CBSE Class 8 Science Revision Notes Chapter 4 Electricity: Magnetic and Heating Effects

Electric current can produce a magnetic field around a conductor and heat the conductor through resistance. Electric cells and batteries generate current through chemical reactions between electrodes and an electrolyte.

Electricity can produce effects beyond lighting a lamp or running an appliance. A current-carrying wire creates a magnetic field, while resistance in the wire converts some electrical energy into heat.

These CBSE Class 8 Science Revision Notes Chapter 4 cover magnetic effects, electromagnets, heating effects and electric cells. The notes follow the current 2026–27 chapter for quick and clear revision.

Key Takeaways

• Magnetic effect: A current-carrying conductor produces a magnetic field around it.
• Electromagnet: A current-carrying coil behaves like a temporary magnet.
• Heating effect: Resistance converts some electrical energy into heat energy.
• Electric cell: Chemical reactions between electrodes and an electrolyte generate electricity.

Access Class 8 Science Chapter 4 Electricity: Magnetic and Heating Effects Notes in 30 Minutes

Revise the chapter in three parts:

First 10 minutes: Magnetic effect, magnetic field and Oersted’s discovery
Next 10 minutes: Electromagnets, their strength, poles and applications
Final 10 minutes: Heating effect, Voltaic cells, dry cells and rechargeable batteries

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Magnetic Effect of Electric Current in Class 8 Science Chapter 4 Notes

Electricity and magnetism are closely linked. A wire carrying electric current produces a magnetic effect around it.

Compass Needle Observation

A magnetic compass is placed close to a wire connected to an electric cell and switch.

When the switch is turned on:

• Current flows through the wire.
• The compass needle moves from its original direction.
• The current-carrying wire produces a magnetic effect.

When the switch is turned off:

• Current stops flowing.
• The compass needle returns to its original position.
• The magnetic effect disappears.

This observation proves that an electric current produces magnetism.

What Is a Magnetic Field?

A magnetic field is the region around a magnet or current-carrying conductor where its magnetic effect can be felt.

A compass needle placed in this region may deflect. The magnetic field around the wire exists only while current flows.

Magnetic Effect of Electric Current

When electric current flows through a conductor, it produces a magnetic field around the conductor.

This phenomenon is called the magnetic effect of electric current.

The magnetic effect is used in:

• Electromagnets
• Electric bells
• Electric motors
• Fans
• Loudspeakers
• Lifting cranes

Hans Christian Oersted’s Discovery

Hans Christian Oersted discovered the link between electricity and magnetism in 1820.

He noticed that a compass needle placed near an electric circuit moved whenever the circuit was opened or closed. Further experiments confirmed that current produces a magnetic field.

Electromagnets in CBSE Class 8 Science Chapter 4 Notes

A coil of wire carrying electric current behaves like a magnet. Its magnetic effect becomes stronger when an iron core is placed inside the coil.

What Is an Electromagnet?

An electromagnet is a current-carrying coil that behaves like a magnet.

It remains magnetic only while electric current flows through the coil. When current stops, the magnetic effect disappears.

How Is an Electromagnet Made?

A simple electromagnet can be made using:

• An iron nail
• Insulated copper wire
• An electric cell
• Connecting wires
• A switch

The insulated wire is wound tightly around the nail to form a coil. The wire ends are then connected to the electric cell.

When current flows, the nail attracts iron or steel paper clips. When the circuit is disconnected, the clips fall.

Role of the Iron Core

A coil without an iron core can also produce a magnetic field. However, inserting an iron nail makes the electromagnet stronger.

The iron core increases the magnetic effect and allows the electromagnet to attract more iron objects.

Poles of an Electromagnet

An electromagnet has two poles:

• North pole
• South pole

A compass can be used to identify these poles. Unlike poles attract, while like poles repel.

If the direction of current is reversed, the poles of the electromagnet also reverse.

Factors Affecting Electromagnet Strength

The strength of an electromagnet depends mainly on the current and the number of coil turns.

A battery with more cells may allow the electromagnet to attract more paper clips than a single cell.

Temporary Nature of an Electromagnet

An electromagnet can be switched on and off. This makes it different from a permanent magnet.

Its strength and poles can also be controlled by changing the current or coil arrangement.

Lifting Electromagnets

Strong electromagnets are attached to cranes in factories and scrap yards.

When current is switched on, the electromagnet lifts iron or steel objects. When current is switched off, the magnetic field disappears and the objects are released.

Lifting electromagnets are useful for:

• Moving heavy metal objects
• Sorting metal scrap
• Feeding scrap into furnaces
• Handling dangerous materials safely

Earth’s Magnetic Field

Earth behaves like a giant magnet. The movement of liquid iron deep inside Earth produces electric currents and a magnetic field.

Migratory birds, fish and some animals use Earth’s magnetic field for navigation. It also helps shield Earth from harmful particles coming from space.

Heating Effect in Class 8 Science Revision Notes Chapter 4

A conductor becomes warm when electric current passes through it. This happens because the conductor opposes the flow of current.

What Is the Heating Effect of Electric Current?

When current passes through a conductor, part of the electrical energy changes into heat energy.

This warming of a conductor is called the heating effect of electric current.

What Is Resistance?

Resistance is the opposition offered by a conductor to the flow of electric current.

Different materials have different resistance. Nichrome offers more resistance than a copper wire of the same dimensions.

Because of its higher resistance, nichrome produces more heat and is commonly used in heating appliances.

Nichrome Wire Observation

A nichrome wire connected to an electric cell becomes warm after current passes through it for a short time.

Using two cells instead of one usually produces more heat because a larger current flows through the wire.

The heated wire should only be touched momentarily and under teacher supervision.

Factors Affecting Heat Production

The heat produced in a conductor depends on several factors.

No numerical formula is required at this stage. Students should understand the relationship between these factors and heat production.

What Is a Heating Element?

A heating element is a wire, rod or coil that becomes hot when electric current flows through it.

Nichrome and similar materials are used because they offer suitable resistance and can produce significant heat.

Appliances Using the Heating Effect

The heating effect is used in:

• Electric room heaters
• Electric stoves
• Electric kettles
• Electric irons
• Immersion rods
• Hair dryers
• Incandescent lamps
• Industrial furnaces

In some appliances, the heating element becomes red hot.

Heating Effect in an Incandescent Lamp

The filament of an incandescent lamp becomes very hot when current flows. It glows and produces light.

Some electrical energy is also lost as heat. This is why such lamps may become warm during use.

Industrial Use of Electrical Heating

Electric furnaces produce very high temperatures in steel industries.

They can melt scrap steel and convert it into usable material. Electrical heating avoids the direct handling of burning fuels during this process.

Harmful Effects of Electrical Heating

Unwanted heating may cause:

• Loss of energy in wires
• Damage to plugs and sockets
• Melting of plastic parts
• Overheating of appliances
• Electrical fires

Suitable wires, plugs and sockets must be used according to the current rating of a circuit.

Magnetic Effect and Heating Effect of Electric Current

A current-carrying conductor may show both magnetic and heating effects.

A nichrome wire carrying current may become warm and also produce a magnetic field.

Cells and Batteries in Electricity: Magnetic and Heating Effects Notes

Electric cells and batteries provide portable electrical energy. They produce electric current through chemical reactions.

How Does an Electric Cell Generate Electricity?

An electric cell contains materials that react chemically. This reaction produces electrical energy.

A basic cell contains:

• Two electrodes
• An electrolyte
• Positive and negative terminals

When the circuit is completed, electric current flows through the external circuit.

Voltaic Cell Class 8 Notes

A Voltaic cell is also called a Galvanic cell. It is one of the earliest types of electric cells.

Structure of a Voltaic Cell

A simple Voltaic cell contains:

The electrodes are partly dipped in the electrolyte.

Working of a Voltaic Cell

A chemical reaction occurs between the electrodes and electrolyte.

When the circuit is connected, current flows from the positive terminal through the external circuit towards the negative terminal.

Over time, the chemicals are used up. The cell then becomes dead and cannot supply current.

Alessandro Volta and Luigi Galvani

The Voltaic or Galvanic cell is named after Alessandro Volta and Luigi Galvani.

Galvani observed movement in a frog’s leg when it touched two different metals. Volta later showed that electricity came from the combination of metals and liquid rather than the animal tissue.

This work led to the invention of the first battery.

Lemon Cell Activity

A simple electric cell can be made using lemons, iron nails and copper wires.

The lemon juice acts as the electrolyte. Copper and iron act as the two electrodes.

Several lemon cells may be connected to light an LED. If the LED does not glow, reversing its terminals may correct the direction of connection.

Pure water may not work as effectively because it does not provide the same electrolyte conditions as lemon juice.

Dry Cell Class 8 Notes

Dry cells are commonly used in torches, clocks and other portable devices.

They are called dry cells because the electrolyte is a thick moist paste rather than a freely flowing liquid.

Structure of a Dry Cell

A dry cell contains:

• A zinc container
• A carbon rod
• Paste-like electrolyte
• A metal cap

The zinc container acts as the negative terminal. The metal cap connected to the carbon rod acts as the positive terminal.

Features of a Dry Cell

• It is portable.
• It does not contain a freely flowing liquid.
• It is generally used once.
• It cannot normally be recharged.
• It must be replaced after its chemicals are used up.

Rechargeable Batteries Class 8 Notes

Rechargeable batteries can be charged and reused several times.

They are used in:

• Mobile phones
• Watches
• Cameras
• Tablets
• Laptops
• Inverters
• Electric vehicles

Rechargeable batteries reduce repeated disposal and may save money over time.

Do Rechargeable Batteries Last Forever?

Rechargeable batteries wear out after repeated charging and use.

Their ability to store charge slowly decreases. This is why an older mobile phone battery may require charging more frequently.

Lithium-Ion Batteries

Lithium-ion batteries are widely used in modern electronic devices. They rely on materials such as lithium and cobalt.

These materials are valuable and available in limited locations. Therefore, battery recycling is important.

Solid-State Batteries

Scientists are developing solid-state batteries that use solid materials instead of liquid or paste-like electrolytes.

These future batteries may:

• Charge faster
• Last longer
• Provide better safety
• Support cleaner transport technology

Difference Between Voltaic, Dry and Rechargeable Cells

Battery Safety and Recycling

Used batteries may still contain acids and metals such as lead, cadmium, nickel or lithium.

Throwing them into ordinary household waste may:

• Harm soil and water
• Cause fires
• Waste valuable materials
• Create health and environmental risks

Used batteries should be taken to authorised e-waste or battery recycling centres.

Important Terms in Class 8 Chapter 4 Science Notes

Quick Revision of Electricity: Magnetic and Heating Effects

• Current flowing through a wire produces a magnetic field.
• A compass needle detects the magnetic effect.
• A current-carrying coil acts as an electromagnet.
• An iron core strengthens an electromagnet.
• More coil turns and more current increase magnetic strength.
• Reversing current reverses electromagnet poles.
• Resistance causes electrical energy to change into heat.
• Nichrome is commonly used in heating elements.
• Voltaic cells use two electrodes and a liquid electrolyte.
• Dry cells contain a moist paste electrolyte.
• Rechargeable batteries can be charged and reused.
• Old batteries must be sent for safe recycling.

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