# Heat-theory Physics-day-1 Target ssc cgl 2018 Tier 1

Heat-theory Physics-day-1 Target ssc cgl 2018 Tier 1

Heat and Temperature

• A reliable measure of the hotness of an object is its temperature.
• Temperature is measured by a device called thermometer.

MEASURING  TEMPERATURE

• The thermometer that measures our body temperature is called a clinical thermometer.
• A clinical thermometer consists of a long, narrow, uniform glass tube. It has a bulb at one end. This bulb contains mercury. Outside the bulb, a small shining thread of mercury can be seen.
• The scale we use is the celsius scale, indicated by °C.
• India has adopted the celsius scale and we should read that scale. The other scale with the range 94–108 degrees is the Fahrenheit scale (°F). It was in use earlier.
• A clinical thermometer reads temperature from 35°C to 42°C.
• The normal temperature of human body is 37°C.

Is the body temperature of every person 37°C?

The temperature of every person may not be 37°C. It could be slightly higher or slightly lower. Actually, what we call normal temperature is the average body temperature of a large number of healthy persons.

The clinical thermometer is designed to measure the temperature of human body only. The temperature of human body normally does not go below 350C or above 420C. That is the reason that this thermometer has the range 350C to 420C.

CAUTION

Do not use a clinical thermometer for measuring the temperature of any object

other than the human body. Also avoid keeping the thermometer in the sun or near a flame. It may break.

LABORATORY  THERMOMETER

Different types of thermometers are used for different purposes. The maximum and minimum temperatures of the previous day, reported in weather reports, are measured by a thermometer called the maximum-minimum thermometer. The range of a laboratory thermometer is generally from –10°C to 110°C

Why does the mercury not fall or rise in a clinical thermometer when taken out of the mouth?

A kink in a thermometer prevents mercury level from falling on its own.

There is a lot of concern over the use of mercury in thermometers. Mercury is a toxic substance and is very difficult to dispose of if a thermometer breaks. These days, digital thermometers are available which do not use mercury.

Specific Heat

• It is the amount of energy needed to change the temperature of 1kg of substance by 10C.
• Water has the maximum specific heat.
• Radon has the lowest specific heat capacity.

Latent Heat

• The amount of heat absorbed or released as a result of a phase change is called latent heat.
• Latent heat of vapourisation is maximum at 00C and it decreases when we further increase the temperature.

TRANSFER OF HEAT

Heat is transferred or propagated by three processes. They are…

Conduction

• This is the process in which heat is transferred without bodily movement of the particles of the medium.
• All metals are good conductors of heat.
• Almost all the liquids and gases are poor conductor of heat.
• Best conductor of heat….Silver.

Convection

• This is the process in which heat is transferred by the bodily movement of the particles of medium.
• Convection does not takes place in solids.
• Convection takes place in liquids and gases.
• Mercury is the only liquid which is heated by convection.
• Earth’s atmosphere is heated by convection.
• Land and sea breezes are an example of convection.

• This is the quickest way of transmission the heat.
• This process does not require any medium for the transmission of heat.
• Heat from the Sun reaches the Earth by radiation.

LAND AND SEA BREEZE

The people living in the coastal areas experience an interesting phenomenon. During the day, the land gets heated faster than the water. The air over the land becomes hotter and rises up. The cooler air from the sea rushes in towards the land to take its place. The warm air from the land moves towards the sea to complete the cycle. The air from the sea is called the sea breeze. To receive the cooler sea breeze, the windows of the houses in coastal areas are made to face the sea.

At night it is exactly the reverse. The water cools down more slowly than the land. So, the cool air from the land moves towards the sea. This is called the land breeze.

What you have learn

• Our sense of touch is not always a reliable guide to the degree of hotness of an object.
• Temperature is a measure of the degree of hotness of an object.
• Thermometer is a device used for measuring temperatures.
• Clinical thermometer is used to measure our body temperature. The range of this thermometer is from 35°C to 42°C. For other purposes, we use the laboratory thermometers. The range of these thermometers is usually from –10°C to 110°C.
• The normal temperature of the  human body is 37°C.
• The heat flows from a body at a higher temperature to a body at a lower temperature.
• There are three ways in which heat can flow from one object to another.
• These are conduction, convection and radiation.
• In solids, generally, the heat is transferred by conduction. In liquids and gases the heat is transferred by convection. No medium is required for transfer of heat by radiation.
• The materials which allow heat to pass through them easily are conductors of heat.
• The materials which do not allow heat to pass through them easily are called insulators.
• Dark-coloured objects absorb radiation better than the light-coloured objects. That is the reason we feel more comfortable in light coloured clothes in the summer.
• Woollen clothes keep us warm during winter. It is so because wool is a poor conductor of heat and it has air trapped in between the fibres.

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