Heat Science class-10

HEAT

Heat Energy:

The total sum of kinetic energy of the molecules contained in a body is called the heat energy. It flows from a hot body to a cold body.

 

The quantity of heat present in an object depends on:

i) The number of atoms of the matter. i.e. it’s mass.

ii) The average kinetic energy of the molecules.

 

Unit of heat energy:

Heat is measured in Joule in SI system. But in CGS system, heat is measured in calorie.

So, 1 calorie = 4.2 Joules

 

One Calorie heat:

The heat energy required to raise the temperature of one gram of pure water by 1°C or 1K is called one calorie heat.

 

One Joule heat energy:

One Joule of heat energy can be defined as the amount of heat energy required to raise the temperature by 1°C of an object having mass kg and specific heat capacity of 1 J/kg°C.

 

Effects of heat:

i) Heat changes the volume of a body.

ii) Heat changes the physical state of a body.

iii) Heat changes the temperature of a body.

iv) Heat changes the solubility of a substance.

v) Heat brings chemical change in a body.

 

Importance of heat:

i) It is used for cooking food, drying grains, wet clothes.

ii) It is used for running vehicles, machines in industries and water cycle.

iii) It causes wind.

iv) It is necessary to run different life processes normally in human body, animals and plants.

 

Temperature:

The measurement of hotness or coldness of an object is called temperature. The intensity of molecular vibration can be said to be temperature. The normal temperature of the human body is 37°C (98.6°F). SI unit of temperature is Kelvin (K). The temperature of a body is measured by using a thermometer.

 

The relation between temperature and kinctic energy of the molecules of a body:

There is directly proportional relationship between intensity of molecular vibration and temperature.

i.e. Higher the kınetic energy, more the temperature and vice-versa.

So, Kinetic energy ∝ Temperature

 

Thermometer:

A device which is used for measuring the temperature of a body is called thermometer. Generally mercury or colored alcohol is used in it as liquid.

 

Principle of thermometer:

The thermometer is made on the basis of expansion of the substance when it is heated. In other words, increase in the volume of a liquid is directly proportional to the rise in temperature.

 

Advantages of mercury as a thermometric liquid:

a) Mercury is a good conductor of heat. It remains in liquid state in the wide range of temperature.

b) It has uniform rate of expansion and contraction.

c) It does not stick to the inner wall of the capillary tube because it is not adhesive.

d) It is silvery white in colour, So, it can be seen easily in the capillary tube and easy to read.

e) The freezing point of mercury is -39°C and its boiling point is 357°C. i.e. it remains in heat liquid state over a large range.

 

Disadvantage of mercury as a thermometric liquid:

The heezing point of mercury is -39°C, So, a mercury thermometer cannot be used measuring the temperature below -39°C.

Therefore, mercury thermometer cannot be used measure the temperature in very cold regions.

 

Advantages of alcohol as a thermometric liquid:

a) The freezing point of alcohol is -115°C. Due to this reason, alcohol thermometer car used to measure very low temperature in very cold regions.

b) It is cheaper than mercury.

c) Its expansion rate is about six times more than that of mercury, So, it is highly sensitive liquid.

 

Disadvantage of alcohol as a thermometric liquid:

a) It is a bad conductor of heat and it has low density.

b) Its rate of expansion and contraction is not uniform.

c) It sticks to the inner wall of the capillary tube. So, it cannot measure the accurate temperature of the objects.

d) The boiling point of alcohol is 78°C. So, it cannot be used for measuring the temperature above 78°C. Due to this reason, the alcohol thermometer is not used to measure temperature in very warm places.

 

Importance of thermometer:

a) It is used to measure temperature of a body.

b) It is used to note the maximum and minimum temperature of the environment.

c) It is used to know and note the temperature of the water in the aquarium.

d) It is used to measure the human body temperature.

e) It is used to measure the room temperature.

 

Disadvantage of water as a thermometric liquid:

Water is not used as a thermometric liquid in a thermometer because :

a) Pure water is a bad conductor of heat.

b) It sticks to the inner wall of capillary tube.

c) Its expansion rate is not uniform.

d) It vaporizes easily.

 

Relation between Celsius, Fahrenheit and Kelvin scales:

(C-0)/100 = (F-32)/180 = (K-273)/100

Where,

C = Celsius (or centigrade) scale

F = Fahrenheit scale

K = Kelvin (or absolute) Scale.

 

 

Types of thermometers:

a) Clinical thermometer

b) Digital thermometer

c) Maximum and minimum thermometer

d) Laboratory thermometer

 

 

Clinical thermometer:

The thermometer used to measure human body temperature is called the clinical thermometer. They are available in both Celsius and Fahrenheit scales. The Celsius clinical thermometer’s scale extends from 35°C to 42°C while the Fahrenheit clinical thermometer’s scale extends from 4°F to 108°F. In this thermometer mercury is used as thermometric liquid.

 

Maximum and minimum thermometer:

The thermometer that is used to measure the maximum and minimum atmospheric temperature of 24 hours of a particular place is called maximum and minimum thermometer. In this thermometer mmercury is used as thermometric liquid.

 

Laboratory thermometer:

A thermometer used to measure the temperature of different bodies of the laboratory is called Iaboratory thermometer. Mercury is used as thermometric liquid. The temperature scale has a wide range from -10°C to 110°C in Celsius scale.

 

Specific heat capacity:

The amount of heat energy required to increase the temperature of an object of 1 kg mass by 1°C (or 1K) is called the specific heat capacity of that object. Its SI unit is J/kg°C.

 

Advantages of highest specific heat capacity of water:

i) Hot water bag is used during injury.

ii) Water is used as coolant. i.e. to cool the machines and hot engines.

iii) During high fever a cioth souked with water is used on forehead.

 

The relation between specific heat capacity and increasing temperature:

The specific heat capacity of a substance is inversely proportional to the temperature.

Specific heat capacity ∝ 1/Temperature

 

Heat equation:

The quantity of heat lost or gained by the object is equal to the product of its mass, specific heat capacity and change in temperature, This is called heat equation.

i.e. Q = m*s*dt

 

The quantity of heat energy lost or gained by an object is equal to the product of the following things:

i) Mass of the object (m)

ii) Specific heat capacity of the object (s)

iii) Change in the temperature of the object (dt)

 

Calorimetre:

The instrument which is used to measure the heat lost or gained by an object is called calorimetre.

 

Calorimetry:

Calorimetry is defined as the measurement of heat lost or gained by an object.

 

Principle of calorimetry:

The heat loss due to radiation being neglected, the quantity of heat energy lost by one object is equal to the quantity of heat energy gained by other objects.

i.e. The quantity of heat energy lost by one object = The quantity of heat energy gained by other objects.

 

 

 

 

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