Where Mindful Learning Begins
Where Mindful Learning Begins
Class 8 Science Curiosity NCERT Solutions Chapter 4 — every Probe and Ponder prompt, all 6 Activities, all 11 "Keep the Curiosity Alive" exercise questions, and all 3 "Discover, Design, and Debate" project prompts, solved and explained on one page.
This chapter shows how electric current produces a magnetic field (the basis of electromagnets), why current-carrying wires get hot (the basis of heaters and irons), and how cells and batteries — from Voltaic cells to lemon cells to rechargeable Li-ion batteries — generate electricity in the first place.
Chapter 4 opens with a science-exhibition model of a lifting electromagnet, and uses it to ask: how can an electric circuit, with no magnet involved, pick up iron paper clips? The chapter answers this in three linked stages — first showing that current-carrying wires produce a magnetic field (deflecting a compass), then that coiling the wire and adding an iron core turns it into a strong, controllable electromagnet, then that current flowing through any conductor also generates heat (more so in high-resistance wires like nichrome), and finally, how cells and batteries — from simple lemon cells to rechargeable Li-ion batteries — actually generate that current through chemical reactions. Every Activity and exercise is solved here exactly as the textbook presents it.
A current-carrying wire creates a magnetic field — the discovery that connects electricity and magnetism.
Resistance in a conductor converts electrical energy to heat — the basis of heaters, irons, and kettles.
From Voltaic cells and dry cells to lemon cells and rechargeable Li-ion batteries — all powered by chemical reactions.
Magnetic effect of electric current
A current-carrying conductor produces a magnetic field around it — detectable by the deflection of a magnetic compass. The field disappears the moment the current stops.
How to strengthen an electromagnet
| Change | Effect on strength |
|---|---|
| Increase the number of cells (more current) | Stronger magnetic field |
| Increase the number of turns in the coil | Stronger magnetic field |
| Insert an iron core inside the coil | Much stronger magnetic field |
| Reverse the direction of current | Reverses the electromagnet's poles |
Heating effect of electric current
Every conductor resists current flow to some degree; this resistance converts part of the electrical energy into heat. High-resistance wires like nichrome heat up far more than low-resistance wires like copper for the same current — which is why nichrome is used in heaters, irons, and kettles.
Types of electric cells
| Type | Electrolyte | Key feature |
|---|---|---|
| Voltaic (Galvanic) cell | Liquid — weak acid or salt solution | Two different metal electrodes; becomes "dead" once chemicals are used up |
| Dry cell | Thick moist paste (not liquid) | Zinc container (negative terminal), carbon rod (positive terminal); single-use |
| Rechargeable battery (e.g. Li-ion) | Varies by type | Can be recharged and reused many times before wearing out |
Sample answer: even without an electric lamp, we can detect whether current is flowing in a circuit by placing a magnetic compass near the wire — if current flows, the compass needle deflects due to the magnetic field the current produces; if the needle stays still, no current is flowing. This chapter shows exactly this idea in Activity 4.1.
Yes, temporary magnets can be made — an electromagnet, made by wrapping a wire coil around an iron nail and connecting it to a cell, behaves like a magnet only while current flows through it, and loses its magnetism the moment the circuit is switched off. Heat in electrical appliances (like heaters, irons, and kettles) is generated because current flowing through a resistant wire, such as nichrome, converts some electrical energy into heat — this is different from burning fuel, since no combustion is involved. A cell or battery is "dead" when the chemical reaction inside it has used up its usable chemicals and it can no longer supply current — not all cells can be recharged: dry cells are single-use, while rechargeable batteries (like Li-ion batteries) can be recharged and reused many times, though they too eventually wear out.
This is an open reflection prompt meant to set up the chapter's four big ideas — detecting current via magnetism, making temporary magnets, the heating effect, and how cells work — all of which are explained in full through the Activities below.
Observation: when the switch is turned ON and current flows through the wire, the compass needle deflects from its original north-south direction. When the switch is turned OFF and current stops, the needle returns to its original direction. Repeating this shows the same pattern each time — deflection while current flows, and return to normal the moment it stops.
Why this happens: the compass needle is itself a tiny magnet, and it deflects whenever a magnetic field acts on it. Since the needle only deflects while current flows through the wire, this shows that a current-carrying wire produces a magnetic field around it — this is called the magnetic effect of electric current. The region around the wire (or a magnet) where this magnetic effect can be felt is called a magnetic field.
Observation: when the coiled wire is connected to the cell (current flowing), the iron nail attracts and picks up the iron paper clips, just like a magnet. When the wire is disconnected from the cell (current stopped), the clips fall off — the nail is no longer magnetic.
Why this happens: this confirms that the magnetic effect of electric current, seen in Activity 4.1, can be used to temporarily magnetise an iron nail simply by wrapping a current-carrying coil around it. The magnetism exists only as long as current flows through the coil.
Without the iron nail: when current is passed through the cylindrical coil alone, it behaves like a weak magnet, causing some deflection in the compass needles at either end. Disconnecting the wire returns the needles to their original position.
With the iron nail inserted: the deflection of the compass needles becomes much greater, and the coil now attracts iron paper clips placed near the nail's ends — showing it has become a much stronger magnet. When the current is stopped, the coil (with or without the nail) loses its magnetic effect entirely.
Conclusion: a current-carrying coil that behaves as a magnet is called an electromagnet. Inserting an iron core inside the coil makes the electromagnet significantly stronger, which is why most practical electromagnets are built with an iron core.
Finding the polarity: since unlike poles of two magnets attract each other, if the north pole of the compass is attracted towards end A of the electromagnet, then end A must be acting as the south pole. Repeating this at end B shows its polarity is opposite to end A — confirming that, just like a bar magnet, an electromagnet has two distinct poles, North and South.
Going further — effect of more cells and more turns: a single cell provides only a small current, producing a weak magnetic field, so the compass shows less deflection and the coil attracts only a few clips. A battery with more cells provides a larger current, producing a stronger magnetic field — more compass deflection and more clips attracted. Increasing the number of turns in the coil also strengthens the electromagnet, for the same reason: more turns means a stronger magnetic field for the same current.
Going further — reversing the current direction: reversing the direction of current flow through the coil reverses the polarity of the electromagnet — the end that was previously the south pole becomes the north pole, and vice versa.
Conclusion: an electromagnet's strength can be increased by increasing the current (more cells) or the number of turns in the coil, or both — and its poles can be reversed simply by reversing the direction of current.
Observation: before the switch is turned on, the nichrome wire feels at room temperature. After the switch is moved to ON for about 30 seconds and then switched OFF, the wire feels noticeably warm when touched momentarily (with care, and not for long, as instructed under Safety First).
Why this happens: when electric current flows through any conductor, it faces some resistance. Nichrome offers much higher resistance than a copper wire of the same size and length, and this resistance converts part of the electrical energy into heat energy. This warming of a current-carrying conductor is called the heating effect of electric current.
Going further — with 2 cells instead of 1: for the same duration, the wire heats up more with a battery of 2 cells than with a single cell. This is because the amount of heat generated depends on the magnitude of the electric current — a larger current (from more cells) produces more heat. The total heat generated also depends on the wire's material, thickness, length, and the duration for which current flows.
Observation: once the copper wires and iron nails are correctly connected across all the lemons and the LED is joined at the two free ends, the LED glows — but only if its terminals are connected the correct way round (the LED's longer/positive wire to the battery's positive end, shorter/negative wire to the negative end). If it doesn't glow at first, reversing the LED's connections should make it light up.
Why this happens: a glowing LED confirms that the lemon setup is working as an electric cell. Here, the copper wires and iron nails act as the two different metal electrodes, and the acidic lemon juice acts as the electrolyte that enables the chemical reaction needed to generate current — exactly the same principle as a Voltaic cell, just using easily available household materials instead of manufactured electrodes.
The discovery made in Activity 4.1 — that a current-carrying wire deflects a nearby compass needle — is exactly the discovery made by Danish physics professor Hans Christian Oersted (1777–1851) in 1820.
According to the story, Oersted noticed during a lecture demonstration that whenever an electrical circuit nearby was closed or opened, the needle of a magnetic compass lying close by would deflect. Rather than dismiss this as a coincidence, he investigated it carefully, and once he was certain that an electric current indeed produced a magnetic field, he published his findings.
This publication led other scientists to repeat his experiment and confirm the results, sparking further research into the deep connection between electricity and magnetism — a connection that eventually led to electromagnets, electric motors, and generators.
Deep inside the Earth, the movement of liquid iron in the core generates electric currents, which in turn create Earth's magnetic field — this is why a freely suspended magnet always settles along the north-south direction. Many migratory birds, fish, and animals use this field to navigate, and it also shields Earth from harmful particles from space.
Just as electricity can produce magnetism (as seen in this chapter), a moving magnet can also produce an electric current — a deeper connection explored in higher grades, which forms the basis of devices from electric motors to power generators.
To prevent unnecessary or dangerous heating in household switchboards, it's important to use wires, plugs, and sockets rated for the specific electric current they'll carry — mismatched ratings are a common cause of overheating and electrical fires.
Beyond household appliances, the heating effect of electric current is used industrially — for example, in steel manufacturing, specially designed high-temperature electric furnaces melt and recycle scrap steel into usable steel.
The Voltaic (or Galvanic) cell is named after two Italian scientists. In the late 1700s, Luigi Galvani noticed a dead frog's leg twitched when touched with two different metals, and believed the electricity came from the frog itself. Alessandro Volta disagreed — using saltwater-soaked paper instead of a frog's leg, he still got a current, proving the electricity came from the combination of metals and liquid, not the frog. This insight led to the invention of the first battery.
Common metal pairs used in Voltaic cells include zinc/copper, zinc/silver, aluminium/copper, iron/copper, magnesium/copper, and lead/copper. Some metals, like copper, tend to act as the positive electrode, while others, like zinc, act as the negative electrode — a result of their differing chemical properties, explored further in higher grades.
Today's most common rechargeable battery, the lithium-ion (Li-ion) battery, relies on metals like lithium and cobalt that are mined in limited parts of the world. Scientists are now developing solid-state batteries, which replace liquid/paste electrolytes with solid materials — expected to be safer, faster-charging, and longer-lasting.
Even a battery that has stopped working isn't completely inert — it can still contain acids and metals like lead, cadmium, nickel, or lithium, which may cause fires or environmental harm if thrown in regular garbage. Many of these materials are valuable and recyclable, which is why e-waste recycling facilities exist for proper battery disposal.
(i) The solution used in a Voltaic cell is called an electrolyte.
(ii) A current-carrying coil behaves like a magnet (specifically, an electromagnet).
(i) False. Dry cells are actually far more portable than Voltaic cells, since they use a solid/paste electrolyte instead of a liquid one, allowing them to be sealed compactly — this is exactly why dry cells, not Voltaic cells, are used in everyday portable devices.
(ii) True. A coil only behaves as an electromagnet while current flows through it; the moment the current stops, the magnetic effect disappears.
(iii) False. A battery with 2 cells provides a larger current than a single cell, creating a stronger magnetic field — so the electromagnet with 2 cells attracts more clips, not fewer.
Answer: (c) Both (i) and (ii) are correct. Any current-carrying conductor, including nichrome wire, produces both a magnetic effect (deflecting a nearby compass) and a heating effect (warming the wire due to resistance) — these are two separate but simultaneous effects of the same flowing current, not mutually exclusive ones.
| Column A | Matches with |
|---|---|
| (i) Voltaic cell | (d) Generates electricity by chemical reactions |
| (ii) Electric iron | (c) Works on heating effect of electric current |
| (iii) Nichrome wire | (a) Best suited for electric heater |
| (iv) Electromagnet | (b) Works on magnetic effect of electric current |
Answer: (ii) it generates more heat for a given current. Nichrome has a notably higher resistance than metals like copper of the same size, and this higher resistance is exactly what converts more electrical energy into heat for the same current flow — which is why it's chosen specifically for heating elements. Option (iv) is incorrect since nichrome is a conductor, not an insulator, and option (iii) isn't the scientific reason it's used, even if it happens to be true in some cases.
(i) Path of the current: current flows out from the positive terminal of the cell, through the connecting wire to one end of the coil (end A), around each turn of the coil, out through the other end (end B), through the switch, and back to the negative terminal of the cell, completing the circuit.
(ii) Why the compass needle moves: as current flows through the coil, it produces a magnetic field around it (the magnetic effect of electric current). The compass needle is itself a small magnet, so it aligns with this new magnetic field near it, causing it to deflect from its usual north-south direction.
(iii) If the battery terminals are reversed: reversing the terminals reverses the direction of current flow, which reverses the direction of the magnetic field produced by the coil. As a result, the compass needle would deflect in the opposite direction to before (assuming the same current strength).
Most likely reason: after being left connected for an extended time, the cell has weakened considerably (cells weaken quickly when connected continuously, as noted in Activity 4.2) — the current flowing through the coil has dropped significantly. Since the strength of an electromagnet's magnetic field depends directly on the amount of current flowing, a much-weakened current produces too weak a magnetic field to lift the iron paper clips, even though some current is still technically flowing.
Why the wire is still warm despite this: heat builds up gradually in a wire over the time current has been flowing, and takes time to dissipate — so even after the magnetic effect has become too weak to lift clips, residual heat from the sustained current flow can still make the wire feel warm to the touch.
Answer: the LED will glow in the lemon juice setup (a), not the pure water setup (b).
Reason: lemon juice is a weak acid, rich in ions that can conduct electric current and enable the chemical reaction between the two metal electrodes (iron nail and copper strip) needed to generate current — making it an effective electrolyte. Pure water, in contrast, has very few free ions and is a poor conductor of electricity, so it cannot function as an effective electrolyte, and no meaningful current is generated to light the LED.
Answer: yes, the coil will still deflect the compass, but the deflection will be less than before.
Reason: a current-carrying coil produces a magnetic field and behaves as an electromagnet even without an iron core, as shown in Activity 4.3 (before the nail was inserted). Removing the iron nail doesn't stop the coil from generating a magnetic field — current is still flowing through it — but the iron core is what makes the electromagnet significantly stronger. Without it, the magnetic field is weaker, so the compass needle will deflect by a smaller amount than when the iron nail was in place.
Answer: (iv) In all four circuits.
Reason: the magnetic effect of electric current arises simply from current flowing through any conductor — it doesn't depend on which particular metal the conductor is made of. Iron, copper, aluminium, and nichrome are all conductors, so as long as current flows through each coil, each one will produce a magnetic field around it and deflect a nearby compass needle. The material of the wire affects properties like resistance (and therefore how much the wire heats up), but not whether the magnetic effect occurs at all.
These three prompts are hands-on investigations rather than fixed-answer questions. Here's the expected pattern of results and guidance on writing up each one.
Expected observation: as the number of turns increases from 25 to 100 (keeping the cell, and therefore the current, the same), the compass needle should deflect by a progressively greater amount, and the coil should be able to attract progressively more iron paper clips.
Conclusion to draw: for a fixed current, increasing the number of turns in a coil increases the strength of the electromagnet — more turns concentrate the magnetic field more effectively, producing a stronger overall magnetic effect. This matches the principle explored in Activity 4.4, where more turns (alongside more cells) was shown to strengthen an electromagnet.
Equal length, different thickness (e.g. 0.3 mm vs 0.6 mm): the thinner wire (0.3 mm) should heat up more. A thinner wire has a smaller cross-sectional area, which gives it higher resistance for the same length — and higher resistance means more electrical energy converted to heat for the same current.
Equal thickness, different length: the longer wire should heat up more. A longer wire of the same thickness has greater overall resistance (resistance increases with length), so again, more electrical energy is converted to heat.
Report conclusion: both thickness and length affect a wire's resistance, and therefore how much it heats up for a given current — thinner and longer wires offer more resistance and generate more heat than thicker and shorter wires of the same material.
Guidance: following the same method as the lemon cell in Activity 4.6, try inserting two different metal electrodes (such as copper and iron/zinc) into other fruits or vegetables — like a potato, tomato, or orange — and testing whether an LED lights up when connected. More acidic fruits and vegetables (like lemon, tomato, or orange) tend to work better as electrolytes than less acidic ones (like a potato), since a stronger acid provides more ions to conduct the current and drive the chemical reaction.
On metal pairs: as noted in the chapter, different metal pairs (such as zinc/copper, iron/copper, or aluminium/copper) may produce different results, since some metals act as positive electrodes and others as negative electrodes depending on their chemical properties — a good report would note which combinations produced the brightest or most reliable glow.
Now that magnetic and heating effects are covered, move on to the forces that speed objects up, slow them down, or change their direction, revisit Chapter 3, or book a free demo class for personalised coaching.
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