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Download 9th Class Physics Notes (Unit # 8) for Federal Board (FBISE) Islamabad

Download 9th Class Physics Notes (Unit # 8) for Federal Board (FBISE) Islamabad

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Download 9th Class Physics Notes (Unit # 8) for Federal Board (FBISE) Islamabad


Define the terms heat and temperature.
Heat:
Heat is the energy that is transferred from one body to the other in thermal contact with each other as a result of the difference of temperature between them.
Temperature:
Temperature of a body is the degree of hotness or coldness of the body.

Define the terms thermal conduct and thermal equilibrium.
Thermal Conduct:
In heat transfer and thermodynamics, a thermodynamic system is said to be in thermal contact with another system if it can exchange energy with it through the process of heat.
Thermal Equilibrium:
When two objects A and B are in thermal contact and there is no net transfer of thermal energy from A to B or from B to A, they are said to be in thermal equilibrium.

Define the terms energy in transit and internal energy.
Energy in transit:
The form of energy that is transferred from a hot body to a cold body  is called heat. Thus Heat is therefore called as the energy in transit.
Internal energy:
The sum of kinetic energy and potential energy associated with the atoms, molecules and particles of a body is called its internal energy.

Describe the feature of liquid-in-glass thermometer OR Describe  the feature of mercury-in-glass thermometer?
Liquid-in-glass thermometer:
A liquid in glass thermometer has a bulb with a long capillary tube of uniform and fine bore. A suitable liquid is filled in the bulb. When the bulb contacts a hot object, the liquid in   it expands and rises in the tube.  The glass stem of a thermometer is thick and acts as a cylindrical lens. This makes it easy to see the liquid level in the glass tube.
Uses:
Thus mercury is one of the most suitable thermometric materials. Mercury-in­glass thermometers are widely used in laboratories, climes and houses to measure temperatures in the range from -10oC to 150oC.

What do you mean by lower and upper fixed points in thermometer?
  • A thermometer has scale on its stem. This scale has two fixed points. The lower fixed point is marked to show the position of liquid in the thermometer when it is placed in ice.
  • Similarly upper fixed point is marked to show the position of liquid in the thermometer when it is placed in steam at standard pressure above boiling water.

Describe the importance of large specific heat capacity of water.
Water has a large specific heat capacity. For this reason, it is very useful in storing and carrying thermal energy due to its  high  specific  heat  capacity.  The cooling system of automobiles uses water to carry away unwanted thermal energy. In an automobile, large amount of heat is produced by its engine due to which its temperature goes on increasing. The engine would cease unless it is not cooled down. Water circulating around the engine by arrows in maintains its temperature. Water absorbs unwanted thermal energy of the engine and dissipates heat through its radiator.

Define heat capacity.
Heat capacity of a body is the quantity of thermal energy absorbed by it for one kelvin (1 K) increase in its temperature.

Define fusion point and freezing point?
When a substance is changed from solid to liquid state by adding heat, the progress is called melting or fusion. The temperature at which a solid starts melting is called its fusion point or melting point.
Freezing point:
The temperature at which a substance changes from liquid to solid state is called its freezing point. However, the freezing point of a substance is the same as its melting point.

Define thermal expansion.
Thermal  expansion  is  the  tendency  of  matter  to  change  in volume  in response to a change in temperature.

Derive the relation for linear thermal expansion in solids. OR
Show that L = Lo (1+ α Δ T)
Consider a metal rod of length Lo at certain temperature To. Let its length on heating to a temperature T becomes  L Thus                      •
Increase in length of the rod  =     ΔL     =      L - Lo
Increase in temperature       =      ΔT     =      T- To
It is found that change  in  length ΔL of a solid is  directly  proportional to its original length Lo , and the change in temperature ΔT.  That is;
ΔL     α      Lo ΔT
Or     ΔL     =      α Lo ΔT …………………………….    (i)
Or     L - Lo        =      α Lo ΔT
Or     L       =      Lo  + α Lo ΔT
Or     L       =      Lo (1 + α  ΔT) …………………    (ii)
Where α is called the coefficient of linear thermal expansion of the substance.
From equation (i), we get
α      =      ΔL/Lo ΔT
Coefficient of linear expansion (α):
We can define the coefficient of linear expansion a of a substance as the fractional increase in its length per kelvin rise in temperature.
Why gaps are left m railway tracks?
Gapes are left in railway tracks to compensates thermal expansion during hot season. Railway tracks buckled on a hot summer day due to expansion if gaps are not left between sections.

Why gaps are left in bridges with rollers?
Bridges made of steel girders also expand during the day and contract during night. They will bend if their ends are fixed. To allow thermal expansion, one end is, fixed while the other end of the girder rests on rollers in the gap left for expansion.

Why overhead transmission lines (wires on electric  poles) are also given a certain amount  of sag?
Overhead transmission lines are also given a certain amount of sag so that they can contract in winter without snapping.

List the application of thermal expansion?
i.      In thermometers, thermal expansion is used in temperature measurements.
ii.     To open the cap of a bottle that is tight enough, immerse it in hot water for a minute or so. Metal cap expands and becomes loose. It would now be easy to turn it to open.
iii.     To join steel plates tightly together, red hot rivets are forced through holes in the plates.  The end of hot rivet is then hammered. On cooling, the rivets contract and bring the plates tightly gripped.
iv.     Iron rims are fixed on wooden wheels of carts. Iron rims are heated. Thermal expansion allows them to slip over the wooden wheel. Water is poured on it to cool. The rim contracts and becomes tight over the wheel.

Explain that the bi-metallic strip used in thermostat is based on different rate of expansion of different metals on heating.
A bimetal strip consists of two thin strips of different metals such as brass and iron joined together. On heating the strip, brass expands more than iron. This unequal expansion causes bending of the strip.
Uses of bimetal strips:
  • Bimetal thermometers are used to measure temperatures especially in furnaces and ovens.
  • Bimetal strips are also used in thermostats.
  • Bimetal thermostat switch such is used to control the temperature of heater coil in an electric iron.

What is Anomalous expansion of water?
Water on cooling below 4oC begins to expand until it reaches 0°C. On further cooling  its volume  increases  suddenly  as it changes  into ice at 0°C. When ice is cooled below  0°C, it contracts  i.e. its  volume decreases  like solids. This  unusual expansion of water is called the anomalous expansion of water.


Why the coefficient of volume expansion of liquids is greater than solids?
The molecules  of liquids  are free to move in all directions  within the liquid. On heating a liquid, the average amplitude of Vibration of its molecules  increases. The molecules push each other and need more space to occupy. This accounts for the expansion of the liquid when heated. The thermal expansion in liquids is greater than   solids due to  the  weak  forces   between  their  molecules.  Therefore, the coefficient of volume expansion of liquids is greater than solids.

Explain thermal expansion of liquids OR
Differentiate between real and apparent expansion of liquid? Thermal expansion of liquids:
Liquids also expand on heating. As they do not have any shape, therefore only volume expansion is measured in liquids.
There are two types of expansion of liquids:
i)            Real Expansion
ii)           Apparent Expansion
Real Expansion: -
In figure real expansion is from B to C.

Apparent Expansion: -

When we heat a liquid in a flask. Flask expand and level of water first fall from A to B. then it rises upto C. So expansion from A to C is apparent expansion.
In figure, A to B shows the expansion of flask. Real expansion is greater than apparent expansion.
Real expansion =        Apparent Expansion + Expansion of Flask
Co-efficient of Apparent Expansion: -
The apparent increase in volume of one cubic meter of liquid when heated through one - Kelvin temperature is called co-efficient of apparent expansion. It is denoted by (    g)
Co-efficient of Real Expansion: -
The real increase in volume of one cubic meter of liquid when heated through one-Kelvin temperature is called co-efficient of apparent expansion. It is denoted by (    r)

EXERCISES

Why does heat flow from hot body to cold body?
Heat flows from warm to cold because the energy state is higher.

How does heating affect the motion of molecules of a gas?
The larger the temperature of a gas the faster the molecules will move (temperature is proportional to the average kinetic energy of the particles) and the larger the force they will exert, and the higher the pressure (pressure is the force exerted by the particles divided by the area).


What          is   a  thermometer?  Why   mercury     is       preferred    as   a thermometric substance?
A device that is used to measure  the temperature of a body is  called thermometer.
Principle of thermometer:
Mercury thermometers are based on the fact that materials (in this case, the liquid mercury) expand when heated.
Basic properties of thermometric liquid:
A thermometric liquid should have the following properties:
  • It should be visible.
  • It should have uniform thermal expansion.
  • It should have a low freezing point.
  • It should have high boiling point.
  • It should not wet glass.
  • It should be a good conductor of heat.
  • It should have a small specific heat capacity.
Preference of mercury:
Mercury   has uniform thermal expansion, easily   visible, has low freezing point, and has high boiling point and less specific heat.

Explain the volumetric thermal expansion.
The volume  of a solid  also changes  with the change  in temperature and is called volume thermal expansion or cubical thermal expansion.
Consider a solid  of initial  volume Vo  at certain  temperature To .  On  heating the solid to a temperature T, let its volume becomes  V , then

Change in the volume of a solid    ΔV    =      V – Vo
And
Change in temperature                 ΔT     =      T - To
Like linear expansion, the change  in volume ΔV is found to be proportional to its. original volume  Vo and change in temperature ΔT. Thus
ΔV            α      Vo ΔT
Or     ΔV            =      α Vo ΔT …………………………….    (i)
Or     V - Vo        =      α Vo ΔT
Or     V              =      Vo  + α Vo ΔT
Or     V              =      Vo (1 + α  ΔT) …………………    (ii)

Where α is called the coefficient of linear thermal expansion of the substance.
From equation (i), we get
α      =      ΔV/Vo ΔT

Coefficient of volume expansion (B):
Thus, we can define the temperature coefficient of volume expansion as the   fractional    change   in   its volume   per   kelvin   change   in   temperature.   The coefficients of linear expansion and volume expansion are related by the equation:
B      =      3α

Define  specific  heat. How  would  you  find  the  specific  heat  of  a solid?
Specific  heat  of  a substance   is  the amount  of heat  required to  raise  the temperature of 1 kg mass of that substance  through 1K.
It  has  been  observed that  the  quantity  of  heat  ΔQ required   to  raise  the temperature ΔT of a body is proportional to the mass m of the body. Thus
ΔQ    α      m ΔT
or
ΔQ    =      c m ΔT

Here ΔQ is the amount of heat absorbed by the body and c is the constant of proportionality called the specific heat capacity  or simply specific heat.
Mathematically,

c       =      ΔQ/mΔT

Unit of specific heat:
Sl unit of specific heat is Jkg-1K-1

Define and explain latent  heat of fusion.
Heat energy required to change unit mass of a substance  from solid to liquid state at its melting  point without change in its temperature  is called its latent heat of fusion. It is denoted by Hf.

Define latent heat of vaporization.
The quantity  of heat that changes unit mass of a liquid completely  into gas at its boiling point without any change  in its temperature is called its latent heat of vaporization. It is denoted by Hv.

What is meant by evaporation? On what factors the evaporation of liquid depends?
Evaporation is the changing of a liquid into vapours (gaseous state) from the surface of the liquid without heating it.
Evaporation causes cooling:
Evaporation plays an important role in our daily life. Wet clothes dry up rapidly when spread. During evaporation; fast moving molecules escape out from the surface of the liquid. Molecules that have lower kinetic energies are left behind. This lowers the average kinetic energy of the liquid molecules and the temperature of the liquid.  Since temperature of a substance depends on the average kinetic energy of its molecules.  Evaporation of perspiration helps to cool our bodies.
T  α  K. E

Evaporation takes place at all temperature from the surface of a liquid. The rate of evaporation is affected by various factors.



Factors affecting the rate of evaporation:
i.        Temperature:
Why wet clothes dry up more quickly in summer than in winter? At higher temperature, more molecules of a liquid   re moving with high velocities.  Thus, more molecules escape from its surface. Thus, evaporation is faster at high temperature than at low temperature.
ii.       Surface area:
Why water evaporates faster when spread over large area? Larger is the surface area of a liquid, greater number of molecules has the chance to escape from its surface.
iii.      Wind: 
Wind blowing over the surface of a liquid sweeps away the liquid molecules that have just escaped out. This increases the chance for more liquid molecules to escape out.
iv.      Nature of the liquid:

Does spirit and water evaporate at the same rate? Liquids differ in the rate at which they evaporate. Spread a few drops of ether or spirit on your palm. You feel cold, why?

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Download Class Notes: Download 9th Class Physics Notes (Unit # 8) for Federal Board (FBISE) Islamabad
Download 9th Class Physics Notes (Unit # 8) for Federal Board (FBISE) Islamabad
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