AR1  ·  DC Circuits Exercise 9  ·  Introduction

Electromagnetism

// Magnets  ·  Electromagnets  ·  Solenoids  ·  DC Motors

Overview

Learning Outcomes

  • Define magnetism, magnets, and magnetic field, as well as electromagnetism and electromagnets
  • Operate the solenoid component of the AC/DC Training System
  • Explain the principles of operation of DC motors

Topics Covered

01Magnetism, magnets, and magnetic field
02Electromagnetism and electromagnets
03Training system component: the solenoid
04Introduction to DC motors
Magnetism · Topic 01

What Is Magnetism?

  • A material's property to exert a magnetic force (attraction or repulsion) on another material from a distance
  • Ferromagnetic materials are strongly attracted to magnets: primarily iron, nickel, cobalt, and some rare earth metals
  • Every magnet has two poles: North (N) and South (S)
  • Opposite poles attract; like poles repel
Bar magnet with iron filings showing magnetic field Similar poles repulse; dissimilar poles attract
Magnetism · Topic 01 · Magnetic Field
Magnetic field lines diagram around a bar magnet Iron filings aligned along magnetic field lines Compass needle aligned along magnetic field lines Earth as a giant magnet — geographic North is the magnetic south pole

Lines of Force

  • Magnetic fields are visualised using lines of force
  • Lines exit through the North pole and enter through the South pole
  • Iron filings sprinkled around a magnet align along the lines of force, making the field visible
  • A compass needle aligns itself along these lines — the red (N) end points toward Earth's magnetic South
Historical Note
The Earth itself is a giant magnet. Geographic North is actually the Earth's south magnetic pole. Opposite poles attract, so the compass N always points "north."
Electromagnetism · Topic 02

Current Creates a Magnetic Field

Natural magnets are rare. But any wire carrying current produces its own magnetic field.

  • The field is circular and centered on the wire
  • Right-hand rule: wrap your right hand around the wire with the thumb pointing in the direction of current flow. Your fingers indicate the direction of the magnetic field lines
  • Reverse the current direction → field direction reverses
Key Idea
Electricity and magnetism are two aspects of the same fundamental force: electromagnetism.
Right-hand rule: thumb in direction of current, fingers show magnetic field direction
Electromagnetism · Topic 02 · Electromagnets
Wire loop carrying current forming an electromagnet with magnetic field lines

The Electromagnet

Wind the wire in a loop and the circular fields combine to create distinct north and south poles — just like a natural magnet.

  • Current in a loop of wire produces north and south magnetic poles
  • Use the right-hand rule to find the field direction inside the loop → locate N and S
  • More current → stronger magnetic field
  • No current → field disappears instantly — unlike natural magnets
Advantage Over Natural Magnets
An electromagnet can be turned on and off, reversed, and its strength controlled by adjusting the current.
Solenoid · Topic 03

The Solenoid

A single wire loop rarely produces enough field for a real application. Winding the wire into a helix solves this.

  • A wire wound in the form of a helix is called a solenoid
  • Each loop produces a field in the same direction. They all add together
  • Total field ≈ field of one loop × number of loops
  • The center of the solenoid is called the core
Iron Core Upgrade
Replacing the air core with an iron core greatly increases the magnetic field strength. These are what most people mean by "electromagnets."
Wire loop carrying current forming an electromagnet with magnetic field lines Solenoid with current direction and north/south poles labelled
Solenoid · Topic 03 · Plunger

Solenoid with a Plunger

Add a removable iron core — the plunger — and the solenoid produces useful linear mechanical motion.

Solenoid with plunger device
Current ON → solenoid field strongly attracts the plunger into its center
Current OFF → field disappears; a spring returns the plunger to its rest position
Converts electrical energy → linear mechanical motion
Symbol
Solenoid schematic symbol Among others

Real-World Applications

Door bells — plunger strikes a chime
Magnetic locks — plunger engages/releases a latch
Circuit breakers — trips when overcurrent detected
Relays — low-power signal switches high-power circuit
Valves — controls fluid or gas flow
Magnetic brakes — used in cranes, elevators
Magnet cranes — lift and release metallic materials
DC Motors · Topic 04 · Construction

DC Motor Construction

A DC motor converts electrical energy into rotary mechanical motion through the interaction of magnetic fields.

Stator — Fixed outer shell. Contains a pair of magnets with opposite poles facing each other, creating a field across the armature.
Rotor (Armature) — Rotating part. Made of one or more wire loops wound on a metallic armature.
Commutator — Segmented ring connected to the wire loops. Reverses current direction in the loop as the rotor spins.
Brushes — Stationary contacts that slide on the commutator, providing a continuous electrical connection to the rotating loops.
DC motor construction: stator, rotor, commutator, and brushes
Real DC motor armature showing wire loops and commutator segments DC motor stator and rotor separated
DC Motors · Topic 04 · Operation

How the Motor Spins

The interaction between the armature's electromagnet and the stator's magnets produces the rotation.

1 — Current flows in the armature wire loop → creates an electromagnet (N and S poles on the rotor)
2 — The rotor's poles are attracted and repelled by the stator's fixed poles → rotation begins
3 — As the rotor turns, the commutator switches which loop carries current.
4 — This keeps the force in the same rotational direction → continuous spinning
Key Result
The N and S poles of the armature electromagnet oscillate, maintaining the same rotational direction even as the armature rotates.
Animated DC motor loop showing commutation and rotation
Real Motors
Actual DC motors use multiple wire loops and commutator segments for smoother, more consistent torque.
DC Motors · Topic 04 · Speed vs Voltage
DC motor speed vs voltage graph showing direct proportionality

Speed vs Voltage

The rotational speed of a DC motor's rotor is directly determined by the voltage applied to its terminals.

  • Speed is directly proportional ( ∝ ) to voltage — double the voltage, roughly double the speed
  • Higher voltage → stronger current → greater force on armature → faster rotation
  • Speed control is simple: adjust the supply voltage with a variable resistor or power supply
DC Motors · Topic 04 · Applications

DC Motor Applications

DC motors are among the most common motors in the world due to their ease of construction and precise speed control.

Consumer Electronics & Portable Devices
Disk drives and cooling fans in computers
Motors in toys and small electronic devices
Battery-powered tools (the DC source makes them ideal)
Electric Vehicles
Electric bikes, cars, carts, and lifts
Battery-powered forklifts and golf carts
Industrial — Constant Speed
Lathes, drills, boring mills — require stable speed regardless of load
Industrial — High Starting Torque
Cranes and elevators — need high force to start moving heavy loads
Air compressors — high torque needed on startup
Many More Types of DC Motor
DC motors come in many more configurations. Wound field motors replace the permanent magnents with electromagnets. These are further categorized as shunt, series, and compound, depending on how the field winding is connected to the armature winding to provide different speed and torque characteristics.