Activities 5.6 and 5.7 — Electrostatic Force — Exploring Forces

1. 14 marksCuriosity Grade 8, Chapter 5, page 70, Activity 5.6

   Describe Activity 5.6 and what you observe. Why does the activity warn you not to touch the rubbed part with your hand or any metal object?

   Hint. The warning is there because the experiment can fail silently.

   The activity. Take a plastic scale or a plastic straw, a piece of polythene, and small pieces of paper. Rub the scale or straw vigorously with the polythene. Do not touch the rubbed part with your hand or any metal object. Now bring it close to the paper pieces on a table, without touching them (Fig. 5.8).

   What you observe. The paper pieces get pulled towards the plastic scale/straw and stick to it when it is brought close to paper pieces. They jump up to meet it before it arrives — the force acts across the gap.

   Why not to touch the rubbed part. Rubbing builds up static charges on the surface. Touching it with your hand or a metal object lets those charges escape, and then nothing happens when you bring it near the paper. The experiment does not go wrong noisily; it simply stops working, and you would think the effect was not real.

   ✦ Why vigorously is in the instructions. More rubbing builds up more charge, and the effect is small to begin with. A gentle rub often produces nothing you can see.

   Why the paper must not be touched either. If the scale touches the paper, you have an ordinary contact — sticky or otherwise — and you learn nothing. The whole point is that the pieces move before contact is made.

   A dry day works far better than a humid one. The chapter does not say so, but if the activity refuses to work, that is usually why.

2. 23 marksCuriosity Grade 8, Chapter 5, page 70

   What are static charges? What is meant by a charged object?

   Hint. The word 'static' is doing the defining.

   The chapter's account. When two objects of certain materials are rubbed together, electrical charges build up on their surfaces. These charges are called static charges as they do not move by themselves. The object that acquires static charges is said to be a charged object.

   Term	Meaning
   Static charges	Electrical charges built up on a surface by rubbing, which do not move by themselves
   Charged object	An object that has acquired static charges

   What a charged object does. A charged object attracts, that is, exerts a force on uncharged objects made of certain materials, such as small pieces of paper. This force comes into play even when the objects are not in contact.

   ✦ Two careful phrases worth noticing. Objects of certain materials — not every pair of materials gets charged when rubbed, which is exactly what the chapter's first project asks you to investigate. And uncharged objects made of certain materials — a charged object does not attract absolutely everything.

   Why they are called static. The charges stay where the rubbing put them. Contrast this with the A step further box on page 71: When the charges move, they constitute an electric current in an electrical circuit. Charges that stay put give you the effects of this section; charges that move give you Chapter 4.

3. 34 marksCuriosity Grade 8, Chapter 5, page 70, Activity 5.7

   Describe Activity 5.7 with the two balloons. What happens when they are rubbed with the woollen cloth and released, and what happens when the cloth is brought near one of them?

   Hint. Two observations that point in opposite directions — which is the whole point.

   The activity. Inflate two balloons and hang them so that they do not touch each other (Fig. 5.9a). Rub both with a woollen cloth and release them, taking care not to touch the rubbed balloons with your fingers.

   Observation 1 — the two balloons. We observe that the balloons move away from each other as if they are repelling each other (Fig. 5.9b). They hang apart instead of dangling side by side.

   Observation 2 — the cloth. Bring the woollen cloth used for the rubbing close to one of the rubbed balloons. They move towards each other as if they are attracting each other.

   ✦ Why the two observations together are worth more than either alone. Repulsion alone would tell you the balloons had something in common. Attraction to the cloth alone might just look like the paper-pieces effect again. Put side by side, they force a conclusion: the balloons must carry one kind of charge and the cloth the other kind, since the same balloon is repelled by one object and attracted by another.

   Why the balloons must be hung and not held. Held in your fingers, the charge would leak away and they would hang limply side by side. Hanging them also lets them move freely, so a small force produces a visible result — the same reason a compass needle is pivoted rather than glued down.

4. 44 marksCuriosity Grade 8, Chapter 5, page 71

   What does the chapter infer from Activity 5.7 about the kinds of charge and how they behave?

   Hint. Follow the inference in the order the book makes it — the balloons first, then the cloth.

   Step 1 — the balloons carry the same kind of charge. Since the balloons were charged in the same way, we can say that they have acquired similar charges. They were both rubbed with the same woollen cloth in the same way, so whatever one got, the other got too.

   Step 2 — like charges repel. As the similarly charged balloons repelled each other, we can infer that similar (like) charges repel each other.

   Step 3 — the rubbing object gets the opposite charge. Both the rubbing object and the rubbed object get charged but they acquire opposite kind of charges.

   Step 4 — unlike charges attract. Their attraction shows that opposite kind (unlike) of charges attract each other.

   Step 5 — there are two kinds. The two kinds of static charges are said to be 'positive' and 'negative'.

   ✦ Notice how much is being established from two simple observations. That there are exactly two kinds of charge is not something you can see; it is deduced from the fact that a balloon is pushed away by one object and pulled towards another. The names positive and negative are labels attached afterwards — the physics is in the repelling and attracting.

   The pattern to remember: like repel, unlike attract. It is the same rule as for magnetic poles, which is why the chapter placed magnetic force immediately before this section.

5. 53 marksCuriosity Grade 8, Chapter 5, page 71

   Define electrostatic force. Why is it a non-contact force?

   Hint. The definition covers two cases, not one.

   Definition. The force exerted by a charged body on another charged body or an uncharged body is called electrostatic force. It is a non-contact force.

   The two cases in that definition, each demonstrated by one activity:

   Case	Activity	What happens
   Charged body on an uncharged body	5.6	The rubbed scale attracts small pieces of paper, which were never rubbed
   Charged body on another charged body	5.7	Two rubbed balloons repel each other; a rubbed balloon and the cloth attract

   Why non-contact. In both activities the force acts across a gap. The paper pieces jump to the scale before it reaches them, and the balloons swing apart without ever touching. This force comes into play even when the objects are not in contact.

   ✦ The uncharged case is the more surprising of the two. Two charged objects affecting each other is easy to accept. But the paper pieces carry no charge at all, and are still attracted. The chapter states this as an observed fact — A charged object attracts ... uncharged objects made of certain materials — and does not explain the mechanism, which comes in a later grade. Say what happens, and do not invent a reason.

   Like magnetic force, this one can push or pull — repulsion between like charges, attraction between unlike. Gravity, as the next section says, can only pull.

6. 63 marksCuriosity Grade 8, Chapter 5, page 71, A step further

   What connects static charges to Chapter 4 of this book?

   Hint. One word in the phrase 'static charges' is the hinge.

   The chapter's link. When the charges move, they constitute an electric current in an electrical circuit. It is the same current which makes a lamp glow or generates a heating effect or a magnetic effect.

   Charges	What you get	Where in the book
   Static — built up by rubbing, not moving	Electrostatic force; paper pieces attracted, balloons repelled	Chapter 5, this section
   Moving	An electric current	Chapter 4

   ✦ This is a genuinely large idea compressed into three lines. The charges rubbed onto a plastic scale and the current flowing through a nichrome wire are the same thing in two different states — one sitting still on a surface, the other flowing along a conductor. Rubbing a scale and lighting a lamp look like completely unrelated phenomena, and they are not.

   It also explains the word static. The name distinguishes charges that stay where they are from charges in motion, which is why 'static electricity' and 'current electricity' are named the way they are.

   Chapter 4's magnetic and heating effects were both effects of moving charges, though it did not put it that way at the time.

7. 73 marksCuriosity Grade 8, Chapter 5, pages 69-71

   In what way are magnetic force and electrostatic force alike, and in what way do they differ from gravitational force?

   Hint. Two similarities and one sharp difference.

   How magnetic and electrostatic force are alike:

   	Magnetic	Electrostatic
   Contact needed?	No — non-contact	No — non-contact
   Attract only, or both?	Both — like poles repel, unlike attract	Both — like charges repel, unlike attract
   Two kinds involved	North and South poles	Positive and negative charges
   Acts on uncharged/unmagnetised objects?	Yes — a magnet attracts magnetic materials	Yes — a charged body attracts uncharged paper

   The parallel is close enough that the two sections of the chapter are written almost in step with each other.

   How gravitational force differs. Gravitational force is always an attractive force, unlike magnetic force or electrostatic force, which can either be attractive or repulsive.

   ✦ This is the sharpest single fact about gravity in the chapter, and it is easy to read past. There is no anti-gravity, no repelling pair, nothing corresponding to a north pole and a south pole. Every gravitational force ever observed is a pull. That is why a ball thrown upwards always comes back and never gets pushed away, and why the chapter's project asking you to imagine a scenario where the gravity disappears is science fiction rather than an experiment.

   All three are non-contact forces, and that is the point of grouping them in section 5.4.2.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity, Textbook of Science for Grade 8, Chapter 5 'Exploring Forces', book pages 62-79 (hecu105.pdf, 18 pages, Reprint 2026-27), downloaded from ncert.nic.in and read page by page. Every activity number, figure number, quantity and quoted sentence below was checked against that PDF. Fig. 5.17 (exercise 10) was MEASURED, not eyeballed: the page was rendered at 1200 dpi and the three cylinders and their waterlines located by colour segmentation. Taking the waterline at the ellipse's mid-height gives submerged fractions of about 82%, 59% and 32% for objects 1, 2 and 3; taking it at the ellipse's top edge gives 69%, 46% and 19%. Both methods give the same strict ordering 1 > 2 > 3, so object 1 displaces the most water, has the largest buoyant force and therefore the largest weight - answer (ii), w1 > w2 > w3. Fig. 5.13's scale was also read directly: NEWTONS 0 to 10 N alongside GRAMS 0 to 1000 g, so 1000 g lines up with 10 N, consistent with the planet table on page 75. Three deliberate restraints on what is claimed. (1) The chapter is entirely qualitative and contains no formula. Nothing here uses F = ma, W = mg, F = Gm1m2/r^2, a value of g, or Newton's laws of motion - none of which is in this book. Balanced forces are named once and explicitly deferred by the chapter to higher grades, and this file defers them too. (2) The swing question in Probe and ponder is answered by distinguishing weight from the seat's upward push, with an explicit note that the chapter does not explain it and that the full account needs later ideas. (3) Buoyancy is explained purely by comparing two forces, as the chapter does, because density is not defined until a later chapter of the same book; the chapter's own phrase 'less dense than water' is quoted only where the book itself uses it.. Questions are referenced from the NCERT textbook for identification.