Electricity: Magnetic And Heating Effects

Hello students, welcome to today's science lesson. I am so happy to be here with you to learn about one of the most fascinating topics in science — Chapter 4 from your NCERT book, titled "Electricity: Magnetic and Heating Effects". This is a chapter that connects two very important concepts in science — electricity and magnetism — and shows us how they are beautifully linked. Are you ready to explore this wonderful world of electricity and its effects? Let us begin.

So students, let us start with a story. Imagine it is the day of the science exhibition in your school. The school is buzzing with energy, and you are going around with your friends Mohini and Aakarsh, eagerly exploring different models, asking questions, and taking notes. You stop at one exhibit that really fascinates you. It is a working model of a lifting electromagnet displayed by your senior, Sumana. In this model, instead of a hook like a typical crane, there is an iron nail wrapped with a wire, which is connected to a battery. When Sumana closes the circuit, the nail picks up iron paper clips like a magnet. When she opens the circuit, the clips fall off. Mohini and Aakarsh are surprised. They remember learning earlier, in the chapter "Exploring Magnets" in Grade 6, that magnetic materials were attracted by a magnet and that iron was a magnetic material. But in Sumana's model, there is no magnet — only an electric circuit. They are so excited that they want to try it out themselves.

Now students, this is exactly what we are going to learn in this chapter. We will understand how electric current can produce magnetic effects, how we can make electromagnets, how electric current also produces heat, and how batteries generate electricity. Let us start by answering the first big question.

Does an Electric Current Have a Magnetic Effect?

Let us do an activity to find this out. In Activity 4.1, we are going to investigate whether electric current can affect a magnetic compass. You will need a magnetic compass, an electric cell, a cell holder, two drawing pins, a safety pin, two nails, two pieces of connecting wires of different lengths, and two small pieces of cardboard.

First, using two drawing pins, a safety pin, and a cardboard piece, make a switch just like you learned to make in the chapter on electricity in Grade 7. Now, place the cell in the cell holder. Fix two nails to a piece of cardboard as shown in the figure. Fix the middle portion of the longer wire stretched between the nails, such that it is slightly above the surface of the cardboard. Attach one end of that wire to the cell holder and another end to the switch. Connect the second wire between the cell holder and the switch. Now, place the magnetic compass beneath the wire between the two nails.

Now, while watching the compass needle carefully, move the switch to the ON position to allow electric current to flow through the wire. What do you observe? You will notice that when the current flows, the compass needle gets deflected from its original direction. It no longer points to the north-south direction as it normally does. Now, again while watching the compass needle, move the switch to the OFF position. What do you observe this time? The needle returns to its original direction. Move the switch between ON and OFF positions a few more times and carefully observe how the compass needle behaves each time.

Students, this is a very important observation. When the current flows, the compass needle deflects. When the current stops, the needle returns to its original direction. Why does this happen? Let us think about what we already know.

We have learnt earlier, in the chapter "Exploring Magnets" in Grade 6, that the compass needle is a tiny magnet. It deflects when a magnet is brought near it, and this magnetic effect can act through any non-magnetic materials kept in between. But why does the compass needle deflect when the current flows through the wire? The deflection indicates that the current-carrying wire has a magnetic effect on the compass needle. When the current stops, this magnetic effect disappears and the compass needle returns to its original direction.

So students, the region around a magnet or a current-carrying wire where its magnetic effect can be felt — such as by the deflection of a compass needle — is said to have a magnetic field. 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. The magnetic field disappears when the current stops flowing.

Now, let me tell you something interesting. We have learnt about magnets and electric current in earlier grades. Many of us might have thought that there was no link between the two. But now we have found that electricity and magnetic effect are linked! This is a huge discovery.

Be a Scientist

Students, you have just now made the same discovery that was made by the scientist Hans Christian Oersted, who lived from 1777 to 1851, in the year 1820. He was a professor at a university in Denmark. It is said that once while giving a demonstration in his class, he noticed that whenever an electrical circuit was closed or opened, the needle of a magnetic compass lying nearby deflected. He investigated this further, and when he was certain that an electric current indeed produced a magnetic field, he published his findings. This discovery was very important because it led to other scientists repeating his experiment to check if they got the same results, and further investigating the connection between electricity and magnetism. This discovery opened up an entirely new field of science called electromagnetism, which has given us many wonderful technologies.

Now students, the magnetic effect of electric current has many practical applications. Can you think of some devices that use this effect? Devices like electromagnets, electric bells, motors, fans, and loudspeakers all work on the principle of magnetic effect of electric current. We will learn about some of these in this chapter.

Now, let us ask another question. Can we use electric current to make a magnet? The answer is yes, and we call such magnets electromagnets. Let us learn how to make one.

Electromagnets

In Activity 4.2, we are going to explore how to make an electromagnet. You will need about 50 centimeters long length of a flexible insulated wire, an iron nail, an electric cell, and a few iron paper clips.

First, tightly wrap the wire around the nail in the form of a coil, as shown in the figure, and secure it with some adhesive tape. Now, connect the ends of the wire to the cell. Be careful to not connect the wires to the cell for more than a few seconds, otherwise the cell may weaken quickly. Now, bring the nail close to the iron paper clips and lift up. Do the clips hang to the ends of the nail? Yes, they do! Now, disconnect the wire from the cell to stop the flow of electric current in the wire. Do the clips fall down? Yes, they do!

So students, what do we observe? When electric current flows through the coil, the clips cling to the nail. But when the current is stopped, the clips no longer cling to it. This is because the coil with current flowing through it behaves like a magnet, and when the current stops, it loses its magnetic property.

Now, let us investigate these observations in more detail through Activity 4.3. In this activity, we need around 100 centimeters long flexible insulated wire, a piece of chart paper, an iron nail, an electric cell, two magnetic compasses, and a few iron or steel paper clips.

First, roll a piece of chart paper to make a cylinder of diameter roughly equal to the width of a pencil, and secure it with adhesive tape. Now, tightly wind around 50 turns of the insulated wire on the cylinder to form a cylindrical coil. Secure the wire with adhesive tape. Place the compasses near the two ends of the cylindrical coil. Now, connect the two ends of the coil with the terminals of the cell and observe the magnetic compasses. Do you find any deflection in the needles of the compasses? Yes, there is deflection! Now, disconnect the wire from the cell. Do the needles of the compasses come back to their original positions? Yes, they do!

Now, insert an iron nail in the paper cylinder and repeat the steps. Is there any difference in the deflection of the compass needles? Yes, there is much more deflection! Now, place some iron paper clips near the two ends of the nail. Are the clips attracted to the ends of the nail? Yes, they are!

So students, what have we learned from this activity? When current is passed through the cylindrical coil, it behaves like a magnet and deflects the needle of a magnetic compass. When an iron nail is inserted in the core of the coil, then the coil becomes a stronger magnet and the deflection of the magnetic compass needle is much more. It also attracts iron clips. When the current is stopped, the cylindrical coil loses its magnetic effect.

A current-carrying coil that behaves as a magnet is called an electromagnet. For practical applications, most electromagnets have an iron core to make them stronger. The iron core gets magnetized when current flows through the coil, and this makes the electromagnet much more powerful.

Now, students, let us ask another question. Does an electromagnet also have two poles like a bar magnet? Let us find out through Activity 4.4.

Take the electromagnet made in Activity 4.3 and a magnetic compass. Label the two ends of the coil as A and B. Place the magnetic compass near the end A of the coil as shown in the figure. Connect the coil to the cell and observe the compass. Note down which pole of the magnetic compass is attracted to end A.

As we have learnt earlier, when two magnets are brought close to each other, their unlike poles — that is, North and South — attract each other. So, if the north pole of the magnetic compass is attracted towards end A of the electromagnet, then end A is the south pole. Now, repeat this procedure to find the polarity of end B as well. Did you find that the polarity of end B is opposite to the polarity of end A? Yes, it is!

We learnt in Grade 6 that a magnet has two poles. Just like a magnet, an electromagnet also has two poles — North and South. So students, remember that an electromagnet behaves just like a bar magnet when current flows through it. It has a North pole and a South pole, and it can attract magnetic materials like iron.

Think Like a Scientist

Now students, let us do some thinking. What would happen if we repeat Activity 4.3 with different numbers of cells or different numbers of turns in the coil?

Let us think about this. A single cell provides only a small amount of current, so the magnetic field is weak. As a result, the deflection of the compass needle is less, and the coil can only attract a few clips. A battery with more cells gives a larger current compared to that with a single cell. This creates a stronger magnetic field, so the deflection of the compass needle is more, and the coil can attract more clips. The increase in number of turns of the coil also makes the coil a stronger magnet!

Also, if we repeat Activity 4.4 by changing the direction of the current, what would happen? The poles of the electromagnet would reverse! So students, the strength of an electromagnet can be changed by changing the amount of electric current flowing through the coil or the number of turns of the coil, or both. Also, its poles can be reversed by changing the direction of the current.

A Step Further

Now students, let me tell you something interesting about the Earth itself. Do you remember learning earlier, in the chapter "Exploring Magnets" in the book Curiosity, Grade 6, that a freely suspended magnet rests along the north-south direction because our Earth itself behaves like a giant magnet? But why does Earth behave like a magnet? Deep inside the Earth, the movement of liquid iron in the core creates electric currents, which generate a magnetic field. Many migratory birds, fish, and animals use this field to navigate across continents and oceans. The Earth's magnetic field also acts as a shield, blocking harmful particles from space and helping protect life on Earth. Isn't that wonderful?

Now students, let us move on to another important application of electromagnets.

Lifting Electromagnets

Are electromagnets also used in real life for lifting objects? Yes, they certainly are! Lifting electromagnets are strong electromagnets that may be hung to cranes. The crane operator can control the magnet by switching the current ON and OFF. When the current is turned ON, the electromagnet lifts the iron or steel objects. When the current is switched OFF, the magnetic field disappears, and the objects are released. Lifting electromagnets are widely used in factories and scrap yards to move, lift, and sort heavy metal items efficiently. This is exactly what Mohini and Aakarsh saw in Sumana's model at the science exhibition.

A Step Further

We have learnt that when electric current flows through a conductor like a wire, it produces a magnetic field around it. In the higher grades, you will learn even more about this wonderful link between electricity and magnetism, including the exciting idea that just as electricity can produce magnetism, a moving magnet can also lead to an electric current. This deep connection between electricity and magnetism is vital to our daily lives, as it forms the basis of many devices, from electric motors to power generators. We will learn about these in detail in higher classes.

Now students, let us move on to the next important topic. We have seen that electric current can produce magnetic effects. But can electric current also produce heat? Let us find out.

Does a Current Carrying Wire Get Hot?

In Activity 4.5, we are going to observe what happens when electric current flows through a wire. While doing the activity for electromagnet, did you also notice that the wire ends got warm? Why would that happen? Let us investigate.

In this activity, we will use a special kind of wire called a nichrome wire. Nichrome is an alloy of nickel and chromium, and it has a high resistance to the flow of electric current.

You will need a cardboard piece of about 10 centimeters length and 10 centimeters width, two nails, a nichrome wire of thickness about 0.3 millimeters and length of 10 centimeters, an electric cell, a cell holder, a switch, and connecting wires.

First, mount the nails on the cardboard about 5 centimeters apart. Tie the nichrome wire between these nails and make the connections as shown in the figure with the switch in the OFF position. Now, touch the nichrome wire. What do you feel? It should feel at room temperature, just like any other wire. Now, move the switch to the ON position for about 30 seconds and then move it back to OFF. Touch the nichrome wire momentarily. What difference do you feel? You will notice that the nichrome wire feels warm now, maybe even hot!

Remember to be careful. Do not hold the nichrome wire for an extended period to avoid any injuries. Also, do not touch the wire immediately after the experiment. Repeat the last two steps to confirm the observation.

So students, what did we observe? The nichrome wire feels warm when current is passed through it. This happens because when electric current flows through any conductor, it faces some opposition or resistance to its flow. Different conductors offer different levels of resistance to the flow of current. A nichrome wire, for example, offers higher resistance compared to a copper wire of the same size and length. This resistance causes some of the electrical energy to be converted into heat energy. When an electric current passes through a conductor, it gets heated. This warming is known as the heating effect of electric current.

Think Like a Scientist

Now students, let us think about what would happen if we used a battery of 2 cells instead of just 1 cell. This activity should be carried out strictly under the supervision of a teacher. Repeat Activity 4.5 with a battery of 2 cells. What do you notice? For the same duration, does the wire heat up more with one cell or two cells?

The amount of heat generated is more in the experiment with 2 cells. This is due to the fact that the heat generated depends on the magnitude of the electric current. The heat generated in a wire depends on the material, thickness, length of the wire, and the duration for which the current flows.

Now students, think about the filament of an incandescent lamp. In Grade 7, we have learnt that an incandescent lamp glows because its filament is heated by an electric current. Many household appliances such as electric room heaters, electric stoves, electric kettles, electric irons, water heating immersion rods, and hair dryers work on the same principle of the heating effect of electric current. All these devices contain a rod or a coil of wire, called a heating element. In some appliances where this element is visible, it can be seen glowing red hot.

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.

So students, the heating effect of electric current is very useful in many everyday appliances. But sometimes, it can cause problems, like energy loss in wires during transmission. Overheating in appliances may cause damage to plugs and sockets where plastic parts may melt, or even lead to fires. In household circuits, there are safety devices placed in the circuit to minimise such incidents.

A Step Further

To prevent unnecessary heating in household switchboards, it is important to use appropriate wires, plugs, and sockets that are rated for the specified electric current of the connections. This is why we should always use good quality electrical accessories in our homes.

Ever Heard Of...

Beyond household use, the heating effect of electric current has several industrial applications. One notable example is in steel manufacturing industries, where a specially designed high-temperature furnace — an enclosed space built to generate heat — uses electric current to produce heat. This is used to melt and recycle scrap steel, converting it into usable steel. This is just one example of how the heating effect of electric current is used in industries.

Now students, let us think about the portable sources of electricity, such as cells and batteries. Using these, we could light up a small lamp, make a magnet, and heat up a wire. But have you ever wondered what is inside these cells and batteries that produces electricity? Let us learn about this next.

How Does a Battery Generate Electricity?

Let us start with one of the earliest types of electric cells ever made.

Voltaic Cell

A Voltaic cell, also known as a Galvanic cell, is shown in the figure. It contains two metal rods made of different materials and a liquid called an electrolyte, placed in a glass or plastic container. The rods, called electrodes, are partly dipped in the electrolyte, which is usually a weak acid or salt solution. A chemical reaction between the rods and the electrolyte produces electricity. When the circuit is connected, electric current flows from the positive terminal through the circuit to the negative terminal. Over time, the chemicals get used up, and the cell stops working. It is then called "dead" and cannot supply any more electricity.

Ever Heard Of...

The Voltaic or Galvanic cells get their names from two Italian scientists, Alessandro Volta and Luigi Galvani. In the late 1700s, Galvani noticed that a dead frog's leg kicked when touched with two different metals — copper and iron. It was already known by then that electricity could stimulate muscular motion, and Galvani thought the electricity came from the frog itself. But Volta had a different idea. He believed the electricity came from the metals, and not the frog. To test this, he used saltwater-soaked paper instead of the frog's leg and still got an electric current. This showed that it was the combination of metals and liquid that generated electric current — leading to the invention of the first battery! This is a great example of how scientific discoveries often build on the work of previous scientists.

Now students, can we also make our own Voltaic cell using easily available materials? Let us try Activity 4.6.

Take five or six juicy lemons, copper wires or strips that are 1 to 2 millimeters thick, and iron nails. Also take one LED and some connecting wires. Insert the copper wire and the iron nail in one of the lemons, keeping them apart by a small distance as shown in the figure. Repeat the above step for all the remaining lemons. Now, join the copper wires and nails as shown in the figure. Connect the LED between the copper wire of the first lemon and the iron nail of the last lemon, using connecting wires. What do you observe? Does the LED glow?

If the LED does not glow, reverse its connections. Does the LED glow now? Remember that we have learnt earlier that current can pass through the LED only when the positive terminal — which is the longer wire — of the LED is connected to the positive terminal of the battery, and the negative terminal — which is the shorter wire — of the LED is connected to the negative terminal of the battery.

A glowing LED indicates that your cell is working. In this cell, the metal electrodes are the copper wires and the iron nails. The electrolyte is the lemon juice, which helps conduct electricity. You may also use salt solutions instead of lemon juice.

A Step Further

Some common metal pairs for Voltaic cells are zinc and copper, zinc and silver, aluminium and copper, iron and copper, magnesium and copper, and lead and copper. Some metals, like copper, act as positive electrodes, yet some other metals, like zinc, act as negative electrodes. This is due to their chemical properties. We will learn more about this in the higher grades.

Now students, Voltaic cells were an important discovery, but they are not convenient for everyday use because they contain liquid. Instead, dry cells are one of the most widely used electric cells today. They are called "dry" because the electrolyte is not a liquid but a thick moist paste.

Dry Cells

The structure of a dry cell is shown in the figure. It consists of a zinc container which acts as the negative terminal and a carbon rod at the centre covered with a metal cap that acts as the positive terminal. The carbon rod is surrounded by the paste-like electrolyte.

The dry cell is a single-use cell, meaning once it is used up, it has to be disposed of. For several applications, rechargeable batteries are increasingly being used now.

Rechargeable Batteries

Rechargeable batteries can be recharged and reused multiple times. This prevents wastage and saves money over time as well.

There are many different kinds of rechargeable batteries that are used for different applications — from small batteries used in watches and phones to batteries used in laptops and tablets, to bigger batteries that run inverters or drive electric vehicles. However, rechargeable batteries also do not last forever. After being charged and used many times, they slowly wear out.

Oh, so this is the reason why after a year or two, the phone battery requires charging more often!

A Step Further

Today, the lithium-ion battery is the most common type of rechargeable battery, found in almost all devices that use batteries. These batteries rely on special metals like lithium and cobalt, which are mined and processed in limited parts of the world. Because of this, countries are now racing to secure supplies, recycle old batteries, and develop new technologies.

Scientists are also working on the next big leap: solid-state batteries, which replace the liquid or paste-like electrolytes with solid materials. These future batteries would be much safer, charge faster, and last longer. Improved rechargeable batteries are very important as the world moves to developing environmentally friendly sources of electrical power.

Now students, let us summarize what we have learned so far in this chapter with some snapshots.

Snapshots

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.

A current-carrying coil that behaves as a magnet is called an electromagnet. For practical applications, most electromagnets have an iron core to make them stronger.

Generation of heat in conductors due to flow of electric current is known as the heating effect of electric current.

A cell or a battery is a device that generates electric current because of chemical reactions taking place inside it.

Rechargeable batteries can be recharged and reused multiple times.

Now students, let us work on the exercises and questions given at the end of the chapter. These are very important for your understanding and will help you revise what we have learned.

Keep the Curiosity Alive

Question 1: Fill in the blanks.

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

The answer is electrolyte. In a Voltaic cell, the electrolyte is the liquid — usually a weak acid or salt solution — that helps conduct electricity and takes part in the chemical reaction that produces electric current.

(ii) A current-carrying coil behaves like a magnet.

The answer is magnet. When electric current flows through a coil, it produces a magnetic field and behaves like a magnet. This is called an electromagnet.

Question 2: Choose the correct option.

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

This is False. Dry cells are actually more portable than Voltaic cells because they do not have any liquid that can spill. That is why we use dry cells in torches, remote controls, and many other portable devices.

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

This is True. An electromagnet works only when current flows through it. When the current stops, it loses its magnetic properties. This is one of the key differences between a permanent magnet and an electromagnet.

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

This is False. With more cells, we have more current flowing through the coil, which makes the electromagnet stronger. So it will attract more clips with 2 cells than with a single cell.

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

The correct answer is (c) Both (i) and (ii) are correct. When electric current flows through a nichrome wire, two things happen. First, because nichrome has high resistance, the electrical energy is converted to heat energy, so the wire becomes warm. Second, as we learned earlier, any current-carrying wire produces a magnetic field around it, so a magnetic compass placed near it will be deflected. Both statements are correct.

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

Column A has four items: (i) Voltaic cell, (ii) Electric iron, (iii) Nichrome wire, (iv) Electromagnet.

Column B has four items: (a) Best suited for electric heater, (b) Works on magnetic effect of electric current, (c) Works on heating effect of electric current, (d) Generates electricity by chemical reactions.

Let us match them one by one.

Voltaic cell generates electricity by chemical reactions. So (i) matches with (d).

Electric iron works on heating effect of electric current. It converts electrical energy into heat to iron clothes. So (ii) matches with (c).

Nichrome wire is best suited for electric heater because it has high resistance and produces a lot of heat when current flows through it. So (iii) matches with (a).

Electromagnet works on magnetic effect of electric current. It becomes a magnet only when current flows through it. So (iv) matches with (b).

So the matches are: (i)-(d), (ii)-(c), (iii)-(a), (iv)-(b).

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.

The correct options are (ii) and (iii). Nichrome is commonly used in heating devices because it has high resistance, which means it generates more heat for a given current compared to metals like copper. Also, it is relatively cheaper than some other metals. It is not a good conductor in the sense that it resists current flow, but that is exactly why it produces heat. It is definitely not an insulator.

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.

This is a thoughtful question that asks us to compare modern electric heating with traditional methods. Let me explain the reasons.

First, electric heating devices are much cleaner. When we burn firewood or charcoal, they produce smoke and pollutants that can cause air pollution and health problems. Electric heaters do not produce any smoke or harmful gases.

Second, electric heating devices are more efficient. They convert most of the electrical energy into heat, whereas burning fuel is less efficient and much of the energy is wasted.

Third, electric heating is more convenient and easy to control. You can simply switch on an electric heater or stove, and it starts producing heat immediately. You can also control the temperature easily. With firewood, you need to gather fuel, light it, and maintain the fire.

Fourth, from a societal impact perspective, widespread use of firewood contributes to deforestation and environmental degradation. Using electricity, especially from renewable sources, is more sustainable in the long run.

Fifth, electric heating devices are safer in many ways. They do not produce open flames, reducing the risk of fires. They also do not produce harmful gases like carbon monoxide that can cause poisoning.

However, it is important to note that the source of electricity also matters. If the electricity is generated from fossil fuels, then the environmental benefits are reduced. But as we move towards cleaner sources of energy, electric heating becomes even more beneficial.

Question 7: Look at the figure 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.

Let me answer each part.

(i) The path of electric current would be from the positive terminal of the battery, through the coil, and back to the negative terminal of the battery. The direction of current is from positive to negative outside the cell.

(ii) The compass needle moves because when electric current flows through the coil, it produces a magnetic field around it. This magnetic field interacts with the magnetic field of the compass needle, causing it to deflect from its original north-south direction. This is the magnetic effect of electric current that we learned about.

(iii) If we reverse the battery terminals, the direction of current through the coil reverses. This will reverse the polarity of the electromagnet. So if end A was previously the South pole, it will become the North pole, and vice versa. This will cause the compass needle to deflect in the opposite direction. The deflection will still occur, but in the opposite sense.

Now students, let us move on to the next set of questions.

Question 8: Suppose Sumana forgets to move the switch of her lifting electromagnet model to OFF position (in the 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.

This is an interesting question. Let me think about what might have happened.

There could be several possible reasons:

First, the battery or cell might have become weak or dead. When the switch is left ON for a long time, the cell continues to supply current, and eventually the chemicals inside get used up. When the cell becomes weak, the current flowing through the coil decreases, and the magnetic field becomes weaker. Eventually, it may become too weak to lift the paper clips.

Second, the wire might have become hot due to the heating effect of electric current. When current flows through a wire with resistance, it produces heat. If the wire gets too hot, its resistance might increase, which would reduce the current flowing through it. This would weaken the magnetic field. Also, if the wire gets extremely hot, it might damage the insulation or even melt, breaking the circuit.

Third, the iron nail itself might have become demagnetized over time. Although electromagnets are usually temporary magnets, sometimes if they are kept ON for very long, the magnetic domains in the iron nail might rearrange in a way that reduces its magnetism.

The fact that the wire is still warm suggests that current is still flowing to some extent, but perhaps not enough to create a strong magnetic field. This could be because the cell is weak or the wire has heated up and increased its resistance.

Now students, let us look at the next question.

Question 9: In the figure 4.12, in which case will the LED glow when the switch is closed?

This question refers to a figure showing different circuit arrangements. Without the actual figure, I cannot give you the specific answer, but let me explain the general principle.

For an LED to glow, it must be connected in the correct orientation. The LED has two terminals — a positive terminal (longer wire) and a negative terminal (shorter wire). The longer terminal must be connected to the positive terminal of the battery, and the shorter terminal must be connected to the negative terminal of the battery. If the LED is connected in reverse, it will not glow.

Also, the circuit must be complete for current to flow. If the switch is open or there is a break in the circuit anywhere, the LED will not glow.

So to answer this question, you would need to look at each circuit diagram and check if the LED is connected the right way and if the circuit is complete when the switch is closed.

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?

Yes, the coil will still deflect the compass even without the iron nail. This is because any current-carrying coil produces a magnetic field around it. However, the deflection will be less than before.

When we insert an iron nail inside the coil, the iron core gets magnetized by the magnetic field of the coil. This adds to the overall magnetic field, making the electromagnet stronger. Without the iron core, we only have the magnetic field produced by the coil itself, which is weaker. So the compass needle will deflect, but the deflection will be less compared to when the iron nail was inside.

Question 11: We have four coils, of similar shape and size, made up from iron, copper, aluminium, and nichrome as shown in the figure 4.13. 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

This is a very interesting question. Let me think about this carefully.

The deflection of a compass needle occurs because of the magnetic effect of electric current. When electric current flows through any conductor, it produces a magnetic field around it. This is true for all conductive materials.

So, whether the coil is made of iron, copper, aluminium, or nichrome, when current flows through it, it will produce a magnetic field and deflect the compass needle. The only condition is that current must be able to flow through the material. All these materials — iron, copper, aluminium, and nichrome — are conductors of electricity, so current can flow through them.

Therefore, the correct answer is (iv) In all four circuits. All coils will cause deflection of the compass needle because all of them conduct electricity and produce magnetic fields when current flows through them.

However, there is one thing to note. Iron is a magnetic material, so when we put an iron core inside a coil, it gets magnetized and makes the electromagnet stronger. But even without being in a coil, a straight iron wire carrying current will still produce a magnetic field. So yes, all four circuits will show deflection.

Now students, let us move on to the "Discover, design, and debate" section. These are activities that will help you explore the concepts further.

Discover, Design, and Debate

Activity 1: 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 a conclusion about the effect of number of turns of the coil on the strength of the electromagnet.

This is a great activity to understand how the number of turns affects the strength of an electromagnet. When you perform this activity, you will observe that as you increase the number of turns in the coil, the deflection of the compass needle increases. This is because more turns mean a stronger magnetic field. The magnetic field produced by each turn of the coil adds up, so more turns result in a stronger magnet.

So the conclusion is: The strength of an electromagnet increases with an increase in the number of turns of the coil.

Activity 2: Take two thin nichrome wires of equal length and different thickness (approximately one of these wire thickness to be double 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 seconds in each case. Momentarily touch these wires. Which wire heats up more? Now repeat the same activity with two nichrome wires of same diameter but of different lengths. Prepare a brief report of your activity.

This activity helps us understand what factors affect the heating effect of electric current.

For the first part, with wires of equal length but different thickness: The thinner wire will heat up more. This is because the thinner wire has higher resistance. Resistance is inversely proportional to the area of cross-section. So a thinner wire has more resistance, and more resistance means more heat is generated for the same current.

For the second part, with wires of same diameter but different lengths: The longer wire will heat up more. This is because resistance is directly proportional to the length of the wire. A longer wire has more resistance, so it produces more heat.

Activity 3: Try to make an electric cell using various fruits and vegetables. Also try with electrodes of different metals. Prepare a brief report.

This is a fun activity that demonstrates the working of a simple Voltaic cell. Fruits and vegetables like lemons, potatoes, and oranges contain juices that act as electrolytes. When you insert two different metals into these, a chemical reaction occurs that produces electricity. You can test your cell by connecting an LED or a small bulb.

You will find that different combinations of metals produce different amounts of voltage. Some combinations that work well are copper and zinc, copper and iron, or copper and aluminium. The lemon cell we discussed earlier is a good example.

A Step Further

Now students, let me tell you something important about batteries and the environment. Even when a battery stops working, it is not completely "dead". It could still contain materials like acids, and metals like lead, cadmium, nickel, or lithium, which may cause fires or be harmful for the environment if the battery is thrown in regular garbage. Further, many materials used in these batteries are valuable and could be recycled and reused. These days, there are many places with special e-waste recycling facilities, where used batteries can be disposed of. If you are not sure, ask your teacher. Recycling batteries is good for the planet and the people.

Now students, we have covered the entire chapter. Let me give you a brief summary of everything we have learned in this lesson.

Summary

In this chapter, we learned about the magnetic and heating effects of electric current.

We started with the story of Mohini and Aakarsh who saw a lifting electromagnet at their school science exhibition. This led us to investigate whether electric current has a magnetic effect.

We learned that when electric current flows through a conductor, it produces a magnetic field around it. This is called the magnetic effect of electric current. This discovery was made by Hans Christian Oersted in 1820.

We learned how to make an electromagnet by wrapping insulated wire around an iron nail and passing current through it. We discovered that an electromagnet has two poles like a bar magnet, and its strength can be changed by changing the amount of current or the number of turns in the coil. We also learned about the practical applications of electromagnets, like lifting magnets used in factories and scrap yards.

Then we moved on to the heating effect of electric current. We learned that when electric current flows through a conductor with resistance, it produces heat. This is called the heating effect of electric current. We explored how factors like the material, thickness, length of the wire, and the amount of current affect the heat generated. We learned about various household appliances that work on this principle, like electric heaters, electric irons, and kettles.

Finally, we learned about how batteries generate electricity. We studied the Voltaic cell, which uses chemical reactions to produce electricity. We learned how to make a simple cell using lemons and different metals. We also learned about dry cells and rechargeable batteries, and their applications in everyday life.

We completed various exercises that tested our understanding of all these concepts. We learned about the importance of proper disposal of batteries to protect the environment.

Students, this is a very important chapter that connects electricity and magnetism. The principles we learned here form the basis for many technologies that we use in our daily lives, from electric motors to generators, from electric bells to modern smartphones. I hope you enjoyed this lesson and learned something new. Remember to keep curious and keep exploring!

Thank you for listening. See you in the next lesson!