Grade 8 Chapter 4: Current Electricity and Magnetism

Introduction

Electricity and magnetism are two fundamental forces of nature that are deeply interconnected and play a crucial role in our modern world. From lighting our homes to powering electronic devices, understanding current electricity and its magnetic effects is essential. This chapter will explore the basic concepts of electric charge, current, potential difference, the working of various electric cells, and the fascinating relationship between electricity and magnetism.

Current Electricity

The flow of electric charge is called electric current. Electric charge is carried by electrons, which are negatively charged particles. The movement of these electrons in a conductor constitutes electric current.

Electric Charge

• There are two types of electric charges: positive and negative.
• Like charges repel each other (positive repels positive, negative repels negative).
• Unlike charges attract each other (positive attracts negative).
• The SI unit of electric charge is Coulomb (C).

Electric Current

Electric current is the rate of flow of electric charge. It is denoted by 'I'.

Electric Current (I) = Charge (Q) / Time (t)

The SI unit of electric current is Ampere (A).

Potential Difference (Voltage)

For electric current to flow, there must be a potential difference between two points in a conductor. Potential difference is the work done to move a unit positive charge from one point to another in an electric field.

Potential Difference (V) = Work Done (W) / Charge (Q)

The SI unit of potential difference is Volt (V).

Electric Cell

An electric cell is a device that converts chemical energy into electrical energy, providing a potential difference to drive electric current in a circuit.

1. Dry Cell

A dry cell is a common type of electric cell used in flashlights, radios, remote controls, etc. It is called 'dry' because it uses a paste electrolyte instead of a liquid one.

Structure of a Dry Cell:

• Carbon rod: Acts as the positive terminal (anode).
• Zinc casing: Acts as the negative terminal (cathode).
• Electrolyte paste: A paste of ammonium chloride (NH₄Cl) and zinc chloride (ZnCl₂) between the carbon rod and the zinc casing.
• Manganese dioxide (MnO₂) and carbon powder: Packed around the carbon rod to act as a depolarizer.
• Outer protective layer: Prevents leakage and provides insulation.

Working: Chemical reactions within the cell produce a potential difference between the carbon rod and the zinc casing, allowing current to flow when connected to a circuit.

2. Lead-Acid Cell (Storage Battery)

A lead-acid cell is a rechargeable battery commonly used in automobiles and UPS systems. It consists of several cells connected in series.

Structure of a Lead-Acid Cell:

• Positive plate: Made of lead dioxide (PbO₂).
• Negative plate: Made of pure lead (Pb).
• Electrolyte: Dilute sulfuric acid (H₂SO₄).

Working: During discharge, chemical reactions occur, producing electricity. During charging, the chemical reactions are reversed, storing energy. The potential difference of a single lead-acid cell is approximately 2V.

Magnetic Effects of Electric Current

When electric current flows through a conductor, it produces a magnetic field around it. This phenomenon is known as the magnetic effect of electric current.

Electromagnet

An electromagnet is a temporary magnet created by passing electric current through a coil of wire wound around a soft iron core. The magnetic field exists only as long as the current flows.

Factors affecting the strength of an electromagnet:

• Number of turns in the coil (more turns, stronger magnet).
• Amount of current flowing through the coil (more current, stronger magnet).
• Nature of the core material (soft iron core increases strength).

Solenoid

A solenoid is a coil of wire wound in the shape of a helix (like a spring). When current passes through it, it behaves like a bar magnet, producing a magnetic field. The strength of the magnetic field inside a solenoid is uniform.

Electric Bell

An electric bell is a common application of the magnetic effect of electric current. It works on the principle of electromagnetism.

Working of an Electric Bell:

1. When the switch is pressed, current flows through the electromagnet.
2. The electromagnet gets magnetized and attracts the armature (a soft iron strip with a hammer attached).
3. The hammer strikes the gong, producing sound.
4. As the armature moves towards the electromagnet, it breaks the contact with the contact screw.
5. The circuit is broken, the electromagnet loses its magnetism, and the armature springs back to its original position, making contact again.
6. This makes and breaks the circuit repeatedly, causing the hammer to strike the gong continuously as long as the switch is pressed.

Summary

• Electric current is the flow of electric charge, measured in Amperes.
• Potential difference (voltage) drives the current, measured in Volts.
• Electric cells convert chemical energy to electrical energy; examples include dry cells (carbon rod, zinc casing, ammonium chloride paste) and lead-acid cells (lead dioxide, lead plates, sulfuric acid).
• Electric current produces a magnetic field (magnetic effect of electric current).
• An electromagnet is a temporary magnet formed by current flowing through a coil around a soft iron core; its strength depends on turns, current, and core material.
• A solenoid is a helical coil that acts like a bar magnet when current flows.
• An electric bell uses an electromagnet to repeatedly attract and release an armature, causing a hammer to strike a gong.

References

1. Maharashtra State Board Science and Technology Standard Eight Textbook (Specific Edition/Year) - Chapter 4: Current Electricity and Magnetism.
2. Maharashtra State Board 8th Standard Science Syllabus.
3. Balbharati Science and Technology Textbook.