In-text — Dry Cells and Rechargeable Batteries — "Electricity: Magnetic and Heating Effects"

1. 13 marksCuriosity Grade 8, Chapter 4, page 57, section 4.3.2

   Why are dry cells called 'dry'? Describe the structure of a dry cell as shown in Fig. 4.9.

   Hint. The name is a contrast with the Voltaic cell of the previous section.

   Why 'dry'. They are called 'dry' because the electrolyte is not a liquid but a thick moist paste. The name is a direct contrast with the Voltaic cell, whose electrolyte is a liquid standing in an open container.

   Structure (Fig. 4.9).

   Part	What it is
   Zinc container	The outer casing — it acts as a negative terminal
   Carbon rod	At the centre, covered with metal cap that acts as the positive terminal
   Metal cap	The raised cap on top of the carbon rod — the positive terminal you can see
   Electrolyte	The paste-like electrolyte surrounding the carbon rod

   ✦ Why the paste matters so much. Because there is no liquid to spill, the cell can be sealed, carried in a pocket, and used in any position. The chapter's reason for introducing dry cells is exactly this: Voltaic cells were an important discovery, but they are not convenient for everyday use. You could not put a beaker of acid inside a torch.

   And notice which parts are the electrodes. The two electrodes are still two different materials — zinc and carbon — just as the Voltaic cell needs two different materials. The container is not merely a container here; it is one of the electrodes.

2. 23 marksCuriosity Grade 8, Chapter 4, page 57

   What does it mean to say a dry cell is a 'single use' cell? What has replaced dry cells in many applications, and why?

   Hint. Think about what happens once the chemicals inside are used up.

   Single use. The dry cell is a single use cell, meaning once it is used up, it has to be disposed of. The chemical reaction inside runs in one direction only. When the chemicals are spent, the cell is dead and cannot be brought back — connecting it to a charger will not restore it.

   What has replaced it. For several applications, rechargeable batteries are increasingly being used now.

   Why, in the chapter's words: Rechargeable batteries can be recharged and reused multiple times. This prevents wastage and saves money over time as well.

   	Dry cell	Rechargeable battery
   Reused?	No — single use	Yes, many times
   After it runs down	Disposed of	Recharged
   Waste	One cell per use	One battery for many uses
   Lasts forever?	No	No — it wears out after many cycles

   ✦ Two savings, not one. The chapter names both: less wastage, because one battery does the work of many dry cells, and less money over time, even though the rechargeable battery costs more to buy in the first place.

   But 'disposed of' does not mean 'thrown in the bin'. The chapter's closing box is explicit that used cells belong at an e-waste facility.

3. 33 marksCuriosity Grade 8, Chapter 4, pages 57-58, section 4.3.3

   What are rechargeable batteries? Give the range of applications the chapter mentions.

   Hint. Fig. 4.10 shows five, and the text names more, from very small to very large.

   What they are. Batteries that can be recharged and reused multiple times, rather than being thrown away when they run down. This prevents wastage and saves money over time as well.

   The range of applications the chapter gives: from small batteries used in watches and phones to batteries used in laptops and tablet to bigger batteries that run inverters or drive electric vehicles. Fig. 4.10 shows them in laptops, mobile phones, cameras, inverters and vehicles.

   ✦ Why the chapter stresses the range. There are many different kinds of rechargeable batteries that are used for different applications — a watch battery and an electric-car battery are not the same device made larger. Each application needs a different size, a different amount of stored energy and a different rate of supply, so different kinds of battery are made for them.

   The largest example on the list is the important one. Electric vehicles run entirely on their batteries, which is why the chapter returns at the end to the supply of lithium and cobalt and to the search for better batteries — improved rechargeable batteries are very important as the world moves to developing environmentally friendly sources of electrical power.

4. 43 marksCuriosity Grade 8, Chapter 4, page 57

   Do rechargeable batteries last forever? Explain the chapter character's remark: 'Oh, so this is the reason why after a year or two, the phone battery requires charging more often!'

   Hint. Recharging restores the battery, but not quite completely, and not indefinitely.

   No. However, rechargeable batteries also do not last forever. After being charged and used many times, they slowly wear out.

   What the character has noticed. A new phone battery holds enough charge for a day. After a year or two of daily charging and discharging, the same battery holds less, so it runs down sooner and has to be plugged in more often. The battery has not suddenly failed — it has gradually worn out through use.

   The word to hold on to is slowly. This is not the sudden death of a dry cell, which works and then does not. A rechargeable battery declines by degrees, and you notice it as an inconvenience long before it stops working altogether.

   ✦ How this fits the three kinds of cell in the chapter:

   	Voltaic cell	Dry cell	Rechargeable battery
   Recharge?	No	No	Yes
   End of life	Chemicals used up	Chemicals used up	Wears out after many charge cycles
   Convenient to carry?	No — liquid electrolyte	Yes — paste electrolyte	Yes

   The chapter does not say how many charges a battery lasts, and neither should you. It depends on the type of battery and how it is used, and no figure is given in the text.

5. 54 marksCuriosity Grade 8, Chapter 4, page 58, A step further

   What is the most common type of rechargeable battery today? Why are countries racing to secure supplies and recycle old batteries?

   Hint. The reason is in where the materials come from, not in how the battery works.

   The most common type. Today, the lithium-ion (Li-ion) battery is the most common type of rechargeable battery, found in almost all devices that use batteries.

   Why the race. These batteries rely on special metals like lithium and cobalt, which are mined and processed in limited parts of the world. Because of this, countries are now racing to secure supplies, recycle old batteries, and develop new technologies.

   The reasoning in three steps:

   1. Li-ion batteries are in almost everything — phones, laptops, tools, electric vehicles, grid storage.
   2. They need lithium and cobalt, and those are mined and processed in only a few parts of the world.
   3. So a country that cannot obtain them cannot make the batteries its industry and transport depend on. Hence three responses at once: secure supplies, recycle old batteries to recover the metals, and develop technologies that need less of them.

   ✦ Recycling appears here for a second, quite different reason. At the end of the chapter, recycling batteries is presented as an environmental duty — keeping harmful metals out of the ground. Here it is a supply strategy: an old battery is a source of lithium and cobalt that does not have to be mined. The same action, two independent arguments for it.

   The chapter names no countries and gives no quantities, and this answer follows it. Which nations hold which reserves is a matter of current geography and politics, and any specific figures would need to be checked against a current source.

6. 63 marksCuriosity Grade 8, Chapter 4, page 58, A step further

   What are solid-state batteries, and what advantages are they expected to have?

   Hint. The name tells you what has been replaced.

   What they are. Scientists are also working on the next big leap: solid-state batteries, which replace the liquid or paste-like electrolytes with solid materials.

   The name comes straight from that change. Every cell in this chapter has needed an electrolyte — a liquid in the Voltaic cell, a moist paste in the dry cell. A solid-state battery uses a solid electrolyte instead.

   The expected advantages. These future batteries would be much safer, charge faster and last longer.

   ✦ Note the tense the chapter uses, and keep it. Scientists are working on, would be, future batteries. These are batteries under development and the advantages are expected, not demonstrated in everyday use. It would be wrong to write that solid-state batteries are safer and faster, as though you could buy one today. This is an honest report of research in progress.

   Why the work matters. Improved rechargeable batteries are very important as the world moves to developing environmentally friendly sources of electrical power. Sources such as solar and wind do not produce steadily all day, so storing what they produce is part of using them at all — and that storage is a battery problem.

7. 74 marksCuriosity Grade 8, Chapter 4, page 61, A step further

   Why should used batteries not be thrown into regular garbage? What should be done with them instead?

   Hint. The chapter gives two separate reasons — one about harm, one about value.

   The chapter's warning. Even when a battery stops working, it is not completely 'dead'. It could still contain materials like acids, and metals like lead, cadmium, nickel, or lithium, which may cause fires, or be harmful for the environment if the battery is thrown in regular garbage.

   Two independent reasons, not one:

   Reason	What is at stake
   Harm	Acids and metals such as lead, cadmium, nickel and lithium may cause fires or damage the environment if they end up in ordinary waste
   Waste of value	Many materials used in these batteries are valuable and could be recycled and reused

   What to do instead. These days, there are many places with special 'e-waste' recycling facilities, where used batteries can be disposed of. If you are not sure, ask your teacher.

   ✦ Read the first sentence of that box again — it is the key to the whole passage. 'Dead' in this chapter has a narrow meaning: the chemical reaction that produced electricity has stopped. It does not mean the battery is now an inert lump, because the materials are all still there, still reactive and still valuable. That is why a battery is finished as a source of current long before it is finished as a piece of chemistry, and why it needs handling rather than discarding.

   The chapter's closing line: Recycling batteries is good for the planet and the people.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity, Textbook of Science for Grade 8, Chapter 4 'Electricity: Magnetic and Heating Effects', book pages 46-61 (hecu104.pdf, 16 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. Figures read at high resolution: Fig. 4.4a (compass, coil ends A and B, and cell polarity - the cell's - terminal is on the left and + on the right, and the red north tip of the needle has swung towards end A, so end A is the south pole for the connections shown); Fig. 4.12 (a: iron nail and copper strip in lemon juice, b: same electrodes in pure water); Fig. 4.13 (four coils of iron, copper, aluminium and nichrome, each with a cell and a switch). Three deliberate restraints on what is claimed. (1) The chapter gives the heating effect entirely qualitatively - 'the heat generated depends on the magnitude of the electric current' and on 'the material, thickness, length of the wire, and the duration' - and gives no formula. No formula is supplied here either; Joule's law, Ohm's law, power and kWh belong to a later grade and appear nowhere in this book. (2) The two wire-heating projects are answered as experiments to be performed, not as results to be reproduced: the thickness comparison is explained, and the length comparison is explicitly left open because two effects work against each other and the chapter offers no prediction. (3) Solid-state batteries are reported in the chapter's own tense - under development, advantages expected - and the lithium and cobalt supply question is described without naming countries or quantities, since the chapter names none.. Questions are referenced from the NCERT textbook for identification.