Activity 4.6 — Voltaic Cells and the Lemon Cell — "Electricity: Magnetic and Heating Effects"

1. 14 marksCuriosity Grade 8, Chapter 4, page 55, section 4.3.1

   Describe the construction of a Voltaic cell (Fig. 4.7). Name its parts.

   Hint. Two rods, one liquid, one container — and the rods must not be the same metal.

   Construction. A Voltaic cell, also known as a Galvanic cell, contains two metal rods made of different materials and a liquid called an electrolyte, placed in a glass or plastic container.

   Part	Description
   Electrodes	The two metal rods, partly dipped in the liquid. They must be of different materials
   Electrolyte	The liquid — usually a weak acid or salt solution
   Container	Glass or plastic, holding the electrolyte

   In Fig. 4.7 the cell is shown connected by wires to an electric lamp, with the current flowing round the external circuit.

   ✦ The word 'different' is doing real work in that definition. Two rods of the same metal in the same electrolyte will not produce a current. It is the combination of two dissimilar metals with a liquid that generates electricity — which is exactly what Volta established, and what Activity 4.6 reproduces with a copper wire and an iron nail in a lemon.

   Why it is called a cell and not a battery. A cell is one such unit. Join several together, as the six lemons are joined in Activity 4.6, and you have a battery.

2. 23 marksCuriosity Grade 8, Chapter 4, page 55

   What produces the electricity in a Voltaic cell? Why does a cell eventually become 'dead'?

   Hint. No wire is being moved and no fuel is being burnt. Something chemical is going on.

   What produces the electricity. A chemical reaction between the rods and the electrolyte produces electricity. When the circuit is connected, electric current flows from the positive terminal through the circuit to the negative terminal.

   The Snapshots state the general principle: A cell or a battery is a device that generates electric current because of chemical reactions taking place inside it.

   Why it goes dead. Over time, the chemicals get used up, and the cell stops working. It is then called 'dead' and cannot supply any more electricity.

   ✦ This is what makes a cell fundamentally different from a wire. A copper wire carries current for as long as you like without changing. A cell is being consumed the whole time it works — it is spending something, and when the store runs out there is nothing to be done about it. A Voltaic or dry cell cannot be refilled by connecting it to a charger.

   And it explains the answers to two other questions in this chapter. Why does the electromagnet in exercise 8 stop lifting clips? Because the cell has been left connected and its chemicals are used up. Why must you not leave the coil connected in Activity 4.2? Same reason.

3. 35 marksCuriosity Grade 8, Chapter 4, page 55, Ever heard of ...

   Galvani and Volta disagreed about where the electricity in the frog's-leg experiment came from. What did each think, how did Volta settle it, and what does the episode show about how science works?

   Hint. The whole disagreement turns on one question — is the frog necessary?

   The observation. In the late 1700s Luigi Galvani noticed that a dead frog's leg kicked when touched with two different metals — copper and iron. It was already known by then that electricity could stimulate muscular motion.

   The two explanations.

   	Galvani	Volta
   Where does the electricity come from?	From the frog itself	From the metals, not the frog

   How Volta settled it. To test this, he used saltwater-soaked paper instead of the frog's leg and still got an electric current.

   ✦ Why that single change was decisive. Both men were looking at the same experiment, and both explanations fitted it. What Volta did was remove the one thing the two explanations disagreed about. If the electricity needed the frog, taking the frog away must stop it. It did not stop. So the frog was not the source.

   The conclusion, in the chapter's words: This showed that it was the combination of metals and liquid that generated electric current — leading to the invention of the first battery!

   What the episode shows about science. A disagreement between two scientists is not settled by argument or by seniority. It is settled by designing an experiment whose result the two explanations predict differently, and then performing it. Volta's saltwater-soaked paper is a model of how that is done.

   And Galvani was not simply foolish. His observation was real and important — it is why the cell is called Galvanic as well as Voltaic. He read it wrongly; he did not imagine it.

4. 44 marksCuriosity Grade 8, Chapter 4, page 56, Activity 4.6

   Describe how the lemon cell is made in Activity 4.6.

   Hint. Six lemons, each with two different metals in it, all joined in a chain.

   Materials. Five or six juicy lemons, copper wires or strips 1–2 mm thick, iron nails, one LED, and some connecting wires.

   Procedure.

   1. Insert a copper wire and an iron nail into one lemon, keeping them apart by a small distance (Fig. 4.8a).
   2. Do the same for every remaining lemon.
   3. Join the copper wires and nails as shown in Fig. 4.8b — the copper of one lemon to the nail of the next, and so on down the chain.
   4. Connect the LED between the copper wire of the first lemon and the iron nail of the last lemon, using connecting wires.

   Result. A glowing LED indicates that your cell is working.

   ✦ Why the two metals must not touch. They are the two electrodes, and the current in the external circuit must run from one electrode out through the LED and back to the other. If the copper and the nail touched inside the lemon, the current would take that short path instead and the LED would not glow.

   Why five or six lemons rather than one. A single lemon cell produces only a very small effect. Joined in a chain, they act like several cells in a battery, and together they are enough to light an LED — which needs far less than a filament lamp.

5. 53 marksCuriosity Grade 8, Chapter 4, page 56, Activity 4.6

   In the lemon cell, what are the electrodes and what is the electrolyte? What else could be used in place of lemon juice?

   Hint. Match each part of the lemon cell to the corresponding part of Fig. 4.7.

   The parts, matched to the Voltaic cell of Fig. 4.7:

   Voltaic cell	Lemon cell
   Two electrodes of different metals	The copper wire and the iron nail
   Electrolyte — a weak acid or salt solution	The lemon juice, which helps conduct electricity
   Glass or plastic container	The lemon itself holds the juice

   An alternative. The chapter says: You may also use salt solutions instead of lemon juice. That fits the definition given for a Voltaic cell, whose electrolyte is usually a weak acid or salt solution — lemon juice is the acid case, salt water the salt case.

   ✦ The lemon is not a battery in itself. It is easy to think the lemon is somehow storing electricity. It is not — it is only supplying the electrolyte, the liquid in which the reaction happens. Take the two metals out and the lemon does nothing; put the two metals into salt water and it works just as well. The chapter's final project invites you to test exactly this, by trying other fruits and vegetables.

   A prediction you can now make, which exercise question 9 asks you to check: pure water is not an acid or a salt solution, so it should not work as an electrolyte.

6. 63 marksCuriosity Grade 8, Chapter 4, page 56, Activity 4.6

   The activity says that if the LED does not glow, you should reverse its connections. Why would that make a difference?

   Hint. An LED is not like a lamp — it cares which way round it is joined.

   Because an LED only passes current in one direction. The chapter recalls the rule from an earlier grade: current can pass through the LED only when the positive terminal (longer wire) of the LED is connected to the positive terminal of the battery, and negative terminal (shorter wire) of the LED is connected to the negative terminal of the battery.

   So if the LED does not glow the first time, one likely reason is simply that it is the wrong way round. Swap its two connections and, if the cell is working, it lights.

   How you tell the LED's terminals apart: the longer wire is positive, the shorter wire is negative.

   ✦ Why the activity is worth doing carefully. Reversing the LED is a diagnostic step, not a fix — it tells you which problem you have. If it glows after reversing, the cell was fine all along. If it still does not glow, the problem lies somewhere else: the metals may be touching inside a lemon, a joint may be loose, the lemons may not be juicy enough, or too few are connected.

   A lamp would not behave this way. A filament lamp glows whichever way round it is connected, because it works by the heating effect and heat does not care about direction. The LED's one-way behaviour is a different property altogether — which is also why an LED is used here rather than a lamp: it lights on a much smaller current.

7. 73 marksCuriosity Grade 8, Chapter 4, page 56, A step further

   Name some common metal pairs used in Voltaic cells. Why is one metal of a pair the positive electrode and the other the negative?

   Hint. The chapter gives the list, and is honest about how far it can explain it.

   Common metal pairs, as the chapter lists them: zinc/copper, zinc/silver, aluminium/copper, iron/copper, magnesium/copper, and lead/copper.

   Which is which. Some metals — like copper — act as positive electrodes, yet some other metals — like zinc — act as negative electrodes.

   Why. This is due to their chemical properties. We will learn more about this in the higher grades.

   ✦ Take the chapter at its word here. It states the fact and names the reason in the broadest terms — chemical properties — and explicitly defers the explanation. That is an honest position, not an omission, and the right answer at this stage says exactly the same. Inventing a fuller explanation would be pretending to knowledge the chapter has not given you.

   A pattern worth noticing in the list. Copper appears in five of the six pairs, and in every one of those it is the positive electrode. In the lemon cell of Activity 4.6 the pair is iron/copper, which is on the list — the copper wire is the positive electrode and the iron nail the negative, which is why the LED is connected from the copper of the first lemon to the nail of the last.

   Each pair also needs a suitable electrolyte; the metals alone are not a cell.

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.