Simple phenomena of magnetism Revision Notes

Every magnet has two distinct ends called poles: a north pole (N) and a south pole (S). These poles are the regions where the magnetic force is strongest.

The north pole of a magnet is defined as the pole that, when freely suspended, points towards the Earth's geographic north. The opposite end is the south pole.

Magnetic poles always come in pairs; it is impossible to isolate a single pole (monopole) in a magnet.

• Magnets have two poles: north (N) and south (S).
• Poles are the strongest points of magnetic force.
• Poles always exist in pairs; no isolated poles exist.

Magnetic poles exert forces on each other that can be either attractive or repulsive.

Like poles repel each other: north repels north, and south repels south.

Unlike poles attract each other: north attracts south and vice versa.

• Like poles repel (N-N or S-S).
• Unlike poles attract (N-S).
• The force acts along the line joining the two poles.

When two magnets are brought close, the forces between their poles determine their interaction.

If opposite poles face each other, the magnets attract and may stick together.

If like poles face each other, the magnets repel and push apart.

• Magnets attract when opposite poles face each other.
• Magnets repel when like poles face each other.
• The force depends on the distance between the poles: closer poles exert stronger forces.

Magnetised materials behave like magnets; they have aligned magnetic domains and exhibit magnetic poles.

Unmagnetised materials do not show magnetic properties because their magnetic domains are randomly oriented.

Magnetic materials such as iron, cobalt, and nickel can be magnetised by placing them in a magnetic field or by contact with a magnet.

• Magnetised materials have aligned magnetic domains and act like magnets.
• Unmagnetised materials have random domain orientation and show no magnetic effect.
• Only certain materials are magnetic (ferromagnetic).

A magnet can attract magnetic materials even if the materials are not magnetised.

When a magnetic material is brought near a magnet, the magnetic domains in the material may align temporarily, causing attraction.

Non-magnetic materials (e.g., wood, plastic) do not experience forces from magnets.

• Magnets attract magnetic materials whether magnetised or not.
• Magnetic materials can be temporarily magnetised by a nearby magnet.
• Non-magnetic materials are unaffected by magnets.

Magnetic materials contain regions called magnetic domains. Each domain is a small area where the magnetic moments of atoms are aligned in the same direction, producing a tiny magnetic field.

In an unmagnetised piece of magnetic material, these domains are randomly oriented, so their magnetic effects cancel out, and the material shows no overall magnetism.

When an external magnetic field is applied, the domains tend to align with the field, increasing the net magnetic effect of the material. This process is called magnetisation.

• Magnetic domains are groups of aligned atomic magnetic moments.
• Random domain orientation results in no net magnetism.
• External magnetic fields cause domains to align, magnetising the material.

Induced magnetism occurs when a magnetic material is placed in an external magnetic field and becomes temporarily magnetised.

The external magnetic field causes the magnetic domains inside the material to align in the direction of the field.

This alignment causes the material to behave like a magnet, attracting magnetic materials or being attracted to magnets itself.

• Induced magnetism is temporary magnetisation caused by an external magnetic field.
• The strength of induced magnetism depends on the strength of the external field and the material's properties.
• Materials like soft iron are easily magnetised and demagnetised, showing strong induced magnetism.

Once the external magnetic field is removed, the magnetic domains lose their alignment and return to their original random orientations.

As a result, the material loses its magnetism and no longer behaves like a magnet.

This temporary magnetism distinguishes induced magnets from permanent magnets, which retain their magnetism without an external field.

• Induced magnetism lasts only while the external field is present.
• Magnetic domains return to random orientation when the field is removed.
• Permanent magnets retain domain alignment without an external field.

Materials that can be magnetised by induction are usually ferromagnetic, such as iron, cobalt, and nickel.

Soft iron is commonly used in experiments to demonstrate induced magnetism because it magnetises easily and loses magnetism quickly when the field is removed.

Non-magnetic materials like copper or plastic do not show induced magnetism because their atomic structure does not support domain alignment.

• Ferromagnetic materials show induced magnetism.
• Soft iron is a typical example of a material with strong induced magnetism.
• Non-magnetic materials do not become magnetised by induction.

To demonstrate induced magnetism, bring a piece of soft iron close to a permanent magnet.

The soft iron will be attracted to the magnet, showing it has become temporarily magnetised.

If the soft iron is moved away from the magnet, it will lose its magnetism and no longer attract small magnetic objects like iron filings or paper clips.

• Use a permanent magnet and soft iron to show induced magnetism.
• Observe attraction of soft iron to the magnet as evidence of induced magnetism.
• Remove the magnet to see the temporary nature of the effect.

Magnets are objects that produce a magnetic field, which can attract magnetic materials like iron. There are two main types of magnets relevant here: temporary magnets and permanent magnets.

Temporary magnets are materials that become magnetised only when placed in a magnetic field and lose their magnetism quickly when the field is removed.

Permanent magnets retain their magnetism over a long period without needing an external magnetic field.

• Temporary magnets: magnetised only in presence of a magnetic field.
• Permanent magnets: retain magnetism without external influence.

Temporary magnets are usually made from soft iron. Soft iron is a material that is easily magnetised but does not retain magnetism well.

Permanent magnets are typically made from steel, an alloy of iron with carbon and other elements, which can maintain magnetic domains aligned for a long time.

The difference in atomic structure and domain alignment explains their magnetic behaviour.

• Soft iron: easily magnetised, poor retention of magnetism.
• Steel: harder to magnetise, good retention of magnetism.

When a soft iron object is placed in a magnetic field, its magnetic domains align temporarily, making it a magnet. When the external field is removed, the domains return to random orientation, and magnetism is lost quickly.

Steel’s magnetic domains, once aligned, remain aligned due to the material’s hardness and internal structure, so it remains magnetised as a permanent magnet.

This difference means temporary magnets are useful when magnetism is needed only briefly, while permanent magnets provide a constant magnetic field.

• Temporary magnets lose magnetism quickly after removal of external field.
• Permanent magnets retain magnetism for a long time.

Temporary magnets are used in devices like electromagnets, where magnetism can be switched on and off by controlling the external magnetic field (usually an electric current).

Permanent magnets are used in applications requiring a steady magnetic field, such as fridge magnets, magnetic compasses, and electric motors.

The choice of magnet depends on whether magnetism needs to be temporary or permanent.

• Temporary magnets: electromagnets, relays, magnetic lifting cranes.
• Permanent magnets: compasses, loudspeakers, fridge magnets.

The following table summarises the key differences between temporary magnets made of soft iron and permanent magnets made of steel.

Materials can be classified based on their response to magnetic fields into magnetic and non-magnetic materials. Magnetic materials are attracted by magnets and can themselves become magnetised, whereas non-magnetic materials show no attraction to magnets and cannot be magnetised.

Understanding these differences is essential for applications involving magnets, such as in electric motors, transformers, and magnetic storage devices.

• Magnetic materials respond to magnetic fields by attraction and can become magnets.
• Non-magnetic materials do not respond to magnetic fields and cannot be magnetised.

Magnetic materials are typically metals that contain atoms with unpaired electrons. The most common magnetic materials are iron, nickel, and cobalt. These materials have domains—small regions where the magnetic moments of atoms are aligned in the same direction.

When placed in a magnetic field, these domains can align with the field, causing the material to be attracted to the magnet. Some magnetic materials can retain this alignment after the external field is removed, becoming permanent magnets.

The unit of magnetic field strength is the tesla (T), but the attraction of materials depends on their magnetic permeability, a measure of how easily they become magnetised.

• Common magnetic materials: iron, nickel, cobalt.
• Contain magnetic domains with aligned atomic magnetic moments.
• Can be magnetised permanently or temporarily.
• Attracted to magnets due to alignment of domains.

Non-magnetic materials do not have unpaired electrons arranged in a way that produces magnetic domains. Examples include wood, plastic, glass, and most liquids and gases.

These materials do not experience a net force in a magnetic field and are not attracted to magnets. They cannot be magnetised because their atomic magnetic moments cancel out or are randomly oriented.

Non-magnetic materials are often used as insulators or structural materials where magnetic properties are undesirable.

• Examples: wood, plastic, glass.
• No magnetic domains or net magnetic moments.
• Not attracted to magnets and cannot be magnetised.
• Used where magnetic interference must be avoided.

Magnetism arises from the motion of electrons, specifically their spin and orbital angular momentum, which produce tiny magnetic moments.

In magnetic materials, these moments can align in regions called domains, producing a net magnetic field.

In non-magnetic materials, the magnetic moments of electrons are paired or randomly oriented, cancelling each other out and resulting in no net magnetism.

• Electron spin and orbital motion create magnetic moments.
• Magnetic domains form in magnetic materials.
• Random or paired moments in non-magnetic materials cancel out.

To test if a material is magnetic, bring a strong magnet close to it and observe if there is attraction.

Materials like iron nails will be attracted, indicating they are magnetic.

Materials like plastic rulers or wooden blocks will show no attraction, indicating they are non-magnetic.

• Use a magnet to test attraction.
• Attraction indicates magnetic material.
• No attraction indicates non-magnetic material.

A magnetic field is defined as the region or area around a magnetic pole where a magnetic force can be detected. Within this region, another magnetic pole experiences a force either of attraction or repulsion depending on the nature of the poles involved.

This force arises due to the interaction between magnetic poles, which are the ends of a magnet labelled as north (N) and south (S). The magnetic field is a vector field, meaning it has both magnitude and direction at every point in space around the magnet.

• Magnetic field: area around a magnetic pole where a force acts on another magnetic pole.
• Force can be attractive or repulsive depending on pole types.
• Magnetic field is a vector quantity with direction and magnitude.

Magnetic fields are produced by magnetic materials (such as permanent magnets) and by electric currents flowing in conductors. The magnetic field is strongest near the magnetic poles or near the current-carrying wire and decreases with distance from the source.

The field arises because moving electric charges (currents) generate magnetic effects, and permanent magnets have aligned magnetic domains producing a steady magnetic field.

• Produced by magnetic poles or electric currents.
• Field strength decreases with distance from the source.
• Permanent magnets have aligned domains creating a steady field.

Within a magnetic field, a magnetic pole experiences a force. Like poles (north-north or south-south) repel each other, while unlike poles (north-south) attract.

The force on a magnetic pole placed in a magnetic field depends on the strength of the field and the pole strength. The direction of the force is along the magnetic field lines, acting to move the pole from one point to another within the field.

• Like magnetic poles repel; unlike poles attract.
• Force magnitude depends on magnetic field strength and pole strength.
• Force direction is along magnetic field lines.

Magnetic fields are commonly represented by magnetic field lines. These lines show the direction of the magnetic field at any point and are drawn such that the tangent to a line at any point gives the field direction there.

Field lines emerge from the north pole and enter the south pole of a magnet. The density or closeness of these lines indicates the strength of the magnetic field: closer lines mean a stronger field.

To draw magnetic field lines around a bar magnet, place a compass at various points around the magnet and mark the direction the compass needle points. Joining these directions smoothly forms the field lines.

• Field lines show magnetic field direction and strength.
• Lines go from north pole to south pole outside the magnet.
• Closer lines indicate stronger magnetic field.
• Compass can be used to trace field lines.

The strength of a magnetic field decreases as the distance from the magnetic pole or current source increases. This is because the magnetic influence spreads out over a larger area.

The direction of the magnetic field at any point is the direction of the force that would act on a north magnetic pole placed at that point.

Understanding this variation is important for predicting magnetic forces and for practical applications such as designing electromagnets and magnetic sensors.

• Magnetic field strength decreases with distance from source.
• Field direction at a point is direction of force on north pole.
• Important for predicting forces and designing magnetic devices.

A magnetic field is a region around a magnet where magnetic forces can be detected. It is a vector field, meaning it has both magnitude and direction at every point.

Magnetic field lines are an abstract representation used to visualise the magnetic field. They show the direction a north magnetic pole would move if placed in the field and indicate the field's strength by their spacing.

Magnetic field lines are continuous closed loops; they never start or end abruptly. This reflects the fact that magnetic monopoles do not exist; instead, magnetic poles always come in north-south pairs.

• Magnetic field lines represent the magnetic field around a magnet.
• They have direction and magnitude (strength).
• Field lines are continuous closed loops.

A bar magnet has two poles: a north (N) pole and a south (S) pole. The magnetic field lines emerge from the north pole and curve around to enter the south pole.

Outside the magnet, the field lines travel from the north pole to the south pole. Inside the magnet, the field lines continue from the south pole back to the north pole, completing closed loops.

The pattern of the field lines is symmetrical about the magnet's axis. The lines are densest near the poles, indicating the magnetic field is strongest there, and spread out further away from the magnet, where the field is weaker.

• Field lines emerge from the north pole and enter the south pole outside the magnet.
• Inside the magnet, field lines run from south to north, forming closed loops.
• Field lines are densest near the poles, showing stronger magnetic fields.

The direction of a magnetic field line at any point shows the direction of the force that a north magnetic pole would experience if placed at that point.

By convention, magnetic field lines are drawn pointing away from the north pole and towards the south pole outside the magnet.

This direction helps predict the behaviour of magnetic materials and other magnets placed in the field, such as attraction or repulsion depending on pole orientation.

• Field line direction shows force direction on a north magnetic pole.
• Lines point from north to south outside the magnet.
• Direction helps predict magnetic interactions.

To draw magnetic field lines around a bar magnet: start by marking the north and south poles on the magnet.

Draw lines emerging from the north pole, curving around the magnet, and entering the south pole.

Inside the magnet, draw lines connecting the south pole back to the north pole to complete the loops. Ensure lines do not cross each other and are denser near the poles to indicate stronger fields.

• Mark north and south poles on the magnet.
• Draw lines from north to south outside the magnet.
• Complete closed loops inside the magnet from south to north.
• Avoid crossing lines and show denser lines near poles.

The spacing between magnetic field lines indicates the strength of the magnetic field at that location.

Where the lines are close together, the magnetic field is stronger; where they are farther apart, the field is weaker.

Near the poles of a bar magnet, the lines are closest, showing the strongest magnetic field in these regions.

• Closer field lines mean stronger magnetic field.
• Field strength is greatest near the poles.
• Field lines spread out and weaken with distance from the magnet.

A magnetic field is a region around a magnet where magnetic forces can be detected. Every magnet has two poles: a north pole and a south pole. These poles are the sources of magnetic forces.

The north pole of a magnet is defined as the pole that, when freely suspended, points towards the Earth's geographic north. The south pole is the opposite end.

Magnetic forces arise from interactions between these poles: like poles repel, unlike poles attract.

• Magnetic field: region where magnetic forces act.
• Magnet has north and south poles.
• North pole points towards Earth's geographic north.

The direction of the magnetic field at any point is defined as the direction of the force that would act on the north pole of a small magnet placed at that point.

If a tiny north pole is placed at a point in the field, the force on it points in the direction of the magnetic field.

This definition is consistent with the convention that magnetic field lines point away from the north pole of a magnet and towards the south pole.

• Magnetic field direction = force direction on a north pole.
• Field lines point from north to south outside a magnet.
• Force on north pole aligns with magnetic field direction.

Magnetic field lines are imaginary lines drawn to represent the magnetic field. They show both the direction and relative strength of the field.

The direction of the field at any point is tangent to the field line at that point, indicating the force direction on a north pole.

Field lines are drawn from the north pole to the south pole outside the magnet and continue through the magnet from south to north, forming closed loops.

• Field lines represent magnetic field direction and strength.
• Direction at a point is tangent to the field line.
• Lines form closed loops from north to south poles.

A compass needle is a small magnet with a north and south pole. When placed in a magnetic field, the north pole of the compass needle experiences a force and aligns with the magnetic field direction.

By placing a compass at different points around a magnet, the direction of the magnetic field can be mapped by noting the direction the compass needle points.

This practical method confirms that the magnetic field direction is the direction of the force on a north pole.

• Compass needle aligns with magnetic field direction.
• North pole of compass points along magnetic field.
• Used to map magnetic field directions around magnets.

Understanding the direction of magnetic fields is essential in designing electric motors, generators, and magnetic sensors where forces on magnetic poles determine motion and function.

It also helps in predicting the behaviour of magnets in magnetic materials and in Earth's magnetic field navigation.

Correctly identifying the magnetic field direction allows for accurate analysis of magnetic forces and interactions in physics problems.

• Magnetic field direction crucial in motor and generator design.
• Predicts magnet behaviour in materials and navigation.
• Essential for analysing magnetic forces in physics.

A magnetic field is a region around a magnet where magnetic forces can be detected. Magnetic field lines are an imaginary representation used to show the direction and strength of the magnetic field.

Magnetic field lines always emerge from the north pole of a magnet and enter the south pole. They never cross each other.

The density (closeness) of magnetic field lines indicates the strength of the magnetic field: closer lines mean a stronger magnetic field, while lines spaced further apart indicate a weaker field.

• Magnetic field lines show the direction of magnetic force on a north magnetic pole.
• Lines emerge from the north pole and enter the south pole.

A compass contains a small magnetised needle that can rotate freely and aligns itself with the magnetic field at its location.

To plot magnetic field lines around a bar magnet: place the magnet on a flat surface and position a compass at various points around it.

At each point, observe the direction the compass needle points; this direction is tangent to the magnetic field line at that point and points from the north pole towards the south pole of the magnet (the needle’s north end points along the field). Mark the direction on a sheet of paper and move the compass to the next point, repeating the process to trace out the field lines around the magnet.

• Compass needle aligns with magnetic field direction.
• Plot points around the magnet and mark needle direction.

Iron filings are tiny pieces of iron that become magnetised in the presence of a magnetic field and align along the magnetic field lines.

To visualise the magnetic field pattern, place a sheet of paper over a magnet and sprinkle iron filings evenly on the paper.

Gently tap the paper to allow the filings to move and align themselves along the magnetic field lines, revealing the pattern of the magnetic field around the magnet.

• Iron filings align along magnetic field lines.
• Sprinkle filings on paper over magnet and tap gently.

The direction of the magnetic field at any point is the direction in which the north pole of a compass needle points when placed at that point.

This means the compass needle’s north end points along the magnetic field line from the magnet’s north pole towards its south pole.

By placing a compass at different points in the field, you can find the direction of the magnetic field at those points, which is essential for plotting field lines accurately.

• Compass needle’s north end points along magnetic field direction.

When plotting magnetic field lines with a compass, use a sheet of paper under the magnet and mark the needle direction at multiple points around the magnet to get a clear pattern.

Draw smooth curves through the marked directions to represent magnetic field lines, ensuring lines emerge from the north pole and enter the south pole.

For iron filings, ensure the magnet is stable and the paper is flat to get a clear pattern; avoid excessive tapping to prevent scattering filings too widely.

• Mark compass needle direction at many points around magnet.

Permanent magnets are materials that produce a persistent magnetic field without the need for an external power source. They are usually made from ferromagnetic materials such as steel or alloys like neodymium-iron-boron.

The magnetic field of a permanent magnet arises from the alignment of magnetic domains within the material, which remain aligned after magnetisation.

Permanent magnets have fixed magnetic poles (north and south) and produce a constant magnetic field around them, represented by magnetic field lines emerging from the north pole and entering the south pole.

• Produce a constant magnetic field without external energy.
• Made from materials like steel or rare-earth alloys.
• Magnetic domains remain aligned permanently.
• Have fixed north and south poles.

Electromagnets generate a magnetic field only when an electric current flows through a coil of wire, usually wound around a soft iron core.

The magnetic field strength of an electromagnet can be controlled by varying the current or the number of turns in the coil.

When the current is switched off, the magnetic field disappears, making electromagnets temporary magnets with controllable magnetism.

• Magnetic field produced by electric current in a coil.
• Core usually made of soft iron to enhance magnetism.
• Magnetic field strength is variable and controllable.
• Magnetism ceases when current is off.

Permanent magnets are used in devices where a constant magnetic field is required without continuous energy input.

Examples include: refrigerator magnets that hold notes on a fridge door; compass needles that align with Earth's magnetic field for navigation; loudspeakers where magnets interact with coils to produce sound; and magnetic locks that use permanent magnets to secure doors.

In electric motors and generators, permanent magnets provide a steady magnetic field to interact with coils carrying current, enabling conversion between electrical and mechanical energy.

• Refrigerator magnets for holding items.
• Compass needles for navigation.
• Loudspeakers to convert electrical signals to sound.
• Magnetic locks for security.
• Electric motors and generators for energy conversion.

Electromagnets are used where the magnetic field needs to be switched on and off or varied in strength.

Examples include: electric bells where the electromagnet attracts a striker to hit a bell when current flows; magnetic cranes that lift heavy ferrous materials by switching the magnet on and off; relays that use electromagnets to open or close electrical circuits; and MRI (Magnetic Resonance Imaging) machines that use strong electromagnets to produce detailed images of the body.

Electromagnets are also used in maglev trains, where magnetic forces are controlled to levitate and propel the train with minimal friction.

• Electric bells for sound signalling.
• Magnetic cranes for lifting heavy metal objects.
• Relays to control electrical circuits.
• MRI machines for medical imaging.
• Maglev trains for frictionless transport.

Permanent magnets provide a constant magnetic field without energy consumption but cannot be switched off or adjusted.

Electromagnets require electrical energy but offer the advantage of controllable magnetic fields, including the ability to switch the magnetism on and off.

The choice between them depends on the application requirements such as energy availability, need for variable magnetic strength, and safety considerations.

• Permanent magnets: constant field, no power needed.
• Electromagnets: variable field, require power.
• Electromagnets can be switched off; permanent magnets cannot.
• Application depends on control and energy needs.

A magnetic field is a region around a magnet or a current-carrying conductor where magnetic forces can be detected. It is a vector field, meaning it has both magnitude and direction at every point.

Magnetic fields are produced by moving electric charges, such as electrons in atoms or electric currents in wires, and by permanent magnets due to the alignment of magnetic domains within the material.

The presence of a magnetic field can be detected by the force it exerts on magnetic materials or other magnets placed within the field.

• Magnetic field: a vector field around magnets or currents.
• Produced by moving charges or aligned magnetic domains.
• Detectable by forces on magnetic poles or materials.

When two magnets are brought close, their magnetic fields overlap and interact. The interaction of these fields results in a magnetic force between the magnets.

This force arises because each magnet's magnetic field exerts a force on the magnetic poles of the other magnet.

The magnetic force is a vector quantity; its direction and magnitude depend on the relative orientation and distance between the magnets.

• Magnetic forces arise from overlapping magnetic fields.
• Each magnet's field acts on the other's poles.
• Force depends on orientation and separation distance.

Magnetic poles come in two types: north (N) and south (S).

Opposite poles (N and S) attract each other, while like poles (N and N or S and S) repel each other.

This behaviour is a direct consequence of the interaction between their magnetic fields: field lines emerge from north poles and enter south poles, causing forces that pull opposite poles together and push like poles apart.

• Magnetic poles: north and south.
• Opposite poles attract; like poles repel.
• Field line patterns explain force directions.

Magnetic fields are represented by field lines drawn around magnets. These lines show the direction of the magnetic field at any point: from north to south outside the magnet.

The density of field lines indicates the strength of the magnetic field; closer lines mean a stronger field.

Magnetic forces act along the line joining the poles, with direction depending on whether the poles attract or repel.

• Field lines show direction and strength of magnetic fields.
• Lines run from north to south outside magnets.
• Force direction depends on pole interaction.

Magnetic forces arising from field interactions are fundamental to many devices such as electric motors, generators, magnetic levitation trains, and magnetic storage media.

Understanding how magnetic fields interact allows engineers to design systems that use attraction and repulsion effectively.

In practical situations, the strength of magnetic forces decreases with increasing distance between magnets, approximately following an inverse square or inverse cube law depending on configuration.

• Magnetic forces used in motors, generators, maglev trains.
• Design relies on controlling magnetic field interactions.
• Force strength decreases with distance between magnets.

A magnetic field is a region around a magnet or a current-carrying conductor where magnetic forces can be detected. Magnetic field lines are imaginary lines drawn to represent the direction and strength of the magnetic field at various points.

Magnetic field lines always form closed loops, emerging from the north pole of a magnet and entering the south pole outside the magnet, continuing through the magnet from south to north internally.

These lines provide a visual tool to understand magnetic effects without showing the field itself, which is invisible. The direction of the magnetic field at any point is tangent to the field line at that point.

• Magnetic field lines show the direction of the magnetic field.
• They form continuous closed loops from north to south poles outside the magnet.
• The tangent to a field line at any point gives the field direction there.

The strength of a magnetic field at a point is related to the force a magnetic pole would experience there. This strength is visually represented by the density or spacing of magnetic field lines.

Where the magnetic field lines are close together, the magnetic field is stronger. Conversely, where the lines are spaced further apart, the magnetic field is weaker.

This means that the number of field lines per unit area perpendicular to the lines is proportional to the magnetic field strength.

• Closer magnetic field lines indicate a stronger magnetic field.
• Wider spacing of lines indicates a weaker magnetic field.
• Magnetic field strength ∝ number of field lines per unit area.

To draw magnetic field lines around a bar magnet, start by drawing lines emerging from the north pole and curving around to enter the south pole.

The lines should be denser near the poles where the magnetic field is strongest and more widely spaced further away where the field weakens.

When interpreting such diagrams, the relative spacing of lines at different points indicates the relative strength of the magnetic field at those points.

• Field lines start at the north pole and end at the south pole outside the magnet.
• Lines are denser near the poles, showing stronger fields.
• Spacing between lines indicates relative field strength.

The magnetic field is strongest near the poles of a magnet because the magnetic forces are concentrated there.

As you move away from the poles, the magnetic field spreads out over a larger area, causing the field lines to be spaced further apart and the field strength to decrease.

This spreading out of field lines with distance is why the magnetic force weakens with increasing distance from the magnet.

• Magnetic field strength is greatest near the poles.
• Field lines spread out with distance, reducing field strength.
• Magnetic force decreases as distance from the magnet increases.

Magnetic field lines can be visualised experimentally using iron filings sprinkled around a magnet; the filings align along the field lines, showing their pattern and density.

A compass needle placed at various points around a magnet aligns tangentially to the magnetic field lines, indicating the field direction.

Understanding the spacing of magnetic field lines helps in designing devices like electromagnets and electric motors, where controlling field strength is important.

• Iron filings reveal magnetic field patterns by aligning along field lines.
• Compass needles show the direction of magnetic fields at different points.
• Field line density guides practical applications involving magnetic fields.