A measure of the average kinetic energy of molecule

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Why is it colder in the night than in the day?

The sun is the greatest heat source.

As the sun comes up, it warms earth.

As the sun goes down, the heat is taken away and it cools off.

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What will happen to this cold snowman throughout the day as the sun warms it up?

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What would happen to the temperature of the boiling water in this kettle if I added ice cubes?

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How is the change in temperature of the snowman and the boiling water related?

The temperature of both the snowman and the boiling water changed to a temperature that was not really cold or really hot, but rather somewhere in between.

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“Temperature”

How hot or cold an object is.

Measured in degrees Celsius.

Temperature and heat are NOT THE SAME.

It’s a measure of the average kinetic energy of an individual molecule.

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HeatThe amount of thermal energy an object has. It’s measured in joules or J.

A cup of hot tea has heat energy , it is due to the kinetic energy (internal energy) of its particles.

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The small beaker of water boils first

The large beaker contains more water molecules so it needs more thermal energy (heat) to reach the same temperature (100°C) as the small one.

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A swimming pool at 30°C is at a lower temperature than a cup of tea at 100°C.

BUTthe swimming pool contains more water, so it stores more thermal energy or heat.

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ThermometersA thermometer (in Greek: thermos means "hot" and metron “meansure") It’s a device that measures temperature  using a physical Principle. The Galilean thermometer:The first thermometer (1953) was really a thermoscope. It is not called a thermometer, because the scale was arbitrary. The egg-sized globe at the top is the sensor. The gas within it expands or contracts, and the liquid level rises and falls.

http://www.youtube.com/watch?v=W_xc-6662f810

Types of Thermometer

1- Contact thermometers:

(1) Liquid-in-glass thermometers.

(2) Electric thermometers:

a) Thermistors.

b) Thermocouples.

(3) Liquid crystal thermometers.

*2- Non-contact thermometers:

Infrared (IR) thermometers.

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1. Liquid-in-glass thermometer Liquid thermometers have been around for almost 300 years. These rely on the expansion of a liquid

(Mercury or Alcohol) with temperature.

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Liquid-in-glass thermometer

The liquid is contained in a sealed glass bulb and it expands into the fine bore in the thermometer stem. Temperature is read using a scale etched

along the stem. The relationship between the temperature

and the column's height is linear over the small temperature range for which the thermometer is used.

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Advantages

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Disadvantages:

Limited temperature-range coveredNot very accurate.Requires visual reading and is not easy to be automated.

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Thermometer Calibration

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Thermistors, invented by Samuel Ruben 1930, are very temperature sensitive. Their resistance decreases as the temperature increases, so the Electric current increases, (and vice versa).

2. Electrical thermometersa) Thermistors = THERMal resISTORS

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Resistance versus Temperature GraphFor a Thermistor

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Thermistors

Example Applications:

1.Temperature measurement.

2.Time delay (self heating from large current ‘opens’ the thermistor so it can be used as a slow switch). Heating = I2 R, where R is the resistance and I is the current.

3.Surge suppression when a circuit is first energized. Current needs to flow through the thermistor for awhile to heat it so that it ‘opens’, and acts again as a switch.

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Thermistors can be classified into two types depending on the sign of k.

a) If k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor.

b) If k is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor, Posistor.

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ELECTRIC SIGNAL

HAIR DRIER

THERMISTOR

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b) Thermocouples A thermocouple is a pair of junctions formed from two dissimilar metals. One at a reference temperature (e.g. 20⁰C) and the other junction at the temperature to be measured. A temperature difference will produce an electric voltage that depends on this temp.difference.  

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Why use thermocouples to measure temperature?

◦They are inexpensive.◦They are rugged and reliable.◦They can be used over a wide temperature range.

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3. Liquid crystal thermometersA liquid crystal thermometer (or plastic strip

thermometer) is a type of thermometer that contains heat-sensitive liquid crystals in a plastic strip that change color to indicate different temperatures.

In medical applications, liquid crystal thermometers may be used to read body temperature by placing against the forehead, usually called “forehead thermometers”. 

When it’s cold When it’s used to measure temp.

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Temperature scales Celsius Scale:Celsius is the metric scale for measuring temperature.

Water freezes at 0ºC and boils at 100ºC.

Fahrenheit Scale: water freezes at 32 °F, and boils at 212 °F

[F = 1.8C + 32]25

Kelvin scaleIn the Kelvin scale temperature is measured in Kelvin units (K)

Formula (273+ºC)= Kelvin

Absolute zero (0 K) :The temperature in which all molecular motion stops

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Units for measuring heat Like any type of energy, the SI unit for heat is the Joule. Calorie: the name calorie is used for two units of

energy:a) The small calorie or gram calorie (symbol: cal) is

the amount of energy needed to raise the temperature of one gram of water 1⁰C.

b) the large Calorie, or ”Kg calorie”, “nutritionist's calorie”, (symbol: Cal) is the amount of energy needed to raise the temperature of 1 Kg of water by 1⁰C.

The large calorie is thus equal to 1000 small calories or one kilocalorie (Cal = 1000 cal =1 Kcal ).

1 Cal is about 4.2 kilojoules (4.186 KJ), and 1 cal = 4.2 J

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Thermal EquilibriumTwo bodies are said to be at thermal equilibrium if they are at the same temperature. This means there is no net exchange of thermal energy between the two bodies. The top pair of objects are in contact, but since they are at different temps, they are not in thermal equilibrium, and energy is flowing from the hot side to the cold side.

hot coldheat

26 °C 26 °C

No net heat flow

The two purple objects are at the same temp and, therefore are in thermal equilibrium. There is no net flow of heat energy here.

Specific Heat CapacityS.H.C. is defined as the amount of thermal energy needed to raise a unit mass of substance a unit of temperature. Its symbol is C (or s.h.c.).For example, the specific heat of water is : C = 1 cal / (g · ºC), or 4.186 J / (g · ºC), or 4186 J/Kg ºC ≈ 4200 J/Kg ºC) Water has a very high specific heat, so it takes more energy to heat up water than it would to heat up most other substances, of the same mass, by the same amount of temp. Oceans and lakes act like “heat sinks” storing thermal energy absorbed in the summer and slowly releasing it during the winter. Large bodies of water thereby help to make local climates less extreme in temperature from season to season.

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Specific Heat EquationQ = m C T

Q = thermal energy (J, or KJ, or cal, or Cal …) m = mass (g, or Kg …) T = change in temp. (ºC, or ºF, or Kelvin) C = specific heat capacityExample1: The specific heat of silicon is 703 J / (kg · ºC). How much energy is needed to raise the temperature of a 7 kg chunk of silicon by 10ºC ?

Solution:703 Jkg · ºCQ = 7 kg * [ ]*10 ºC = 49 210 J

☺ Visual experiments: Measure Specific Heat Capacity of Ethanol:

http://www.chm.davidson.edu/vce/calorimetry/SpecificHeatCapacityofEthanol.html30

Example2: How much energy does it take to raise the temperature

of 50 g of copper by 10 ºC? Example 3:

If we add 30 J of heat to 10 g of aluminum, by how much will its temperature increase?

Example 4: 216 J of energy is required to raise the temperature of

aluminum from 15o to 35oC. Calculate the mass of aluminum.(S.H.C. of aluminum is 0.90 JoC-1g-1).

Example 5: The initial temperature of 150g of ethanol was 22oC.

What will be the final temperature of the ethanol if 3240 J was needed to raise the temperature of the ethanol?(Specific heat capacity of ethanol is 2.44 JoC-1g-1).

Answers: 192.5 J, 3 ºC, 12 g, 30.9 ºC. 31

Some Materials’ Specific Heat Capacity (J/g ºC)

Water 4.18 6 Air 1.01  Ice 2.03  Glass 0.84 Al 0.385C Aluminum 0.902  Graphite 0.720 NaCl 0.864  Mercury 0.14 Granite 0.79 Fe 0.451 Concrete 0.88 Cu 0.385 Wood 1.76 Au 0.128 C2H5OH (ethanol) 2.46 (CH2OH)2  (antifreeze) 2.42

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Heat (Thermal) CapacityH.C. of an object is the energy required to change the temperature of this object by 1 degree.Equation: orThermal capacity Q, Thermal capacity 1/ TThe SI unit for heat capacity is J/KIt can also be expressed in KJ/K, J/ºC, Cal/ºC …E.g. Night storage heaters (page 106 in your Course book).Exercise: Solve the previous examples (1-5) to calculate the thermal capacity for each object.Answers: 4921 J/ºC, 19.25 J/ºC, 10 J/ ºC, 10.8 ºC, 364 J/ ºC .

Homework: make a comparison table for the thermal (heat) capacity and the s.h.c. (C).

Heat capacity = Q / T Heat capacity = m * C

☺ Visual experiment: T vs. Q to calculate heat capacity: http://www.chm.davidson.edu/vce/calorimetry/heatcapacity.html

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Latent Heat

The word “latent” comes from a Latin word that means

“Hidden.” When a substance changes phases (liquid solid or

gas liquid) energy is transferred without a change in

temperature. For example, to turn water ice into liquid water,

energy must be added to bring the water to its melting point,

0ºC. This is not enough, additional energy is required to

change 0 ºC ice into 0 ºC water. The energy supplied to water

increases its internal energy but does not raise its temp., it

breaks down the bonds between, the particles. This energy is called latent heat of fusion.

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Latent Heat (L)Latent heat of fusion: the energy needed to convert

a solid into a liquid at the melting temperature. Specific Latent heat of fusion: the energy needed

to convert 1Kg of a solid into liquid at the melting temperature.

Latent heat of vaporization: the energy needed to change a liquid into vapor (gas) at the boiling temperature.

Specific Latent heat of vaporization: the energy needed to change 1Kg of a liquid into vapor (gas) at the boiling temperature.

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Latent Heat FormulaS.L.H. = energy supplied / mass i.e. Lf = Q/m & Lv = Q/m, or Q = m Lf & Q = m Lv

Q = thermal energy Units examples: J, KJ, Cal., Kcal. m = mass Kg, g Lf = Specific Latent heat of fusion J/Kg, KJ/Kg, J/g, Cal/Kg Lv = Specific Latent heat of vaporization J/Kg, KJ/Kg, Cal./Kg Example: Gold melts at 1063 ºC, what is the amount of heat needed to melt 5 grams of solid gold at this temp. given that Lf (the specific latent heat of fusion) for gold is 6440 J / kg. ?

Answer: Q = 32 J.

Homework: make a comparison table for the thermal (heat) capacity, the s.h.c. (C), the latent heat & the specific latent heat.

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Specific Latent Heat & Specific Heat

Substance Specific Heat (in J / kg · ºC)Ice 2090Liquid water 4186Steam 1970

Example: Superman can vaporize a 1800 kg ice-monster with his heat ray vision. The ice-monster was at -20 ºC. After being vaporized the steam was heated to 135 ºC. How much energy did Superman expend?

For Water: Lf = 3.33 ×105 J/kg; Lv = 2.26 × 106 J/kg

Information you will need:

ICE-20 ºC

ICE0 ºC

WATER0 ºC

VAPOR100 ºC

WATER100 ºC

VAPOR135 ºC

Heating Melting Heating Boiling Heating S.H.C. Lf S.H.C. Lv S.H.C.

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Solution steps1. The energy needed for heating ice from -20ºC to the melting point:Q1 = m*C*T = (1800 kg) (2090 J/kg·ºC) (20 ºC) = 75240000 J

2. The energy needed for turning ice into water at 0ºC:

Q2 = m*Lf = (1800 kg) (3.33 × 105 J / kg) = 5994 × 105 J

3. The energy needed for heating water from 0ºC to the boiling point: Q3 = m*C*T = (1800 kg) (4186 J/kg·ºC) (100ºC) = 753480000 J

4. The energy needed for turning water into steam at 100 ºC:

Q4 = m*Lv = (1800 kg) (2.26 × 106 J/kg) = 4068 × 106 J

5. The energy needed for heating steam to 135 ºC:

Q5= m*C*T = (1800 kg) (1970 J/kg·ºC) (135 ºC) = 478710000 J

Total energy expended by Superman = 75240000 + 5994 × 105 + 753480000 + 4068 × 106 + 478710000 = (75.24 × 106) + (599.4 × 106) + (753.48 × 106) + (4068 × 106) + (478.71 × 106) = 5974.83 × 106 J 38

Thermal ExpansionAs a material heats up its particles move or vibrate more

vigorously, and the average separation between them increases. This results in small increases in lengths and volumes.

Buildings, railroad tracks, bridges, and highways contain thermal expansion joints to prevent cracking and warping due to expansion.

Factors which affect the expansion of solids:Original length & temperature raise (direct), material type.

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What do you see in these pictures?

What is meant be expansion?It is the difference between the original size of an object and its size when its heated (or cooled).

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Some real-life problems due to expansion On a hot day concrete runway sections in airport

expands and this cause cracking. To solve this problem we leave small gabs between sections.

On a hot day concrete bridges expand. To solve this problem, we leave small gab at one end and support the other end with rollers.

Telephone wire contract on cold days. To solve this problem, we leave wires slack so that they are free to change length.

On a hot day railway lines expand.  To solve this problem, gaps are left between sections of railway lines to avoid damage of the rails as the expand in hot weather.

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Bimetallic Strip

A bimetallic strip is a strip of two different metals — often steel on one side and brass on the other. When heated the strip curves because the metals have different coefficients of thermal expansion. Brass’s coefficient is higher, so for a given temperature change, it expands more than steel. This causes the strip to bend toward the steel side. The bending would be reversed if the strip were made very cold. cold strip

hot strip

handle steel (brass on other side)

Top view

steel side

brass sideSide view

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Thermostats Bimetallic strips are used in thermostats, at least in some

older ones. When the temperature changes, the strip bends, making or breaking an electrical circuit, which causes the furnace to turn on or shut off.

Some applications to thermostat in industry :electric irons, home heating/cooling systems, ovens, refrigerators, fire alarms, fish tanks, car thermostat .

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Expansion of liquidsWhat do you see in the picture below?

Explain what happen when liquids are heated?When a liquid is heated, its molecules gain kinetic energy and vibrate more vigorously.  As the vibration become larger, the molecules are pushed further apart and the liquid expands slightly in all directions.

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List the factors which affect the expansion of liquids?temperature & liquid volume (direct),

liquid type.*The effect of temperature on volume and

density of water:

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Expansion of gasesCompare the expansion of gases to that of solids and liquids?The expansion of gases is much more larger than that of solids or liquids under the same rise in temperature.The effect of temperature on gas volume under constant pressure The volume of a gas is directly proportional to the Kelvin temperature under constant pressure (Charlie's Law).When the temperature of a gas is increased, the molecules move faster and the collisions become more violent thus they spread away from each other causing the volume to increase. 

inside and

outside pressures are balanced.

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Amal Sweis