NCERT Solutions for Class 8 Science Chapter 15 Some Natural Phenomena

Select the correct option in Questions 1 and 2.

Q.1 Which of the following cannot be charged easily by friction?

Answer :

(b) A copper rod

EXPLANATION

Copper rod cannot be charged easily by friction because it is a conductor. Charging any body by friction only takes place in case of non-conducting materials.

Q.2 When a glass rod is rubbed with a piece of silk cloth the rod
(a) and the cloth both acquire a positive charge.
(b) becomes positively charged while the cloth has a negative charge.
(c) and the cloth both acquire a negative charge.
(d) becomes negatively charged while the cloth has a positive charge

Answer :

When a glass rod is rubbed with a piece of silk cloth, the rod
(b) becomes positively charged while the cloth has a negative charge.

EXPLANATION

This happens because when a glass rod is rubbed with a piece of silk cloth, then the glass rod loses some electrons and become positively charged while the silk cloth gains those extra electrons and becomes negatively charged.

Q.3 Write T against true and F against false in the following statements.
(a) Like charges attract each other.
(b) A charged glass rod attracts a charged plastic straw.
(c) Lightning conductor cannot protect a building from lightning.
(d) Earthquakes can be predicted in advance.

Answer :

(a) Like charges attract each other. ( F )

(b) A charged glass rod attracts a charged plastic straw. ( T )

(c) Lightning conductor cannot protect a building from lightning. ( F )

(d) Earthquakes can be predicted in advance. ( F )

EXPLANATION

Like charges repel each other and unlike charges attract each other.

A charged glass rod attracts a charged plastic straw because when they are brought near each other then the glass rod accepts excess electrons and becomes negatively charged and the plastic straw give away electrons and becomes positively charged.

Lightning conductor can protect a building from lightning as it will conduct/transfer the electric charges from the clouds directly towards the ground and will not allow the high volt charges to find a path through the electrical appliances and wires of the building, thus preventing the damage of the electrical appliances and wires and protecting from any hazardous situation like fire.

Earthquakes cannot be predicted in advance. Neither the USGS nor any other scientists have ever predicted a major earthquake. USGS scientists can only calculate the probability that a significant earthquake will occur in a specific area within a certain number of years.

Q.4 Sometimes, a crackling sound is heard while taking off a sweater during winters. Explain.

Answer :

A crackling sound is heard while taking off a sweater during winters. This happens due to electrostatic forces.

When the sweater gets rubbed with our body, then due to friction our body loses some electrons to the sweater, thus making the sweater negatively charged and leaving the body to become positively charged.

Now attraction takes place between negatively charged sweater and positively charged body as a result of electrostatic forces.

Q.5 Explain why a charged body loses its charge if we touch it with our hand.

Answer :

A charged body loses its charge if we touch it with our hand. This happens with two cases.

If we touch a positively charged body, then as it lacks more electrons so electrons from the earth moves towards the positively charged body through our body. The body loses its identity of being positively charged and becomes neutral.

If we touch a negatively charged body, then as it has more electrons so electrons from the body moves towards the earth through our body. The body loses its identity of being negatively charged and becomes neutral.

This phenomenon is known as electric discharge.

Q.6 Name the scale on which the destructive energy of an earthquake is measured. An earthquake measures 3 on this scale. Would it be recorded by a seismograph? Is it likely to cause much damage?

Answer :

The destructive energy of an earthquake is measured by the Richter scale.

This scale has readings from 1 to 10.

Seismograph can measure an earthquake of magnitude 3 on the Richter scale.

If the Richter scale gives a reading of magnitude 3, then the earthquake is not likely to cause much damage. Generally, earthquakes of magnitude, higher than 5 is considered destructive in nature.

Q.7 Suggest three measures to protect ourselves from lightning.

Answer :

Three measures to protect ourselves from lightning are as follows :-

(i) We should never touch any electrical wires, telephone cables, metallic rods, water which may be charged or can conduct the charges.

(ii) We should try to stay indoor or under covered area and if travelling in car or bus, we should stay inside them by closing the doors and windows.

(iii) We should avoid walking outside and standing under a tree or electric pole during lightning.

Q.8 Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon?

Answer :

A charged balloon will repel another charged balloon if both the balloons carry same type of charges i.e., either they both are positively charged or they both are negatively charged. Because like charges repel each other.

On the other hand, an uncharged balloon is attracted by another charged balloon in two cases.
If the charged balloon is positively charged and is brought near an uncharged body, then both will attract each other because the uncharged body has more electrons than positively charged body and as we know unlike charges attract each other, so they will attract each other.

If the charged balloon is negatively charged and is brought near an uncharged body, then both will attract each other because the uncharged body has less electrons and more protons than negatively charged body and as we know unlike charges attract each other, so they will attract each other.

Q.9 Describe with the help of a diagram an instrument which can be used to detect a charged body.

Answer :

An electroscope is used to detect whether a body is charged or not.

It consists of a metal rod in which one end of the rod is fixed with two leaves of aluminium foil, while the other end has a metal disc. The leaves of aluminium foil are placed inside a beaker and the beaker is covered with a insulating rubber cork to prevent the entry of air into the beaker.

When the metal disc is touched with a charged body either positively charged body or negatively charged body, it is observed that the aluminium strips move away from each other.

This happens because the metallic disc conducts/transfers the charges towards the aluminium leaves, and as same charges get accumulated in the two aluminium leaves, thus they repel each other and move away from each other. This shows that conduction is taking place in the electroscope from the metallic disc to the aluminium leaves through the metallic rod.

Even if the charged body is removed from touching the metallic disc, then also the aluminium leaves are seen to be present, away from each other as the same charges are still present in them.

Q.10 List three states in India where earthquakes are more likely to strike.

Answer :

3 states in India where earthquakes are more likely to strike :-

(i)Gujarat

(ii)Assam

(iii)Jammu & Kashmir

Q.11 Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself?

Answer :

The following precautions that we take to protect ourselves are as follows:

(i) We will try to find an open field away from tall buildings, installations, tall trees, electric wires and poles.

(ii) While travelling in a bus or a car, we will try not to come out during an earthquake and will try to move the car or bus to an open field.

(iii) We will try to avoid staying near any glass made structures which can also cause harm during earthquake.

Q.12 The weather department has predicted that a thunderstorm is likely to occur on a certain day. Suppose you have to go out on that day. Would you carry an umbrella? Explain.

Answer :

No, I won't carry an umbrella because its stretcher(to which the nylon material of umbrella is attached and forms a canopy) and shaft( the part above the handle to which all the stretchers are attached) are metallic in nature and thus can conduct the electric charges.

As we know that thunderstorm are accompanied with lightning, so if umbrella is used then it may become the medium through which the electric charges will travel from the cloud, to the earth.

In this case it may give electric shocks to me.