# CONCEPTUAL PHYSICS HEWITT, 1999 BLOOM HIGH SCHOOL Chapter 21 Temperature, Heat and Expansion.

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CONCEPTUAL PHYSICS HEWITT, 1999 BLOOM HIGH SCHOOL Chapter 21 Temperature, Heat and Expansion

21.1 Temperature Temperature- a measure of how hot or cold something is  Demonstrated by the expansion or contraction of a liquid  Red thermometer- colored alcohol  Silver thermometer- mercury Celsius scale- 100 degrees difference between freezing and boiling of pure water  0°C corresponds to the freezing point of pure water  100°C corresponds to the boiling point of pure water Fahrenheit scale- 180 degrees difference between freezing and boiling of pure water  0°F corresponds to a mixture of ice, water, ammonium chloride and salt  100°F corresponds to human body temperature

Kelvin Kelvin- 100 degrees difference between freezing and boiling of pure water  273K corresponds to the freezing point of pure water  373K corresponds to the boiling point of pure water  0K (absolute zero) represents zero kinetic energy of a substance Fahrenheit is an English unit Celsius is not SI, but a derived unit Kelvin is SI (metric)  We don’t use “degrees” with Kelvin

Temperature Conversions °F = (1.8 x °C) +32 °C = (°F-32) x 0.555 Kelvin = °C + 273.2

Temperature & Kinetic Energy Temperature is related to the motion of molecules  Proportional to the average kinetic energy  Not a measure of the total KE of the molecules Example: There is twice as much energy in 2L of 90°C H 2 O as 1L of 90°C H 2 O  Even though the temperature is the same, there is twice as much mass

21.2 Heat Heat- the energy transfer that takes place because of the temperature difference between two objects  Energy always flows from high to low  Matter does not contain heat Thermal energy- energy resulting from heat flow  Chapter 8  Internal energy- term used in this chapter for the same thing Thermal contact- when heat flows due to direct contact

21.3 Thermal Equilibrium Thermal equilibrium- when objects in contact reach the same temperature  No heat flows from one to the other any more A thermometer is read at thermal equilibrium  When the alcohol or mercury stops moving  The glass of the thermometer is too small to affect the temperature

21.4 Internal Energy Internal energy- grand total of all of the energies in the substance  Translational energy (moving in a straight line)  Rotational energy of the molecule  Kinetic energy within the molecule  Potential energy due to forces within molecule As a substance absorbs energy, the temperature increases As a substance gives off energy, temperature decreases and one or more of the energies decrease

21.5 Measurement of Heat Temperature- a measure of heat transfer, not heat content Calorie (cal)- amount of heat required to increase 1g of H 2 O by 1°C  Metric, derived unit of heat energy  kilocalorie (kcal or Cal)- 1000 calories  Found on food wrappers Joule (J)- 4.18J are required to increase 1g of H 2 O by 1°C  Standard SI unit of heat energy Hungry Man Classic Fried Chicken Dinner

21.6 Specific Heat Capacity Specific heat capacity (or just specific heat)- the quantity of heat required to raise the temperature of 1g by 1°C  Ability to store internal energy A measured value for each substance (see table)

Using Specific Heat (chemistry review) LabelSymbolUnit Specific Heatccal/(g°C) or J/(g°C) HeatQcal or J Massmg Change in temp. TT °C q=mc  T cal=(g)(cal/(g°C))(°C) J=(g)(J/(g°C))(°C)

21.7 The High Specific Heat Capacity of H 2 O Absorbs more than would be expected Slow to gain heat Slow to lose heat Air near water areas are cooler than far inland because water resists a change in temperature

“Cooler near the lake”

21.8 Thermal Expansion As temperature increases, kinetic energy of the molecules increase and they “wiggle” further apart Gases expand the most when heated  Liquids expand less than gases  Solids expand less than liquids

Your Thermostat & You Bimetallic strip- a strip made of two metals that expand at different rates  Brass & iron, typically  One side expands at a different rate than the other Thermostat- practical application of bimetallic strip to turn on and off your furnace/AC  In the diagram, closing the circuit (touching) turns the furnace on

Thermostat Cat

21.9 Expansion of Water All liquids expand when heated Ice cold (0°C) H 2 O does the opposite! As H 2 O warms from 0°C to 4°C, it contracts  H 2 O is more dense at 4°C than 0°C, therefore sinks to the bottom of the container

Solid versus Liquid H 2 O In liquid H 2 O, some of the O’s are attracted to the H’s  H 2 O molecules are able to slide past each other with minimal interaction In solid H 2 O, each O is attracted to two H’s  This causes large, fixed gaps, which decreases density  Ice floats because of this decreased density  Lakes/ponds freeze from the top down

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