General Science Notes Chapter 4 Current Electricity and Magnetism Class 8 Maharashtra Board

Notes For All Chapters – General Science Class 8
1. Introduction

• Electricity is the flow of electric charge.
• It is used in homes, schools, industries, and transport.
• Electric current flows through wires and powers devices like bulbs and fans.

2. Electric Charge and Current

• Everything around us has electric charge inside its atoms.
• Charges can be positive (protons) or negative (electrons).
• When electrons move in a circuit, electric current is formed.

Electric Current:

• The flow of electric charge in a circuit is called electric current.
• SI Unit: Ampere (A).
• Formula:1 Ampere=1 Coulomb1 second1 \text{ Ampere} = \frac{1 \text{ Coulomb}}{1 \text{ second}}1 Ampere=1 second1 Coulomb​

3. Electrostatic Potential and Potential Difference

• Electrostatic potential is the ability of a charge to do work.
• Potential difference is the difference in electric potential between two points in a circuit.
• Example: Water flows from a higher level to a lower level, just like electricity flows from higher potential to lower potential.
• SI Unit: Volt (V).

4. Electric Cell and Battery

(a) Electric Cell:

• A device that produces electricity from chemical reactions.
• Types: Dry cell, Lead-acid cell, Ni-Cd cell, Lithium-ion cell.

(b) Dry Cell:

• Used in radios, clocks, and torches.
• Parts of a Dry Cell:
  • Zinc container: Negative terminal.
  • Graphite rod: Positive terminal.
  • Electrolyte: A paste of ammonium chloride (NH₄Cl) and zinc chloride (ZnCl₂).
  • Manganese dioxide (MnO₂): Helps in chemical reactions.
• Working:
  • Chemical reactions between zinc and electrolyte produce electrons.
  • Electrons flow from the negative terminal (zinc case) to the positive terminal (graphite rod).
  • This flow of electrons creates an electric current.

(c) Lead-Acid Cell:

• Used in cars, trucks, and UPS systems.
• Parts:
  • Lead (Pb): Negative terminal.
  • Lead dioxide (PbO₂): Positive terminal.
  • Electrolyte: Dilute sulfuric acid (H₂SO₄).
• Working:
  • Chemical reactions between lead and sulfuric acid generate electricity.
  • It can be recharged after use.

5. Electric Circuit

• A closed path in which electric current flows.
• Parts of an Electric Circuit:
  • Battery (Cell): Provides energy.
  • Wires: Connect the components.
  • Switch: Turns the circuit ON or OFF.
  • Bulb or Load: Uses electricity.

Types of Circuits:

1. Closed Circuit: Current flows, and the bulb glows.
2. Open Circuit: Current does not flow, and the bulb does not glow.

6. Connecting Cells in a Battery

Series Connection:

• Cells are connected one after another.
• Increases voltage.
• Example: If each cell is 1.5V, then 3 cells = 4.5V.

Parallel Connection:

• All positive terminals are connected together, and all negative terminals are connected together.
• Increases battery life.

7. Magnetic Effect of Electric Current

• When current flows through a wire, a magnetic field is created around it.
• Discovered by: Hans Christian Oersted.
• Experiment:
  • A magnetic needle moves when a current flows through a nearby wire.
  • This shows that electricity creates magnetism.

Electromagnet:

• A coil of wire wound around an iron piece that acts as a magnet when current flows.
• Used in cranes, motors, and electric bells.

8. Electric Bell

An electric bell works using an electromagnet.

Parts:

• Electromagnet: Creates a magnetic field.
• Iron Strip (Armature): Moves to strike the gong.
• Striker: Hits the bell to produce sound.
• Battery & Switch: Provide electricity.

Working of an Electric Bell:

1. When the switch is ON, current flows through the electromagnet, making it magnetic.
2. The magnet pulls the iron strip, moving the striker to hit the bell.
3. The circuit breaks, and the iron strip returns.
4. The process repeats, producing continuous ringing.

9. Uses of Electricity and Magnetism

Electricity is used in:

• Homes (lights, fans, fridges).
• Industries (machines, welding).
• Transport (trains, electric cars).

Magnetism is used in:

• Electromagnets: In cranes, electric motors.
• Electric Bells: In homes and schools.
• Generators & Transformers: In power stations.

10. Important Definitions

1. Electric Current: The flow of electrons in a circuit.
2. Potential Difference: The difference in electric potential between two points.
3. Battery: A combination of two or more cells.
4. Electromagnet: A temporary magnet formed by electric current.
5. Electric Circuit: A closed path in which electricity flows.