1. Permanent magnets

 

 

Permanent Magnets

What Is a Permanent Magnet?

A permanent magnet is an object that produces its own persistent magnetic field without needing an electric current or another external magnetic field.

Permanent magnets can exert forces on:

• other magnets
• certain magnetic materials
• moving electric charges under suitable conditions

Common permanent magnets include:

• bar magnets
• horseshoe magnets
• refrigerator magnets
• neodymium magnets
• permanent magnets inside motors and speakers

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Magnetic Poles

Every ordinary permanent magnet has two magnetic poles:

north pole (N)

and:

south pole (S)

The magnetic force is usually strongest near these poles.

For a bar magnet, the poles are located near the two ends.

A freely suspended magnet tends to align approximately with Earth's magnetic field, which is the principle behind a magnetic compass.

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Interactions Between Magnetic Poles

Magnetic poles exert forces on one another.

The basic rule is:

like poles repel

unlike poles attract

Therefore:

N facing N → repel

S facing S → repel

N facing S → attract

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Like Poles Repel

If the north poles of two magnets are brought together, they experience forces pushing them apart.

The same happens when two south poles are brought together.

Therefore:

N ↔ N → repulsion

S ↔ S → repulsion

The magnets do not need to touch for the force to act.

Magnetic forces can act across a distance through a magnetic field.

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Unlike Poles Attract

If the north pole of one magnet is brought near the south pole of another magnet, they attract.

Therefore:

N →← S

The force pulls the magnets toward one another.

This attraction also occurs without direct contact because each magnet interacts with the magnetic field produced by the other.

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Why Do Magnetic Poles Attract or Repel?

Every magnet produces a magnetic field around itself.

When two magnets are brought together, their magnetic fields interact.

The resulting magnetic forces depend on their relative orientation.

With unlike poles facing:

• the magnetic field pattern connects smoothly between the magnets
• the magnets experience forces toward one another

With like poles facing:

• the field patterns oppose each other in the region between the magnets
• the magnets experience forces away from one another

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Magnetic Fields

A magnetic field is a region in which a magnetic material, magnet, or moving electric charge can experience a magnetic force.

Magnetic fields are represented using:

magnetic field lines

For a bar magnet, field lines outside the magnet are conventionally drawn:

north → south

Inside the magnet, they continue:

south → north

forming continuous closed loops.

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Magnetic Field Around a Bar Magnet

A bar magnet produces a characteristic magnetic field.

The field is generally strongest near the poles.

On a field diagram, stronger fields are represented by field lines that are closer together.

Therefore:

closer field lines → stronger magnetic field

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Magnetic Field Lines Never End

Unlike the electric field lines associated with isolated electric charges, magnetic field lines form:

continuous closed loops

Outside a bar magnet:

N → S

Inside the magnet:

S → N

This reflects an important property of magnetism: ordinary isolated magnetic north or south poles have not been observed.

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Cutting a Magnet

Suppose a bar magnet is cut in half.

You do not obtain:

one isolated north pole

and:

one isolated south pole.

Instead, you obtain:

two smaller magnets

Each has its own:

• north pole
• south pole

If the pieces are divided again, each new piece still has both poles.

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Magnetic vs Non-Magnetic Materials

Not all materials respond strongly to magnets.

Materials that are strongly attracted to magnets and can often be magnetized are called ferromagnetic materials.

Important examples include:

• iron
• nickel
• cobalt
• many steels
• some alloys

These materials can respond very strongly to magnetic fields.

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Ferromagnetic Materials

Ferromagnetism is the strong magnetic behavior found in materials such as iron, nickel, cobalt, and certain alloys.

Inside these materials, groups of atomic magnetic moments can align together.

These aligned regions are called:

magnetic domains

Domain behavior helps explain why some materials can become permanent magnets.

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Non-Magnetic Materials

Many common materials are not strongly attracted to ordinary magnets.

Examples include:

• wood
• plastic
• glass
• paper
• rubber

Many metals are also not ferromagnetic.

For example, ordinary pieces of:

• copper
• aluminium
• gold
• silver

are not attracted strongly to a permanent magnet under everyday conditions.

This means:

metal does not automatically mean magnetic

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Are All Metals Magnetic?

No.

This is a common misconception.

Iron is strongly magnetic.

Copper is not ferromagnetic.

Aluminium is not ferromagnetic.

Gold is not ferromagnetic.

Therefore, whether a material looks metallic does not determine whether it is strongly magnetic.

Its magnetic properties depend on its atomic and electronic structure.

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Steel and Magnetism

Steel usually contains iron, so many types of steel are ferromagnetic.

However, different steels have different magnetic properties depending on their composition and structure.

Some steels are useful for making permanent magnets because they can retain magnetization.

Other iron-rich materials may magnetize easily but lose much of their magnetization when an external magnetic field is removed.

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Magnetic Domains

A magnetic domain is a region within a ferromagnetic material in which many atomic magnetic moments are aligned in approximately the same direction.

A piece of ferromagnetic material can contain many domains.

Each domain behaves somewhat like a tiny magnetic region.

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Domains in an Unmagnetized Material

In an unmagnetized piece of iron, individual domains may be internally aligned, but different domains point in different directions.

For example:

↑ ↙ → ↓ ↗ ←

The magnetic effects of the different domains largely cancel.

Therefore, the entire object may have little or no overall magnetization.

It still contains magnetic domains—the domains simply do not produce a strong net magnetic field.

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Domains in a Magnetized Material

When a ferromagnetic material becomes magnetized, domains favorably aligned with the applied magnetic field grow or reorient.

The material becomes much more strongly aligned overall.

For example:

→ → → → → →

The individual magnetic contributions reinforce one another.

The object therefore develops a strong overall magnetic field.

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Magnetizing a Material

A ferromagnetic material can sometimes be magnetized by exposing it to a strong magnetic field.

For example, repeatedly moving a magnet along a suitable piece of steel in the same direction can increase its magnetization.

The external magnetic field encourages domain alignment.

Afterward, some materials retain much of this alignment.

They can therefore become:

permanent magnets

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Temporary Magnets

Some magnetic materials become strongly magnetized when placed in an external magnetic field but lose much of that magnetization when the field is removed.

These materials can act as:

temporary magnets

Soft iron is a useful example.

It magnetizes and demagnetizes relatively easily.

This makes it useful in devices such as:

• electromagnets
• transformers
• relays

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Permanent vs Temporary Magnetism

A good permanent-magnet material retains substantial domain alignment after the external magnetizing field is removed.

A temporary magnetic material loses much of its alignment more easily.

Therefore:

permanent magnet → difficult to demagnetize

temporary magnetic material → easier to magnetize and demagnetize

Different applications require different magnetic properties.

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Magnetic Domains and Atomic Magnetism

Magnetism ultimately originates from the behavior of charged particles, especially electrons.

Electrons contribute magnetic effects through their quantum properties, including their magnetic moments.

In ferromagnetic materials, interactions between neighboring atoms can cause many atomic magnetic moments to align.

This collective alignment produces magnetic domains.

Therefore, the domain model connects the large-scale behavior of magnets with microscopic atomic behavior.

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Why Iron Can Be Attracted to Either Pole

Suppose an unmagnetized iron object is placed near the north pole of a magnet.

The magnet's field causes the iron to become temporarily magnetized.

The side nearest the magnet develops an effective opposite magnetic pole.

This produces attraction.

If the south pole is brought near instead, the iron's induced magnetization changes accordingly.

Therefore, an unmagnetized iron object can be attracted to:

either pole of a magnet

This is different from the interaction between two permanent magnets.

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Magnetic Induction

The process in which a magnetic material becomes magnetized because it is placed in a magnetic field is called:

magnetic induction

For example:

magnet → iron nail → temporary magnetization

The nail may then attract another iron object.

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Attraction Is Not Enough to Identify a Magnet

Suppose object A attracts object B.

Can we conclude that both are permanent magnets?

No.

A permanent magnet can attract an unmagnetized ferromagnetic object through magnetic induction.

A stronger test is repulsion.

If two objects repel when particular ends face each other, both must already possess magnetic polarity.

Therefore:

repulsion provides stronger evidence of permanent magnetism than attraction.

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Testing Magnetic Poles

Suppose you know one end of a reference magnet is north.

Bring it near one end of an unknown magnet.

If they repel:

unknown end = north

If they attract, the unknown end may be south—but attraction alone requires care if the object has not already been established as a permanent magnet.

Once the object is known to be a magnet:

N attracts S

and:

N repels N.

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Compasses

A compass contains a small permanent magnet that can rotate freely.

The compass aligns with the local magnetic field.

The north-seeking end points in the direction of the magnetic field.

Compasses can therefore be used to map the magnetic field around a permanent magnet.

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Mapping a Magnetic Field

A magnetic field can be investigated using small compasses.

A simple procedure is:

1. Place a bar magnet on a surface.
2. Place a compass near the magnet.
3. Observe the direction of the compass needle.
4. Mark the field direction.
5. Move the compass to another location.
6. Repeat.
7. Connect the directions to form field lines.

The arrows outside the magnet point:

N → S

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Iron Filings

Iron filings can also reveal the shape of a magnetic field.

When placed near a magnet, the filings become temporarily magnetized and tend to align with the local field.

This creates a visible pattern resembling magnetic field lines.

Iron filings show the shape of the field well, but small compasses are more useful for determining the direction of the field.

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Magnetic Field Strength

The magnetic field around a permanent magnet is not equally strong everywhere.

It is generally:

• strongest near the poles
• weaker farther away

On a field-line diagram:

closer lines → stronger field

widely spaced lines → weaker field

The magnetic effect generally decreases as distance from the magnet increases.

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Horseshoe Magnets

A horseshoe magnet is shaped so that its north and south poles are close together.

This produces a relatively strong magnetic field in the gap between the poles.

This design can concentrate the magnetic field in a useful region.

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Demagnetization

A permanent magnet can lose some or all of its magnetization if its domain alignment is disrupted.

Possible causes include:

• strong heating
• mechanical shock
• exposure to a strong opposing magnetic field
• certain alternating magnetic fields

These processes can make the domain arrangement less aligned.

The overall magnetic field then becomes weaker.

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Heating a Magnet

Increasing temperature increases thermal motion within a material.

At sufficiently high temperatures, this can disrupt the magnetic ordering responsible for ferromagnetism.

Above a material's Curie temperature, long-range ferromagnetic ordering is lost.

Cooling the material again does not automatically restore its previous permanent-magnet arrangement.

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Permanent Magnets in Motors

Permanent magnets are commonly used in electric motors.

They provide a magnetic field that interacts with magnetic fields produced by electric currents.

This interaction produces forces and can generate rotational motion.

Applications include:

• small fans
• toys
• computer drives
• electric tools
• vehicle motors

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Permanent Magnets in Speakers

Speakers commonly use permanent magnets.

A current passes through a coil located in the permanent magnet's magnetic field.

The interaction produces a force on the coil.

As the current changes direction and magnitude, the coil and attached speaker cone move back and forth.

This produces sound waves.

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Permanent Magnets in Generators

Permanent magnets can also be used in generators.

Relative motion between:

• a magnet
• a conducting coil

can produce an induced potential difference.

This converts:

mechanical energy → electrical energy

Small generators and some renewable-energy systems use permanent magnets.

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Neodymium Magnets

Neodymium magnets are powerful permanent magnets made from an alloy containing neodymium, iron, and boron.

They can produce strong magnetic fields despite their relatively small size.

Applications include:

• motors
• headphones
• speakers
• computer equipment
• sensors
• generators

Their strength also means they require careful handling.

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Earth as a Magnetic Environment

Earth has a large-scale magnetic field.

A compass aligns with this field.

Earth's magnetic field is more complicated than the field of a simple bar magnet, but the bar-magnet model provides a useful first approximation.

This field allows magnetic compasses to be used for navigation.

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Comparing Magnetic and Non-Magnetic Materials

Strictly speaking, many materials have weak magnetic responses, but they are not strongly ferromagnetic like iron.

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Permanent Magnets vs Electromagnets

A permanent magnet produces a magnetic field without requiring continuous electrical current.

An electromagnet produces its magnetic field mainly because electric current flows through a conductor, often a coil.

Permanent magnet

• does not require continuous electrical power
• field is normally always present
• difficult to switch off
• often has relatively fixed strength

Electromagnet

• requires current
• can be switched on and off
• strength can often be adjusted
• polarity can be reversed by reversing current

Both are extremely important in modern technology.

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Common Mistakes

Mistake: Saying magnets have only one pole.
Ordinary magnets have both north and south poles.

Mistake: Saying north attracts north.
Like poles repel; unlike poles attract.

Mistake: Assuming all metals are magnetic.
Many metals, including copper and aluminium, are not ferromagnetic.

Mistake: Assuming only iron is magnetic.
Nickel, cobalt and many alloys can also be ferromagnetic.

Mistake: Saying an unmagnetized piece of iron contains no magnetic domains.
It contains domains, but their overall orientations largely cancel.

Mistake: Saying magnetization creates magnetic domains.
Domains already exist in suitable ferromagnetic materials; magnetization changes their relative alignment and structure.

Mistake: Saying magnetic field lines end at the south pole.
They form continuous closed loops.

Mistake: Assuming cutting a magnet produces an isolated north and south pole.
Each piece becomes a smaller magnet with both poles.

Mistake: Assuming attraction proves that both objects are permanent magnets.
A permanent magnet can attract unmagnetized ferromagnetic material.

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Key Terms

Permanent magnet — object that maintains a persistent magnetic field without continuous electrical power.

Magnetic pole — region of a magnet where its magnetic effects are especially strong.

North pole — pole from which external magnetic field lines are conventionally drawn outward.

South pole — pole into which external magnetic field lines are conventionally drawn.

Magnetic field — region where magnetic forces can act.

Magnetic field line — representation showing magnetic-field direction.

Ferromagnetic material — material capable of strong magnetic ordering, such as iron, nickel or cobalt.

Magnetic domain — region in a ferromagnetic material where many atomic magnetic moments are aligned.

Magnetization — development of an overall magnetic alignment in a material.

Magnetic induction — magnetization produced by exposure to an external magnetic field.

Temporary magnet — material that becomes magnetized relatively easily but loses much of its magnetization when the external field is removed.

Demagnetization — reduction or loss of a material's overall magnetization.

Curie temperature — temperature above which a ferromagnetic material loses its long-range ferromagnetic ordering.

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Key Takeaways

• Permanent magnets produce persistent magnetic fields without requiring continuous electrical current.
• Every ordinary permanent magnet has a north and south pole.
• Like magnetic poles repel.
• Unlike magnetic poles attract.
• Magnetic forces can act without physical contact.
• Outside a magnet, magnetic field lines are conventionally drawn from north to south.
• Magnetic field lines form continuous closed loops.
• The magnetic field is generally strongest near a magnet's poles.
• Cutting a magnet produces smaller magnets, each with north and south poles.
• Iron, nickel, cobalt and many steels are ferromagnetic.
• Not all metals are strongly magnetic.
• Ferromagnetic materials contain magnetic domains.
• In an unmagnetized material, different domains are arranged so their magnetic effects largely cancel.
• Magnetization increases the overall alignment of magnetic domains.
• Some materials retain this alignment and become permanent magnets.
• Magnetic induction can temporarily magnetize a ferromagnetic object.
• Attraction alone does not prove that both objects are permanent magnets.
• Repulsion is strong evidence that two interacting objects possess magnetic poles.
• Heating, mechanical shock or opposing magnetic fields can weaken a permanent magnet.
• Permanent magnets are widely used in motors, speakers, generators, sensors and many electronic devices.

Check Your Understanding

1. Define a permanent magnet.
2. Name the two poles of a magnet.
3. What happens when two north poles are brought together?
4. What happens when a north pole and south pole are brought together?
5. State the rule describing the interaction of magnetic poles.
6. In which direction are magnetic field lines drawn outside a bar magnet?
7. Where is the magnetic field of a bar magnet generally strongest?
8. What happens if a bar magnet is cut in half?
9. Give three examples of ferromagnetic materials.
10. Explain why saying "all metals are magnetic" is incorrect.
11. Define a magnetic domain.
12. Describe the domains in an unmagnetized ferromagnetic material.
13. Describe what happens to domains when a material becomes magnetized.
14. Explain why some materials can become permanent magnets while others behave mainly as temporary magnets.
15. Explain why an unmagnetized iron nail can be attracted to either pole of a permanent magnet.
16. Why is repulsion a better test for identifying a permanent magnet than attraction?
17. Explain how iron filings can be used to investigate a magnetic field.
18. Explain how a compass can be used to determine magnetic-field direction.
19. Describe two ways in which a permanent magnet can become demagnetized.
20. Explain how the magnetic-domain model helps account for the properties of permanent magnets.