HEAT DEFINITION | KIRCHHOFF’S LAW

HEAT

HEAT | PHYSICS |~GENERAL STUDIES 4 U
HEAT | PHYSICS
 

Heat is a form of energy which produces in us the sensation of warmth.Air is a non-conductor of heat.Heat is due to the kinetic energy of the molecules constituting the body.Heat’s units are calorie,kilo calorie or joule.
Calorie It is the amount of heat required to raise the temperature of 1 g of water through
10 C (from 100 C to 110 C).It is represented by cal. 1 cal = 4.18 J

TEMPERATURE

An instrument used to measure the temperature of a body is called a thermometer.
Mercury is generally used as thermometric substance,because it is not easily obtained in nature in pure state and clearly visible in capillary tube but has fairly uniform expansion with increase of temperature.

Celsius scale The melting point of ice at standard atmospheric pressure is regarded as 00 C and the boiling point of water as 1000 C.This scale was designed by Anders Celsius, in the year 1710.

Fahrenheit scale The melting point of ice is regarded as 32F and the boiling point of water as 212F.This scale was designed by Gabriel Fahrenheit in the year 1717.

Reaumer scale The melting point of ice is regarded as 00 R and the boiling point of water as 800 R. This scale was introduced by RA Reaumer in year 1730.

The normal temperature of a human body is 370 C or 98.4 F.
-400 is the temperature at which Celsius and Fahrenheit thermometers read same.

TOTAL RADIATION PYROMETER

Radiation pyrometer measures the temperature of a body by measuring the radiation emitted by the body.It cannot measure temperature below 8000 C because at low temperature,emission of radiation is very small and cannot be detected.Sun’s temperature is measured by it.

SPECIFIC HEAT 

It is the amount of heat required to raise the temperature of a unit mass of the substance by 10 C.
It is usually represented by C.
SI unit of specific heat is joule per kilogram per degree Celsius.It may be written J/kg0 C.
Specific heat of gold = 130 J/kg0 C, specific heat of water = 4180 J/kg0 C.
The specific heat of water is maximum.

THERMAL CAPACITY

It is the amount of heat required to raise the temperature of a body through 10 C.Its units are J/0 C or cal/0 C or kcal/0 C.

WATER EQUIVALENT

It is the mass of water, which absorbs or emits the same amount of heat as is done by the given body for the same rise or fall in temperature.
The water equivalent of a body is denoted by W and its unit is kg in SI system and g in CGS system.

PRINCIPLE OF CALORIMETRY

Calorimeter is a cylindrical vessel made of copper.
The heat lost by the hot body must be equal to the heat gained by the cold body.
i.e., Heat gained = Heat lost
Work and heat are two equivalent forms of energy.

THERMAL EXPANSION

It is the increase in size on heating.
A solid can undergo three types of expansions.

Linear expansion                        (expansion in length)
Superficial expansion                (expansion in area)
Cubical expansion                      (expansion in volume)

SOME PRACTICAL APPLICATIONS

When hot tea is poured into a glass tumbler made of soft glass,it cracks because of thermal expansion of glass.
Telegraph wires are given enough gap to allow the wires for contraction in winter.

An ordinary pendulum clock runs faster in winter but slower in summer,because in summer the length of pendulum increases while in winter it decreases.

A metallic ball before heating can pass easily through the ring, but after heating the ball does not pass through the ring because of the thermal expansion of ball.

A gap is provided between the iron rails of the railway track so that rails can easily expand during summer and do not bend.In the construction of bridges,ends of steel gerders are not fixed but placed on rollers to allow free expansion and contraction in summer and winter respectively to avoid any damage to the bridge.

A tightly fitted cork can be removed from a glass bottle without breaking it on heating its mouth carefully.The glass wall expand and the cork will come out easily.

EXPANSION OF LIQUIDS

Liquids do not have their definite length and surface area.
Liquids always need a container.When they are heated,they expand and at the same time the container also expands.
The expansion of a liquid,when the expansion of its container has not been taken into account is called apparent-expansion.
The expansion of a liquid when the expansion of its container is taken into account, is called real expansion.

ANOMALOUS EXPANSION OF WATER

Almost every liquid expands with the increase in temperature.But when temperature of water is increased from 00 C to 40 C its volume decreases if the temperature is increased above 40 C its volume starts increasing, clearly density of water is maximum at 40 C.

CHANGE OF STATE (FUSION AND VAPORIZATION)

The process in which a solid changes into a liquid on heating is called fusion (melting).
Change of state takes place i.e., ice changes into water.
The process in which a liquid changes into a vapour on heating is called vaporization.
Change of state takes place i.e., water changes into water vapour or steam.

LATENT HEAT

It is the amount of heat absorbed by a unit mass of the substance to change its state without change in temperature.

SI unit of latent heat is joule per kilogram.
Latent heat of fusion is 80 cal/g.
Latent heat of vaporization is 540 cal/g.
Ice at 00 C is more effective in cooling a substance that water at 00 C.
If Q is quantity of heat absorbed ,then Q= mL.

where, m = mass of substance
L = Latent heat of substance

Steam contains more heat,in the form of latent heat than boiling water.
The burns caused by steam are much more severe than those caused by boiling water though both of them are at the same temperature of 1000 C.

EVAPORATION

It is the slow process of conversion of a liquid into its vapour even below its boiling temperature.
Cooking is caused by evaporation due to expansion.
Factors affecting the evaporation of a liquid are

  1. The area of the liquid surface.
  2. The nature of the liquid.
  3. Temperature of the liquid.
  4. Dryness of air.
  5. The movement of the air.
  6. Temperature of air or surrounding.

RELATIVE HUMIDITY

The amount of water vapour in air is called as humidity.
The amount of water vapour in the air varies as ir depends of the rate of evaporation.
The amount of water vapour present in 1 m3 air is called absolute humidity.

The ratio of amount of water vapour (m) actually present in a certain volume of air a given temperature to the amount of water vapour (M) required to saturate it, is called relative humidity,RH.

i.e.,                      RH  = m/M✕100%

TRANSMISSION OF HEAT

Transfer of heat from one place to other place is called transmission of heat.
These are of three types

  1. Conduction
  2. Convection
  3. Radiation

In solids transmission of heat takes by conduction process.
In liquids and gases heat takes place by convection process.
Heat from the sun reaches the earth by radiation.

NEWTON’S LAW OF COOLING

The rate of loss of heat by a body is directly proportional to the difference in temperature between the body and the surrounding.

KIRCHHOFF’S LAW

Kirchhoff’s law signifies that good absorbers are good emitters.If a shining metal ball with some black spot on its surface is heated to a high temperature and seen in dark,the shining ball becomes dull but the black spots shines brilliantly because black spot absorbs radiation during heating and emit in dark.

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