9th Class || Short Questions/Answers of chapter 8

Why metallic handle of a door is colder than the wood of the same door when touched?

Because, the transfer of heat from our hand to the metallic handle is faster, as compared to wood, thus loosing more thermal energy resulting in our hand feeling colder.

Which type of the clothes do the people of desert wear and why?

The people of the desert wear loose-fitting, long, heavy robes, and head wrappings. 

The loose-fitting cloth prevents sweat from evaporating quickly and allows the air to circulate, hence help the body to retain fluids. These clothes also protect people living in deserts from hot winds and dust storms.

Why does temperature of sea shore cities remain moderate during most of the year?does temperature of land areas vary more during winters and summers? Why

Water has a higher heat capacity than soil and rock, so the ocean takes much longer to heat and to cool than the land. Coastal areas will generally have more moderate temperatures than in land areas because of the heat capacity of the ocean. 

Warm ocean currents tend to raise winter temperatures, while cool currents can lower summer temperatures. Prevailing winds also affect world temperatures. The difference in temperature between the land and the sea in different seasons determines the temperature of the prevailing wind.

During the process of sweating (perspiration), we feel cooling during a hot day. Why? 

That's because cooling your body via sweating relies on a principle of physics called "heat of vaporization." It takes energy to evaporate sweat off of your skin, and that energy is heat. As your excess body heat is used to convert beads of sweat into vapor, you start to cool down.

How a bimetallic strip, made up of copper and iron is used as automatic switch in different devices? Give one example.

Thermal expansion of bimetallic strips used as a heat operated switch in the circuit of automatic equipment's like iron box, fire alarms, electric heater etc. Some thermometers work on the principle of expansion of liquids In automobile engines useful work is done by the expansion of gases. 

How is anomaly in the expansion of water help marine life to survive in extremely cold areas?

The anomalous expansion of water helps preserve aquatic life during very cold weather. When temperature falls, the top layer of water in a pond contracts, becomes denser and sinks to the bottom. A circulation is thus set up until the entire water in the pond reaches its maximum density at 4°C.

Examples of anomalous expansion of water

Bottle Burst: If you put a fully-filled water bottle in the refrigerator and its temperature is below 4 degrees Celsius, then according to the anomalous behavior of water, the water inside the bottle will expand. Therefore, due to no space in the bottle for the expansion of water molecules, water will exert force on the walls of the bottle and it will burst.

Breaking of Rocks: Rocks break during winter because as the temperature decreases below, water inside the rocks starts expanding and hence exert a large amount of force on the rocks and results in the breaking of rocks. 

Aquatic Life: This unique property of water helps aquatic life to survive. With a further drop in temperature, water on top forms ice and becomes a bad conductor of heat. This stops the heat from escaping the water body and helps aquatic life to survive. Hence, aquatic life can survive even when the temperature reaches or falls below. 

Why rollers are used at the ends of steel bridges?

One end of girder is fixed in concrete, but the other end is not fixed into concrete. It is supported on rollers. This is because when temperature increases in summer, the steel girder in a bridge expands and the rollers slide to allow the expansion otherwise the bridge may break.