CONCEPTUAL PHYSICSHEWITT, 1999

BLOOM HIGH SCHOOL

Chapter 21Temperature, 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 unitCelsius is not SI, but a derived unitKelvin is SI (metric)

We don’t use “degrees” with Kelvin

Temperature Conversions

°F = (1.8 x °C) +32°C = (°F-32) x

0.555Kelvin = °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 H2O as 1L of 90°C H2O 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 thingThermal 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 H2O 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 H2O 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)

Label Symbol UnitSpecific Heat c cal/(g°C) or

J/(g°C)Heat Q cal or J

Mass m g

Change in temp.

DT °C

q=mcDT cal=(g)(cal/(g°C))(°C)J=(g)(J/(g°C))(°C)

21.7 The High Specific Heat Capacity of H2O

Absorbs more than would be expectedSlow to gain heatSlow to lose heatAir 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) H2O does the opposite!

As H2O warms from 0°C to 4°C, it contracts H2O is more dense at 4°C

than 0°C, therefore sinks to the bottom of the container

Solid versus Liquid H2O

In liquid H2O, some of the O’s are attracted to the H’s H2O molecules are able to slide past

each other with minimal interaction

In solid H2O, 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