Electricity and Magnetism Lecture

Introduction

• Topic: Electricity and Magnetism (2 parts - Electricity and Magnetism).
• Important since changes in syllabus from 2023.
• Removed topic: Digital Electronics (covers logic gates now covered under communication in Waves).
• New addition: Kilowatt hour (unit of measuring electrical energy).

Electricity

Core Topics

• Static Electricity
  • Introduction of charges: positive, negative, attraction, repulsion.
  • Charging, electric fields.
• Current Electricity
  • Circuits: Definitions and components (Voltage, Current, Resistance, Resistors, Series and Parallel connections).
  • Potential dividers.
  • Electrical safety: Fuses, circuit breakers, earth wires.

Static Electricity

• Charge: Property of matter that experiences a force in an electric field.
  • Atoms: Protons (+), Neutrons (neutral), Electrons (-).
  • Like charges repel; opposite charges attract.
  • Unit of charge: Coulomb.
• Electric Field: Region around a charge where force is experienced.
  • Field lines: Out of + charge, into - charge.
  • Strength: Closer lines = stronger field.
• Charging by friction: Transfer of electrons through rubbing.
  • Example: Cloth and rod (electron transfer based on material).
• Induction: Charging conductors without direct contact.
  • Example: Using a ground wire to remove electrons and charge a metal sphere positively.

Current Electricity

• Purpose of a circuit: Transfer energy from source to device.
• Circuit Symbols: Battery, switch, light bulb.
• Quantities and Units:
  • Voltage (V): Energy per charge (unit: Volt), potential difference (PD).
  • Current (I): Rate of flow of charges (unit: Ampere). Direction: Conventional current (+ to -), actual flow (- to +).
  • Resistance (R): Opposes current (unit: Ohm). High resistance = low current, low resistance = high current.
  • Ohm's Law: V = IR.
• Measurement Instruments:
  • Voltmeter (parallel connection).
  • Ammeter (series connection).
• Important Equations:
  • P = VI (Power calculation).
  • Energy consumed: E = Pt (Energy = Power x Time). Cases: Household consumption, kilowatt hour (kWh).

Resistors

• Types:
  • Fixed (constant resistance).
  • Variable (resistance changes with position: sliding contact).
  • Thermistor (resistance changes with temperature, inversely proportional).
  • LDR (Light Dependent Resistor, changes with illumination).
• Diodes: Allow current in one direction only (used in converting AC to DC).

Circuits

• Series Connection:
  • Total resistance increases (R_total = R1 + R2 + ...).
  • Current is same through the circuit.
  • Voltage splits across components.
• Parallel Connection:
  • Total resistance decreases.
  • Voltage is same across each branch.
  • Current splits depending on resistance.
• Potential Dividers: Splitting voltage for components, use fixed/variable resistors, thermistors, LDRs.

Electrical Safety

• Fuse: Protects by melting at high current.
• Circuit Breaker: Uses electromagnet to break circuit under excess current.
• Earth Wire: Routes electricity away from metal cases to prevent electric shocks.

Magnetism

Basics

• Magnet Properties: North and South poles, attraction/repulsion of poles.
• Ferrous Materials: Iron and Steel (iron temporary, steel permanent magnetization).
• Magnetic Field: Region around a magnet where forces are experienced (lines go from North to South).
  • Field strength: Closer lines = stronger field.
• Viewing Fields: Using iron filings or compasses.

Electromagnetism

• Current-carrying wires produce magnetic fields (circular around wire).
  • Right Hand Grip Rule: Thumb (current), curve (field direction).
• Coils: Produce fields similar to bar magnets, control field strength with current, turns, iron core.
• Electromagnetic Devices:
  • Relay: Low-voltage circuit controls high-voltage circuit.
  • Speakers: Current drives coil inside speaker, creating sound waves.

Magnetizing and Demagnetizing

• Methods: Stroking, hammering, using DC/AC coils.
• Implications of changing current/fields around steel.

Applications and Principles

• Generator Principle (Induction): Moving a wire through a magnetic field induces a current.
  • Factors: Speed of movement, number of loops, magnet strength.
  • Angle of cutting: Perpendicular max voltage, parallel zero voltage.
• DC Motors: Coil, magnets, battery, split ring for directional change (Fleming's Left Hand Rule).
• AC Generators: Coil rotates in magnetic field, generating alternating current.
• Transformers: Step up/down voltage based on coil turns ratio. Calculations: VP/VS = NP/NS.
  • High voltage transmission reduces energy loss by lowering current.

Conclusion

• Importance of understanding basics and applications.
• Revisions in the curriculum should be noted for comprehensive preparation.