CBSE Class 8 Science Chapter 15: Some Natural Phenomena NCERT Solutions

The correct option is (b) A copper rod. Materials like plastic, rubber, and wool are insulators, meaning they do not allow electric charges to flow easily. When rubbed, these materials can accumulate static charges. A copper rod, however, is a conductor. Electric charges can move freely through conductors, so any charge generated by friction would quickly dissipate, making it difficult to charge easily by friction.

The correct option is (b) becomes positively charged while the cloth has a negative charge. When a glass rod is rubbed with silk, electrons are transferred from the glass rod to the silk cloth. Since electrons are negatively charged, the glass rod loses electrons and becomes positively charged. The silk cloth gains these electrons and becomes negatively charged. This process demonstrates how friction can cause charge transfer between different materials.

(a) False. Like charges repel each other. For example, two positive charges will push away from each other, and two negative charges will also repel each other.

(b) True. A charged glass rod typically has a positive charge. If a plastic straw is charged with a negative charge (which can happen if it's rubbed with certain materials), then unlike charges attract each other, and the rod will attract the straw.

(c) False. A lightning conductor is specifically designed to protect buildings from lightning by providing a safe path for the electrical charge to reach the ground.

(d) False. Currently, earthquakes cannot be reliably predicted in advance with sufficient accuracy to allow for timely warnings.

During winter, the air is usually dry. When you wear a sweater, especially one made of wool or synthetic fibers, friction occurs between the sweater and your clothes or skin. This friction causes a transfer of electrons, leading to the accumulation of static electric charges on the surface of the sweater. When you take off the sweater, these accumulated charges can suddenly discharge through the air, creating small sparks and a crackling sound. This is similar to how static electricity builds up and discharges in other situations.

The human body is a conductor of electricity. When you touch a charged body with your hand, the excess electric charges on the body find a path to flow through your body to the Earth. The Earth acts as a large reservoir that can accept or supply charges to neutralize the object. This process, known as earthing or grounding, allows the charges to transfer from the charged body to the Earth, thus neutralizing the body and causing it to lose its charge.

The scale on which the destructive energy of an earthquake is measured is called the Richter Scale. Yes, an earthquake measuring 3 on the Richter Scale would certainly be recorded by a seismograph, as seismographs are designed to detect even very small tremors. However, an earthquake of magnitude 3 is considered to be of low intensity. It is unlikely to cause significant damage to buildings or pose a major threat to life.

Here are three measures to protect ourselves from lightning:

1. Seek Shelter Indoors: During a thunderstorm, the safest place to be is inside a substantial building or a hard-top vehicle. Avoid staying in open fields, under isolated tall trees, or near water bodies.
2. Avoid Contact with Conductors: Inside a building, stay away from electrical appliances, corded phones, and metal objects like pipes or window frames, as lightning can travel through these conductive materials.
3. Stay Low if Outdoors and No Shelter is Available: If you are caught outdoors with no shelter, crouch down low to the ground. Minimize your contact with the ground and avoid lying flat. This reduces your risk of being struck directly or through ground currents.