Singapore H2 Physics Wiki

Solenoids and Electromagnets

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Solenoids and Electromagnets

Overview

A solenoid is a long coil of insulated wire. When electric current flows through it, the solenoid produces a magnetic field similar to that of a bar magnet.

If a soft iron core is inserted, the system becomes an electromagnet.

This topic is important because solenoids and electromagnets are widely used in:

  • relays
  • cranes
  • doorbells
  • electric starters
  • valves
  • motors
  • transformers

This page expands ideas introduced in Magnetic Fields.

Definition

A solenoid is a long cylindrical coil of closely wound wire.

An electromagnet is a solenoid, usually with a soft iron core, that becomes strongly magnetic when current flows.

Why It Matters

Solenoids and electromagnets show how electrical energy can produce useful, controllable magnetic effects in devices and machines.

Key Representations

What Is a Solenoid?

A solenoid consists of many circular loops wound closely together into a cylindrical coil.

When current flows:

  • each loop produces a magnetic field
  • the fields combine
  • a strong resultant field forms inside the coil

Why a Solenoid Behaves Like a Bar Magnet

The magnetic field pattern of a current-carrying solenoid resembles a bar magnet.

Similarities

  • one end behaves as a North pole
  • the other behaves as a South pole
  • field lines emerge from North and enter South externally
  • field lines return through the interior

Reason

Each loop contributes a field in the same internal direction, reinforcing one another.

Hence the solenoid acts as a large combined magnetic dipole.

Field Pattern of a Solenoid

Inside Solenoid

  • field lines nearly parallel
  • equal spacing
  • approximately uniform field

Outside Solenoid

  • weaker curved returning lines
  • similar to bar-magnet external field

For a long solenoid:

where:

  • = turns per unit length
  • = current

Identifying North and South Poles

Use the right-hand grip rule.

  • curl fingers in direction of conventional current around turns
  • thumb points:
    • along the internal magnetic field
    • toward the North pole

Alternative Face Rule

Look at one end of the solenoid:

  • anticlockwise current at that end gives North pole
  • clockwise current at that end gives South pole

Soft Iron Core

Placing a soft iron core inside a solenoid greatly increases field strength.

Why:

  • iron becomes magnetised in the applied field
  • its domains align
  • the core contributes additional magnetic field

Result:

  • much stronger electromagnet than an air-core solenoid

Permeability Qualitative Idea

Permeability describes how easily a material supports magnetic field formation.

  • air has relatively low permeability
  • soft iron has high permeability

Hence magnetic field lines are concentrated more effectively in iron than in air.

For H2 Physics, a qualitative understanding is usually sufficient.

Why Soft Iron Instead of Steel?

Soft Iron

  • magnetises easily
  • loses magnetism quickly when current stops

Useful for electromagnets.

Steel or Hard Magnetic Materials

  • harder to magnetise
  • retains magnetism

Useful for permanent magnets, not ideal when rapid switching is needed.

Temporary vs Permanent Magnet

Temporary Magnet

Produced only while external field or current acts.

Example:

  • electromagnet with soft iron core

Permanent Magnet

Retains magnetisation after the field is removed.

Example:

  • bar magnet
  • hardened steel magnet

Factors Affecting Electromagnet Strength

Increase magnetic strength by:

1. Increasing Current

Larger current gives stronger field.

2. Increasing Number of Turns

More turns per unit length increases field.

3. Using Soft Iron Core

Strong enhancement through magnetisation.

4. Reducing Air Gaps

Keeping the magnetic path compact improves effectiveness in many devices.

Practical Uses

Relay

Small control current energises an electromagnet, which closes or opens another circuit.

Useful for switching high-power circuits safely.

Electromagnetic Crane

Large electromagnet lifts scrap steel.

Switch current off to release the load.

Doorbell or Buzzer

Electromagnet attracts an armature, making contact repeatedly.

This produces the ringing action.

Motor Starter or Solenoid Switch

Electromagnet moves mechanical contacts or engages gears.

Common in automotive starter systems.

Valves or Actuators

Solenoids move plungers to control fluid flow.

Short Worked Examples

Example 1

Two identical solenoids, but one carries twice the current.

Since:

its field is doubled.

Example 2

Two electromagnets carry the same current, but one has an iron core.

The iron-core electromagnet is much stronger.

Example 3

A device requires magnetism only while powered.

Choose a soft iron core rather than a permanent magnet.

Common Mistakes

  1. Using the wrong pole identification
  2. Assuming outside field is zero
  3. Thinking iron creates a field without current
  4. Confusing soft iron with permanent-magnet material
  5. Forgetting turns per unit length

Summary

Solenoid

Long current-carrying coil producing a bar-magnet-like field.

Electromagnet

Solenoid with a soft iron core.

Key Relation

Stronger Electromagnet Requires

  • larger current
  • more turns
  • soft iron core

Main Advantage

It can be switched on or off and controlled electrically.

Graph View

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