CBSE Class 8 Science Some Natural Phenomena Lightning Earthquakes and Static Charge Assignment Set 01

Answer: 1. The device to detect electric charge is the electroscope.
2. The instrument that measures and records an earthquake is the seismograph.
3. A lightning conductor is made of copper.
4. The epicentre is the point on the earth's surface directly above the focus of the earthquake.
5. Earthing carries extra charge safely to the ground, so it protects buildings and appliances from shocks.
6. Seismic (fault) zones are weak zones of the earth's crust, near plate boundaries, where earthquakes occur often.
7. Benjamin Franklin showed that lightning is electric discharge in clouds.
8. Like charges repel and unlike charges attract.
9. Cyclones, floods and droughts can be predicted to some extent.
10. a) The silk cloth gets a negative charge. b) The glass rod gets a positive charge.

Teacher's Note: Use the key term in your answer.

Answer: 1. (b) a copper rod; 2. (d) electrons; 3. (d) Richter scale; 4. (d) Rajasthan; 5. (a) heat energy (which comes from the electrical energy); 6. (c) an earthquake under the sea; 7. (b) nitrogen fixation; 8. (d) earthing; 9. (b) equal and unlike charges; 10. (a) two.

Teacher's Note: Give an example if the question allows.

Answer: The human body is a conductor, so the charge flows through it to the ground, and the body becomes neutral.

Teacher's Note: Write a short, clear answer.

Answer: Stay indoors and keep away from tall trees and open fields, and fit lightning conductors on buildings. Electric discharge is the sudden flow of a large charge through air between two charged bodies or a charged cloud and the earth, shown by sparks, lightning and thunder.

Teacher's Note: Remember this for your exam.

Answer: An earthquake is the shaking of the ground due to the movement of the earth's plates. A tsunami is a huge sea wave that occurs when an earthquake or volcano erupts under the sea.

Teacher's Note: Learn this point and write it in your own words.

Answer: Charging by induction is charging a body by bringing a charged body near it, without touching. An earthquake is the sudden shaking of the ground. It occurs when the tectonic plates move and the stress built up in rocks is suddenly released as waves.

Teacher's Note: Use the key term in your answer.

Answer: An electroscope has a metal rod with a knob at the top and two thin metal leaves at the bottom, in a glass jar.

Teacher's Note:

Answer: The focus is the point deep inside the earth where the earthquake starts. The epicentre is the point on the surface straight above the focus, where the shaking is strongest.

Teacher's Note: Write a short, clear answer.

Answer: Lightning is a huge spark of electricity in the sky. Clouds are charged when ice and water particles rub against each other in strong winds: the top gets a positive charge and the bottom a negative one. When the charge difference is very large the air can no longer insulate, and a large spark jumps, which is lightning, with thunder from the heated air.

Teacher's Note: Remember this for your exam.

Answer: The ground shakes and cracks, buildings, roads and bridges fall, landslides, floods (from broken dams) and fires occur, tsunamis may form, and many people are killed or left homeless.

Teacher's Note: Learn this point and write it in your own words.

Answer: An electroscope has a metal rod with a knob at the top and two gold or aluminium leaves at the bottom, held inside a glass jar through an insulating cork. When a charged body touches the knob, the charge passes to both leaves, which get the same charge, repel each other and spread apart. The wider the spread the more the charge.

Teacher's Note:

Answer: The earth's crust is broken into plates that move slowly. When two plates push or slide against each other, stress builds up in the rocks. When it becomes too much, the rocks suddenly slip along a fault and release energy as seismic waves, which shake the ground.

Teacher's Note: Give an example if the question allows.

Answer: A lightning conductor is a thick copper strip with sharp points at the top, fixed above the building and joined to a copper plate buried in the earth. When lightning strikes, the pointed rod attracts the charge and carries it safely through the copper strip into the earth, so the building is saved.

Teacher's Note:

Answer: Go inside a building or a car, keep away from windows, switch off electrical appliances, do not shelter under tall trees or hold metal objects, do not stand in open fields or near water, and crouch low with your feet together if you are caught in the open.

Teacher's Note: Remember this for your exam.

Answer: Conduction: a charged rod touches a neutral metal body, and the charge flows to it, giving it the same charge. Induction: a charged rod is brought near a neutral metal body without touching it; the far end gets the same charge and the near end the opposite. If the far end is earthed and the rod removed, the body is left with a charge opposite to the rod.

Teacher's Note: Learn this point and write it in your own words.

Answer: Movement of the earth's plates and slipping along faults is the main cause. Volcanic eruptions, landslides, big explosions, collapse of mines and building large dams can also cause earthquakes.

Teacher's Note: Use the key term in your answer.

Answer: By friction (rubbing two bodies), by conduction (contact with a charged body) and by induction (bringing a charged body near, without contact).

Teacher's Note: Give an example if the question allows.

Answer: Electric discharge is the flow of charge through air when the charge difference is large. In a thundercloud, rubbing of ice and water particles makes the upper part positive and the lower part negative. The charge of the lower part induces an opposite charge on the earth below. When the charge difference is too high, a huge spark flows as lightning.

Teacher's Note: Write a short, clear answer.

Answer: The crust is the thin outer solid layer, about 5 to 70 km thick. The mantle lies below it and is about 2,900 km thick, made of hot, partly molten rock. The core is the innermost part, which is made of iron and nickel, with a liquid outer core and a solid inner core.

Teacher's Note: Remember this for your exam.

Answer: Stay indoors; do not use a telephone with a cord or electrical appliances; do not take shelter under a tree; keep away from metal poles, fences and water; and if you are in a car, keep the windows closed.

Teacher's Note: Learn this point and write it in your own words.