Class 7 Science Chapter 3 Important Questions

By using thermometer, we can determine the temperature of any object.
Generally, thermometer is a device that measures the temperature or hotness of an object.

Our touching sense is not reliable because we can’t say the temperature of any object by touching. We just feel the flow of any temperature either that is hot or cold. The other reason is that our senses do not indicate the accurate flow of temperature by the skin. We just feel the hotness or coldness by our skin.
A classic experiment proves that our senses are not reliable
If we put one hand in a bowl of very warm water and the other hand in a bowl of cold. After keeping the hands in the two mugs for 2–3 minutes, put both the hands simultaneously in mix water or room temperature water mug. we cannot decide whether an object is hot or cold. Sometimes it may deceive us.

Temperature is that thermal condition (hotness) of a substance which determines whether heat is applied to the other bodies or not.

The thermometer that measures our body temperature is called a clinical thermometer.

Wash the thermometer, preferably with an antiseptic solution. Hold it firmly and give it a few jerks. The jerks will bring the level of mercury down. Ensure that it falls below 35°C. Now place the bulb of the thermometer, under your tongue.
After one minute, take the thermometer out and note the reading. This is your body temperature. The temperature should always be stated with its unit, °C (the Celsius).
About Division: The temperature difference indicated between the two bigger marks. Suppose the bigger marks read one degree and there are five divisions between them. Then, one small division can read 1/5 =0.2 degree Celsius.

The normal temperature of human body is 37°C. This temperature is stated with its unit.

The temperature of every person may not be 37°C. It could be slightly higher or slightly lower. Actually, the 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 because the temperature of human body normally does not go below 35 degree Celsius or above 42 degree Celsius. That is the reason that this thermometer has the range 35 degree Celsius to 42 degree Celsius.

The maximum and minimum temperatures of any day, reported in weather reports. Temperature of any day is measured by a thermometer called the maximum -minimum thermometer.

A laboratory thermometer is a tool used in laboratories to measure the temperature of an object with high accuracy. It can be partially or fully immersed in the substance being measured.

The mercury does not fall or rise in a clinical thermometer when taken out of the mouth because of the presence of kink. It 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. Digital thermometers are designed to be more accurate because it consists digital circuit board which converts the temperature into numerical values with decimals.

The word heat to mean something that feels warm, but science defines heat as the flow of energy from a warm object to a cooler object. Heat energy is the result of the movement of tiny particles called atoms, molecules or ions in solids, liquids and gases.
In other words, heat is a form of energy which spontaneously passes to its surroundings.

We are advised not to hold the thermometer bulb while reading it, as the level of mercury increases from the actual reading due to our body temperature.

Take a rod or flat strip of a metal, say of aluminium or iron. Fix a few small wax pieces on the rod. These pieces should be at nearly equal distances. Clamp the rod to a stand. If you do not find a stand, you can put one end of the rod in between bricks.
Now, heat the other end of the rod. These wax pieces begin to fall. Heat is transferred from the end nearest to the flame to the other end. The process by which heat is transferred from the hotter end to the colder end of an object is known as conduction. In solids, generally, the heat is transferred by the process of conduction.

The materials which allow heat to pass through them easily are conductors of heat. For examples, aluminium, iron and copper.
The materials which do not allow heat to pass through them easily are poor conductors of heat such as plastic and wood. Poor conductors are known as insulators.

Take a steel or aluminium container with some water. When water is heated, the water near the flame gets hot. Hot water rises up. The cold water from the sides moves down towards the source of heat.
This water also gets hot and rises and water from the sides moves down. This process continues till the whole water gets heated. This mode of heat transfer is known as convection.

An umbrella guards against the sun’s ultraviolet rays to keeps away from sunburn. When we go out in the sun, its ultraviolet rays directly reach us that makes various skin diseases or makes hot.
To avoid further damage from the sun, you are advised to use an umbrella while going out.

The air gets trapped between two layers of warm clothes. Air acts as insulator of heat. This layer prevents our body heat to escape in the surroundings. More layers of thin clothes will allow more air to get trapped and as a result we will not feel cold. For wearing more layers of clothing during winter keeps us warmer than wearing just one piece of clothing.

For the setting of curd at places where temperature is below room temperature, the container in which curd is to be made, must be kept in a thermally insulated cover or it can be wrapped either by a woollen material or a jute sack so that temperature is maintained for the setting of curd.
The container can also be kept in the sun or near the gas stove while cooking food for the setting of curd.