Heat and temperature

Heat and Temperature

Prepared by:

Victor R. Oribe

What is heat?

Up to the middle of 19th century, it was thought that heat was an invisible, weightless substance called Caloric Fluid.

When wood, for instance is burned, it is hot because it produces large quantities of caloric fluid which is transferred to other substance close by.

The burned wood, having lost its caloric fluid, is said to have cooled off.

The theory of Caloric Fluid was disproved around 1800 by Count Benjamin Thompson Rumford, an American-born physicist.

Count Benjamin Thompson Rumford concluded that heat was produced from friction alone.

It was only about 40 years later that an English physicist, James Prescott Joule proved that a measurable amount of mechanical energy could always be transformed into definite amounts of heat.

He concluded that heat must be just another form of energy.

Sources of Heat

Natural Sources1. The Sun

When the sun’s energy reaches the earth, 30% of its is immediately reflected back out into space.

Only 70% remains to interact with the earth, and warm it up.

Of the 70%, 20% is energy to heat both the surface of the earth, as well as the ocean.

It is the sun’s heat energy that makes plants and animals to provide themselves with energy.

These plants provide food for animals.

Human eat plants and animals to provide themselves with energy.

Much of the heat on earth may be traced back to the sun.

2. The interior of the Earth

A great amount of heat also comes from buried hot rocks and lava flow from craters and crevices of erupting volcanoes.

The hot spring in Tiwi, Albay, Los Banos, Laguna and in Tayabas Quezon are proofs that the earth’s interior is much hotter than its surface.

Artificial Sources

1. Chemical Action

When fuels like coal, kerosene, wood, and gasoline are burned, they are used as artificial sources of heat.

The oxygen we breathe in burns the food we eat, producing enough body heat to keep our body temperature close to 370 C.

2. Mechanical Action

Heat energy produced by a friction.

3. Electrical Energy

Electricity is a very common source of heat at home.

Oven toaster and electric iron are heated by the resistance of the heating coils to the electricity passing through them.

4. Nuclear Energy

The tremendous amount of energy from nuclear reactions can be used to change water into steam, which is then used to operate a turbine to generate electricity.

How much have you learned?1. Heat is one of the topics in Thermodynamics.

What is thermodynamics?2. Use kinetic molecular theory as a basis to explain

how heat is produced.3. Solar energy is used by plants to manufacture

their own food. Give two other examples how solar energy is used.

4. Make a short description of the following kinds of thermometer:

A. liquid-in-glass thermometerB. Platinum-resistance thermometerC. ThermocoupleD. OpticalE. thermograph

Temperature Scale

The most common thermometer used in the laboratory is the Mercury Thermometer.

Colored alcohol is another liquid commonly used in thermometer.

Celsius Scale

Commonly known as Centigrade Scale

This scale was devised in 1741 by a Swedish astronomer, Anders Celsius, based on the properties of water.

On the Celsius scale, the freezing point of water is fixed at 0 degree Celsius (00 C)

The boiling point of pure water under normal atmospheric pressure, 760 mm Hg (at sea level), is 100 degrees Celsius (1000 C).

The normal temperature of the human body is 370 C

Fahrenheit Scale

The Fahrenheit scale was devised by Daniel Gabriel Fahrenheit, A German physicist.

Unites States of America and England use the Fahrenheit scale for weather observation and other purposes.

On Fahrenheit scale, 320 F is the freezing point of water and 2120 F is the boiling point.

Kelvin Scale or Absolute Zero

Named after Sir William Thompson (Lord Kelvin) who devised it.

This temperature scale is often used when gases are involved.

Its zero point (0 K) corresponds to -2730 C and is referred to as Absolute Zero or the temperature at which the molecular energy of matter is minimal.

On the Kelvin scale, the freezing point of water is 273 K and the boiling point of water is 373 K.

To convert temperature readings from one scale to another, the equation below are used:

Converting Units of Temperature

oC to oF

oF = (9/5)(oC) + 32

38.7 oC

oFoF =

=

(1.8) (OC) + 32

(1.8)

(38.7)

+ 32

= 69.66

+ 32= 101.7

oF to oC oC = 5/9 ( oF – 32)

101.7 oF

o CoC = = .555 (101.7 –

32)= (.555) (69.7)

= 38.7

5/9( oF – 32)

oC to K

38.7 oC

K

K = 273 + oC

K

= 273

+ 38.7

= 273+ oC

= 311.7

Convert the following 1) 37o C to oF 1) 98.6oF2) 350oF to oC 2) 176.7oC3) 100oC to K 3) 373 K4) 95o C to oF 4) 203 oF5) 130oF to K 5) 327.4 K6) 39oC to oF 6) 102 oF7) -160oF to oC 7) -107oC8) 80.0oF to oC 8) 27 o C9) 27oC to K 9) 300 K

10) 120oC to oF 10) 248 oF

How Heat is Measured?Thermometer only gives us an idea of the total potential energy and kinetic energy of the molecules in an object.

It does not tell us how much heat a particular object absorbs or gives off.

In fact there is no instrument that directly measures the amount of heat energy that a body gives off or absorbs

If you want to calculate the amount of heat given off by a piece of charcoal or a certain kind of gas, then you have to determine the change in temperature it produces on a definite volume of water.

Units of HeatCommon units used for measuring heat energy:

a. Calorie (cal)b. kilocalorie (kcal)c. joule (J)

The SI unit for heat, as for any other form of energy is the Joule (1 N.m = 1 joule)

Today, the calorie is defined in terms of the joule: that is 1 cal = 4.184 J.

This is the mechanical equivalent of heat1 Kcal = 1000 calories

Effects of Heat

1. Generally when a substance absorbs heat, its temperature rises.

2. Solid usually melt or change to the liquid state when heated.

3. Liquid may absorb enough energy when heated to change to vapor state.

4. Almost all objects expand when heated.

5. A change in the heat content of a substance can cause chemical changes.

6. Heat causes many changes in bodily functions of living organisms

Enzymes are protein-like substances formed in human cells that act as catalysts in initiating or speeding up specific chemical reactions.

Each has a characteristics shape that allows it to hold two or more molecules close enough together so they can react chemically.

If the temperature is increased, then the speed of moving molecules is also increased.

Bombardment by high speed molecules in the cell can cause the enzymes to lose its shape.

It can no longer perform its vital function.

Two glasses stack together.

If we twist them apart, one or both of them may break.

What is the safest way to separate the two glasses that stick together?

One way is to pour some cold water into the inner glass and dip the outer glass in hot water.

Within a short while, the two glasses can easily be pulled apart.

Why? Generally most substances expands when heated and contract when cooled.

A change in temperature produces a change in length in almost all solid.

Experimental results have shown that change in length is directly proportional to change in temperature and to the original length of the object.

Different solids of the same length differ in their expansion for the same change in temperature.

In liquids the effect of heat is an increase in their volume while the effect of cooling is a decrease in their volume.

Expansion of Gases

Why do gases expand at a greater rate than solids and liquids?

Change of Phase

Whenever the temperature of a substance changes, there is an accompanying change in its molecular structure.

If the change in molecular structure is such that it becomes noticeable, then we say that the substance is undergoing a Change of Phase.

Heat Transfer

Heat may be transferred in three ways:

a. Conductionb. Convectionc. Radiation

a. ConductionTransmission of heat from particle to particle

Conduction of heat in an object will take place only when one part of the object has a higher temperature than another part.

The part with a higher temperature will transfer its heat energy to that part with a lower temperature.

Among solids, metal conduct heat the best.

But metals differ in their conductivity.

Silver is very good conductor of heat than iron and lead. Why?

Solid are the best conductors of heat.

b. ConvectionThe heat energy is distributed by the liquid or gaseous molecules in circling currents.

c. RadiationTransfer of heat through electromagnetic waves.

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QUIZ

1. The absolute zero temperature is _____a. -273 0 Cb. -320 Cc. 00 Cd. 273 K

2. The space between the walls of the double-walled thermos bottle is evacuated to ______

a. increase volumeb. reduce convectionc. reduce heat transfer by

conductiond. both b and c

3. Dry ice has a temperature of -110.2 0 F. What would its temperature be on the Celsius scale?

a. -7110 Cb. -198.3 0 Cc. -142.20 Cd. -790 C

4. Room temperature is often identified as 680 F. What temperature is this on the Kelvin scale?

a. 20 Kb. 100 Kc. 293 Kd. 341 K

5. The transfer of energy from a source to an object by direct contact between them is _____

a. conductionb. convectionc. insulationd. radiation

6. As more heat is added to a sample of water boiling in a stove, its temperature ______

a. decreasesb. increasesc. increases then decreasesd. remains the same

7. When water reaches 40 C, ____.a. it will expand only if heated.b. its volume will remain constant.c. it will expand whether it is cooled

or heated.d. it will contract until it freezes into

ice, and then expand.

8. When does vaporization happen? It _____________.

a. will occur only at night.b. takes place at very great pressures.c. occurs at the boiling point temperatured. takes only at a definite temperature

may occur at a definite temperature or at any temperature.

9. Which of the following statements is true about boiling? It ____________.

a. is lower than evaporation.b. takes place at a definite

temperature.c. is the same for all liquids at the

same temperature.d. takes place when bubbles begin to

appear in the liquid.

10. The rate of evaporation may be increase by _______

a. raising the temperature of the liquid.

b. increasing the surface area of the liquid.

c. decreasing the air pressure on the surface of the liquid.

d. any of the above.

11. What is condensation? It ____a. is an exothermic process.b. is the opposite process of

melting.c. is due to the increase in the

kinetic energy of the particle of the liquid.

d. refers to the change from solid state to gaseous state form of matter.

Two cars of the same model, the same brand, and the same size, are both locked and parked under the sun in an open parking lot. The two cars are parked from 9:00 A.M. to 5:00 P.M. One of the cars is ivory white in color and the other is dark blue.

12. Under the same condition, at 12:00 noon, _____a. the dark-colored will be hotter inside.b. the ivory-colored car will be hotter inside.c. since both are locked, the sun will not affect

either car.d. since both are under the same sun, they will

both be equally hot.

Match Column A with Column B

Column A_____13. William Thompson_____14. Anders Celsius_____15. D. Gabriel Fahrenheit_____16. B.T. Rumford _____17. J. Prescott Joule

Column Aa. Centigrade tempt. Scaleb. Absolute Tempt. Scalec. Fahrenheit Tempt. Scaled. Heat is produced by frictione. Concluded that heat is a

form of energy.

18-21. Enumerate the different artificial sources of heat.

18.19.20.21.

22-24. Enumerate the different methods of heat transfer.

22.23.24.

25. True or FalseLand breeze and sea breeze

occurs because of conviction current.

Answer

1. The absolute zero temperature is _____a. -273 0 Cb. -320 Cc. 00 Cd. 273 Kd. 273 K

2. The space between the walls of the double-walled thermos bottle is evacuated to ______

a. increase volumeb. reduce convectionc. reduce heat transfer by

conductiond. both b and cd. both b and c

3. Dry ice has a temperature of -110.2 0 F. What would its temperature be on the Celsius scale?

a. -7110 Cb. -198.3 0 Cc. -142.20 Cd. -790 Cd. -790 C

4. Room temperature is often identified as 680 F. What temperature is this on the Kelvin scale?

a. 20 Kb. 100 Kc. 293 Kd. 341 Kc. 293 K

5. The transfer of energy from a source to an object by direct contact between them is _____

a. conductionb. convectionc. insulationd. radiation

a. conduction

6. As more heat is added to a sample of water boiling in a stove, its temperature ______

a. decreasesb. increasesc. increases then decreasesd. remains the samed. remains the same

7. When water reaches 40 C, ____.a. it will expand only if heated.b. its volume will remain constant.c. it will expand whether it is cooled

or heated.d. it will contract until it freezes into

ice, and then expand.d. it will contract until it freezes into ice, and then expand.

8. When does vaporization happen? It _____________.

a. will occur only at night.b. takes place at very great pressures.c. occurs at the boiling point temperatured. takes only at a definite temperature

may occur at a definite temperature or at any temperature.

c. occurs at the boiling point temperature