Class 8 Science
Short Answer Type Questions
Question 1: What do you understand by charging by rubbing? Give suitable examples.
Answer: Some objects get electrically charged when they are rubbed against one another. For example; plastic refill is rubbed with polythene both of them become charged.
Question 2: Explain the structure of lightning conductor.
Answer: This is a simple device which protects a building from being hit by lightning. It is composed of a vertical metallic rod which usually has a trident at the top. The base of the metallic rod is attached to thick metallic wire; which goes very deep inside the earth. This metallic wire provides a passage for earthing.
Question 3: How do tectonic plates create earthquake?
Answer: The tectonic plates are in continuous motion. They keep on brushing against each other or striking each other. When a tectonic plate brushes against another or strikes another tectonic plate; it results in vibrations in the tectonic plates. These vibrations are experienced by us in the form of an earthquake.
Question 4: Explain the structure of seismograph.
Answer: Seismograph is a device which records the seismic activities. It is composed of an oscillator, writing device and a roll of paper. The writing device is attached to the oscillator. In case of an earthquake, the oscillator begins to oscillate. This creates oscillation in the writing device; which plots wave-like patterns on the paper. The wave-like pattern is then analysed by seismologist to interpret the various features of an earthquake.
Question 5: When a comb is rubbed against hair, the comb attracts the bits of paper. Why?
Answer: When a comb is rubbed against hair, the comb attracts the bits of paper. This happens because the comb acquires a charge on rubbing.
Long Answer Type Questions
Question 1: Explain the working of electroscope.
Answer: When the brass disc of the electroscope is touched with a charged object, electric charge gets transferred to the gold leaf through the rod. This results in the gold leaves moving away from each other. This happens because both the leaves have similar charges.
Question 2: Explain the formation of lightning.
Answer: Formation of lightning takes place in following steps:
- During a thunderstorm, the air currents move upward and water droplets move downward. These movements take at very fast pace and it results in separation of charges in the clouds.
- The positive charges accumulate at the upper edges of the clouds and the negative charges accumulate at the lower edges of the clouds. Scientists are yet to understand the exact reason for this.
- At the same time, the ground becomes positively charged.
- Air is a bad conductor of electricity under normal circumstances. But when the amount of charge becomes too much in the clouds, the air is not in a position to hold on to this charge. As a result, the electric charge is transferred to the ground. This appears like a bright streak of light across the sky; which lasts for a few seconds.
Question 3: Write some safety tips during lightning.
Answer: Safety during lightning:
- No open place is safe during lightning and hence a home or any other building is the safest place during lightning.
- The first sound of thunder is like a warning call for lightning. Hence, one should rush to a building soon after hearing a sound of thunder. One should come out of home only a few minutes after the last sound of thunder.
- You should not carry an umbrella (with a metallic handle) during thunderstorm. Metallic handle can be a potential target for lightning.
- Tall trees and other tall structures are more prone to be hit by lightning. If you are caught in the open, try to hide under a short tree.
- If you are in open and have no place to hide, then crouch on the ground and keep your head between your knees and hands.
- Don’t take shower during lightning.
- Use of phone should be avoided during lightning.
- Television should be disconnected from the antenna.
- Avoid operating the electric switches during lightning.
Question 4: Suggest some tips for a quake safe building.
Answer: Following provisions make a quake safe building:
- Buildings should be made quake proof. Engineers and architects are the experts who can give suitable advice in this regard.
- Light-weight materials should be used for making the buildings in earthquake prone areas.
- Cupboards should be fixed to the walls so that they do not fall on someone during an earthquake.
- People should be made to practice drills for earthquake. Such drills should be frequently organized in schools and at public places.
- In case of an earthquake, you should hide under a table or any other similar structure. If you are in bed, then keep a pillow over your head and do not move out of the bed. If you are in an open area, try to move away from buildings and other structures.
- Proper installation of fire-fighting equipments should be ensured in buildings. Fire due to short circuit and gas leakage is quite common during an earthquake.