Electricity: Magnetic and Heating Effects
Chapter 4 · Curiosity — Science Textbook for Grade 8 · Complete Solutions
This page provides step-by-step, detailed answers to every in-text (Probe & Ponder, Activity reflection, "Think like a scientist") question and every end-of-chapter exercise question from Chapter 4 — Electricity: Magnetic and Heating Effects (NCERT Curiosity, Grade 8). Diagrams are recreated for clarity wherever needed.
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In-Text Questions
Probe and Ponder · Page 46
1. If we don't have an electric lamp while making an electric circuit with an electric cell, is there any other way through which we can find out if current is flowing in the circuit?
Answer
Yes. We can use a magnetic compass. If we place a magnetic compass below or near a wire connected in the circuit, the compass needle will get deflected from its original (North–South) direction whenever current flows through the wire. This happens because a current-carrying wire produces a magnetic field around it (magnetic effect of electric current). If the compass needle does not deflect, it means no current is flowing. We could also check for the heating effect, since a current-carrying wire (like a nichrome wire) becomes warm, indicating that current is flowing.
2. Is it possible to make temporary magnets? How can these be made?
Answer
Yes, temporary magnets can be made by passing electric current through a coil of insulated wire — this is called an electromagnet. Such a magnet exists only as long as current flows through the coil; once the current is switched off, the magnetic effect disappears. The strength of this temporary magnet can also be increased by inserting an iron core inside the coil, increasing the number of turns of wire, or increasing the current (using more cells).
3. We can generate heat by burning fossil fuels and wood; but how is heat generated in various electrical appliances?
Answer
In electrical appliances, heat is generated through the heating effect of electric current. When electric current flows through a conductor, the conductor offers some resistance to the flow of current. This resistance causes a part of the electrical energy to be converted into heat energy, making the conductor (heating element) hot. Appliances like electric heaters, kettles, irons, and water heaters use special high-resistance wires (such as nichrome) as heating elements for this purpose.
4. How do we know if a cell or a battery is dead? Can all cells and batteries be recharged?
Answer
A cell or battery is "dead" when the chemicals inside it have been completely used up by the chemical reactions that generate current, so it can no longer supply electricity — for example, a torch bulb connected to it will not glow, or a device will not switch on. Not all cells/batteries can be recharged. Dry cells (like the ones used in torches and remotes) are typically single-use and cannot be recharged. Rechargeable batteries (like Li-ion batteries used in phones and laptops) can be recharged and reused many times by passing current back into them, but even these eventually wear out permanently after many charge-discharge cycles.
Activity 4.1 — Let us Investigate · Page 47–48
Observe the compass needle as the switch is moved ON and OFF. What do you observe?
Answer
When the switch is moved to the ON position, current starts flowing through the wire placed above the compass, and the compass needle gets deflected from its original North–South direction. When the switch is moved to OFF, the current stops flowing, and the needle returns to its original direction. Repeating this a few times shows the same pattern each time: deflection when ON, return to normal when OFF. This shows that a current-carrying wire produces a magnetic field, which is strong enough to affect the tiny magnet inside the compass — this is the magnetic effect of electric current, discovered by Hans Christian Oersted.
In-text Bubble · Page 48
"We have learnt about magnets and electric current in earlier grades. I used to think that there was no link between the two. But now we found that electricity and magnetic effect are linked!"
Explanation
This reflects the key discovery of the chapter: a flowing electric current is not just an electrical phenomenon — it also creates a magnetic field around the conductor. This was demonstrated in Activity 4.1, where the compass needle deflected only when current flowed, proving the deep connection between electricity and magnetism (first discovered by Oersted in 1820).
In-text Bubble · Page 49
Can we use electric current to make a magnet?
Answer
Yes. When electric current is passed through a coil of wire (especially one wound around an iron core, like a nail), the coil behaves like a magnet — this is called an electromagnet. It attracts magnetic materials like iron paper clips only as long as current flows through it; once the current is stopped, it loses its magnetism. This was demonstrated in Activity 4.2 and Activity 4.3.
Activity 4.2 — Let us Explore · Page 49
Do the paper clips hang to the ends of the nail when current flows, and do they fall down when current is stopped?
Answer
Yes. When the wire-wound nail is connected to the cell and current flows through the coil, the nail behaves like a magnet and the iron paper clips cling to it. When the wire is disconnected from the cell (current stops), the nail loses its magnetism and the clips fall down. This confirms that the magnetic effect exists only while current flows — the nail and coil together form a temporary magnet called an electromagnet.
Activity 4.3 — Let us Experiment · Page 49–50
Compare the deflection of the compass needles with and without an iron nail inside the coil. What do you conclude?
Answer
When current flows through just the cylindrical coil (without the iron nail), the compass needles near its two ends show a small deflection, indicating a weak magnetic field. When an iron nail is inserted into the core of the coil and current is passed again, the deflection of the compass needles becomes much greater, and the coil is now able to attract iron paper clips at its ends. When the current is disconnected, the coil (with or without the nail) loses its magnetic effect and the compass needles return to their original position.
Conclusion: A current-carrying coil behaves like a magnet (an electromagnet), and inserting a soft iron core inside the coil makes the electromagnet considerably stronger, because the iron core itself gets magnetised by the coil's magnetic field and adds to the overall magnetic strength.
Conclusion: A current-carrying coil behaves like a magnet (an electromagnet), and inserting a soft iron core inside the coil makes the electromagnet considerably stronger, because the iron core itself gets magnetised by the coil's magnetic field and adds to the overall magnetic strength.
In-text Bubble · Page 50
Does an electromagnet also have two poles like a bar magnet?
Answer
Yes. Just like a permanent bar magnet, an electromagnet also has two poles — a North pole and a South pole — at its two ends, as confirmed in Activity 4.4 using a magnetic compass. The polarity of the two ends is always opposite to each other.
Activity 4.4 — Let us Investigate · Page 50–51
Find the polarity at ends A and B of the electromagnet using a magnetic compass. Is the polarity of end B opposite to end A?
Answer
When the compass is placed near end A and the coil is connected to the cell, one pole of the compass needle (say, North) gets attracted towards end A. Since unlike poles attract, this means end A acts as the South pole of the electromagnet. When the same procedure is repeated at end B, the opposite pole of the compass needle is attracted, showing that end B is the North pole.
Yes — the polarity at end B is found to be opposite to the polarity at end A, confirming that an electromagnet, like a bar magnet, has two distinct (opposite) poles at its two ends.
Yes — the polarity at end B is found to be opposite to the polarity at end A, confirming that an electromagnet, like a bar magnet, has two distinct (opposite) poles at its two ends.
Think Like a Scientist · Page 51
Repeat Activity 4.3 with (i) 2 and 4 cells with the same coil, (ii) 2 cells but different number of turns of the coil. What do you observe? Also repeat Activity 4.4 by changing the direction of current.
Answer
- Effect of number of cells: With a single cell, the current is small, so the magnetic field is weak — the compass deflection is less, and the coil attracts only a few clips. With 2 cells, the current increases, producing a stronger magnetic field, greater deflection, and more clips attracted. With 4 cells, the current (and hence the magnetic field strength) increases further, giving even greater deflection and more clips attracted — so strength of electromagnet increases with current.
- Effect of number of turns: Keeping the current (2 cells) constant, increasing the number of turns of the coil also makes the electromagnet stronger, producing greater compass deflection and attracting more clips.
- Effect of reversing current direction: When the direction of current is reversed (by reversing the battery terminals), the poles of the electromagnet get interchanged — the end that was earlier the South pole becomes the North pole, and vice versa. The compass needle now deflects in the opposite direction compared to before.
In-text Bubble · Page 52
Are electromagnets also used in real life, for lifting objects?
Answer
Yes. Lifting electromagnets are strong electromagnets hung from cranes and are widely used in factories, junkyards, and scrap yards to lift, move, and sort heavy iron/steel objects. The crane operator switches the current ON to magnetise the electromagnet and lift the object, and switches it OFF to demagnetise it and release the object at the desired location.
In-text Bubble · Page 52
While doing the activity for electromagnet, did you also notice that the wire ends got warm? Why would that happen?
Answer
Yes, this happens because when electric current flows through any conductor (including the coil wire), the conductor offers some resistance to the flow of current. This resistance converts a part of the electrical energy into heat energy, causing the wire to become warm. This is known as the heating effect of electric current, and it occurs in every current-carrying conductor to some extent, though the amount of heating depends on the material, thickness, and length of the wire.
Activity 4.5 — Let us Observe · Page 52–53
Touch the nichrome wire before and after passing current through it. What difference do you feel?
Answer
Before the switch is turned ON, the nichrome wire feels at room temperature (cool/normal). After moving the switch to ON for about 30 seconds and then touching the wire momentarily, the wire feels distinctly warm/hot. Repeating the steps confirms the same observation each time. This happens because the nichrome wire has high electrical resistance; when current flows through it, this resistance converts electrical energy into heat energy, raising the wire's temperature — demonstrating the heating effect of electric current.
Think Like a Scientist · Page 53
Repeat Activity 4.5 with a battery of 2 cells. For the same duration, does the wire heat up more with one cell or two cells?
Answer
The wire heats up more with 2 cells than with a single cell, for the same duration of time. This is because a battery of 2 cells provides a larger electric current than a single cell, and the amount of heat generated in a conductor increases with the magnitude of current flowing through it. In general, the heat generated in a wire depends on the material of the wire, its thickness and length, the magnitude of current, and the duration for which the current flows.
In-text Bubble · Page 53
"Oh, now I understand why the incandescent torch lamp sometimes used to get warm when we did the activity of making it glow using an electric cell."
Explanation
An incandescent lamp glows because its thin metal filament has high resistance; when current flows through it, the filament heats up so much that it glows (lights up). Since heating occurs along with glowing, the lamp (and its filament) also feels warm to touch after being lit for some time — this is a direct everyday example of the heating effect of electric current.
Activity 4.6 — Let us Construct · Page 56
Connect the LED to the lemon cell setup. Does the LED glow? What happens if its connections are reversed?
Answer
When the copper wires and iron nails of five—six lemons are connected in series and joined to an LED with the correct polarity, the LED glows, showing that the lemon cells together generate enough electricity to light it. This happens because the lemon juice acts as an electrolyte, and the copper wire and iron nail act as two different electrodes; a chemical reaction between the electrodes and the acidic lemon juice produces electric current — exactly like a Voltaic cell. If the LED does not glow initially, it is because its positive (longer) and negative (shorter) terminals are connected the wrong way round; reversing the LED's connections (so the positive terminal connects to the copper-wire/positive side, and the negative terminal connects to the iron-nail/negative side) makes the LED glow, since an LED allows current to flow in only one direction.
In-text Bubble · Page 56
Can we also make our own Voltaic cell using easily available materials?
Answer
Yes — as demonstrated in Activity 4.6, a simple Voltaic cell can be made using everyday materials like lemons (or potatoes, or salt water), a copper wire/strip, and an iron nail. The acidic lemon juice acts as the electrolyte, and the copper and iron act as the two different electrodes. Such homemade cells can generate enough current to light a small LED, especially when several lemon cells are connected together (in series).
In-text Bubble · Page 57
"Oh, so this is the reason why after a year or two, the phone battery requires charging more often!"
Explanation
Rechargeable batteries (like the Li-ion battery in a phone) do not last forever — after being charged and discharged (used) many times over months and years, the chemical materials inside them gradually wear out and lose some of their ability to store charge. This is why an older phone's battery holds charge for a shorter time and needs to be recharged more frequently compared to when it was new.
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Exercise Questions (Keep the Curiosity Alive)
Question 1
Fill in the blanks:
(i) The solution used in a Voltaic cell is called ________.
(ii) A current carrying coil behaves like a _______.
Answer
(i) The solution used in a Voltaic cell is called an electrolyte.
(ii) A current carrying coil behaves like a magnet (electromagnet).
(ii) A current carrying coil behaves like a magnet (electromagnet).
Question 2
Choose the correct option:
(i) Dry cells are less portable compared to Voltaic cells. (True/False)
(ii) A coil becomes an electromagnet only when electric current flows through it. (True/False)
(iii) An electromagnet, using a single cell, attracts more iron paper clips than the same electromagnet with a battery of 2 cells. (True/False)
Answer
(i) False — Dry cells are actually more portable than Voltaic cells, since they use a paste-like electrolyte (not liquid) sealed in a compact container, making them convenient and spill-proof for everyday use.
(ii) True — A coil behaves as an electromagnet only while current flows through it; once the current stops, it loses its magnetism.
(iii) False — A battery of 2 cells provides more current than a single cell, producing a stronger magnetic field, so the electromagnet with 2 cells attracts more clips, not fewer.
(ii) True — A coil behaves as an electromagnet only while current flows through it; once the current stops, it loses its magnetism.
(iii) False — A battery of 2 cells provides more current than a single cell, producing a stronger magnetic field, so the electromagnet with 2 cells attracts more clips, not fewer.
Question 3
An electric current flows through a nichrome wire for a short time.
(i) The wire becomes warm.
(ii) A magnetic compass placed below the wire is deflected.
Choose the correct option:
(a) Only (i) is correct (b) Only (ii) is correct (c) Both (i) and (ii) are correct (d) Both (i) and (ii) are not correct
Answer
Correct option: (c) Both (i) and (ii) are correct
Any current-carrying conductor, including a nichrome wire, simultaneously exhibits both effects of electric current: the heating effect (the wire becomes warm due to its resistance) and the magnetic effect (a magnetic field is produced around it, deflecting a nearby compass needle). Both effects occur together whenever current flows, regardless of the type of wire.
Any current-carrying conductor, including a nichrome wire, simultaneously exhibits both effects of electric current: the heating effect (the wire becomes warm due to its resistance) and the magnetic effect (a magnetic field is produced around it, deflecting a nearby compass needle). Both effects occur together whenever current flows, regardless of the type of wire.
Question 4
Match the items in Column A with those in Column B.
Answer
A Voltaic cell generates electricity through chemical reactions between electrodes and electrolyte; an electric iron is a household device that uses the heating effect; nichrome wire (being a high-resistance material) is the heating element that works on the heating effect of current; and an electromagnet works on the magnetic effect of electric current.
| Column A | Matches With | Column B |
|---|---|---|
| (i) Voltaic cell | → | (d) chemical reactions |
| (ii) Electric iron | → | (a) Best suited for electric heater |
| (iii) Nichrome wire | → | (c) Works on heating effect of electric current |
| (iv) Electromagnet | → | (b) Works on magnetic effect of electric current |
Question 5
Nichrome wire is commonly used in electrical heating devices because it
(i) is a good conductor of electricity. (ii) generates more heat for a given current. (iii) is cheaper than copper. (iv) is an insulator of electricity.
Answer
Correct option: (ii) generates more heat for a given current
Nichrome is chosen for heating elements because it has a much higher electrical resistance than ordinary conductors like copper. Since the heat produced by a current-carrying wire increases with its resistance, nichrome generates significantly more heat than copper for the same current — exactly what is needed in heating appliances. (It is not an insulator — it does conduct electricity, just with higher resistance than copper — and cost is not the primary reason for its use.)
Nichrome is chosen for heating elements because it has a much higher electrical resistance than ordinary conductors like copper. Since the heat produced by a current-carrying wire increases with its resistance, nichrome generates significantly more heat than copper for the same current — exactly what is needed in heating appliances. (It is not an insulator — it does conduct electricity, just with higher resistance than copper — and cost is not the primary reason for its use.)
Question 6
Electric heating devices (like an electric heater or a stove) are often considered more convenient than traditional heating methods (like burning firewood or charcoal). Give reason(s) to support this statement considering societal impact.
Answer
- Cleaner and healthier: Electric heating devices do not produce smoke, soot, or harmful gases, unlike burning firewood/charcoal, which causes indoor air pollution and respiratory health problems, especially for women and children who often handle traditional cooking/heating.
- Convenience and time-saving: Electric devices can be switched on/off instantly and have adjustable heat settings, saving time compared to collecting fuel and starting/maintaining a fire.
- Environmental benefit: Burning wood/charcoal contributes to deforestation and releases carbon dioxide and particulate matter, while electric heating (especially from renewable electricity sources) causes far less environmental damage.
- Safety: Electric appliances (with proper safety devices) reduce risks of open flames, accidental fires, and burns associated with traditional fuel-based heating.
- Better resource management: Reduces dependence on forests for firewood, helping conserve natural resources for the wider community.
Question 7
Look at Fig. 4.4a. If the compass placed near the coil deflects: (i) Draw an arrow on the diagram to show the path of the electric current. (ii) Explain why the compass needle moves when current flows. (iii) Predict what would happen to the deflection if you reverse the battery terminals.
(i) Path of current
As shown in the diagram above, current flows out of the positive (+) terminal of the cell, through the connecting wire, through the coil wound around the nail (from one end to the other), and back into the negative (–) terminal of the cell, completing the circuit.
(ii) Why the compass needle moves
When current flows through the coil, it produces a magnetic field around the coil (magnetic effect of electric current), turning the coil into an electromagnet with a North and a South pole. The compass needle is itself a tiny magnet; when placed near this magnetic field, it experiences a force and aligns itself according to the new magnetic field instead of Earth's magnetic field, causing it to deflect from its original North–South direction.
(iii) Effect of reversing battery terminals
If the battery terminals are reversed, the direction of current flow through the coil reverses, which reverses the polarity of the electromagnet (the end that was North becomes South, and vice versa). As a result, the compass needle will now deflect in the opposite direction compared to before, though the amount (magnitude) of deflection will remain the same since the current's strength is unchanged.
Question 8
Suppose Sumana forgets to move the switch of her lifting electromagnet model to OFF position. After some time, the iron nail no longer picks up the iron paper clips, but the wire wrapped around the iron nail is still warm. Why did the lifting electromagnet stop lifting the clips? Give possible reasons.
Answer
- Cell getting weak/discharged: Since the switch was left ON for a long time, the cell's chemicals get used up continuously, and the cell gradually becomes weak (or "dies"). As the current decreases, the strength of the electromagnet's magnetic field also decreases, until it becomes too weak to lift the paper clips.
- Heating effect alongside: The wire is still warm because current was flowing through it for a long time (heating effect of electric current occurs regardless of whether the magnet is strong or weak) — this confirms that some current is still flowing, but it has reduced too much to produce sufficient magnetic force, even though enough to still generate some heat.
- This situation is a good reminder of the instruction in Activity 4.2 — not to connect the wire to the cell for more than a few seconds, as continuous use weakens the cell quickly.
Question 9
In Fig. 4.12, in which case will the LED glow when the switch is closed? (a) Iron nail + Copper strip in lemon juice, (b) Iron nail + Copper strip in pure water.
Answer
The LED will glow only in case (a) — with lemon juice, not in case (b) with pure water.
This is because a Voltaic-type cell needs an electrolyte — a liquid that can conduct electricity through ions — between the two different electrodes to generate a chemical reaction and produce electric current. Lemon juice is acidic and contains ions, so it acts as a good electrolyte and the chemical reaction between the copper strip and iron nail produces enough current to light the LED. Pure water, on the other hand, does not contain enough free ions (it is a poor conductor) and cannot support the chemical reaction needed to generate current, so the circuit in (b) will not produce sufficient current and the LED will not glow.
This is because a Voltaic-type cell needs an electrolyte — a liquid that can conduct electricity through ions — between the two different electrodes to generate a chemical reaction and produce electric current. Lemon juice is acidic and contains ions, so it acts as a good electrolyte and the chemical reaction between the copper strip and iron nail produces enough current to light the LED. Pure water, on the other hand, does not contain enough free ions (it is a poor conductor) and cannot support the chemical reaction needed to generate current, so the circuit in (b) will not produce sufficient current and the LED will not glow.
Question 10
Neha keeps the coil exactly the same as in Activity 4.4 but slides the iron nail out, leaving only the coiled wire. Will the coil still deflect the compass? If yes, will the deflection be more or less than before?
Answer
Yes, the coil will still deflect the compass needle, because a current-carrying coil produces a magnetic field on its own, even without an iron core inside it (as seen in Activity 4.3, before the nail was inserted). However, the deflection will be less than before. This is because the iron nail, when present inside the coil, gets magnetised and significantly strengthens the overall magnetic field of the electromagnet. Without the iron core, the coil's magnetic field is comparatively weaker, so the compass needle will deflect by a smaller angle.
Question 11
We have four coils of similar shape and size, made of iron, copper, aluminium, and nichrome. When current is passed through the coils, compass needles placed near the coils will show deflection in:
(i) Only circuit (a) (ii) Only circuits (a) and (b) (iii) Only circuits (a), (b), and (c) (iv) In all four circuits
Answer
Correct option: (iv) In all four circuits
The magnetic effect of electric current is produced by the flow of current itself, not by the specific material of the wire. As long as a material is a conductor and allows current to flow through it when connected to a cell, a magnetic field will be produced around it. Iron, copper, aluminium, and nichrome are all electrical conductors (though nichrome and iron have higher resistance than copper and aluminium), so current flows through all four coils when connected to a cell, and all four will produce a magnetic field that deflects a nearby compass needle.
Note: While all four coils will show deflection, the iron coil may show a relatively higher deflection if it behaves partly like a magnetic core material, and the amount of current (and hence deflection) may slightly differ across coils due to their different resistances, but deflection will occur in all four cases as long as current flows.
The magnetic effect of electric current is produced by the flow of current itself, not by the specific material of the wire. As long as a material is a conductor and allows current to flow through it when connected to a cell, a magnetic field will be produced around it. Iron, copper, aluminium, and nichrome are all electrical conductors (though nichrome and iron have higher resistance than copper and aluminium), so current flows through all four coils when connected to a cell, and all four will produce a magnetic field that deflects a nearby compass needle.
Note: While all four coils will show deflection, the iron coil may show a relatively higher deflection if it behaves partly like a magnetic core material, and the amount of current (and hence deflection) may slightly differ across coils due to their different resistances, but deflection will occur in all four cases as long as current flows.