Electricity Magnetic and Heating Effects Class 8 Notes Science Chapter 4

Students can rely on these Class 8 Science Notes Chapter 4 Electricity Magnetic and Heating Effects Class 8 Notes for complete exam preparation.

Class 8 Science Chapter 4 Electricity Magnetic and Heating Effects Notes

Class 8 Science Curiosity Chapter 4 Notes

Class 8 Science Chapter 4 Notes – Electricity Magnetic and Heating Effects Notes Class 8

→ Magnetic effect is the ability of electric current to produce a magnetic field around a wire or coil. It is called the magnetic effect of electric current.

→ Magnetic field is the invisible region around a magnet or current-carrying wire where magnetic effect can be felt.

→ Electromagnet is a temporary magnet made by passing electric current through a coil of wire wrapped around an iron core.

→ Heating effect The production of heat in a electric device due to the flow of electric current is called the heating effect of electric current.

→ Voltalc cell is an early type of electric cell that produces current through a chemical reaction between two different metals and an electrolyte.

→ Dry cell is a compact electric cell with a paste-like electrolyte, commonly used in torches, remotes and toys. It cannot be recharged.

→ Rechargeable battery is a battery that can be used multiple times by charging it again with electricity, used in phone laptops and electric vehicles.

→ Ramesh saw that a crane in a junkyard lifts heavy iron objects only when powered. What effect of current is used in this case?

→ Which scientist first discovered that electric current can produce a magnetic field?

→ Why does a coil of wire with an iron nail inside it behave like a magnet when electric current passes through it?

→ Name one factor that can increase the strength of an electromagnet.

→ Jeetu uses a thin wire instead of a thick one in an electric circuit. Will the wire heat up more or less?
→ Give a factor on which the heating effect of electric current depends.

→ Give one example of the heating effect of electric current used in industries.

→ Name any two devices that work on the heating effect of electric current.

→ Though copper is a better conductor, why is it not used as a heating element in electric heaters?

→ During a science activity, Gazal saw that the middle rod in a dry cell was covered with a cap. What is this rod made of, and what is its function?

→ Name one device that commonly uses small rechargeable batteries.

→ Give one reason why rechargeable batteries are better for long-term use than dry cells.

→ Prerna read that a dry cell does not contain liquid. What does it contain instead?

Magnetic Effect of Electric Current

When electric current flows through a wire, it creates a magnetic field around the wire. This is known as the magnetic effect of electric current.

The presence of this magnetic field can be observed using a compass needle, which deflects when placed near a current-carrying wire as shown in the figure.

This effect only lasts as long as the current is flowing. When the current is switched off, the magnetic field disappears.

The magnetic effect of electric current is used in many devices around us like electric bells, electromagnets, fans, mixers, loudspeakers, and machines having electric motors.

Hans Christian Oersted, a Danish scientist, discovered in 1820 that electric current produces a magnetic field. He observed a compass needle move when current was switched on, proving that electricity can create magnetic effect around a wire. This discovery helped scientists understand the connection between electricity and magnetism.

Electromagnet

When electric current flows through a coil of wire, the coil starts acting like a magnet and can deflect the needle of a nearby compass.

A coil that shows magnetic properties only when current flows through it is called an electromagnet.
Just like a magnet, an electromagnet also has two poles – North and South.

Ways to Increase the Strength of an Electromagnet

1. Use more number of cells in the battery.
2. Increase the number of turns in the coil.
3. Insert an iron core inside the coil.

Advantages of Electromagnets over Permanent Magnets

• The magnetism of an electromagnet can be switched ON or switched OFF as desired, while it is not possible with a permanent magnet.
• By increasing the number of turns in the coil and by increasing the current passing through the coil, an electromagnet can be made very strong. On the other hand, a permanent magnet cannot be made so strong.

The Earth acts like a big magnet, which is why a compass needle points north-south. This is because of the movement of liquid iron inside the Earth’s core. This movement of liquid iron creates electric currents, which in turn produce a magnetic field. Many migratory birds, fish, and other animals use this field to find their way while travelling. The Earth’s magnetic field also protects us by blocking harmful particles from space.

Lifting Electromagnets

Strong electromagnets in cranes lift heavy iron and steel in factories and scrap yards. They work only when current flows, making them safe and useful. The link between electricity and magnetism is used in many everyday machines.

Heating Effects of Electric Current

• The production of heat in a electric device due to the flow of electric current is called the heating effect of electric current.
• When the appliances like (electric iron, bulb or room heater) are switched ON after connecting to the electric supply, then a coil of wire present inside the device, becomes red hot and releases the heat.
• Nichrome wire is used to make coils or elements because nichrome has a very high melting point and very high resistance.

Never touch a lighted electric bulb connected to the mains electricity supply as it may be very hot and can damage your hands.

The amount of heat depends on the following factors

1. Magnitude of current : More current means more heat.
2. Material of wire : High-resistance materials like nichrome produce more heat.
3. Thickness of wire : Thinner wires have more resistance and generate more heat.
4. Length of wire : Longer wires resist current more and produce more heat.
5. Duration of current flow : Longer flow time results in more heat.

Uses of Heating Effect of Electric Current

• The heating effect is used in many household appliances such as electric heaters, stoves, irons, kettles, and hair dryers.
• These devices contain a heating element (usually a coil or rod) that becomes hot when current passes through it.
• In industries, the heating effect is used in electric furnaces, especially in steel manufacturing, to melt and recycle scrap steel into usable metal.

Precautions While Using Heating Devices

• Excessive heating, however, can be harmful. It may cause energy loss during transmission and can damage wires, plugs and sockets.
• In extreme cases, overheating may lead to electrical fires if safety is not ensured.
• To prevent such risks, it is important to use properly rated wires, plugs and safety devices in household electrical systems.
• Electrical components must always match the specified current ratings to avoid hazards due to overheating.

How Does a Battery Generate Electricity?

Voltaic Cell

• A Voltaic cell, also called a Galvanic cell, is a device that produces electricity through a chemical reaction.
• It has two metal plates made of different materials, which are called electrodes.

• These plates are partly dipped in a liquid solution called an electrolyte, usually a weak acid or salt solution, kept in a glass or plastic container.
• When the two electrodes react with the electrolyte, they produce electric current. The current flows from the positive terminal to the negative terminal through the circuit.
• After some time, the chemicals inside the cell get used up, and the cell stops working. Such a cell is called a dead cell.
• Voltaic cells are the basic idea behind batteries used in torches, remote controls and toys.
• Some common metal pairs for Voltaic cells are zinc-copper aluminium-copper, iron-copper, magnesium-copper and lead-copper. Some metals like copper act as positiveele^trodes, yet some other metal like zinc act as negative electrodes.

The names Voltaic and Galvanic come from two Italian scientists — Alessandro Volta and Luigi Galvani. In the 1700s, Galvani noticed that a dead frog’s leg moved when touched with two different metals. He thought the electricity came from the frog. But Volta believed it came from the metals. To prove it, he used saltwater-soaked paper instead of the frog and still got electricity showing that the current was due to the metals. This experiment helped in the invention of the first battery.

Dry Cells

• Dry cells are commonly used in electric cells found in torches, clocks, and toys. Unlike voltaic cells, dry cells are more convenient and easier to carry. They are called ‘dry’ because they use a thick paste instead of a liquid as the electrolyte.
• A dry cell has a zinc container that acts as the negative terminal and a carbon rod in the center, which works as the positive terminal.
• The carbon rod is surrounded by the moist paste-like electrolyte.
• Dry cells are usually non-rechargeable, so they must be thrown away after use.
• Today, in many devices like mobile phones and cameras, rechargeable batteries are preferred because they can be used again and again by charging them with electricity.

Rechargeable Batteries

Rechargeable batteries can be used again and again by charging them with electricity. This helps to reduce battery waste and saves money over time.

These batteries come in many types and sizes. Small ones are used in watches, phones and tablets, while larger ones are used in laptops, inverters and even electric vehicles.
However, rechargeable batteries don’t last forever. After many charge and use cycles, they start to lose their strength

and need to be charged more often. That’s why a phone battery doesn’t last long after a year.

Lithium-ion Batteries

Lithium-ion (Li-ion) batteries are the most commonly used rechargeable batteries today. They are found in devices like smartphones, laptops and electric vehicles.

Need for Recycling Lithium-ion Batteries

• They use valuable metals like lithium and cobalt, which are limited and found only in certain parts of the world.
• Recycling old batteries helps recover these metals, prevents environmental harm and supports sustainable use of natural resources.

Future of Batteries

1. Scientists are developing solid-state batteries that use solid materials instead of liquid or paste electrolytes.
2. These advanced batteries will be safer, more durable, and charge faster than current ones.
3. Better rechargeable batteries are essential as the world moves towards clean and green energy.

Safe Disposal and Recycling of Used Batteries

1. Even used batteries contain harmful metals like lead, lithium and nickel that can damage the environment or cause fires.
2. These materials are also valuable and can be recycled and reused.
3. Old batteries should be taken to e-waste collection centres, not thrown in dustbins.
4. Recycling batteries helps protect the environment and conserve resources.