = 1.8T C + 32 (Celsius to Fahrenheit) T C = T F 32 1.8 (Fahrenheit to Celsius) Section 5.1

Converting a Temperature Example # 1 Convert the Celsius temperature of -30 o C into Fahrenheit. EQUATION: T F = 1.8TC + 32 Section 5.1

Thermal Energy and Heat Temperature a measure of the AVERAGE kinetic energy of the individual particles of a substance. Temperature Perception of how cold or hot something is. Thermal energy TOTAL energy of all of the particles

Heat Kinetic and Potential energy both exist at the molecular level. Kinetic motion of molecules Potential bonds that result in the molecules oscillating back and forth Heat is energy that is transferred from one object to another as a result of a temperature difference. Heat thermal energy moving from a warmer object to a cooler object, trying to reach thermodynamic equilibrium. Section 5.2

Heat Unit SI - Calorie Since heat is energy, it has a unit of joules. (J) A more common unit to measure heat is the calorie. Calorie - the amount of heat necessary to raise one gram of pure water by one Celsius degree at normal atmospheric pressure 1 cal = 4.186 J (or about 4.2 J) Kilocalorie heat necessary to raise 1kg water by 1 o C 1 food Calorie = 1000 calories (1 kcal) 1 food Calorie = 4186 J (or about 4.2 kj) Section 5.2

Heat Unit British - Btu British thermal unit (Btu) the amount of heat to raise one pound of water 1 o F 1 Btu = 1055 J = 0.25 kcal = 0.00029kWh A/C units are generally rated in the number of Btu s removed per hour. Heating units are generally rated in the number of Btu s supplied per hour. Section 5.2

Conceptual Question 1: Which of the following phrases best describes heat? 1. Energy in transit because of a temperature difference 2. The same as temperature 3. The internal energy of a substance 4. Unrelated to energy 0% 0% 0% 0%

Conceptual Question 2: How does the energy is transferred? 1. Energy is transferred from the cold object to the hot object 2. Energy is transferred from the hot object to the cold object 3. The temperature is transferred from cold object to hot object 4. The temperature is transferred from hot object to cold object 0% 0% 0% 0%

Example 2 How much energy have you gained after eating a peanut that contains 10 Calories? 5 18 Section 5.2

Heat vs. Temperature http://www.youtube.com/watch?v=nvxld PHGYhg

Expansion/Contraction with D s in Temperature In general, most matter, solids, liquids, and gases will expand with an increase in temperature (and contract with a decrease in temperature.) Water is an exception to this rule (ice floats!) Section 5.2

Thermal- Expansion Joints in a Bridge These joints allow for the contraction and expansion of the steel girders during the winter and summer seasons. Section 5.2

Behavior of Water Strange! The volume of a quantity of water decreases with decreasing temperature but only down to 4 o C. Below this temperature, the volume increases slightly. Most dense point With a minimum volume at 4 o C, the density of water is maximum at this temperature and decreases at lower temperatures. Section 5.2

Behavior of Water: Structure of Ice Solid water takes up more volume (Ebbing, Darrell D., General Chemistry, Sixth Edition. Copyright 1999 by Houghton Mifflin Company) a) An illustration of the open hexagonal (six-sided) molecular structure of ice. b) This hexagonal pattern is evident in snowflakes. Section 5.2

Behavior of water Liquid water below 4 C is bloated with ice crystals. Upon warming, the crystals collapse, resulting in a smaller volume for the liquid water. Above 4 o C liquid water expands as it is heated because of greater molecular motion. Section 5.3

Conceptual Question 3: A bimetallic strip used in thermostats relies on the fact that different metals have different 1. Specific heat capacities 2. Energy at different temperatures 3. Rates of thermal expansion 4. All of these 0% 0% 0% 0%

Specific Heat (Capacity) If equal quantities of heat are added to equal masses of two metals (iron and aluminum, for example) would the temperature of each rise the same number of degrees? -- NO! Different substances have different properties. Specific Heat the amount of heat necessary to raise the temperature of one kilogram of the substance 1 o C Section 5.3

Specific Heat (Capacity) The greater the specific heat of a substance, the greater is the amount of heat required to raise the temperature of a unit of mass. Put another way, the greater the specific heat of a substance the greater its capacity to store more heat energy Water has a very high heat capacity, therefore can store large amounts of heat. Section 5.3

Conceptual Question 4: Water is useful in the cooling systems of automobiles, and other engines because it 1. Absorbs great quantity of heat for small increases of temperature 2. Absorbs great quantity of heat for a big increases of temperature 3. Absorbs a small quantity of heat for small increases of temperature 4. Absorbs a small quantity of heat. 0% 0% 0% 0%

Specific Heats of Some Common Substanc es The three phases of water are highlighted. Section 5.3

Specific Heat Capacity Youtube: http://www.youtube.com/watch?v=hyplu sd-tym Demonstration:

Specific Heat Depends on Three Factors The specific heat or the amount of heat necessary to change the temperature of a given substance depends on three factors: 1) The mass (m) of the substance 2) The heat (c) of the substance 3) The amount of temperature change (DT) Section 5.3

Using Specific Heat H = mcdt H = amount of heat to change temperature m = mass c = specific heat capacity of the substance DT = change in temperature The equation above applies to a substance that does not undergo a phase change. Section 5.3

Example: Using Specific Heat How much heat in kcal does it take to heat 80 kg of bathwater from 12 o C to 42 o C? GIVEN: m = 80 kg, c = 1.00 kcal/kg. C o (known value for water) Section 5.3

Example: How much it costs to heat the bath water electrically? Heat needed = 2.4 x 103 kcal Convert to kwh 1kcal = 0.00116kWh At 12 cents per kwh, it will cost?? 5 36 Section 5.3

Using Specific Heat Confidence Exercise How much heat needs to be removed from a liter of water at 20 o C so that is will cool to 5 o C? GIVEN: 1 liter water = 1 kg = m DT = 15 o C; c = 1.00 kcal/kg. C o Section 5.3