Reviewing Class 8 Science Notes Chapter 4 Electricity: Magnetic and Heating Effects Class 8 Notes regularly helps in retaining important facts.
Class 8 Science Chapter 4 Electricity: Magnetic and Heating Effects Notes
Class 8 Electricity: Magnetic and Heating Effects Notes
Class 8 Science Chapter 4 Notes – Electricity: Magnetic and Heating Effects Notes Class 8
Magnetic Effects of Electric Current
→ The region around a magnet where its magnetic effect can be felt 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.
→ The magnetic field around a current-carrying conductor can be shown by the deflection of a compass brought near the conductor.
→ The needle of the compass deflects when the switch is on and the current is passing through the wire (as shown in the Figure).
→ When the switch is turned off, the current in the wire stops flowing, and the needle of the compass comes to its original position. This shows the magnetic effect of an electric current.
→ The scientist Hans Christian Oersted (1777 – 1851), Professor at a university in Denmark, observed in 1820 that there exists a connection between electricity and magnetism.
→ Practical applications of the magnetic effect of electric current are seen in daily life. Examples are electromagnets, electric bells, electric motors, fans, loudspeakers, and many battery-operated toys.
→ Applications of electromagnets are common in industrial activities that use magnetic substances. For example, heavy iron scrap is fed into furnaces at very high temperatures to melt it and make iron ingots and iron rods. This is a hazardous activity and cannot be done with human hands. Cranes fitted with very strong electromagnets are used to lift the iron scrap and to feed the same into the furnaces.
→ Electromagnets, in experiments, are made by wrapping an insulated wire around an iron nail. The insulated wire can also be wrapped around a chart paper folded into a cylinder shape. An iron nail is then placed inside the rolled chart paper.
→ When current passes through the wire, the nail acts as a magnet. When the current flow is stopped, the nail loses magnetism.
→ When we increase the current flowing through the coil (use a battery of two or more cells in place of one cell), the magnet becomes stronger.
→ When the number of turns in the coil is increased (length of the conducting wire increases), the magnet becomes stronger.
→ The electromagnets, like bar magnets, have two poles, the North pole and the South pole. If the direction of the flow of current is reversed, the poles interchange their positions. The end of the magnet that has the North pole becomes the South pole, and the South pole becomes the North pole on reversing the direction of current in the coil.
→ Freely suspended magnets rest along the north-south direction because our Earth behaves like a giant magnet.
→ The movement of liquid iron deep inside the core of the Earth creates electric currents that have a magnetic field. This magnetic field helps fish, many aquatic animals, and migratory birds to navigate across oceans and continents. This magnetic field also acts as a shield, protecting life on Earth from the harmful particles from space.
Heating Effects of Electric Current
→ When electric current passes through a conductor, it gets heated. This warming is called the heating effect of electric current.
→ When electric current flows through any conductor, the conductor faces some opposition or resistance to its flow. This resistance converts some electric energy to heat energy.
→ Different conductors have different resistances. Nichrome offers more resistance to the flow of current compared to copper. Thus, current flowing through a nichrome wire has a greater heating effect than copper wire.
→ Nichrome is a material suitable for making heating elements. When electric current passes through wires of nichrome and other such materials, the wires become hot. Some of the heating elements are shown in the Figure:
(A) is the heating element of an electric iron, (B) is the element of a room heater, and (C) and (D) are the heating elements of water heaters.
→ The amount of heat produced in a wire depends on
- The nature of the material
- Thickness of wire (thin wires offer more resistance and thus more heat) and
- The length of the wire (Resistance offered and heat generated are directly proportional to the length of the wire).
→ While making elements of heating appliances from wires of these materials, the length of the wires is increased by making coils of the wire. Also, the wires are kept thin to produce more heat.
→ The amount of heat produced by the electric current flowing through the conductors also depends on the magnitude of the electric current and the duration of time for which the current is passed.
→ A battery of two or more cells produces more heat when current is passed through a nichrome wire compared with a single cell. Also, the wire will be heated more if an electric current is passed for 1 minute compared to the passage of current for 30 seconds.
→ The filament of an incandescent bulb is made of tungsten. When an electric current is passed through this filament, it gets heated, and on heating, tungsten glows to give out light. Some of the electricity is used up in heating the filament of the bulb. This reduces the efficiency of these bulbs.
→ Fluorescent tubes are more efficient than incandescent bulbs as they produce more light and less heat.
→ These days most efficient of all these lights are the Light Emitting Diode (LED) bulbs and are now preferred over other lights.
→ More electric appliances based on the heating effect of electric current are electric stoves, hair dryers, electric kettles, etc.
→ Example of the use of the heating effect of electric current in industry is high temperature furnaces used to melt iron scrap, etc., to make steel in factories.
→ Heating effects of electric current may sometimes be harmful, too. The overheating of electric wires may result in the melting of plastic sockets and plugs.
→ Energy loss during transmission of electric current is also a problem related to the heating effect of electric current.
Portable Sources of Electricity: Cells and Batteries
→ Voltaic cell (named after the scientist Alessandro Volta), also known as Galvanic cell (named after the scientist Luigi Galvani) is one of the earliest electric cells.
→ In a plastic or glass container, a weak acid or salt solution (called an electrolyte) is taken.
→ Two plates of different metals (zinc and copper, zinc and silver, iron and copper, aluminium and copper, magnesium and copper, or lead and copper) are partly inserted in the electrolyte.
→ These plates are called electrodes. One of the metal plates acts as the positive electrode and the other acts as the negative electrode. This depends upon the chemical nature of the metal. Copper is generally positive electrode and zinc negative electrode.
→ When the circuit is connected (as shown in the Figure), electric current flows from the positive terminal to the negative terminal.
→ When the chemicals get used up, the cell becomes “Dead”, which means it stops working and cannot supply any more electricity.
→ Working of a voltaic cell can also be shown using juicy lemon’ cut pieces as electrolyte and iron nails and copper wires/strips (1-2 mm thick).
→ When the lemons, iron nails, and copper strips are joined together as shown in the figure, it serves as a voltaic cell, and the current flows when the circuit is completed.
→ If the LED in the circuit glows, the voltaic cell works. If it does not glow, it may start glowing on changing its terminals in the circuit.
→ In voltaic cells, the liquid used as electrolyte makes its use and portability inconvenient. Therefore, dry cells are the most commonly used electric cells.
→ A dry cell has a zinc container that has a thick, moist paste-type electrolyte filled in it. Inside the electrolyte, a carbon rod is placed. The bottom of the zinc container serves as a negative terminal.
→ The top of the carbon rod is a little above the zinc container and covered with metal that acts as a positive terminal.
→ The dry cell, once used up, has to be disposed of. It cannot be made to work again.
→ Now Rechargeable batteries are becoming popular. These can be recharged and used multiple times.
→ Batteries used in smart watches, phones, and their accessories are small-sized chargeable batteries.
→ Batteries used in laptops, backup devices like inverters, and in motor vehicles are bigger batteries that are rechargeable.
→ Rechargeable batteries also do not last forever. After using and recharging many times, these wear out and need to be discarded.
→ The next generation of batteries came with the invention of Lithium-ion batteries. These give out better output of electricity and last longer.
→ The scientists are now aiming to produce solid-state batteries that will use solids as electrolytes.
→ These batteries are expected to be much safer, charge faster, and last longer.
→ Improved portable sources of electricity are needed to move to a world of a safe environment.
Electricity Magnetic and Heating Effects Notes
→ Electrode: Metallic plates or carbon rods that are part of the electric cell and act as positive and negative terminals of the cell.
→ Electrolyte: The liquid or paste form of chemicals inside an electric cell that are used to give out electricity due to their reaction with the electrodes.
→ Electromagnet: The magnet that has a magnetic force when current is passing through a conductor and loses the magnetic force when the current flow stops.
→ Magnetic field: The region around a magnet where its magnetic effect can be felt.
→ Rechargeable batteries: The batteries that can be charged multiple times and used as a portable source of electricity.
→ Resistance: The property of certain metal conductors to oppose the flow of current through them.
→ Voltaic cell: One of the earliest forms of batteries that uses mild acids or salt solutions in liquid form as an electrolyte.
→ 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 the flow of electric current is known as the heating effect of electric current.
→ A cell or a battery is a device that generates an electric current because of chemical reactions taking place inside it.
→ Rechargeable batteries can be recharged and reused multiple times.
It was the day of the science exhibition, and the school was buzzing with energy. Mohini and Aakarsh, along with their friends, went from one exhibit to another, eagerly exploring different models, asking questions, and taking notes. One simple model fascinated them. It was a working model of a lifting electromagnet, which was displayed by their senior, Sumana. In it, instead of a hook like a typical crane, there was an iron nail wrapped with a wire, which was connected to a battery. When Sumana closed the circuit, the nail picked up iron paper clips like a magnet. When she opened the circuit, the clips fell off. Mohini and Aakarsh were surprised. They remembered learning earlier (in the chapter ‘Exploring Magnets’, Curiosity, Grade 6) that magnetic materials were attracted by a magnet and that iron was a magnetic material. But in Sumana’s model, there was no magnet, only an electric circuit. They were so excited that they wanted to try it out themselves.
Does an Electric Current Have a Magnetic Effect? Class 8 Notes
Activity 1: Let us investigate
Collect a magnetic compass, an electric cell, a cell holder, two drawing pins, a safety pin, two nails, two pieces of connecting wires (one longer and one shorter), and two small pieces of cardboard. Using two drawing pins, a safety pin, and a cardboard piece, make a switch (as you made it earlier in the chapter ‘Electricity: Circuits and their Components’ in Curiosity, Grade 7). Place the cell in the cell holder. Fix two nails to a piece of cardboard as shown in Figure a. 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. Place the magnetic compass beneath the wire between the two nails.
While watching the compass needle, move the switch to the ‘ON’ position to allow electric current to flow through the wire. What do you observe? Now again, while watching the compass needle, move the switch to the ‘OFF’ position. What do you observe this time? Move the switch between ‘ON’ and ‘OFF’ positions a few more times. Carefully observe how the compass needle behaves each time. You may have noticed that when the current flows, the compass needle gets deflected from its original direction. When the current stops, the needle returns to its original direction.
As we have learnt earlier (in the chapter ‘Exploring Magnets’ in Curiosity, Grade 6), the compass needle is a tiny magnet which 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. 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 an 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. The magnetic effect of electric current has many practical applications, such as in devices like electromagnets, electric bells, motors, fans, loudspeakers, and more.
You have just made the same discovery that was made by the scientist Hans Christian Oersted (1777-1851) in 1820, that is, the discovery that electricity and magnetism are linked. He was a professor at a university in Denmark. It is said that once, while giving a demonstration, he noticed that whenever an electrical circuit was closed or opened, the needle of a magnetic compass, lying nearby, deflected. He investigated this, and when he was certain that an electric current indeed produced a magnetic field, he published his findings. This led to other scientists repeating his experiment to check if they got the same results, and further investigating the connection between electricity and magnetism.
Electromagnets Class 8 Notes
Activity 2: Let us explore
Take around 50 cm long length of a flexible insulated wire, an iron nail, an electric cell, and a few iron paper clips. Tightly wrap the wire around the nail in the form of a coil, as shown in the Figure, and secure it with an adhesive tape.
Connect the ends of the wire to the cell. Take care not to connect the wires to the cell for more than a few seconds; otherwise, the cell may weaken quickly. Bring the nail close to the iron paper clips and lift up. Do the clips hang to the ends of the nail? Disconnect the wire from the cell to stop the flow of electric current in the wire. Do the clips fall? When electric current flows through the coil, the clips cling to it. But when the current is stopped, the clips no longer cling to it. Let us now try to investigate these observations in detail through Activity 3.
Activity 3: Let us experiment
Take around 100 cm long flexible insulated wire, a piece of chart paper, an iron nail, an electric cell, two magnetic compasses, and a few iron/steel paper clips. Roll a piece of chart paper to make a cylinder of diameter roughly equal to the width of a pencil. Secure it with adhesive tape. Tightly wind around 50 turns of the insulated wire on the cylinder to form a cylindrical coil as shown in Figure a. Secure the wire with an adhesive tape.
Place the compasses near the two ends of the cylindrical coil. Connect the two ends of the coil with the terminals of the cell as shown in Figure c and observe the magnetic compasses. Do you find any deflection in the needles of the compasses? Disconnect the wire from the cell. Do the needles of the compasses come back to their original positions? Insert an iron nail in the paper cylinder and repeat the steps. Is there any difference in the deflection of the compass needles? Place some iron paper clips near the two ends of the nail. Are the clips attracted to the ends of the nail?
It is observed that 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, the coil becomes a stronger magnet, and the deflection of the magnetic compass needle is much greater. 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.
Activity 4: Let us investigate
Take the electromagnet made in Activity 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 Figure A. 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 (North-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. 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? We learnt in grade 6 that a magnet has two poles. Just like a magnet, an electromagnet also has two poles: North and South.
Repeat Activity 3 with 2 and 4 cells with the same coil, and 2 cells but with a different number of turns of the coil. What do you observe? 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 as compared to one with a single cell. This creates a stronger magnetic field, so the deflection of the compass needle is greater and the coil can attract more clips. The increase in the number of turns of the coil also makes the coil a stronger magnet! Also, repeat Activity 4 by changing the direction of the current. So, 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.
Do you remember learning earlier (in the chapter ‘Exploring Magnets’ in 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 helps protect life on Earth.
Lifting Electromagnets Class 8 Notes
Lifting electromagnets are strong electromagnets that may be hung on the 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/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.
We have learnt that when an 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.
Does a Current-Carrying Wire Get Hot? Class 8 Notes
Activity 5: Let us observe
In this activity, we will use a special kind of wire, called a nichrome wire. Take a cardboard piece of about 10 cm length and 10 cm width, two nails, a nichrome wire of thickness about 0.3 mm (26–28 gauge) and length of 10 cm, an electric cell, a cell holder, a switch, and connecting wires. Mount the nails on the cardboard about 5 cm apart. Tie the nichrome wire between these nails and make the connections as shown in the Figure with the switch in the OFF position. Touch the nichrome wire. What do you feel? Move the switch to the ON position for about 30 s and then move it back to OFF. Touch the nichrome wire momentarily (Do not hold the nichrome wire). What difference do you feel? Repeat the last two steps to confirm the observation.
You may have observed that 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 an electric current.
This activity should be carried out strictly under the supervision of a teacher. Repeat Activity 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, and length of the wire, and the duration for which the current flows.
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, stoves, irons, immersion rods, water heaters, kettles, 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. 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.
The heating effect of electric current is 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.
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.
How Does a Battery Generate Electricity? Class 8 Notes
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 Figure. It contains two metal plates made of different materials and a liquid called an electrolyte, placed in a glass or plastic container. The plates, called electrodes, are partly dipped in the electrolyte, which is usually a weak acid or salt solution. A chemical reaction between the plates 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.
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 an electric current, leading to the invention of the first battery!
Activity 6: Let us construct
Take five or six juicy lemons, copper wires/ strips (1-2 mm 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 Figure A. Repeat the above step for all the remaining lemons. Join the copper wires and nails as shown in Figure b.
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 (longer wire) of the LED is connected to the positive terminal of the battery, and the negative terminal (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.
Some common metal pairs for Voltaic cells are zinc/copper, zinc/silver, aluminium/copper, iron/copper, magnesium/copper, and lead/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.
Dry Cells Class 8 Notes
Voltaic cells were an important discovery, but they are not convenient for everyday use. 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. The structure of a dry cell is shown in Figure. It consists of a zinc container, which acts as a 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 Class 8 Notes
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.
Today, the lithium-ion (Li-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.