NCERT Solutions for Class 8 Science Chapter 4 – Electricity: Magnetic and Heating Effects

ALLEN offers comprehensive NCERT solutions meticulously developed to augment students' comprehension of the core principles concerning electric current and its effects, with a specific emphasis on the thermal and magnetic phenomena associated with electricity.

The solutions for the chapter are systematically structured in a step-by-step manner to ensure conceptual precision and facilitate efficacious preparation for academic assessments.

Chapter 4 Electricity: Magnetic and Heating Effects delves into the practical utilisation of these effects in quotidian life, exemplified by devices such as incandescent lamps, heating appliances, and electromagnets. Furthermore, the chapter encompasses crucial safety protocols pertinent to the utilisation of electrical energy.

1.0NCERT Solutions for Class 8 Science Chapter 4 PDF Download

This section will give students easy access to download a PDF containing the full set of solutions for Electricity: Magnetic and Heating Effects (NCERT Class 8 Science Chapter 4) to assist them with the best study and revision practices. 

The NCERT Class 8 Science Chapter 4 solutions have been carefully prepared by ALLEN's highly trained faculty, ensuring that they are accurate, clear, and comprehensive to cover the entire syllabus. The solutions are designed to help the student effectively revise the concepts covered in Chapter 4 and to perform confidently in examinations.

2.0NCERT Questions with Solutions Class 8 Science Chapter 4 Detailed Solutions

1. Fill in the blanks:

(i) The solution used in a Voltaic cell is called ________. (ii) A current carrying coil behaves like a ________.

Solution

(i) The solution used in a Voltaic cell is called electrolyte. In voltaic cell the electrodes are dipped into a conducting solution, which allows the flow of ions; this conducting solution is called an electrolyte.

(ii) A current carrying coil behaves like a bar magnet. When current flows through a coil of wire,

2. Choose the correct option:

(i) Dry cells are less portable compared to Voltaic cells. (True/False) 

Solution: False. Dry cells are small, sealed, and can be easily carried in torches, remotes, radios, etc. Voltaic cells use liquid electrolytes, which may leak and are difficult to transport. Hence, dry cells are more portable than Voltaic cells.

(ii) A coil becomes an electromagnet only when an electric current flows through it. (True/False) 

Solution: True. A simple coil of wire has no magnetism on its own. When current flows, it produces a magnetic field around the coil, turning it into an electromagnet. If the current stops, the magnetism disappears.

(iii) An electromagnet, using a single cell, attracts more iron paper clips than the same electromagnet with a battery of 2 cells. (True/False) 

Solution: False. The strength of an electromagnet depends on the current flowing through the coil. More cells provide more current in the electromagnet which makes a stronger electromagnet. Therefore, an electromagnet with 2 cells attracts more paper clips than with 1 cell.

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

Solution - (c) Both (i) and (ii) are correct 

(i) The wire becomes warm: When electric current passes through a nichrome wire, part of the electrical energy changes into heat. Since nichrome offers resistance to the flow of current, the wire gets heated up.

(ii) A magnetic compass placed below the wire is deflected: An electric current always produces a magnetic field around the wire. The magnetic needle of a compass gets affected by this field and changes its direction (deflects).

4. Match the items in Column A with those in Column B.

Solution:

• (A) → (s): A voltaic cell produces electricity through chemical reactions between electrodes and electrolyte.
• (B) →(r): It works on the heating effect of current to press clothes.
• (C) → (p): Nichrome has high resistance and a high melting point, making it best suited for use in electric heaters, toasters, etc.
• (D) → (q): When current flows through a coil, it produces a magnetic field, turning it into an electromagnet.

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.

Solution

(ii) generates more heat for a given current. Nichrome (an alloy of nickel and chromium) has a high resistance compared to metals like copper or aluminium. When current passes through it, due to this resistance, it produces a lot of heat (heating effect of current). It also has a high melting point, so it can remain hot for a long time without melting. That’s why it is used in heaters, toasters, and electric irons.

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.

Solution

Electric heating devices are more convenient because of the following reasons:

1. Electric devices don’t produce smoke, unlike firewood or charcoal. Less smoke means fewer respiratory problems and healthier living.
2. No open flames, reducing the risk of burns or accidental fires.
3. Heats up quickly and doesn’t require gathering fuel like firewood or charcoal.
4. Reduces deforestation and air pollution caused by burning wood or charcoal.
5. Temperature can be adjusted easily, unlike firewood, where it’s harder to control heat.
6. Works well in homes without outdoor space for burning wood or charcoal.

7. Look at the Fig. If the compass is 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.

Solution

(i) [Arrows should be drawn from the positive terminal of the battery, through the coil, and back to the negative terminal.]

(ii) A compass needle is a small magnet that aligns itself with external magnetic fields. When the electric current flows, it creates a magnetic field around the coil and the iron nail. This magnetic field exerts a force on the compass needle, causing it to deflect from its usual alignment with the Earth's magnetic field.

(iii) If you reverse the battery terminals, the compass needle will deflect in the opposite direction. Reversing the battery terminals reverses the direction of the electric current flowing through the coil. According to the rules of electromagnetism, reversing the current direction also reverses the polarity of the magnetic field.

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.

Solution

The lifting electromagnet stopped working because the battery became weak after supplying current for a long time. Due to this, the current reduced and the magnetic field became very weak. Continuous current also heated up the wire, increasing its resistance and further reducing the current. Since the iron nail is only a temporary magnet, it quickly lost its magnetism when the current weakened. Therefore, it could no longer attract the paper clips.

9. In Figure, in which case the LED will glow when the switch is closed?

Solution

In a cell, the liquid used is called an electrolyte, and it is usually an acid or a salt solution.

In case (a), lemon juice acts as the electrolyte. Since lemon juice is a weak acid, it allows electricity to pass through. A chemical reaction happens between the iron nail and the copper strip, so current flows in the circuit and the LED glows.

In case (b), pure water is not a good conductor of electricity. It does not allow current to pass because it is not a good electrolyte. That is why no current flows and the LED does not glow.

10. Neha keeps the coil exactly the same as shown in fig. 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?

Solution

When Neha removes the iron nail and leaves only the coil of wire, the coil still produces a magnetic field as soon as current flows through it. This is because any wire carrying current behaves like a magnet, and when the wire is coiled, the magnetic effect becomes stronger. Therefore, the compass needle will still deflect.

However, when the iron nail was inside the coil earlier, it acted like a core. The nail got magnetized and made the magnetic field much stronger. Without the nail, the coil has no iron core to concentrate and strengthen the field. As a result, the magnetic field produced is weaker, and so the deflection of the compass needle is less compared to when the nail was present.

11. We have four coils, of similar shape and size, made up from iron, copper, aluminium, and nichrome as shown in Fig. 

When current is passed through the coils, compass needles placed near the coils will show deflection.

(i) Only in circuit (a)

(ii) Only in circuits (a) and (b)

(iii) Only in circuits (a), (b), and (c)

(iv) In all four circuits

Solution

(iv) In all four circuits - When electricity flows through a wire, no matter what the wire is made of, it always creates a magnetic field around it. Because of this magnetic field, a compass needle kept near the coil will move (deflect). So, in this case, whether the coil is made of iron, copper, aluminium, or nichrome, the compass needle will show deflection in all four circuits.

The difference is only in how much the needle moves — copper and aluminium (good conductors) give stronger deflection, while nichrome (higher resistance) gives weaker deflection.

3.0Key Topics Covered in Class 8 Science Chapter 4 – Electricity: Magnetic and Heating Effects

1. Electric Current and Cells: Understanding the components of a voltaic cell, including the electrolyte (conducting solution) and electrodes. It also covers the differences between voltaic cells and more portable dry cells.
2. The Heating Effect of Electric Current: How electrical energy changes into heat when passing through a wire with resistance.

• Nichrome Wire: A material with high resistance and a high melting point, making it ideal for heating devices like electric irons, heaters, and toasters.

3. The Magnetic Effect of Electric Current: Understanding that any wire carrying a current produces a magnetic field around it.

• Magnetic Compass Deflection: How current-carrying wires exert force on a compass needle magnet, causing it to change direction.

4. Electromagnets: Coils of wire that behave like magnets (with North and South poles) only when current is flowing.

• Factors Affecting Magnetic Strength:
• Current Strength: Using more cells increases current, creating a stronger electromagnet.
• Iron Core: Placing an iron nail inside a coil concentrates the magnetic field, significantly strengthening the electromagnet.

5. Conduction in Liquids: Testing the conductivity of different liquids, such as lemon juice (which acts as an electrolyte) versus pure water (which is a poor conductor).

4.0Benefits of Studying Class 8 Science Chapter 4 – Electricity: Magnetic and Heating Effects

The academic and real-life application of this chapter will benefit students by:

• Increasing their comprehension of electrical effects
• Raising their awareness of electrical safety
• Developing their practical experience with electrical devices
• Preparing them to study more advanced topics such as physics
• Equipping them to solve test questions based on the application