NCERT Solutions Class 8 Science Chapter 04 Electricity Magnetic and Heating Effects

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 acts like a magnet.
In simple words: In a Voltaic cell, the special liquid that allows electricity to be made is the electrolyte. When electricity flows through a coil of wire, the coil starts to work like a magnet.

Exam Tip: Remember that an electrolyte must be a substance that lets ions move freely. A current-carrying coil becomes magnetic only when electricity is flowing through it.

Question 2. Choose the correct option:
(i) Dry cells are less portable compared to Voltaic cells.
(ii) A coil becomes an electromagnet only when electric current flows through it.
(iii) An electromagnet, using a single cell, attracts more iron paper clips than the same electromagnet with a battery of 2 cells.
Answer:
(i) False - Dry cells are much more portable. Voltaic cells are bulky and not easy to carry around.
(ii) True - Without electricity flowing, a coil has no magnetic properties. It only becomes an electromagnet when current passes through.
(iii) False - More cells mean stronger electric current, which creates a stronger magnet. So a 2-cell battery will lift more paper clips than a single cell.
In simple words: Dry cells are easy to carry. A coil needs electricity to become magnetic. More batteries make a stronger electromagnet.

Exam Tip: Always think about what makes an electromagnet work - it is the electric current flowing through the coil. More current from more cells means a stronger magnet.

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: (c) Both (i) and (ii) are correct
In simple words: Electricity flowing through a nichrome wire does two things - it makes the wire hot, and it makes a magnetic field around the wire that turns a compass needle.

Exam Tip: Nichrome wire shows both effects of electric current - the heating effect (wire gets warm) and the magnetic effect (compass moves). These are two separate and real outcomes.

Question 4. Match the items in Column A with those in Column B:
Answer:

In simple words: A Voltaic cell makes electricity through chemicals. An electric iron and nichrome wire use heat from electricity. An electromagnet uses the magnetic power of electric current.

Exam Tip: Know which devices use the heating effect and which use the magnetic effect of electric current - this is a key concept in this chapter.

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: (ii) generates more heat for a given current
In simple words: Nichrome wire is chosen for heaters because it makes lots of heat from the same amount of electricity compared to other wires like copper.

Exam Tip: Nichrome wire has high resistance, which makes it the best choice for heating devices. High resistance means more heat is produced - that is why it is used in heaters and toasters.

Question 6. Electric heating devices (like an electric heater or 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: Electric heating devices offer many advantages over firewood and charcoal burning. They produce no smoke and leave no ash, making them clean and safe for indoor use. Operating them is simple - just flip a switch on and off with no effort needed. These devices change electrical energy directly into heat with very little waste, so they work very well. People save both time and hard work because there is no need to collect, store, or manage fuel. Health benefits are clear too - no harmful smoke means better air quality inside homes. The environment also gains because we do not burn fuel that releases pollution into the air.
In simple words: Electric heaters are better because they are clean, easy to use, work well, and do not hurt the air we breathe.

Exam Tip: When answering "give reasons" questions, always mention at least two points - here, efficiency, cleanliness, ease of use, and health/environmental benefits are all key marks.

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.
Answer:
(i) The current flows from the positive terminal of the battery through the coil and back to the negative terminal (following the external circuit path shown in the diagram).

(ii) The compass needle moves because the current flowing through the coil generates a magnetic field all around it. This magnetic field interacts with the earth's magnetic field and the compass needle, causing the needle to turn in response to the new field created by the coil.

(iii) If you reverse the battery terminals, the direction of current through the coil reverses. Since the direction of the magnetic field depends on the direction of current, reversing the current will also reverse the magnetic field direction. As a result, the compass needle will point in the opposite direction - the deflection will reverse.
In simple words: The current makes a magnetic field around the coil that pushes or pulls the compass needle. If you flip the battery around, the current goes the other way, so the compass points the opposite direction.

Exam Tip: The relationship between current direction and magnetic field direction is critical - use the right-hand rule if asked to show the field direction or predict how it will change.

Question 8. Suppose Sumana forgets to move the switch of her lifting electromagnet model to OFF position (in introduction story). 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: The most likely reason the electromagnet stopped lifting paper clips is that the cell or battery powering the circuit has become weak or fully used up. When a cell gets old, it cannot deliver enough current anymore. Without sufficient current flowing through the coil, the magnetic field produced becomes too weak to attract the iron paper clips. However, some current is still flowing, even if it is very small. This tiny amount of current still causes some electrical resistance in the wire, which produces a small amount of heat. This leftover heat is what makes the wire feel warm to the touch, even though it is not strong enough to create a magnet anymore.
In simple words: The battery got tired and could not push enough electricity through the wire. Without strong electricity, the coil cannot be a strong magnet and cannot lift things. But even weak electricity still makes a little heat.

Exam Tip: Distinguish between the two effects - a weak electromagnet cannot lift objects, but even a weak current still produces some heat. This shows that both heating and magnetic effects happen together, just at different strengths.

Question 9. In Fig. 4.11, in which case will the LED glow when the switch is closed?
Answer:
Case (a): Iron nail + Lemon juice + Copper strip - The LED will glow. This works because lemon juice acts as an electrolyte. It contains ions that allow electricity to move from the copper strip to the iron nail, completing the circuit and powering the LED.

Case (b): Iron nail + Pure water + Copper strip - The LED will not glow. Even though pure water seems like it should conduct electricity, it actually does not. Pure water is a very poor conductor because it lacks the ions needed to carry electrical current from one metal to the other.
In simple words: Lemon juice lets electricity flow because it has special particles. Pure water does not have these particles, so electricity cannot flow through it.

Exam Tip: Always remember - a good electrolyte has ions that let current move. Pure water has almost no ions, so it blocks current. This is why salt water or lemon juice works but pure water does not.

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 even without the iron nail inside. Any wire carrying electric current produces a magnetic field around it. Since the coil is just wire wrapped in loops, the current flowing through it will create a magnetic field, which will affect the compass. However, the deflection will be noticeably smaller than when the iron nail was inside. The iron nail acts as a core that makes the magnetic field much stronger - it concentrates and amplifies the field. Without this iron core, the electromagnet becomes much weaker, and the compass will show only a small movement compared to before.
In simple words: A coil of wire by itself still makes a magnetic field and moves a compass. But the iron nail inside makes the field much stronger. Without the nail, the field is weak.

Exam Tip: The iron core is not the source of magnetism - it amplifies the magnetic field created by the current. A current-carrying coil produces a magnetic field on its own, but an iron core increases it greatly.

Question 11. We have four coils, of similar shape and size, made up from iron, copper, aluminium and nichrome as shown in Fig. 4.12. When current is passed through the coils, compass needles placed near the coils will show deflection.
(i) Only in circuit (a)
(ii) Only in circuit (a) and (b)
(iii) Only in circuit (a), (b), and (c)
(iv) In all four circuits
Answer: (iv) In all four circuits
In simple words: All four materials - iron, copper, aluminium, and nichrome - let electricity flow through them. Whenever electricity flows through any conductor, it makes a magnetic field around it. So the compass will move near every coil, no matter what metal it is made from.

Exam Tip: The key insight is that magnetic effects come from current flow, not from the metal itself. Every conductor that carries current produces a magnetic field. The material matters for the strength of the field, not for whether a field exists.