12. Class 8 Science Chapter 12 Some Natural Phenomena

Exam Notes: Some Natural Phenomena

12.1 Lightning

Nature of Lightning:

• Lightning is a giant spark of electricity, similar to the sparks you see on electrical poles and loose plugs, but on a much larger scale.
• It is caused by the accumulation of large amounts of electric charges within clouds, often during thunderstorms.

Historical Perception:

• Even though ancient Greeks knew about static electricity, they didn’t understand its cause.
• For centuries, lightning was often attributed to the wrath or anger of gods.

Properties of Electric Charges:

• Rubbing two different materials together can generate electric charges.
• For example, rubbing a plastic refill with a piece of polythene can create a positive charge on the refill and a negative charge on the polythene.

12.2 Charging by Rubbing

Activity 12.1:

• This activity will demonstrate how rubbing different objects with specific materials can generate electric charges.
• You will rub various objects (like a plastic refill) with different materials (like polythene, wool, etc.) and observe the resulting charges.
• The observations will be recorded in Table 12.1 for further analysis.

12.3 Types of Charges and Their Interaction

Activity 12.3:

• This activity will showcase the interaction between charged objects.
• You will observe how objects with similar charges repel each other, while objects with opposite charges attract each other.

12.4 Transfer of Charge

Activity 12.4:

• This activity will demonstrate the transfer of electric charge using an electroscope.
• You will observe how touching a charged object with the electroscope can transfer the charge to the electroscope, causing it to deflect its leaves.
• You will also observe the repulsion or attraction between the charged electroscope and other objects.

12.5 The Story of Lightning

Explanation of Lightning:

• The accumulation of large amounts of positive and negative charges in clouds can lead to a sudden and powerful discharge of electricity, known as lightning.
• Lightning can occur between clouds (intra-cloud lightning) or between clouds and the ground (cloud-to-ground lightning).

Lightning Safety Measures:

• During thunderstorms, it is crucial to take safety precautions to avoid being struck by lightning.
• Avoid open spaces like fields and open vehicles.
• Seek shelter in buildings or enclosed vehicles.

12.6 Lightning Safety

Precautions during Thunderstorms:

• Stay away from open vehicles, fields, and isolated tall objects like trees or metal structures.
• Do not stand under or near trees or power lines.
• If caught outdoors, find low-lying ground and crouch down, making yourself as small a target as possible.

Safety Inside the House:

• Avoid contact with electrical wires and water during thunderstorms.
• Unplug unnecessary appliances and electronics.

12.7 Earthquakes

Definition and Causes:

• An earthquake is a sudden and violent shaking of the ground caused by disturbances within the Earth’s crust.
• These disturbances can be triggered by the movement of tectonic plates, volcanic activity, or even human activities like mining.
• Earthquakes can be difficult to predict accurately, making it crucial to be prepared for them.

Protection Against Earthquakes:

• Design buildings to withstand tremors, especially in earthquake-prone regions.
• Practice earthquake drills and have an emergency plan in place.
• Take precautions inside and outside buildings during earthquakes to minimize injury.

Definitions of Key Terms:

• Crust: The outermost solid layer of the Earth, consisting mainly of rock and soil. It’s relatively thin compared to the Earth’s other layers, ranging from 5 to 75 kilometers deep.
• Discharge: The sudden release of stored energy, often in the form of electricity or heat. In the context of lightning, it refers to the rapid release of electricity built up within clouds.
• Earth’s Plates: Large, rigid segments of the Earth’s crust that move slowly over time. Their movement can cause earthquakes, volcanic eruptions, and other geological phenomena.
• Earthquake: A sudden shaking of the Earth’s surface caused by the release of stress from within the crust, often due to the movement of tectonic plates. They can range in intensity from barely noticeable tremors to catastrophic events.
• Electroscope: A scientific instrument used to detect the presence and type of electric charge. It typically consists of a metal rod with two leaves that will diverge from each other when charged.
• Lightning: A powerful electrical discharge in the atmosphere that causes a brightly lit flash followed by a loud sound (thunder). It can occur within clouds (intra-cloud) or between clouds and the ground (cloud-to-ground).
• Lightning Conductor: A metal rod or wire installed on buildings or other structures to protect them from lightning strikes. It provides a path for the electricity to safely discharge into the ground, preventing damage from direct hits.
• Negative Charge: An electrical charge associated with an excess of electrons in an atom or molecule. When two objects with negative charges come close, they repel each other.
• Positive Charge: An electrical charge associated with a deficiency of electrons in an atom or molecule. When two objects with positive charges come close, they repel each other.
• Richter Scale: A logarithmic scale used to measure the magnitude of earthquakes. An increase of one unit on the scale represents a tenfold increase in the amount of energy released.
• Seismograph: An instrument that records the movement of the ground during an earthquake. It provides valuable data about the location, intensity, and frequency of earthquakes.
• Thunder: The loud sound produced by the rapid expansion of air heated by lightning. It typically travels slower than the flash of light, appearing seconds after the lightning is seen.
• Thunderstorm: A storm with lightning and thunder, usually accompanied by heavy rain and strong winds. They are caused by rising warm air and convection currents.
• Transfer of Charge: The movement of electric charge from one object to another. This can occur through various mechanisms, such as rubbing, induction, and conduction.
• Tsunami: A large, fast-moving ocean wave caused by an underwater earthquake, volcanic eruption, or landslide. Tsunamis can be highly destructive, reaching significant heights and flooding coastal areas.
• Tremor: A slight shaking or shuddering of the ground, often caused by a minor earthquake or other geological activity.

Test Your Knowledge of Natural Phenomena: With these MCQ!

Question 1: Lightning is most similar to:

• a) Fire caused by a chemical reaction
• b) Sparks from loose electrical wires
• c) Explosions from volcanic eruptions
• d) The aurora borealis

Answer: (b) Sparks from loose electrical wires

Explanation: Both lightning and sparks from loose electrical wires involve the sudden discharge of electricity, creating a bright flash and sometimes a crackling sound.

Question 2: Which statement about rubbing materials and generating charges is INCORRECT?

• a) Rubbing two different materials can create charges.
• b) The type of material determines the kind of charge generated.
• c) Rubbing wool with silk creates positive charges on the wool.
• d) Rubbing plastic with fur creates negative charges on the plastic.

Answer: (c) Rubbing wool with silk creates positive charges on the wool. (It actually creates negative charges on the wool.)

Explanation: Wool rubbed with silk generates negative charges on the wool, not positive charges.

Question 3: During Activity 12.3, what would you observe when bringing two objects with the same type of charge close together?

• a) They will attract each other.
• b) They will repel each other.
• c) They will have no effect on each other.
• d) They will glow brightly.

Answer: (b) They will repel each other.

Explanation: Objects with the same type of charge repel each other, while objects with opposite charges attract each other.

Question 4: In Activity 12.4, touching a charged electroscope with another object can:

• a) Increase the deflection of the leaves.
• b) Decrease the deflection of the leaves.
• c) Make the leaves spin rapidly.
• d) Have no effect on the leaves.

Answer: (a) Increase the deflection of the leaves.

Explanation: Touching a charged electroscope with an object can transfer charge, causing the leaves to deflect further.

Question 5: Which type of lightning occurs between clouds and the ground?

• a) Intra-cloud lightning
• b) Cloud-to-ground lightning
• c) Ball lightning
• d) Sheet lightning

Answer: (b) Cloud-to-ground lightning

Question 6: The safest place to be during a thunderstorm is:

• a) Standing under a tree
• b) Running in an open field
• c) Inside a sturdy building
• d) Taking shelter in a small shed

Answer: (c) Inside a sturdy building

Explanation: Buildings offer the best protection from lightning strikes during thunderstorms. Avoid open spaces, trees, and metal structures.

Question 7: An earthquake is caused by:

• a) Explosions in the Earth’s core
• b) Building a very tall skyscraper
• c) Movement of tectonic plates
• d) Heavy rainfall

Answer: (c) Movement of tectonic plates

Explanation: The sudden movement of tectonic plates beneath the Earth’s surface is the primary cause of earthquakes.

Question 8: Buildings in earthquake-prone areas should be:

• a) Made of wood and straw
• b) Built very tall and thin
• c) Designed to withstand tremors
• d) Located near rivers and cliffs

Answer: (c) Designed to withstand tremors

Explanation: Earthquake-resistant buildings incorporate structural elements and materials that can absorb and redirect seismic waves, minimizing damage during earthquakes.

Question 9: Which instrument measures the strength of an earthquake?

• a) Thermometer
• b) Seismograph
• c) Electroscope
• d) Richter Scale (This isn’t an instrument, but a scale used to classify earthquake strength)

Answer: (b) Seismograph

Explanation: Seismographs record the seismic waves generated by earthquakes, allowing scientists to measure their intensity and location. The Richter Scale then uses this data to classify the earthquake’s magnitude.

Question 10: In case of an earthquake indoors, it is best to:

• a) Run outside into the street
• b) Stand under a doorway
• c) Climb onto a table
• d) Hide in a corner closet

Answer: (b) Stand under a doorway

Explanation: Doorways offer some protection from falling debris during an earthquake. Avoid windows, mirrors, and outside exits while indoors.