Complete Physics Notes on Basic Concept of Electricity and Magnetism for BPSC and Other Competitive Exams in 2025

Complete Physics Notes on Basic Concept of Electricity and Magnetism    for BPSC and Other Competitive Exams in 2025

General Science-4 (Physics)

Basic Concept of Electricity and Magnetism

1. Electricity

Electricity deals with electric charges, their forces, fields, and motion.

• Electric Charge:
  • Concept: A fundamental property of matter that causes it to experience a force when placed in an electromagnetic field.
  • Types: Positive (protons) and Negative (electrons).
  • Principle: Like charges repel, opposite charges attract.
  • SI Unit: Coulomb (C).
  • Quantization of Charge: Charge exists in discrete packets, i.e., q=ne, where ‘n’ is an integer and ‘e’ is the elementary charge (1.6×10−19 C).
• Electric Current (I):
  • Concept: The rate of flow of electric charge. Conventionally, it’s defined as the flow of positive charge. In metals, it’s the flow of electrons.
  • Formula: I=tQ​ (Current = Charge / Time)
  • SI Unit: Ampere (A). 1 Ampere=1 Coulomb/second.
  • Measurement: Measured by an Ammeter, which is always connected in series in a circuit.
• Electric Potential and Potential Difference (V):
  • Electric Potential: The amount of work done per unit positive charge to bring it from infinity to a point in an electric field.
  • Potential Difference (Voltage): The work done per unit charge in moving a positive charge from one point to another in an electric field. It’s the “push” or “pressure” that causes current to flow.
  • Formula: V=QW​ (Potential Difference = Work Done / Charge)
  • SI Unit: Volt (V). 1 Volt=1 Joule/Coulomb.
  • Measurement: Measured by a Voltmeter, which is always connected in parallel across the two points where the potential difference is to be measured.
• Electric Resistance (R):
  • Concept: The opposition offered by a material to the flow of electric current.
  • Factors Affecting Resistance:
    1. Length (L): R∝L (Resistance increases with length).
    2. Area of Cross-section (A): R∝A1​ (Resistance decreases with a wider area).
    3. Nature of Material (Resistivity, ρ): Different materials have different inherent resistance. Good conductors have low resistivity, insulators have high resistivity.
    4. Temperature: Resistance of metals generally increases with temperature; resistance of semiconductors decreases with temperature.
  • Formula: R=ρAL​
  • SI Unit: Ohm (Ω).
• Ohm’s Law:
  • Concept: States that the current flowing through a conductor between two points is directly proportional to the voltage across the two points, provided the temperature and other physical conditions remain constant.
  • Formula: V=IR (Voltage = Current × Resistance)
  • Ohmic vs. Non-Ohmic Conductors: Conductors that obey Ohm’s Law are ohmic (e.g., metallic conductors). Those that don’t (e.g., semiconductors, diodes) are non-ohmic.
• Electric Circuits:
  • Series Circuit: Components are connected end-to-end, forming a single path for current.

• Current: Same through all components.
• Voltage: Divides across components (Vtotal​=V1​+V2​+…).
• Equivalent Resistance: Sum of individual resistances (Req​=R1​+R2​+…).
• Parallel Circuit: Components are connected across the same two points, providing multiple paths for current.

• Current: Divides among branches (Itotal​=I1​+I2​+…).
• Voltage: Same across all components.
• Equivalent Resistance: Req​1​=R1​1​+R2​1​+… (Always less than the smallest individual resistance).
• Electric Power (P):
  • Concept: The rate at which electrical energy is consumed or dissipated in a circuit.
  • Formulas:

• P=VI (Power = Voltage × Current)
• P=I2R
• P=RV2​

• SI Unit: Watt (W).
• Commercial Unit of Energy: Kilowatt-hour (kWh). 1 kWh=3.6×106 Joules (Also known as “unit” of electricity consumed).
• Heating Effect of Electric Current (Joule Heating):
  • Concept: When current flows through a resistor, electrical energy is converted into heat energy.
  • Formula (Heat Produced, H): H=I2Rt (where t is time).
  • Applications: Electric heaters, geysers, toasters, electric bulbs (filament glows due to heating).

2. Magnetism

Magnetism deals with magnetic fields and their effects on materials and moving electric charges.

• Magnets: Materials that produce a magnetic field. They have two poles: North (N) and South (S).
  • Principle: Like poles repel, opposite poles attract.
  • Magnetic Field Lines: Imaginary lines representing the direction and strength of a magnetic field. They emerge from the North pole and enter the South pole outside the magnet, forming continuous closed loops. They never intersect each other.
• Magnetic Effect of Electric Current (Electromagnetism):
  • Oersted’s Discovery: A current-carrying conductor produces a magnetic field around it.
  • Electromagnet: A temporary magnet created by passing electric current through a coil of wire (solenoid) wound around a soft iron core. The strength of the electromagnet depends on the current, number of turns in the coil, and the core material.
  • Applications: Electric bells, cranes (to lift heavy iron objects), motors, generators, MRI machines.
• Electromagnetic Induction (Faraday’s Law):
  • Concept: The phenomenon of producing an electric current (induced current) in a conductor by changing the magnetic field passing through it.
  • Applications: Electric generators (convert mechanical energy to electrical energy), transformers.
• Electric Motor vs. Electric Generator:
  • Motor: Converts electrical energy into mechanical energy (based on the force experienced by a current-carrying conductor in a magnetic field).
  • Generator: Converts mechanical energy into electrical energy (based on electromagnetic induction).