Scientific Reasons From Past Papers 15


➽ A bird perches on a bare high power line and nothing happens to it. A man standing on the ground touches the same line and gets a fatal shock. Why?
Answer: When bird perches on a live high power line, no current passes through the body of bird because there is no potential difference between live wire and the body of bird as the potential of bird’s body is the same as that of wire.
When a man standing on the ground touches the same wire then due to a large potential difference between his hands and feet, a large current flows through his body to the earth.

➽ During lightning it is safest to be inside a car rather than under a tree or in the open. Give reason. Answer: When a person is in the open or under the tree, the lightning passes through his body to the earth. On the other hand, for a person inside a car, the car provider shielding and the electric field inside the car is zero, thus lightning does not affect the person inside it. The lightning actually passes through the metallic body of the car to the earth without affecting the person sitting inside it.

➽ Why is earthing desirable for electric appliances?
Answer: Earthing helps the current move into the earth in the event of short-circuit, without giving a
shock to the user.

➽ Why does a perspiring man feel relief when air flows by his side?
Answer: The air flowing by the side of a perspiring man quickens the pace of evaporation of perspiration from the body of that person, and the resultant loss of heat from his body causes the cooling sensation which provides relief to him.

➽ Why is magnet always made of soft iron?
Answer: Magnet is always made of soft iron because ‘magnetisation’ and ‘demagnetisation’ both are possible in soft iron.

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