Thermal Properties of Matter Physics Class 9th Notes Chapter 8. Heat is the form of energy transferred between two (or more) systems or a system and its surroundings by virtue of temperature difference.
Physics Class 9th Notes Chapter 8 Thermal Properties of Matter
Q.1) Ordinary electric fan increases the kinetic energy of the air molecules caused by the fan blades pushing them means the air temperature increases slightly rather than cool the air? why use it?
Answer:
Ordinary fans work on the principle of convection. The moving air increases the rate of evaporation or perspiration from our bodies, so we get a cooling sensation. The flowing air takes away the heat from the body and is replaced with the colder air.
Q.2) Why are small gaps left behind the girders mounted in walls?
Answer:
Usually, small gaps are left behind the girders mounted in walls to give room for expansion. One end of the iron is fixed while the other end is allowed to expand into the left out gaps in summer.
Q.3) Why you should not put a closed glass jar into a campfire. What could happen if you tossed an empty glass jar, with lid on tight, into fire?
Answer:
A closed jar in a campfire tends to increase the kinetic energy of the air molecules inside the jar. As a result, the air expands, putting pressure on the walls and the lid of the jar. Consequently, the jar could break or it could even explode.
Q.4) Explain why it is advisable to add water to an overheated automobile engine only slowly, and only with the engine running.
Answer:
If we add water quickly to an overheated engine, water comes into contact with the hot metal part of the engine. Metals tend to contract on cooling and expand on heating. Some area of the metal part will cool down very rapidly, while other parts will not. This could cause a crack in the engine or could even cease the pistons.
So water should be added slowly with the running engine. This will create a gradual and uniform rate of decrease in the temperature of the engine.
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Q.5) Explain why burns caused by steam at 100 °C on the skin are often more severe than burns caused by water at 100 ° C?
Answer:
The steam produces more severe burns than boiling water at 100°C because steam has more latent heat. As water absorbs extra heat to change from the liquid state into its gas state (steam) that is up to 100°C, the heat raises the temperature of the water. But at 100 °C, as the water changes to steam, more heat is added without changing the temperature.
That extra heat is called the heat of vaporization, it is stored in the steam as a latent heat that will be released when the steam liquefies. And that extra heat is passed on into your body as the steam condenses on your skin. That way, the steam produces more severe burn effects than boiling water.
Q.6) Explain why cities like Karachi situated by the ocean tend to have less extreme temperatures than inland cities at the same altitude.
Answer:
Karachi is situated near a sea shore. The specific heat of water is five times the specific heat of the land. It means that the heat require to increase 1oC or 1 K temperature of land is less than that of water. Hence, the land gets heated much more easily than water, it also cools down easily. Hence a large temperature difference arise that give rise to land breeze and sea breeze, which keeps the temperature of the coastal areas moderate.
Q.7) An iron rim that is fixed around a wooden wheel is heated before its fixture. Explain why?
Answer:
In order to make a tight fit, the diameter of the iron rim is made slightly lesser than the diameter of the wooden wheel. When the iron rim is heated, it expands on heating and can be placed around the wooden wheel. When the iron ring cools down it contracts in this way, it produces a tight fit
Q.8) Why is ice at 0oC a better coolant of soft drinks than water at 0oC?
Answer:
The ice is a better coolant of soft drinks than water at 0°C because the latent heat of fusion of ice is large. Which is the amount of heat absorbed by a solid body to convert 1 kg of a substance from solid to liquid. So the heat extracted from the soft drink by ice is more than the heat extracted by water.
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Q.9) Why we feel cool after perspiration ?
Answer:
We feel cool after perspiration because during evaporation more energetic molecules escape from the liquid surface. While molecules with less K.E remain in the liquid, that’s why the cooling effect is produced due to evaporation.
Comprehensive Questions Physics 9th Notes 2021 New
Q.1) Explain the term internal energy and temperature. Use kinetic theory to distinguish between heat, internal energy and temperature.
Answer:
Internal Energy:
"Internal Energy is defined as the sum of kinetic and potential energies associated with the motion of the atoms of substance".
In a monoatomic gas, such as helium, the atoms move around, randomly colliding with each other and the walls of the container. So each atom has some translational kinetic energy. Whereas in a diatomic gas the molecules can also stretch, contract and spin, such molecules can have vibrational and rotational kinetic energy.
In a liquid, molecules are free to move but within the confines of the surface of the liquid. They have some attraction between the molecules, which means that they have some potential energy stored.
In a solid, the molecules are closely packed and move around their mean position, so they have very small kinetic energies. But they have strong potential forces of attraction between their molecules.
Temperature:
"Temperature is the measure of the average kinetic energy of the particles".
It measures the degree of hotness and coldness of a body with respect to some standard. The other contribution of potential energy does not affect the temperature. The kinetic energy may be in the form of translational, vibrational, and rotational kinetic energy. The kinetic molecular theory says that the molecules are always in constant motion. The kinetic energy increases as the temperature rises and vice versa.
Heat:
"Heat is defined as the thermal energy transferred from a hot body to a colder body".
When two objects at different temperatures are held in thermal contact, the temperature of the warmer object decreases while the temperature of the colder object increases. An equilibrium point is reached when both objects attain the same temperature, which is called thermal equilibrium. During this process, the energy is transferred from the warmer object to the colder one.
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Difference between heat, internal energy, and temperature:
Using kinetic molecular theory, we can make a clear distinction between heat, internal energy, and temperature.
i) Temperature is the measure of the average kinetic energy of the individual molecule,
ii) Internal Energy is the measure of the total energy of all molecules within an object.
iii) Heat refers to the transfer of energy from one body to another because of the difference in temperature.
Q.2) How do we measure temperature? Explain liquid in a glass thermometer.
Answer:
Measurement of temperature:
Temperature could be measured in a simple way by using our hand to sense the hotness or coldness of an object. However, the range of temperature that our hand can bear is very small, and our hand is not precise enough to measure temperature correctly. Temperature is measured by a thermometer. A thermometer is put in thermal contact with the body whose temperature is to be measured.
The branch of Physics deals with the measurements of temperature is called thermometry.
Liquid in glass thermometer:
Principle:
The liquid used in a glass thermometer utilizes the principle of variation in volume due to temperature. They use the fact that most fluids expand on heating.
Construction of the thermometer:
The fluid is contained in a sealed glass bulb, and its expansion is measured using a scale etched in the stem of the thermometer. If we consider that the thermometer does not expand then as physical property it utilizes the variation of length of liquid with temperature.
Working of the liquid:
The liquid in a glass bulb expands up a cappilary tube when the bulb is heated. The liquid must be easily seen and must expand (or contract) rapidly and by a large amount over a wide range of temperature. It must not stick to the inside of the tube otherwise the reading will be too high when the temperature is falling.
Commonly used liquids are mercury and alcohol.
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Q.3) What are various temperature scales. Derive mathematical expression to convert between various classes of temperature.
Answer:
Scales of temperature
There are three scales of temperature which are as follow.
a) Celsius or Centigrade scale
b) Fahrenheit scale
c) Kelvin or Absolute scale
Celsius or Centigrade scale (°C)
i) It is the simplest scale of temperature out of all other scales. It is denoted by °C.
ii) The freezing point of water on this scale is 0°C.
iii) The boiling point of water on this scale is 100°C.
iv) The interval between freezing point and boiling point of water is divided into 100 equal parts, each part is called a degree centigrade.
Fahrenheit scale (°F)
i) This scale was introduced by Fahrenheit. It is dented by °F.
ii) The freezing point of water on this scale is 32°F.
iii) The boiling point of water on this scale is 212°F.
iv) The interval between ice point and boiling point is divided into 180 equal parts, each part is called a degree Fahrenheit.
iv) Mostly, it is used for the measurement of the temperature of humans.
Kelvin or Absolute scale (K)
i) This scale was introduced by Lord Kelvin. It is denoted by K.
ii) The freezing point of water i=on this scale is 273K.
iii) The boiling point of water on this scale is 373K.
iv) The interval between freezing point and boiling point is divided into 100 equal parts, each part is called one Kelvin.
Q.8) What is meant by evaporation? On what factors the evaporation of a liquid depends. Explain how cooling is produced by evaporation?
Answer:
Evaporation definition
"The process by which a liquid slowly change to vapors at any temperature without the aid of any external source of heat is known as evaporation".
Factors affecting the rate of evaporation
i. Nature of liquid
ii. Temperature of liquid and the surrounding
iii. Temperature of surrounding
iv. Area of the surface of liquid
v. Presence of water vapors in air
vi. Movement of air
vii. Dryness of the air
viii. Air pressure on the surface of the liquid
i) Nature of liquid
Liquids with low boiling points evaporate quickly than those with high boiling points. For example, the rate of evaporation of alcohol is higher than water because the boiling point of alcohol is lower than the boiling point of water.
ii) Temperature of liquid
The rate of evaporation depends upon the temperature of the liquid. The higher the temperature of the liquid greater will be the rate of evaporation and vice versa. For example, when we push hot iron upon wet clothes, they dry out more quickly and easily. It is because due to high temperature, the molecules of water can evaporate easily in the hot summer season as compared to winter.
iii) Temperature of surrounding
The rate of evaporation depends upon the temperature of the surrounding. Higher the temperature of the surrounding higher will be the rate of evaporation and vice versa. For example, wet clothes dry out quickly in the hot summer season as compare to winter.
iv) Area of the surface of the liquid
The rate of evaporation depends upon the area exposed surface of the liquid. The greater the area of exposed surface greater will be the rate of evaporation and vice versa. This is because the chance of escaping the molecules from the liquid surface is greater.
v) Presence of water vapors in air
The higher the number of water vapors in the air, the smaller will be the rate of evaporation and vice versa. For example, wet clothes dry slowly in the rainy season because a lot of water vapors are present in the air.
vi) Movement of air
The rate of evaporation depends upon the movement of air. Greater the speed of flow of air, the higher will be the rate of evaporation and vice versa. That’s why wet clothes dry out more rapidly on a windy day.
vii) Dryness of the air
Drier the air, more rapid will be the evaporation and vice versa. For example, wet clothes dry quickly on a dry day as compared to a humid day.
viii) Air pressure on the surface of the liquid
The rate of evaporation depends upon air pressure on the surface of the liquid. The greater the air pressure on the surface of the liquid, the lesser will be the rate of evaporation and vice versa.
The cooling effect of evaporation
According to kinetic molecular theory, every liquid contains molecules that possess different kinetic energy. Molecules having more energetic molecules escape from the liquid surface while less energetic molecules remain in the liquid.
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When a liquid evaporates, its molecules convert from the liquid phase to the vapor phase and escape from the surface. The process that drives it is latent heat. In order for the molecule to leave the liquid surface and escape as a vapor, it must take heat energy with it. Since the molecule is taking heat with it as it is leaving, this has a cooling effect on the surface left behind. For example, a spirit spilled on your palm quickly evaporates. As a result, your palm feels cold. Water evaporates much slower than ether and spirit. Evaporation of water also produces cooling. You can feel the chilling effect of the evaporation of water if you sit under a fan and wearing wet clothes. Perspiration in a human body helps to cool the body and to maintain a stable body temperature.
The kinetic theory explains the cooling caused by evaporation. During evaporation, more energetic molecules escape from the liquid surface. Molecules that remain in the liquid have lower kinetic energy. A liquid with molecules of less kinetic energy has a lower temperature. Thus evaporation produces cooling.
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