Temperature & Heat

Physical Science Chapter 5 Cont2

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 - 460o 32o 212o 180

Celsius -273o 0o 100o 100

Kelvin 0o 273o 373o 100

Section 5.1

Converting Temperatures is Easy!

TK = TC + 273 (Celsius to Kelvin)

TC = TK – 273 (Kelvin to Celsius)

TF = 1.8TC + 32 (Celsius to Fahrenheit)

Section 5.1

TF – 32

1.8 • TC = (Fahrenheit to Celsius)

Converting a Temperature Example # 1

Convert the Celsius temperature of -30oC into Fahrenheit.

EQUATION: TF = 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 1oC

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 1oF

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

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

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Conceptual Question 1: Which of the following phrases best describes heat?

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

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Conceptual Question 2: How does the energy is transferred?

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Example 2

How much energy have you gained after eating a peanut that contains 10 Calories?

Section 5.2

Heat vs. Temperature

http://www.youtube.com/watch?v=NVXLDPHGYhg

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 4oC. Below this temperature, the volume increases slightly.

With a minimum volume at 4oC, the density of water is maximum at this temperature and decreases at lower temperatures.

Most dense point

Section 5.2

Behavior of Water: Structure of Ice Solid water takes up more volume

a) An illustration of the open hexagonal (six-sided) molecular structure of ice.

b) This hexagonal pattern is evident in snowflakes.

Section 5.2

(Ebbing, Darrell D., General Chemistry, Sixth Edition. Copyright 1999 by Houghton Mifflin Company)

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 4oC liquid water expands as it is heated because of greater molecular motion.

Section 5.3

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Yellowstone Lake - Frozen

Photo Source: Copyright © Bobby H. Bammel. All rights reserved.

Section 5.2

Do you know that fish live below frozen lakes? How?

1. Specific heat capacities

2. Energy at different temperatures

3. Rates of thermal expansion

4. All of these

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Conceptual Question 3: A bimetallic strip used in thermostats relies on the

fact that different metals have different…

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 1oC

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

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.

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Conceptual Question 4: Water is useful in the cooling systems of automobiles, and other engines because it …

Specific Heats of Some Common Substances

The three phases

of water are

highlighted.

Section 5.3

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Sand (700 J/kg-Co) & Water (4186 J/kg-Co)

Photo Source: Copyright © Bobby H. Bammel. All rights reserved.

Section 5.3

Specific Heat Capacity

Youtube: http://www.youtube.com/watch?v=hyPLusD-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 12oC to 42oC?

GIVEN: m = 80 kg, c = 1.00 kcal/kg.Co (known value for water)

Section 5.3

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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 ??

Section 5.3

Using Specific Heat Confidence Exercise

How much heat needs to be removed from a liter of water at 20oC so that is will cool to 5oC ?

GIVEN: 1 liter water = 1 kg = m

DT = 15oC; c = 1.00 kcal/kg.Co

Section 5.3