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# Heat - PowerPoint PPT Presentation

Heat. Science – 9th Grade. Module Objectives. Nature of Heat Effects of heat Thermal expansions in solids. Linear expansions in solids Applications of thermal expansions Increase in temperature due to heating Specific heat Thermal capacity Melting Evaporation Latent Heat. .

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### Heat

Nature of Heat

Effects of heat

Thermal expansions in solids.

Linear expansions in solids

Applications of thermal expansions

Increase in temperature due to heating

Specific heat

Thermal capacity

Melting

Evaporation

Latent Heat

• .

• Effects of heat on solids

• Linear expansions

• Coefficient of linear expansion

• Quantity of heat

• Unit of heat of specific heat

• Simple numericals

• Latent heat of fusion

• Latent heat of vapourization

• Applications of the idea of latent heat

• Heat is a form of energy

• The molecules in the body will be in the form of motion.

• The motion could be:

• Vibration

• Rotation

• Displacement

• By the virtue of motion, molecules acquire kinetic energy

• Heat in a body = Sum of the kinetic energy of all its constituent molecules.

• Since heat is a form of energy, it must be able to do work

• It must be possible to convert heat into other forms

Units of Heat:

1 Calorie = 4.18 Joule

Effects of Heat

• Various effect of Heat:

• Thermal Expansion

• Increase in temperature

• Change of physical state

• Thermal Expansion

Object increases in size when heated. Substance expand when temperature increases

Expansion is different for different substances

exception is water: water in temperature range 00C(273K) – 40C(277K) does not expand)

Watch this video to understand the thermal expansion of solids ,liquids and gas: http://www.youtube.com/watch?v=EkQ2886Sxpg

• Three types of thermal expansions

• Experiment 1:

• Stretch and tie a thin metal wire.

• Let the length be 30-40 cms

• Tie it between 2 firm supports

• Let the wire be as tight as possible

• Suspend a weight at the middle of wire

• Use a scale to measure the change

• Heat the wire strongly along its length

• The weight is lowered slowly.

• Allow the wire to cool

• The weight raises.

Reason:

Increase in the length of the solid due to heating is called linear expansion

Note:

When the solid is in form of wire or rod is heated, there will be considerable increase in its length

• The initial length of the rod

• Increase in the temperature

• The nature of the material of the rod

• Consider 3 wires of length 1m, 2m, 3mof same material

• Increase the temperature on the wires equally on the 3 wires.

• The length of the wires increases in the ratio 1:2:3respectively

• The increase in the length of the 2m wire is twice that occurs in 1m wire.

• Similarly the increase in the length of the 3m wire is thrice that occurs in 1 m wire

• Consider 3 wires all of length 1m

• Assume that the temperature of each wire is increased as 100c, 200c, 300c

• The Ratio of the increase in the lengths would be in the ratio 1:2:3

• Thus the length of the rod increases as the same ratio as the increase in temperature.

• Consider 3 wires all of same length and thickness

• Assume that the same temperature is applied to each wire

• Let the wires be made of different materials

• Initial length of the wire = l1

• Initial temperature of the wire = t1

• Length of the wire after applying the temperature =l2

• Temperature of the wire after applying the temperature =t2

• Increase in the length of the wire = (l2 –l1)

• Increase in the temperature of the wire = (t2-t1)

• Hence,

• Increase in the length of the rod is directly proportional to the initial length of the rod

• i.e, (l2-l1) αl1 ……. (i)

• Also, increase in the length of the rod is directly proportional to the increase in its temperature

• i.e, (l2-l1) α(t2-t1) ….. (ii)

• Combining (i) and (ii)

• (l2-l1) αl1 (t2-t1) or (l2-l1) = αl1 (t2-t1)

• α=(l2-l1) / l1(t2-t1)

• Here αis a constant of proportionality and is called the coefficient of linear expansion

Coefficient of linear expansion

• Expansion in length of different materials is different, when their temperatures are increased by same amount

• Thus, it is desirable to have a quantity that can explain the linear expansion of a substance in absolute terms

• The coefficient of linear expansion is such a quantity

• α=(l2-l1)/l1(t2-t1)

• Let l1= 1m and (t2-t1)=10c

• Then, α=(l2-l1)

• Ie, when the length of the rod is 1m, Increase in its length when the temperature is increased by 10c is called coefficient of linear expansion of that solid.

Coefficient of linear expansion with example

• Unit of Coefficient of linear expansion is expressed as “per degree celsius” written as “c-1”

• In international system the unit is written as K-1

• Example:

• Coefficient of linear expansion

of iron is 12X10-6 0C-1

• Ie, when temperature is

increased by 10cto a iron rod

of length 1m, the iron rod

increases its length by 12X10-6

• A rod of iron of length 50cms becomes 50.12cm when the temperature is increased from 120c to 2120c. What is the coefficient of linear expansion of iron?

• Initial length of the iron rod l1= 0.50m

• Final length of the rod l2 = 0.5012m

• Change in temperature (t2-t1) = 2120c – 120c = 2000c

• What is α?

• α= (l2-l1)/l1(t2-t1)

• α= (0.5012-0.50)/0.50(200)

• α=0.0012/100 0C-1

• α=0.000012 0C-1

• α=12X10-6 0C-1

• coefficient of linear expansion of iron=12X10-6 0C-1

• There are 2 lines on a glass rod. The distance between these markings is 100cm at 100c and 100.09cm at 1000c. Find the coefficient of linear expansion of glass.

• Initial distance between markings l1=100cm

• The final distance between markings l2=100.09cm

• Increase in temperature (t2-t1) = 1000c -100c = 900c

• coefficient of linear expansion of glass α?

• α = (l2-l1)/l1(t2-t1)

• α = (100.09-100)/100(90)

• α = 0.09 / 9000 0c-1

• α = 0.000010c-1

• α = 10X10-6 0c-1

• coefficient of linear expansion of glass= 10X10-6 0c-1

Coefficient of superficial expansion and coefficient of volume expansion

• Similar to coefficient of linear expansion we can also define coefficient of superficial expansion and coefficient of volume expansion just as we defined coefficient of linear expansion.

• Units of coefficient of superficial expansion and coefficient of volume expansion are also 0C-1 or K-1

• Looping of metal pipes

• In industries hot liquids or hot water is transported through metal pipes

• When hot liquids are carried, the pipes may expand or contract frequently

• So the pipes develop stress and crack.

• Looping reduces stress caused by thermal expansion

• A Thick glass develops crack when boiling water is poured

• Glass is a poor conductor of heat.

• Heat does not flow easily through it

• When the boiling water is poured into glass, the inner surface expands more than outer surface.

• Hence the glass develops cracks due to stress developed.

• Explain why hot glass plate cracks when water is sprinkled over it?

• If the lid of the bottle is tight and difficult to open, we can use the principle of thermal expansion. Explain how?

• Thermal Expansion coefficient of metal is greater than that of glass

• Hence when a stream of water is poured at the center of the lid, tends to loosen the lid

• Roads are constructed by placing large concrete slabs one beside the other

• While placing the slabs a small gap is left between the slabs

• Reason for this?

• During summer, due to thermal expansion the slabs would expand and so there can be cracks. To avoid this small gaps are left between slabs.

. Thermostatswitches

• Devices that automatically break an electric circuit and help the maintenance of a stable temperature is called thermostat switches

• A thermostat consists of a bimetallic strip

• Bimetallic strip bends when the temperature exceeds a certain limit and hence the circuit breaks.

• As the temperature decreases, the bimetallic strip bends back and the circuit is again complete.

• This process repeats enabling the maintenance of a stable temperature.

• Increase in temperature is one of the effects of heat

• Ie, when the body is heated the temperature of the body increases

• When heat is given to the body, the average kinetic energy of the molecules increases.

• When heat is removed, the average kinetic energy of the molecules decreases.

• The change in the temperature of the body involves change in the kinetic energy of the body.

• This means the body either looses or gains heat

Take 3 beakers of water.

Beaker 1 has 50 ml of water

Beaker 2 has 75ml of water

Beaker 3 has 100ml of water

Heat Beaker 1 with a lamp .

Let the amount of time taken to boil beaker 1 be X

Heat Beaker 2 with a lamp .

Let the amount of time taken to boil beaker 2 be Y

Heat Beaker 3 with a lamp .

Let the amount of time taken to boil beaker 3 be Z

What do you infer?

Amount of time taken to boil beaker 3 is greater than the amount of time taken to boil beaker 2.

Amount of time taken to boil beaker 2 is greater than the amount of time taken to boil beaker 1.

Thus the quantity of heat required to increase the temperature by equal amount depends on the mass of the substance

Take 3 beakers of water.

Let all the Beakers have equal amount of water

Measure and note the initial temperature.

Heat the first beaker with spirit lamp for 5 mins

Note the temperature

Heat the second beaker with the lamp for 10 mins

Note the temperature

Heat the third beaker with spirit lamp for 15 mins

Note the temperature

What do you infer?

The beaker that was heated the longest time shows highest increase in temperature

We have given more heat to that beaker that was heated longer

Take 3 beakers with same level of contents in it.

Beaker 1 has water

Beaker 2 has sunflower oil

Beaker 3 has glycerine

Let the masses of the liquid and initial temperature be the same

Heat the 3 Beakers with a spirit lamp for 10 mins .

Note the temperature of the liquids at the end of 10 mins and compare the temperature of the liquids.What do you infer?

The masses of the 3 liquids are the same.

The quantity of heat given is also the same.

Nature of the substance is different.

The increase in the temperature of the substance depends on the nature of the substance.

Mass = m

Heat given = Q

If there is an increase in the temperature then,

t α Q/m

Q αmt

Q = sXmt

Q = mst

Here ‘s’ is a constant.

It is called “specific heat of the substance

• Consider different substances of equal mass at the same temperature – e.g., water, copper, tungsten

• Increase the temperature of each by 10C

• Will all the materials require the same amount of heat?

NO

It is specific to the nature of the substance – expressed as specific heat

Formula for specific heat:

Where, Q = quantity of heat required, m = mass, t = increase in temperature, s = specific heat

• Specific heat,

• If m = 1 kg, t = 10C,

s = Q = quantity of heat required to increase the temperature of 1kg of a substance by 10C is the specific heat of that substance

Unit of specific heat = JKg-1 0C-1

SI unit of specific heat = JKg-1K-1

What are some other units of specific heat that you can think of?

• One point to note: specific heat of a substance is different at different temperatures

• The specific heat values in the table are the average specific heat of the substance

Specific heat of different substances (cont.)

• What do these values of specific heat in the table mean?

For e.g., 1 kg of copper requires 387 joules of heat energy to increase its temperature by 10C

• 3 blocks of copper, glass and iron of equal mass are placed in the sun. After a few hours, which one will be the hottest?

• Try this experiment

• fill 3 cups, with equal masses of iron powder, saw dust, sand - and place them in the sun. After 1 hour, which is the hottest and why?

Specific heat of water

• Water has a high specific heat value - what does this mean?

• It simply means that water absorbs a lot of heat before it gets hot, and when cooling, lets out the heat slowly

• The high specific heat of water is useful to us and the environment in many ways:

• it helps in regulating extremes in the environment –

the temperature of the water will stay relatively the same

from day to night in ponds - it wont become very hot during

the day or very cold at night

• car radiators - used as a coolant

• Water is circulated throughout the engine where

it absorbs the heat. This water is then pumped to a

radiator where the heat is released to the metal

core of the radiator, which then releases the heat

to the surrounding air.

Specific heat of water

• Land and sea breezes

- in summer, temperature is lower near water bodies when compared to a distant place

- in winter, water gives up more heat making surrounding region relatively warmer

• in cold regions, hot water is used to keep the house warmer

• Hot water is used to extinguish certain types of fires

• Watch this video:

Specific heat and physical state

• Specific heat of a substance changes with a change in its physical state

• The specific heat of steam and ice is about half that of water at the same temperature

• This is why ice melts quickly, but water requires a lot of heat to be converted to steam.

• This is also why steam can quickly lose heat and condense into water

• Take 3 cylinders of equal mass and same area of cross section each made up of - aluminium, iron, copper

• Which cylinder penetrates deepest and which, the least? Why?

• This is due to the thermal capacity - capacity of a substance to hold heat

• Thermal capacity is different for different substances

• "The amount of heat required to increase the temperature of a substance by 10C is called its thermal capacity“

• Thermal capacity/Heat capacity,

where m = mass of the substance, s = specific heat

sublimation

heating

• Can a solid bypass the liquid state, and directly change to the gaseous state on heating?

• Yes – Sublimation. Eg. Iodine,

dry ice(solid carbon dioxide – seen in

the picture)

Small pellets of dry ice subliming in air

heating

heating

gas

solid

liquid

melting

Evaporation/vaporization

Removal of heat

gas

Removal of heat

liquid

solid

condensation

deposition

Removal of heat

• When a substance changes its physical state, will the structure of its molecules change?

• No – then what changes?

• attraction and distance between molecules

• kinetic energy of the molecules

• The specific temperature at which the substance changes from solid to liquid state is called "melting point" of that substance

• Why does a substance liquefy on heating?

• Solid state – molecules are rigid, have definite position, vibrate in that position.

On adding heat, kinetic energy of molecules increases.

• Liquid – as more heat is applied to the solid, attraction between molecules becomes lesser than a certain minimum and they move freely – liquid state. Molecules are still attached to one another and hence, they have definite volume and have to be contained

• Gas – attraction between molecules is even lesser than liquids, and they need a container with a lid

• Put a few drops of petrol or alcohol on your hand and wait for a few minutes? What do you observe?

• Place equal quantities of water in a beaker and an earthen pot. Cover the mouths with a piece of cardboard and place in the sun.

• Measure the temperature after an hour - what do you

observe?

• The water in the earthen pot is cooler than the water

in the beaker.

• The earthen pot has many pores from which water

seeps and evaporates.

• As it evaporates, it cools the pot and the water inside

it.

• This is why many people drink water from earthen pots

during summer, due to this cooling effect

• The conversion from liquid to vapor state is known as evaporation

• The rate of evaporation increases as temperature increases, and at a certain temperature, the liquid starts boiling

• The fixed temperature at which a liquid starts boiling is called the boiling point of the liquid

• Increasing rate of evaporation – methods:

Blowing air

• It removes the water vapor crowded

near the surface of the water and makes

room for more evaporation.

• This is why the laundry dries faster on a

windy day

• Increasing rate of evaporation – methods:

• Increasing the heat or boiling the liquid

increases kinetic energy of the liquid.

Molecules and they easily transition to

vapor state

• Decreasing the pressure

The boiling point of a liquid is the temperature at

which its vapor pressure matches the external

pressure. If you lower the external pressure, the

vapor pressure necessary for boiling - and thus the

temperature necessary for boiling - drops.

• Increasing rate of evaporation – methods:

• Increasing the surface area of the liquid - with a larger surface area, more molecules have a chance to make it into the vapor phase

• Melt a few pieces of wax in a beaker, and after it has melted, let it cool.

• After it has solidified, observe the surface of the wax – there is a small depression in the middle. What could be the reason for this?

Latent Heat

• Melt ice in a beaker and note the temperature as heat is added.

• Ice melts, but does the temperature increase?

NO

Temperature remains constant at 00C. What happens to the heat that is given to the beaker?

Latent Heat

• Is the heat disappearing? This goes against the principle of conservation of energy.

• What is actually happening to the heat?

• The heat is used to change the physical state of ice – it is used to convert the ice from the solid to the liquid state

• Hence, it remains hidden (latent) without manifesting as an increase in temperature.

• “The quantity of heat required to completely change 1 kg of a substance from solid state to liquid state without increasing its temperature is called the latent heat of melting of that substance”

Unit of latent heat of melting: joules/kilogram (or) kilojoules/kg

[1 kilojoule = 1000 joules]

Latent Heat

• Latent heat of melting of ice = 336 kj/kg

• 336 kilojoules of heat is required to convert 1 kg of ice fully from solid to the liquid state

• Joseph Black, a professor of chemistry, pharmacology and physiology discovered the phenomenon of latent heat in 1756

• 3 year later, he succeeded in measuring the latent heat of steam

• Joseph Black also established that different substances have different thermal capacities

Latent Heat

• James Watt, who worked as a philosophical instrument maker in the same university used Joseph Black’s discovery to develop the improved steam engine.

Latent Heat

• Consider the experiment:

• As the heat is increased, the water starts boiling at a definite temperature.

• Continue heating. Does the temperature of the water increase?

Latent Heat

• The temperature does not increase because the heat added at its boiling point is utilized to change the state of the water from liquid to vapour state.

• It is specific for a given substance and is different for different liquids

• “The heat required to change 1 kg of a substance from liquid to vapour state at its boiling point is called the latent heat of vaporization of the substance”

• Latent heat of vaporization of water = 2260 kJ/kg

• Measuring latent heat of vaporization:

where, m = mass of the substance, L = latent heat of vaporization, Q = heat required to vaporize the entire mass at its boiling point

Latent Heat

• When you eat ice cream or ice candy, you feel terribly cool.

• But you don’t feel as cold, when you are drinking ice cold water.

• Heat is required by the ice cream/candy to liquefy it. The heat lost from your body is not used to increase the temperature of the ice cream. For cold water, the heat from your body is used to increase the temperature of water.

Latent Heat

• The changes in temperature in ice when it is heated in a vessel is represented by the graph below:

• Which needs more heat?

• Increasing the temperature of 20L of water from 200C to 750C

• Increasing the temperature of 100L of water from 200C to 300C

[Hint: Q = mst]

• A glass contains 150 ml of boiling water at a temperature of 1000C.

A tank contains 150 liters of water at 200C.

Which contains more heat?

[Hint: The same reason why there is more heat in an iceberg than a pot of boiling water]

• Which of these will cause severe burn injuries:

Water at 1000C or steam at the same temperature

• Can ice and boiling water exist at the same temperature?

Salt is often used in the winter to melt ice on the roads. Why?

(For the same reason, a mixture of ice and salt is used in making ice cream)