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

NCERT Solutions For Class 8 Science Chapter 4 - PDF Download

The NCERT Solutions in Class 8 Science Chapter 4 is clear and step-wise answers to every question that the textbook poses to its readers hence helping them understand some of the harder concepts in an easier manner. The PDF version makes it easy to study offline and even revise anywhere to prepare better in the exam.

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NCERT Solutions for Class 8 Science Chapter 4: Solved Exercise Questions

Class 8 NCERT Solutions: Science Chapter 4 Solved Exercise Questions will give step-by-step solutions to all the questions of the textbook exercises. The solutions give guidance to the student to learn the concept of the magnetic and heating effect of electric current and to do well during exams.

Q1. Fill in the blanks:

(i) The solution used in a Voltaic cell is called ________.

(ii) A current carrying coil behaves like a _______ .

Answer:

(i) Electrolyte

The Voltaic cell contains two metal plates made of different materials and a liquid called an electrolyte, placed in a glass or plastic container.

(ii) Magnet

When electric current flows through a conductor (like a wire), it produces a magnetic field around it. This phenomenon is known as the magnetic effect of electric current. Or we can say that the current-carrying wire is behaving like a magnet.

Q2. 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 more portable than Voltaic cells because they do not leak and can be easily carried around.

(ii) True

Without current, it remains just a coil of wire.

(iii) False

More cells mean more current, leading to a stronger electromagnet which attracts more clips.

Q3. 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

Electric current generates both heat (heating effect) and a magnetic field (magnetic effect).

Q4. Match the items in Column A with those in Column B

Answer:

Q5. 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) Correct: generates more heat for a given current.

Nichrome is not a very good conductor (not as good as copper) — it resists current, so it heats up more for a given current.

Other options:

(i) Incorrect: Nichrome is not a better conductor than copper.

(iii) Incorrect: Cost is not the main reason.

(iv) Incorrect: "Insulator" is incorrect; it conducts electricity.

Q6. 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:

Electric heating devices (like an electric heater or a stove) are often considered more convenient than traditional heating methods (like burning firewood or charcoal) because:

1. Electric heat sources are less polluting; they do not emit smoke, ash, or toxic gases indoors like traditional sources of heating and thus minimize air pollution indoors and health hazards.

2. They are user-friendly: one can use them at any time and turn them off.

3. They are safer; there is reduced risk of incidence of accidental fires unlike open flames.

4. They are more efficient; energy can be focused on a certain place where it is required.

5. They are also portable and do not need to store any fuel.

6. There is no need to cut the trees (unlike when wood is used), contributing to the preservation of the environment.

Q7. Look at the Fig. 4.4a. If the compass is placed near the coil deflects:

Figure 4.4(a)

(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) Draw the arrow following from the positive terminal of the cell, into one end of the coil, through the coil (from A to B or B to A, as per connection), and back to the negative terminal of the cell.

(ii) A coil develops a magnetic field around it when an electric current is passed through it. This field interacts with the needle on the magnetic compass causing it to deflect away from the North-South direction, establishing that an electric current creates a magnetic effect around it.

(iii) If the battery terminals are reversed, the direction of current in the coil reverses. This means the polarity of the electromagnet flips—the magnetic field's direction changes, so the compass needle will be deflected in the opposite direction compared to before.

Q8. 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 possible reason could be that the electric cell became ‘dead’ or fully discharged after being connected for a long time. When the battery is drained, not enough current flows through the coil, so the electromagnet becomes too weak to lift the paper clips. The wire may still feel warm due to residual current, but it is not sufficient to generate a strong enough magnetic field.

Q9: In Fig. 4.11, in which case will the LED glow when the switch is closed?

Answer:

With lemon juice as an electrolyte: The LED will glow (with correct connections), because the lemon juice will permit the movement of ions to close the circuit, to permit the chemical reaction necessary in the transmission of current.

Using pure water as an electrolyte: The LED will fail to glow as a result of the fact that pure water is not a good conductor of electricity and does not contain ions to supply the current to flow.

This means that, the LED will only light up using lemon juice as the electrolyte.

Q10. 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 because a current-carrying coil does produce a magnetic field. However, the deflection will be less without the iron nail. With the iron nail inside, the magnetic field is much stronger, so the compass needle moves more.

Q11. 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 circuits (a) and (b)

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

(iv) In all four circuits

Answer:

All four coils — iron, copper, aluminium, and nichrome — are metals that can conduct electricity, and when current flows through any of these coils, a magnetic field is produced around them. So the correct answer is option (iv). In all four circuits, the compass needles will show deflection.

NCERT Solutions for Class 8 Science Chapter 4: Discover, design, and debate

NCERT Solutions for Class 8 Solution English Discover, Design, and Debate chapter 4 solution helps students develop their thinking capacity and its ability to apply scientific terms and concepts in real-life scenarios. It contains engaging and challenging questions and activities to develop innovative and creative thinking, and reasoning abilities to understand the heating effects and magnetic effects of electricity in a better way.

Q1. Make coils of turns 25, 50, 75, and 100. Connect them to the same cell one by one. Note the deflection in a magnetic compass placed in the same position in all the cases. Report your observations. Draw conclusion of the effect of the number of turns of the coil on the strength of the electromagnet.

Answer:

Effect of the Number of Turns on the Strength of an Electromagnet:

The more turns you put into the coil the deflection will be the more. This implies that the greater the number of turns, the stronger the electromagnet.

This is because: The more turns, the more current loops, and this provides a stronger magnetic field.

Conclusion: When the number of turns of wire in the coil is increased, under the same conditions of the electric cell, the strength of an electromagnet is increased.

Q2. Take two thin nichrome wires of equal length and different thickness (approximately one of these wire thickness to be double that of the other, say 0.3 mm and 0.6 mm). Connect them one by one in a circuit which has a switch and a cell, and allow the current to flow for 30 s in each case. Momentarily touch these wires. Which wire heats up more? Now repeat the same activity with two nichrome wires of the same diameter but of different lengths. Prepare a brief report of your activity.

Answer:

Case I: Wire of the same length and different thickness:

Observation:

The thinner wire (0.3 mm) becomes hotter than the thicker one (0.6 mm).

Reason: Thinner wire has more resistance to the flow of the electrical current, hence more heat is produced in the thinner one.

Case II: Wire of the same thickness and different length:

Observation:

The longer wire gets hotter than the shorter one.

Reason: The Longer the wire, the higher the resistance; hence, more heat will be generated.

Conclusion:

A thin wire and a longer wire will get hotter due to the increased resistance, and more electrical energy will be converted into heat.

Q3. Try to make an electric cell using various fruits and vegetables. Also, try with electrodes of different metals. Prepare a brief report.

Answer:

The LED could be slightly illuminated, or the voltmeter indicates a reading, this is because the juices of fruit/vegetables behave as electrolytes, and this will enable electricity to pass through.

It also works with different metals as electrodes (such as copper and zinc). The electricity generated by some fruits/vegetables is higher due to the higher acidity or presence of more ions in the juices.

Conclusion: A wide range of fruits and some vegetables can be utilised as the electrolyte in a basic electric cell, and the kind of metal used as electrodes may affect the amount of electricity that is produced.

Electricity: Magnetic and Heating Effects Class 8 Science Chapter 4: Topics

The fourth chapter of Class 8 Science, Electricity: Magnetic and Heating Effects, engages students with the two magical roles of electric current: the heating effect and the magnetic effect. The chapter is practical and thought-provoking since the effects are applied in common places such as electric irons, heaters, fans and doorbells.

4.1 Does an Electric Current Have a Magnetic Effect?

An electric current running through a conductor generates a magnetic field around the conductor. This phenomenon is called the magnetic effect of electric current or electromagnetism. Some of the devices that make use of this principle are electric bells, motors and magnetic cranes.

4.1.1 Electromagnets

An electromagnet is a temporary magnet produced by the passage of electric current through a coil of wire wound around a soft iron core. The strength of the magnetic varies with the turns of the coil and with the number of turns of the coil and the current. Electromagnets are useful since they can either be turned on or off hence very convenient in industries and domestic appliances.

4.1.2 Lifting electromagnets

These are powerful electromagnets that are used in factories and scrap yards to lift heavy metallic items such as iron sheets, steel rods or scrap cars. As current passes through them, they also become magnets and hold onto metal; when current is turned off, the load is immediately released.

4.2 Does a Current-Carrying Wire Get Hot?

The heating effect of electric current (Joule Law) comes into force when electric current that flows in a conductor produces heat. Heat generated is a function of current, resistance and time. Examples of applications to this principle are electric heaters, toasters and fuses.

4.3 How Does a Battery Generate Electricity?

A battery transforms chemical energy to electrical energy. It is composed of one (or more) cells that generate a potential difference between terminals so that electrons can be made to flow when the device is connected in a circuit.

4.3.1 Voltaic cell

The voltaic cell is a device that generates electricity using chemical reactions involving zinc and copper plates in the presence of an electrolyte, which was invented by Alessandro Volta. This was among the earliest form of generating continuous current.

4.3.2 Dry cells

A dry cell (e.g. in torches, clocks) is a portable, easily carried source of electricity with the electrolyte in paste form, to minimize leakage. It comprises a negative terminal made of zinc and positive terminal, made of carbon.

4.3.3 Rechargeable batteries

These are rechargeable secondary cell batteries (e.g., lithium-ion batteries, lead-acid batteries). When battery is charging, electrical energy is being converted to chemical form, to be released back into electricity when in use. They are widely used in mobile, laptop and electric vehicle.

NCERT Class 8 Science Chapter 4 Overview

This NCERT Class 8 chapter 4 describes how electric currents can generate magnetic fields. When current flows in a wire, it behaves in a specific way described as the magnetic effects of current. Electromagnets are made from coils of wire carrying current and are found in cranes and electric bells, as well as in other machines. The chapter also describes the heating effects of current, where some wires, such as nichrome, get heated when current is allowed to flow. This heating is put into use in electric irons, heaters, or cookers. Different types of cells and batteries, such as Voltaic cells, dry cells, and rechargeable batteries and how they generate electricity are also studied. The last part discusses how one may use batteries and dispose of them without harming the environment.

Approach to solve the questions of Class 8 Science Chapter 4

In order to effectively answer the question of Class 8 Science Chapter 4: Electricity: Magnetic and Heating Effects, the students are required to first concentrate on the basic relative terms in the form of electric current, circuit components and the range of voltage in which electricity can lead to the production of both heat and magnetism. These ideas should be related to practical applications such as electric heaters, bulbs, and electromagnets. As a practical activity, draw simple circuit diagrams using standard symbols and rehearse textbook activities that show the nature of the magnetic effect on a current (e.g. make an electromagnet) and the heating effect (e.g. an electric bulb). The NCERT questions, both multiple-choice and short-answer ones, can be solved to strengthen the learning and increase exam confidence. The keys to mastering this chapter are to comprehend the main principles of the experiments and use them logically, particularly during the preparation of final exams.

Chapter-wise NCERT Solutions of Class 8 Science

The Chapter-wise NCERT Solutions for Class 8 Science are step-by-step explanations of all the questions mentioned in the textbook, which makes it easy to understand and learn concepts and answer questions. These solutions present systematically all chapters so that it is easy to prepare an in-depth examination and revise content promptly.