JK Academy Maharashtra Board Class 7 Science Solutions Chapter 8 Static Electricity
Balbharti Maharashtra State Board Class 7 Science Solutions Chapter 8 Static Electricity Notes, Textbook Exercise Important Questions and Answers.
Maharashtra State Board Class 7 Science Solutions Chapter 8 Static Electricity
Class 7 Science Chapter 8 Static Electricity Textbook Questions and Answers
Exercise
1. Choose the correct option and fill in the blanks.
(always repulsion, always attraction, displacement of negative charge, displacement of positive charge, atom, molecule, steel, copper, plastic, inflated balloon, charged object, gold)
(a) There is ……… between like charges.
Answer: repulsion
(b) ……….. is responsible for generation of electric charge in an object.
Answer: displacement of negative charge
(c) A lightning conductor is made of a ……….. strip.
Answer: copper
(d) ……. does not get electrically charged easily by rubbing.
Answer: steel
(e) There is …………… when opposite electric charges come near each other.
Answer: attraction
(f) A …………… can be detected with an electroscope.
Answer: charged object
2. Explain why it is not safe to go out with an umbrella when there is heavy rain, lightning or thunder.
Answer:
(i) When the negative charge on the bottom of the cloud becomes much larger than the charge on the ground, it starts flowing towards the ground in stages.
(ii) The metal and synthetic cloth of the umbrella may attract this charge and catch fire.
(iii) Therefore it is not safe to go out with an umbrella when there is heavy rain, lightning or thunder.
3. Answer in your own words.
(a) How will you protect yourself from lightning?
Answer: In order to protect ourselves from lightening we should
(i) stay indoors
(ii) If we are outside, we should not stand under a tree.
(iii) We should not carry an umbrella.
(iv) We should ensure that lightening conductors are installed in tall buildings.
(b) How are charges generated?
Answer: All objects are electrically neutral. For some reason, the balance of electrically neutral atoms gets disturbed. For example, when certain objects are rubbed against each other, the negatively charged particles on one object go to the other object. The object to which they go, becomes negatively charged due to an excess of negatively charged particles. Similarly, the object from which the negatively charged particles go away becomes positively charged due to a deficiency of negatively charged particles. It means that, of the two objects being rubbed, one becomes positively charged and the other, negatively charged.
(c) In the lightning conductor, what provision is made for spreading the electricity into the ground?
Answer: The lightning conductor is a device used for protection from a lightning strike. It consists of a long copper strip with one end forked. This end is at the highest part of the building. The other end of the strip is connected to a plate of cast iron. A pit is dug in the ground, coal and salt are filled into the pit and the iron plate is placed upright in the pit. There is also a provision for pouring water into it. This helps to spread the electric charge quickly into the ground and prevent damage.
(d) Why do farmers stick an iron staff into the ground while working in the field in rainy conditions?
Answer: When there are electrically charged clouds in the sky, electricity is attracted towards the earth. The farmers stick an iron rod into the ground to attract this lightning which is then transferred to the ground. Thus the farmers can protect themselves from the lightning.
(e) Why is lightning not seen everyday during the rainy season?
Answer: Where air and clouds rub against each other in the sky, the upper part of some clouds becomes positively charged and the lower sides become negatively charged. When a cloud with a negatively charged base becomes much larger than the charge on the ground, it starts flowing towards the ground in stages. This phenomenon does not occur on a daily basis; therefore lightning is not seen every day during the rainy season.
4. What are the characteristics of a static electric charge?
Answer: Characteristics of a static electric charge are:
(i) The electric charge generated by friction is called frictional electricity. This charge is produced only at the place of friction. Hence, it is called static electricity.
(ii) It remains on the object for a short duration.
(iii) The charges of static electricity are absorbed in moist air.
(iv) Electrically charged objects attract uncharged objects.
(v) There is repulsion between like electric charges.
(vi) There is attraction between unlike electric charges.
(vii) Repulsion is used as a test for identifying an electrically charged object.
5. What is the damage caused by lightning? How will you create awareness to prevent it?
Answer: The damage caused by lightening are:
(i) Trees and property can catch fire.
(ii) A person struck by lightning could get several burnt and even die.
We can create awareness to prevent it by:
(i) Stay indoors whenever there is thunder and lightning.
(ii) Avoid using any plugged electric appliances or devices.
(iii) Do not use any mobile phones out in the open area.
(iv) Advise people to install lightning conductors on their buildings.
Project :
Make an electroscope yourself by using a thin aluminium foil and check which substances become electrically charged.