Revision Series | Science | General Science | 67

Answer- A

• Solar cells convert solar energy into electricity. Silicon is used for making solar cells, which is abundant in nature, but availability of the special grade silicon for making solar cells is limited.
• The entire process of manufacture is still very expensive, silver used for interconnection of the cells in the panel further adds to the cost.
• The principal advantages associated with solar cells are that they have no moving parts, require little maintenance and work quite satisfactorily without the use of any focusing device.
• Another advantage is that they can be set up in remote and inaccessible hamlets or very sparsely inhabited areas in which laying of a power transmission line may be expensive and not commercially viable.

Answer- D

• Interlocking between two surfaces in contact is the reason behind friction.
• In the absence of friction, we will be unable to hold a pen or pencil. It is due to friction that a matchstick catches fire on rubbing. Friction causes heat.
• Kabaddi players rub sand/dust on their hands, which increases friction.
• Substances which decreases friction are called lubricants.
• Water and other fluids also apply friction on the objects that move across them. Friction applied by fluids is called ‘drag’.
• When an object slides over a surface, it applies sliding friction on it. Sliding friction is less than static friction.

Answer- C

Cow dung cakes serve as a steady source of fuel. Since, these fuels are derived from plants and animals hence they constitute – Biomass.

These fuels, however, do not produce much heat on burning but a lot of smoke is given out when they are burnt.

Cow-dung, various plant materials like crops residue, vegetable waste and sewage are decomposed in the absence of oxygen to give Biogas. Since the starting material is mainly cow-dung, it is popularly known as ‘gobar-gas’.

Bio-gas is produced in a dome-shaped plant.

Bio-gas is an excellent fuel as it contains up to 75% methane. It burns without smoke, leaves no residue like ash in wood, charcoal and coal burning. Its heating capacity is high.

Answer- A

Answer- C

Answer- C

• Microorganisms contaminate food items. Preservatives are used to prevent the contamination of food items. Sodium Benzoate and Sodium Metabisulfite are common food preservatives. These chemicals are used for the manufacture of jams and squash.Salt, sugar, edible oil and vinegar are used to inhibit the multiplication of microorganisms in food items. For example, salt, edible oil and vinegar are used to prevent the contamination of pickles; and sugar is used to prevent the contamination of jams and squash.

Answer- B

Answer- c

Hydro power plants convert the potential energy of falling water into electricity.

In order to produce hydel electricity, high-rise dams are constructed on the river to obstruct the flow of water and thereby collect water in larger reservoirs.

The water level rises and in this process the kinetic energy of flowing water gets transformed into potential energy. But, constructions of big dams have certain problems associated with it.

Large ecosystems get destroyed when submerged under the water in dams. The vegetation which is submerged, rots under anaerobic conditions and gives rise to large amounts of methane which is a greenhouse gas.

Answer- A

• Endocrine glands release hormones directly into the bloodstream, and hence are also called ductless glands. Sex hormones are also secreted through endocrine/ductless glands.
• Estrogen and testosterone are sex hormones which are responsible for secondary sexual characters. Estrogen is the female hormone and testosterone is the male hormone, secreted by ovary in females and testes in males respectively.
• This sex hormone is responsible for the development of facial hair, enlargement of voice box (larynx, Adam’s apple) etc in males. Due to an enlarged voice box, boys have a deeper voice and girls have a high pitched voice. Secondary sexual characters like development of breasts and mammary glands in females is caused by estrogen.
• The secretion of these sex hormones is regulated by a separate hormone, secreted by the pituitary. It regulates the maturation of egg/ovum in the ovary and sperms in the testes