Electricity and Magnetism

Introduction

During electrical power cuts (load shedding), people use alternative arrangements like rechargeable emergency lamps, solar panels with inverters, battery-inverter systems, and petrol/diesel generators to maintain electricity supply.

Types of Electric Current

Direct Current (DC)

• Definition: Current that flows in a particular direction continuously
• Characteristics:
  • Magnitude remains constant over time
  • Direction does not change
  • Frequency is zero
• Sources: Dry cells, batteries, solar panels (photocells)
• Applications: Mobile phones, computers, and other electronic device internal circuits
• Current-Time Graph: Shows constant magnitude (e.g., 4A) regardless of time

Alternating Current (AC)

• Definition: Current in which magnitude and polarity change continuously at fixed intervals
• Characteristics:
  • Direction changes periodically
  • Magnitude varies from zero to maximum to zero
  • Has frequency measured in Hertz (Hz)
• Frequency in Nepal: 50 Hz (direction changes 100 times per second)
• Voltage in Nepal: 220V to 240V (average)
• Sources: Dynamo, AC generators
• Applications: Fans, motors, refrigerators
• Cycle: One complete variation from zero to maximum to zero to minimum back to zero

Universal Applications

Electric heaters and filament lamps can operate on both AC and DC.

Conversion

Rectifier: Device used to convert alternating current into direct current

Magnetic Effect of Electric Current

Discovery

• Discoverer: Hans Christian Oersted (1820)
• Observation: Magnetic compass needle deflects near an electric circuit
• Conclusion: Electric current produces a magnetic effect

Magnetic Field Around a Straight Wire

• Formation: When current flows through a conducting wire, a circular magnetic field forms around it
• Direction depends on: Direction of current flow
• Pattern: Circular magnetic field lines around the wire

Maxwell's Right-Hand Thumb Rule:

• Grip the wire with right hand
• Thumb points in direction of current
• Fingers indicate direction of magnetic field
• Current upward → Magnetic field anticlockwise
• Current downward → Magnetic field clockwise

Magnetic Field Around a Solenoid

• Solenoid: Cylindrical coil made of insulated wire
• Magnetic Field Pattern: Similar to a bar magnet
  • Strong at both ends (poles)
  • Weak in the middle
  • Uniform inside the solenoid
• Pole Formation: One end becomes North Pole, other becomes South Pole

Maxwell's Right-Hand Grip Rule:

• Grip solenoid with right hand
• Fingers point in direction of current
• Thumb points to North Pole

Factors Affecting Magnetic Field Strength:

1. Magnitude of current in solenoid
2. Number of turns in the coil
3. Material placed inside (core) - soft iron increases strength

Application: Electromagnets (temporary magnetic field)

Magnetic Flux

Definition

Total number of magnetic lines of force passing through a surface area within a magnetic field

Symbol and Unit

• Symbol: Φ (Phi)
• Unit: Weber (Wb) - named after Wilhelm Eduard Weber

Characteristics

• Density indicates magnitude:
  • Dense lines → Strong magnetic flux
  • Sparse lines → Weak magnetic flux
• Example: Bar magnet has stronger flux at poles than middle

Motor Effect

Definition

Production of motion in a wire placed in a magnetic field when current passes through it

Principle

• Force of attraction and repulsion between two magnetic fields produces motion
• Interaction between permanent magnet field and current-induced magnetic field

Construction

• Coil wound around a core
• Placed between opposite magnetic poles
• AC passed through the coil
• Magnetic field develops and changes direction continuously
• Coil rotates due to field interaction

Methods to Increase Rotation Speed

1. Increase number of turns in coil
2. Increase surface area of coil
3. Use more powerful magnets
4. Use soft iron core

Applications

Fans, water pumps, mixer grinders, DC motors

Electromagnetic Induction

Discovery

• Discoverer: Michael Faraday (1831)
• Discovery: Voltage generated when magnetic force lines are cut perpendicularly by a conducting wire

Definition

Process of inducing electromotive force (emf/voltage) in a conductor when there is a change in magnetic flux linked with that conductor

Key Concept

• Moving magnet in/out of coil → Changes magnetic flux → Induces voltage
• Rotating coil in magnetic field → Changes magnetic flux → Induces voltage
• Mechanical energy converts to electrical energy

Faraday's Law of Electromagnetic Induction

Statement: When there is relative motion between a conductor and magnet, an emf is induced in the conductor, and its magnitude is directly proportional to the rate of change of magnetic flux linked with the conductor.

Factors Affecting Induced Voltage

1. Strength of magnetic field: Stronger field → More voltage
2. Number of turns in coil: More turns → More voltage
3. Speed of motion: Faster motion → More voltage (rate of change of flux)

Dynamo and AC Generator

Dynamo

• Use: Small-scale current generation (bicycles, motorcycles)
• Working:
  • Magnet rotates near a fixed coil
  • Friction between dynamo cap and tire rotates magnet
  • Coil cuts magnetic lines of force
  • Voltage induced depends on: number of turns, magnetic field strength, rotation speed

AC Generator

• Use: Large-scale electricity production for domestic circuits
• Working Principle: Same as dynamo (electromagnetic induction)
• Construction:
  • Rectangular coil in magnetic field
  • Coil rotates and cuts magnetic field
  • Magnetic flux changes
  • EMF induced proportional to rate of change of flux

Large-Scale Electricity Generation

Hydropower Station:

• Water stored in dam flows through tunnel at high pressure
• Rotates turbine at high speed
• Turbine connected to generator produces electricity

Thermal Plant (Coal):

• Coal burnt to produce heat
• Heats water to produce steam
• High-pressure steam rotates turbine
• Generator produces electricity

Diesel Power Plant:

• Diesel engine rotates turbine directly

Wind Power:

• Wind energy rotates turbine

Nuclear Power Plant:

• Heat from nuclear fission of uranium boils water
• Steam rotates turbine
• Not used in Nepal

Nepal's Electricity Capacity (2022 data)

• Hydroelectric potential: 2200 MW
• Thermal plants: 487 MW (Dubahi and Hetauda)
• Under construction: 487 MW
• Proposed projects: 3219 MW (Upper Arun, Uttar Ganga, Dudha Kosi)

Transformer

Definition

Device used to increase or decrease the voltage of alternating current

Principle

Mutual Induction: Inducing emf in a coil by changing current in an adjacent coil

Key Points

• Works ONLY with AC (not DC)
• No mutual induction with DC
• Based on electromagnetic induction

Construction

Components:

1. Primary coil: Input coil (receives AC)
2. Secondary coil: Output coil (induced AC)
3. Core: Laminated soft iron sheets (E, U, I-shaped)

Core Lamination:

• Iron sheets coated with non-conductive paint
• Tied together with nuts and bolts
• Prevents excessive heating from induced currents

Terminology:

• Np: Primary turns (number of windings in primary coil)
• Ns: Secondary turns (number of windings in secondary coil)
• Vp: Primary/input voltage
• Vs: Secondary/output voltage

Transformer Formula

NpNs=VpVs\frac{N_p}{N_s} = \frac{V_p}{V_s}Ns​Np​​=Vs​Vp​​ 

Types of Transformers

1. Step-Down Transformer

• Function: Reduces voltage
• Construction: Ns < Np (fewer secondary turns)
• Example: If Np:Ns = 2:1, and Vp = 220V, then Vs = 110V
• Applications: Mobile chargers, router adapters, laptop adapters

2. Step-Up Transformer

• Function: Increases voltage
• Construction: Ns > Np (more secondary turns)
• Example: If Np:Ns = 1:2, and Vp = 110V, then Vs = 220V
• Applications: Microwave ovens, power transmission lines

Applications of Transformers

Electricity Distribution System:

1. Generation: Electricity produced at power plant
2. Transmission: Voltage increased (e.g., 132 kV, 400 kV) using step-up transformer
3. Distribution: Voltage reduced (e.g., 220V) using step-down transformer at substations
4. Customers: Receive usable voltage

Example: Dhalkebar substation in Dhanusha transmits at 400 kV

Common Devices:

• Mobile charger: 220V → 5.3V
• Microwave oven: 220V → 2100V
• Router adapter: 220V → 12V
• Laptop adapter: Various ratings

Summary Tables

DC vs AC Comparison

Magnetic Rules Summary

Key Concepts to Remember

1. AC has frequency, DC does not
2. Transformers work only with AC, not DC
3. Electromagnetic induction converts mechanical energy to electrical energy
4. Motor effect converts electrical energy to mechanical energy
5. Magnetic flux is proportional to density of magnetic field lines
6. Induced voltage depends on rate of change of magnetic flux
7. Core lamination in transformers prevents excessive heating
8. Nepal uses 50 Hz AC at 220-240V for domestic supply