## Topic outline

• ### Electricity and Circuits

##### Q41. When electric cell has to be replaced with a new one?

Ans. An electric cell produces electricity from the chemicals stored inside it.

When the chemicals in the electric cell are used up, the electric cell stops producing electricity. The electric cell then has to be replaced with a new one.

##### Q42. Conductors and insulators are equally important for us. Give reasons.

Ans. Switches, electrical plugs and sockets are made of conductors. On the other hand, rubber and plastics are used for covering electrical wires, plug tops, switches and other parts of electrical appliances, which people might touch.

##### Q43. What is the function of a switch in an electric circuit?

Ans. The purpose of a switch in a circuit is either to break the circuit or complete it. The switches used in lighting of electric bulbs and other devices in homes work on the same principle although their designs are more complex.

##### Q44. Differentiate between primary cells and secondary cells.

Ans.

 Primary Cells Secondary Cells 1. Primary cells can only be used once because the chemical reactions that supply the electrical current are irreversible. 1. Secondary cells can be charged and reused. Here chemical reaction is reversible. 2. Example: simple voltaic cell, Daniel cell, dry cell etc. 2. Example: Lead accumulator, nickel-iron accumulator and alkali accumulator.

##### Q45. When we touch live wire we get a shock whereas bird sitting on wire does not get shock. Why?

Ans. For a living being to get a ‘shock’ there has to be a considerable flow of current through the body and the circuit needs to be complete for current to flow. We get shock when we accidently touch the live wire because circuit is complete and current passes through our body and enters the earth. But bird sitting on wire does not complete the circuit and hence does not get shock.

##### Q46. Write some purposes for which we use electricity.

Ans. We use electricity for many purposes to make our tasks easier. For example,

i.        We use electricity to operate pumps that lift water from wells or from ground level to the roof top tank.

ii.        For lighting our homes, roads, offices, markets and factories even after sunset.

iii.        For running many gadgets and machines.

##### Q47. How conductors are different from insulators?

Ans.

 Conductor Insulator 1. Materials which allow electric current to pass through them are conductors of electricity. 1. Materials which do not allow electric current to pass through them are insulators. 2. Example: All metals such as silver, aluminium, gold, iron, copper etc. are good conductors and impure water. 2. Example: cotton, rubber, leather, plastic, air, pure water etc.

##### Q48. Why handles of tools like screwdrivers, pliers and electric tester are covered with plastic or rubber?

Ans. Electricians sometime need to touch copper wires or they may accidently come in contact with live wire while working. Since human body is good conductor of electricity, to prevent them from electric shock handles of tools like screwdrivers, pliers and electric tester are covered with plastic or rubber.

##### Q49. Why we should not join the two terminals of an electric cell without connecting them through a switch or a device like a bulb?

Ans. We should not join the two terminals of the electric cell without connecting them through a switch and a device like a bulb because if we do so, the chemicals in the electric cell get used up very fast and the cell stops working.

##### Q50. Fused bulb does not glow. Give reason.

Ans. An electric bulb may fuse due to many reasons. One reason for a bulb to fuse is a break in its filament. A break in the filament of an electric bulb means a break in the path of the current between the terminals of the electric cell. Therefore, a fused bulb does not light up as current do not passes through its filament.

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