Electricity: Magnetic and Heating Effects — NCERT Solutions

Answer: (ii) generates more heat for a given current.

Explanation:

• Nichrome (an alloy of nickel and chromium) has high electrical resistance. Due to this high resistance, when current flows through it, it generates a large amount of heat (heating effect of electric current).
• It also has a very high melting point, so it does not melt or burn even when it becomes red-hot.
• Option (i) is partially true but not the main reason — nichrome is not as good a conductor as copper; its high resistance is actually the key property.
• Option (iii) is incorrect — nichrome is generally more expensive than copper.
• Option (iv) is incorrect — nichrome is a conductor, not an insulator.

Therefore, the correct reason is (ii): it generates more heat for a given current due to its high resistance.

Given: A comparison between electric heating devices and traditional heating methods (firewood/charcoal).

Reasons why electric heating devices are more convenient (considering societal impact):

1. No smoke or air pollution: Electric heaters do not produce smoke, carbon monoxide, or other harmful gases. Burning firewood/charcoal releases smoke that causes indoor and outdoor air pollution, leading to respiratory diseases. This improves public health.

2. No deforestation: Traditional methods require cutting trees for firewood, leading to deforestation and loss of biodiversity. Electric devices eliminate this need, helping conserve forests.

3. Easy to control and safe: Electric devices can be switched on/off instantly and have adjustable temperature settings. There is no risk of uncontrolled fire, making them safer, especially in homes with children.

4. Cleaner and hygienic: Electric cooking/heating does not produce ash or soot, keeping the surroundings clean and reducing health hazards.

5. Time-saving: Electric devices heat up quickly and efficiently, saving time compared to lighting and maintaining a wood or charcoal fire.

6. Reduced burden on women and children: In many rural areas, women and children spend hours collecting firewood. Switching to electric devices frees up their time for education and other productive activities.

Conclusion: Electric heating devices are more convenient, safer, cleaner, and have a positive societal impact by improving health, reducing environmental damage, and saving time and effort.

Given: An electromagnet coil connected to a cell; a magnetic compass is placed near end A of the coil and it deflects.

(i) Direction of electric current:

Since the figure (Fig. 4.4a) cannot be reproduced here, the general principle is:

• Current flows from the positive terminal of the cell, through the external circuit (connecting wire → coil → switch), and back to the negative terminal of the cell.
• An arrow should be drawn on the wire showing current flowing from the positive terminal of the cell into the coil at one end and out at the other end, returning to the negative terminal.

(ii) Why does the compass needle move?

• When electric current flows through the coil, it produces a magnetic field around the coil (magnetic effect of electric current).
• The coil with current behaves like a bar magnet with a North pole at one end and a South pole at the other.
• A magnetic compass needle is itself a small magnet. When placed near the coil, the magnetic field of the coil exerts a force on the compass needle.
• The needle aligns itself along the direction of the magnetic field produced by the coil, causing it to deflect from its original North–South direction.

(iii) Effect of reversing the battery terminals:

• When the battery terminals are reversed, the direction of current in the coil reverses.
• This reverses the direction of the magnetic field produced by the coil.
• As a result, the poles of the electromagnet are interchanged (what was North pole becomes South pole and vice versa).
• The compass needle will now deflect in the opposite direction compared to the original deflection.

Conclusion: Reversing the battery terminals reverses the current, reverses the magnetic field, and hence reverses the direction of deflection of the compass needle.

Given: Sumana's lifting electromagnet model is left switched ON for a long time. After some time, the nail stops picking up iron clips, but the wire is still warm.

Observation to explain: The electromagnet lost its magnetic strength but the wire is still warm (current is still flowing).

Possible Reasons:

1. The cell/battery has weakened (discharged):

• When the circuit is left ON for a long time, the chemical energy stored in the cell gets used up.
• The cell becomes weak and can no longer supply sufficient current.
• A weaker current produces a weaker magnetic field, which may not be strong enough to lift the iron clips.
• However, even a small current can still produce some heat in the wire (heating effect), which is why the wire remains warm.

2. Overheating of the coil:

• Prolonged flow of current heats up the wire (heating effect of electric current).
• Excessive heat can sometimes affect the insulation of the wire or the properties of the coil, reducing its efficiency as an electromagnet.

3. The nail may have become too hot:

• Due to prolonged heating, the iron nail may have reached a temperature at which its magnetic properties are affected (above the Curie temperature, the magnetic domains get disturbed), reducing its ability to act as a magnetic core.

Most likely reason: The cell has discharged (weakened) due to prolonged use, so the current is now too small to produce a strong enough magnetic field to lift the clips. But even this small current is enough to produce warmth in the wire due to the heating effect.

Conclusion: The electromagnet stopped working primarily because the cell weakened after being left ON for too long, reducing the current and hence the magnetic strength, while the heating effect continued with the remaining small current.

Given: Two circuit diagrams (Fig. 4.12a and 4.12b) with an LED connected in a circuit with a switch and a cell.

Concept: An LED (Light Emitting Diode) allows current to pass and glows only when its positive terminal (longer lead/anode) is connected to the positive terminal of the battery and its negative terminal (shorter lead/cathode) is connected to the negative terminal of the battery. If connected in reverse, it does not glow.

Analysis:
(Since the actual figures cannot be seen, the standard answer based on the context given in the chapter is:)

• In case (a): If the longer lead (positive terminal) of the LED is connected to the positive terminal of the cell and the shorter lead (negative terminal) is connected to the negative terminal of the cell, the LED is forward biased and will glow when the switch is closed.

• In case (b): If the LED is connected in reverse (shorter lead to positive terminal of cell), it is reverse biased and will not glow.

Answer: The LED will glow in case (a), where the positive terminal (longer wire) of the LED is connected to the positive terminal of the cell.

Note: The key principle is that current flows through an LED only in one direction (forward bias). Reversing the connections prevents current flow and the LED does not glow.

Given: The iron nail is removed from the coil; only the coiled wire remains. The coil is connected to the same cell.

Concept:

• A current-carrying coil produces a magnetic field and behaves like an electromagnet.
• An iron core (nail) inside the coil gets magnetised and greatly amplifies the magnetic field of the coil.

Answer:

Yes, the coil will still deflect the compass needle even without the iron nail, because:

• When current flows through the coil, it still produces a magnetic field around it (magnetic effect of electric current).
• This magnetic field will still exert a force on the compass needle and cause deflection.

However, the deflection will be LESS than before.

Reason: When the iron nail was inside the coil, it acted as a magnetic core. The iron nail got magnetised and added its own magnetic field to that of the coil, making the total magnetic field much stronger. Without the nail, only the coil's magnetic field acts on the compass, which is weaker. Hence, the deflection of the compass needle will be less compared to when the iron nail was present.

Conclusion: The coil without the iron nail will still deflect the compass, but the deflection will be less than when the iron nail was inside the coil.

Given: Four coils of similar shape and size made of iron, copper, aluminium, and nichrome respectively, connected to the same source.

Concept:

• The strength of an electromagnet depends on the current flowing through the coil and the magnetic properties of the material.
• The heat generated depends on the electrical resistance of the wire (higher resistance → more heat for the same current).
• Conductivity: Copper > Aluminium > Iron > Nichrome (in terms of electrical conductivity; nichrome has the highest resistance).

Analysis:

(A) Which coil would make the best electromagnet?

• The copper coil would be the best electromagnet.
• Copper has the lowest resistance among the four materials, so it allows the maximum current to flow through the coil for a given voltage.
• A higher current produces a stronger magnetic field, making the copper coil the strongest electromagnet.
• Note: Iron is a magnetic material, but when used as the wire of the coil (not as a core), its higher resistance reduces the current, making it less effective than copper as an electromagnet coil.

(B) Which coil would generate the most heat?

• The nichrome coil would generate the most heat.
• Nichrome has the highest electrical resistance among the four materials.
• According to the heating effect of electric current, more resistance means more heat is generated for the same current (or voltage).
• This is why nichrome is used in heating devices like electric heaters and toasters.

Summary Table:

Conclusion:

• Copper coil → Best electromagnet (lowest resistance, highest current, strongest magnetic field).
• Nichrome coil → Generates most heat (highest resistance, maximum heating effect).