Full chapter
Magnetism and electromagnetism
All 7 topics and the revision summary on one page.
01
Magnet properties and observations
A magnet can attract or repel another magnet. The observation tells you about the poles involved, but attraction alone does not prove that an object was already a magnet.
A force is a push or pull. Magnetic forces can act without the objects touching: they are non-contact forces.
Identify poles and predict their interaction
An ordinary bar magnet has a north-seeking pole, N, and a south-seeking pole, S. Its magnetic effect is usually strongest near the poles.
Predict the force on each magnet
Purple arrows show forces on the named magnets. Their lengths are qualitative; no numerical force is supplied.
Unlike facing poles: attraction
Like facing poles: repulsion
A field-direction arrow, used later, tells how a compass aligns at one position. It does not replace these two forces on the magnets.
A freely suspended magnet, away from nearby magnetic disturbances, turns approximately north-south. Its north-seeking end points roughly towards geographic north. A nearby magnet can change this direction, so a compass needle does not always point geographically north.
Distinguish a magnet from a magnetic material
A magnetic material can become magnetised. A magnetised object has a magnetic effect at that time, whether it retains the effect itself or is being magnetised by a nearby source.
A magnet attracts suitable magnetic materials such as iron and the magnetic steel used in these examples. An initially unmagnetised piece can be attracted because the nearby magnet induces magnetism in it.
Being a metal is not sufficient. Ordinary copper and aluminium do not show the strong attraction of soft iron in this stationary-magnet comparison. Different steel alloys can also have different magnetic behaviour.
Use the observation as evidence
An unknown end X repels a known N pole
Like poles repel, so X is a north pole in this simple end-to-end test. The unknown bar is magnetised.
If X instead attracts the known N pole, two explanations are possible: X could be a south pole of a magnet, or it could be the nearer end of an initially unmagnetised magnetic material.
Attraction alone therefore does not establish that the unknown bar was already a permanent magnet. Repulsion identifies a magnetised end, but does not measure how long the material will retain its magnetism.
Smaller pieces still have both poles
Each smaller piece still has two poles
This is a conceptual division of an ordinary bar magnet. The new faces also become poles.
The newly formed ends acquire poles too. Magnetic N and S labels describe magnetic poles; they are not labels for positive and negative electric charge.
Optional check In a simple end-to-end test, end X of an unknown bar repels the known N pole of a reference magnet. What does this establish?
02
Induced magnetism and material choice
A magnetic material can become magnetised in a magnetic field. What remains after that field is removed helps determine the material's use.
Like magnetic poles repel and unlike poles attract. An unmagnetised magnetic material can acquire poles; it does not have to start as a permanent magnet.
Near a strong magnet
Place an initially unmagnetised iron rod near a strong magnet. The rod becomes magnetised without needing to touch the magnet. Its nearer end develops the opposite pole to the nearby inducing pole.
An external magnet induces an opposite near pole
The iron rod was initially unmagnetised. The pole labels below show its induced state while the strong magnet is nearby.
A is the opposite pole to the nearby inducing pole. Reversing the inducing magnet reverses the rod's induced poles.
If the inducing pole is changed from N to S, A becomes N and B becomes S. Reversing the field reverses the induced poles. This process is induced magnetism; it does not require electric charge to transfer from the magnet to the rod.
Compare before, during and after
- Before: the initially unmagnetised rod has little or no retained attraction for a small test piece of iron.
- During: with the inducing magnet nearby, the rod becomes magnetised and can attract the test piece near its far end.
- After: move the inducing magnet far away and test the rod again. Suitable soft iron loses much of the induced effect.
The inducing magnet can also pull directly on the test piece. A comparison with the rod absent, keeping the magnet and test position fixed, helps distinguish that direct effect from the rod's contribution. Keep the geometry and test piece consistent when comparing observations.
After losing most of its magnetism, the iron remains a magnetic material: it can be magnetised again.
Inside a current-carrying solenoid
A solenoid is a coil with many turns of wire. Current is the rate of flow of charge. When current flows in the coil, it creates a magnetic field that can magnetise a suitable material placed inside.
The core sits inside the current-carrying coil
For this example, the coil's supplied on-state polarity is N on the left, S on the right. The core is magnetised in the same orientation.
Current on: the coil magnetises the core
The soft-iron core becomes a temporary magnet: N left and S right, matching the coil's supplied polarity.
Current off: most induced magnetisation is lost
Suitable soft iron loses most of its induced magnetisation when the main inducing field is removed. It is still a magnetic material.
The magnetised soft-iron core adds to the coil's magnetic effect, producing a stronger temporary electromagnet. Here the core is inside the coil's field, so its pole orientation corresponds to the solenoid's supplied orientation.
Switching off removes the main magnetising influence. Soft iron has low retention, so the core's magnetic effect largely disappears. A suitable steel sample magnetised in the coil can instead retain its poles after the current stops.
Choose a material for what should happen afterwards
- Soft iron: a temporary magnet
- It is readily magnetised and loses much of its magnetisation when the inducing field is removed. This makes it useful as the core of a switched lifting electromagnet that must release its load.
- Suitable steel: a permanent magnet
- It is harder to magnetise than soft iron in this comparison, but retains magnetism once magnetised. A compass needle or permanent bar magnet needs that retention so it works away from a magnetising coil.
Match the material to the task
A device picks up and releases small iron pieces
The core should become magnetised when switched on and lose much of that effect when switched off. Soft iron fits both conditions.
Using a material with high retention could leave the pieces attracted after switch-off. A compass has a different requirement: its needle should stay magnetised, so a suitable permanent-magnet material fits that task.
Temporary does not mean the effect is always exactly zero after switch-off, and permanent does not mean impossible to demagnetise. The distinction concerns how readily the material gains and retains magnetism.
Compare retention fairly
Use comparable specimen dimensions, positions and magnetising conditions. Remove the external field before comparing the retained attraction with the same test method. Repeating the comparison helps reveal variation.
The number of pieces lifted also depends on their weight, shape and contact. It can provide evidence about the magnetic effect under those conditions, but is not a direct numerical measurement of field strength or a universal material constant.
Optional check A switched device must pick up small iron pieces when current flows and release them after the current is switched off. Which material choice and reason best fit its core?
03
Finding field direction with a compass
A small compass is a bar magnet free to turn. Its north-seeking end gives the magnetic field direction at the compass's position.
Magnetic forces act without contact. A magnetic field describes the magnetic effect around a source; the field can turn a small magnet until it aligns with the local direction.
Read the north-seeking end
Identify the compass's marked N end and allow the needle to settle. The direction in which that end points is the field direction. Its S end points the other way.
The compass responds to the field where it is placed. It need not point towards the centre of a nearby bar magnet or towards geographic north. The field direction describes the needle's alignment; it is not a rule for the direction in which the centre of the whole compass must move.
Read the north-seeking end at each position
These are four separate settled-compass observations around one bar. The magnet's field is assumed to dominate the background field.
The marked N end indicates the local field direction. Reversing the bar reverses these four ideal directions.
Read four local directions
The same bar, four observation points
- Left of the N pole: the compass N end points left, away from that pole.
- Above the centre: the N end points right.
- Right of the S pole: the N end points left, towards that pole.
- Below the centre: the N end points right.
These are local directions around one source. If the magnet is reversed while the positions stay fixed, the ideal directions reverse too.
Map a field using a plotting compass
- Place a bar magnet on paper on a non-magnetic surface. Draw its outline and label its poles. Keep the magnet and paper fixed.
- Check the marked N end and that the compass turns freely. Place it at a starting point near the magnet and wait for it to settle.
- Mark the positions of the needle ends, identifying N. Draw a short arrow from its S-end mark towards its N-end mark.
- Move the compass so its S end occupies the previous N-end mark. Let it settle and mark the new N-end position.
- Repeat in small steps, then join the marks with a smooth curve and add arrows in the recorded direction.
- Start again at several other positions to build the field pattern.
A small compass better approximates the direction at one position. Record a settled needle rather than a moving or stuck one, and keep nearby magnets and magnetic objects away from the map.
The needle responds to the resultant field, including Earth's field. An ideal bar-magnet diagram neglects that background. Make observations where the bar's effect is strong enough to dominate; far from it, the background can change the observed direction.
What iron filings show
Iron filings on a sheet near a magnet become magnetised and align, revealing the rough shape of the pattern. The visible chains do not identify an arrow direction by themselves. Use the compass N-end convention to add the arrows.
Field lines are drawn representations. The filings help reveal their shape; they are not threads that already existed around the magnet.
Optional check A small compass settles at a marked position near a bar magnet. Its north-seeking end points right. What is the magnetic field direction at that position?
04
Magnetic field patterns
A field pattern combines shape and direction. Identify every pole first, then draw smooth lines with arrows that agree with the compass convention.
The north-seeking end of a settled compass points along the local field. Outside a magnet, field lines leave N and enter S.
Follow the exterior field from N to S
Teal arrows show field direction. These idealised patterns neglect background fields. Internal return paths through the magnets are omitted.
One bar: exterior field from N to S
Outside the bar, arrows leave its north region and enter its south region. A compass's N end aligns with the local tangent.
Unlike facing poles: field crosses the gap
The gap's arrows run from the facing N to S. Outer curves obey the same rule, following the other labelled poles.
Like facing poles: lines bend away from the gap
The curves leave both facing north regions and bend away from the gap. They do not connect one N pole to the other.
Field lines form continuous loops and do not cross. Line spacing is qualitative here; counting the drawn curves in different panels does not measure or compare field strength.
One bar magnet
Draw lines that leave the north pole, curve through the space around the bar and enter the south pole. The compass needle lies tangent to the local line, with its N end following the arrow.
Magnetic field lines continue through the magnet to form loops. Inside the bar their return direction is S to N. Keep the word outside with the familiar N-to-S rule.
Unlike poles facing
With N on the left of the gap and S on the right, lines connect across the air gap with arrows left to right, from N to S. Lines farther from the axis curve around the magnets.
The magnets attract, but that force observation does not change the field-arrow convention. A force on a whole magnet and the local field direction are different things to label.
Like poles facing
For N facing N, lines leave both facing north poles and bend away from the central gap region. They do not join N directly to N. For the corresponding S-facing-S arrangement, reverse the arrows so that lines enter both south poles.
For two identical magnets arranged symmetrically with like poles facing, their fields can cancel at the exact midpoint if background fields are neglected. There is no nonzero field-direction arrow to draw at that cancellation point. A real compass there can still respond to Earth's field or another disturbance.
Check the representation
- Lines do not cross. A crossing would assign two different field directions to one position.
- Arrows agree along a line. They follow a compass N end; outside magnets they leave N and enter S.
- Closer spacing indicates a stronger field qualitatively within one consistently drawn pattern. An illustrator's arbitrary line count does not give an exact field strength.
- The lines represent the field. They are not physical strings pushing one another apart, nor paths that every moving magnet must follow.
Repair a drawing
The gap shape is right, but the arrows run S to N
For unlike facing poles, keep the connecting curves and reverse the arrows so that they run N to S across the gap. The correction follows the compass convention.
If one magnet is then turned around, identify the new facing poles before drawing again. The gap pattern may need to change as well as its arrows.
Optional check Two bar magnets have N facing S across an air gap, with N on the left of the gap. A drawing joins the facing poles with smooth non-crossing lines but points the arrows from right to left. What should be corrected?
05
Magnetic fields from currents
An electric current produces a magnetic field around the conductor. The field's pattern depends on the conductor's shape, while the current affects its strength and direction.
Conventional current follows the direction in which positive charge would flow. In a metal it is opposite to electron motion. A settled compass N end gives the local magnetic field direction.
A straight wire: circular field lines
Look along a straight wire, so that the wire runs perpendicular to the page. Its magnetic field lines form circles centred on the wire in this end view. The field direction at any point is tangent to a circle, rather than pointing radially towards or away from the wire.
- Dot: towards you
- A dot represents an arrow tip coming out of the page towards the reader.
- Cross: away from you
- A cross represents an arrow tail going into the page, away from the reader.
In the wire diagrams these marks label current direction. They are not positive and negative charge signs. Always read the nearby label: a dot or cross can also be used for another quantity perpendicular to the page.
Look along the wire
The central dot means current towards you; the central cross means current away from you. Teal arrows show the field. Each compass's labelled N end follows the local tangent.
Current towards you: anticlockwise field
Current away from you: clockwise field
Keep the viewpoint fixed when comparing directions. The circles represent field lines, not paths travelled by the compasses or by charges around the wire.
If you look from the opposite end of the wire, what appears clockwise changes. State the viewpoint before applying a direction rule.
Predict a compass direction
A compass is just to the right of the wire
For current out of the page, the circular field is anticlockwise, so the local field on the wire's right points up the page.
Reverse the current while keeping the compass and viewpoint fixed. The field becomes clockwise and the compass N end points down the page, assuming the wire's field dominates other fields there.
Increasing current magnitude makes the field stronger at a fixed position for the same wire geometry. Reversing current reverses the field direction. These are separate changes: a stronger current in the original direction does not reverse the field.
Within a consistently drawn pattern, closer field-line spacing can show greater strength qualitatively. The drawn lines do not move faster when current increases, and an arbitrary number of lines is not a numerical field measurement.
A solenoid: the field of a coil
A solenoid is a coil with many turns. The fields due to its turns combine to give a pattern like that of a bar magnet. In the central region of a sufficiently long solenoid, the field is approximately uniform: its lines are nearly parallel and similarly spaced.
Outside, lines leave the solenoid's N end and curve round to its S end. Inside, they return from S towards N, forming continuous loops.
Grip the coil with your right hand, curling your fingers in the conventional-current direction around its turns. Your thumb points towards its north end, along the field inside. Looking directly at one end, anticlockwise current makes that end N; clockwise current makes it S.
Match the winding current to the poles
Brown arrows show conventional current; teal arrows show magnetic field. This projected coil has N on the left and S on the right.
Side view: the near-side current rises
The field returns from S to N inside the coil. Outside, it runs from N to S. The paler wire sections are on the far side of the coil.
Look from the left end
Here the teal dot represents field out of the page. The brown circular arrows represent current around the turns.
This is a qualitative pattern. The turn spacing and the number of drawn field lines do not give a numerical field strength.
Reversing the current swaps the N and S ends and reverses the field throughout the pattern. Increasing the current magnitude strengthens the field when the coil geometry and core remain the same.
A suitable soft-iron core becomes magnetised in the coil's field and strengthens the magnetic effect. It loses much of this induced magnetism when the current is switched off. Its contribution depends on the magnetising field and material.
Investigate the field with a compass
- Use suitable low-voltage equipment. Pass a straight wire through a horizontal card and keep the wire vertical. Looking down at the card gives an end view of the wire.
- With current off, record the compass's background direction. With current on, allow a small compass to settle at several marked positions and draw arrows along its N end.
- Keep the wire, positions and viewpoint fixed. Reverse only the current and compare the new directions. Record the current direction as seen from your chosen view.
- Map a solenoid in the same way, recording its current direction and the compass directions near both ends and along the outside. Compare with the bar-magnet-like pattern.
Keep nearby magnets and magnetic objects away, and use a freely moving compass. The needle responds to the resultant field, including Earth's, so the measured pattern need not match the ideal wire or coil pattern where its field is weak.
To investigate current magnitude, keep the geometry, compass position and background field fixed and record the actual current. A reproducible change in compass deflection can show that the wire's contribution has changed. It does not by itself give a calibrated numerical field strength, and a compass already aligned almost fully with the wire's field may turn very little further.
Stay within the equipment's current ratings and switch off between readings where appropriate to limit heating. Heating or moving the apparatus would change more than the intended variable.
Optional check Viewed end-on, a straight wire initially carries current out of the page. The current is then reversed and increased in magnitude, with the wire unchanged. What happens to its magnetic field?
06
Magnetic force and its direction
A current-carrying conductor in an external magnetic field can experience a force. To predict its direction, identify the field and conventional current separately.
Across the gap between magnet poles, the field points from N to S. Conventional current is opposite to electron motion in a metal. A dot means out of the page and a cross means into it, for the quantity labelled beside the symbol.
Observe the force on a movable wire
Place a straight, movable conducting segment in the gap of a magnet and connect it to a suitable low-voltage d.c. supply, switch and flexible leads. Arrange the current perpendicular to the field and allow the segment to move in the predicted force direction.
Move a current-carrying wire in a magnetic gap
The upper view separates the wire's rear and front ends. Flexible leads allow its straight section to move. Brown arrows show current, teal arrows field, and purple arrows magnetic force.
Only the magnetic force is marked. Weight and support forces still act. The lower end view represents the same wire, not an extra circuit branch.
- With current off, record the segment's position. Gravity and its supports still act; the magnetic force due to current in the segment is absent.
- Close the switch briefly and observe the movement. In the given geometry, the magnetic force is upward.
- Switch off before changing connections. Reverse the current while leaving the magnet arrangement fixed, then switch on and compare: the magnetic force and resulting initial movement reverse.
- Restore the original current direction. Reverse the magnetic field by exchanging the pole positions, keeping the wire geometry comparable. The magnetic force reverses again.
Keep the support arrangement and current magnitude comparable when comparing directions. Current-induced heating can change wire sag or tension, so observe the controlled initial response and limit heating. A movement demonstrates the effect under these conditions; it is not a calibrated measurement of force magnitude.
Use Fleming's left-hand rule
Hold the thumb, first finger and second finger of your left hand mutually perpendicular:
- First finger: field
- Point it along the magnetic field, from N to S across the gap.
- Second finger: current
- Point it along conventional current in the active conductor.
- Thumb: force
- It gives the magnetic force on that conductor.
The right-hand grip rule finds the field produced by a current. Fleming's left-hand rule finds the relative directions of current, an external field and force. Choose the rule for the relationship you are using.
Keep the viewpoint; change one cause
The dot means current out of the page and the cross means current into it. Teal arrows follow the field from N to S. Purple arrows show force on the wire.
Start: field right, current out
Reverse the current only
Reverse the field only
Reverse both
Reversing either current or field reverses the force. Reversing both restores its original direction. No numerical force is represented by arrow length.
| Current in the wire | Gap field | Initial magnetic-force direction |
|---|---|---|
| Out of the page | Right | Up |
| Into the page | Right | Down |
| Out of the page | Left | Down |
| Into the page | Left | Up |
Compare the first and second rows to isolate current reversal. Compare the first and third to isolate field reversal. The last row changes both and returns to the original force direction.
Find a missing direction
The force is downward and the field is rightward
Put your thumb down and first finger right. The second finger points into the page, so that is the required conventional-current direction.
Alternatively, if current is out of the page and force is upward, those two directions require the field to point right. Any two known perpendicular directions can determine the third.
The examples use perpendicular current and field. If a straight segment's current is parallel or opposite to the field, the magnetic force on that segment is zero in this model. Do not force three non-perpendicular directions into the perpendicular-case hand rule.
Optional check In a fixed front view, a wire carries current out of the page in a magnetic field directed right. Its magnetic force is upward. What is the force direction if both the current and the field are reversed?
07
Why a current-carrying coil turns
Opposite sides of a current-carrying coil carry current in opposite directions. In a magnetic field, the forces on those sides can form a pair that turns the coil.
A moment is the turning effect of a force about a pivot or axis. It depends on the force and the perpendicular distance from the axis to the force's line of action. Use Fleming's left-hand rule to find each magnetic force.
Follow the current around one coil
The two active sides are parts of one conducting loop. Current runs along one side and back along the other, so their directions are opposite. View the coil along its axle, with the magnetic field directed right.
Two active sides belong to one connected coil
First trace the current. Then look from the near end along the axle to work out the two forces.
A carries current from far to near. B carries the same current from near to far. The opposite directions are two parts of one circuit.
End view: the forces make a clockwise turning pair
Green arrow: magnetic field. Brown dot/cross: current. Purple arrows: forces. Blue curve: turning sense. The force arrows use one qualitative scale.
Explain the rotation
Field right, left current out, right current in
- For the left side, rightward field and out-of-page current give upward force.
- For the right side, rightward field and into-page current give downward force.
- Upward force on the left gives a clockwise moment about the axle. Downward force on the right also gives a clockwise moment.
- The turning effects add, so the coil tends to turn clockwise in this view.
In the symmetric model, the forces are equal and opposite, so their resultant force is zero. Their lines of action are separated, however, so their moments do not cancel. Zero resultant force does not establish zero turning effect.
Reversing the current with the field fixed reverses both forces and the turning direction at this orientation. Reversing only the field has the same effect. Reversing both preserves the original turning direction.
As the coil turns, the perpendicular distances from the axle to the force lines change. At an orientation where those lines pass through the axle, there is no turning moment from them. Opposite forces produce a turning pair only when their lines of action have the required separation.
Optional check Viewed along its axle, a coil has its left active side carrying current out of the page and its right side carrying current into the page. The field is rightward. Which explanation is correct?
Revision summary
Magnet properties and observations
Like poles repel and unlike poles attract. An ordinary magnet has N and S poles; dividing it produces smaller magnets with both poles. A freely suspended magnet turns approximately north-south when nearby disturbances are small.
A magnetic material can become magnetised. An end repelling a known N pole is N in the simple end-to-end test. Attraction alone may involve an initially unmagnetised material, so it does not prove that the object was already a permanent magnet.
Induced magnetism and material choice
Near a strong magnet, the nearer end of an initially unmagnetised rod develops the opposite pole to the inducing pole. Inside a current-carrying solenoid, a suitable core becomes magnetised in the coil's field and strengthens the magnetic effect.
Soft iron is readily magnetised and has low retention, useful for a switched lifting magnet. Suitable steel is harder to magnetise but retains magnetism, useful for a permanent magnet or compass needle. Compare retention after removing the inducing field, using comparable specimens and a consistent test.
Find direction with a compass
The N end of a settled compass gives the local field direction. Fix the magnet and paper, mark the needle ends and N-end direction, move S to the previous N mark, and repeat. Join the marks smoothly with arrows and start again elsewhere.
Nearby magnetic objects, a stuck needle and Earth's field can affect the map. Iron filings reveal a rough pattern but do not supply the arrow directions.
Magnetic field patterns
- Outside a bar magnet, lines leave N and enter S; they return through it from S to N.
- Unlike facing poles have connecting lines across the gap from N to S.
- Like facing poles have lines bending away from the central gap region. Do not join N to N or S to S. Reverse the arrows for the corresponding S/S rather than N/N pattern.
- Lines do not cross. A compass is tangent to a local line, and closer spacing indicates a stronger field within a consistently drawn pattern.
Fields from currents
- Straight wire
- Field lines are circles centred on the wire. Right thumb follows conventional current; curled fingers give field direction. In a fixed end view, current out of the page gives anticlockwise field and current into it gives clockwise field.
- Solenoid
- Right-hand fingers follow current around the turns and the thumb points towards N. Anticlockwise current viewed from an end makes that end N. Field is approximately uniform in the central region of a long coil, returning inside from S to N and outside from N to S.
- Change the current
- Greater magnitude gives a stronger field with geometry and core fixed. Reversal reverses the wire's field and swaps a solenoid's poles. Do not confuse strength with direction.
In a compass investigation, fix viewpoint and positions, wait for settling, and compare one change at a time. Record actual current and account for background fields. Compass direction alone is not a calibrated numerical field-strength reading.
Force direction and controlled reversals
For mutually perpendicular directions, Fleming's left hand gives first finger = field, second finger = conventional current, thumb = magnetic force. Any two known directions determine the third.
- Field right, current out of the page: force up.
- Field right, current into the page: force down.
- Field left, current out of the page: force down.
- Field left, current into the page: force up.
In the movable-wire experiment, reverse current alone, then restore it and reverse field alone. Either reverses the magnetic force; reversing both restores it. Keep geometry and supports comparable and limit heating. Current off removes this current-dependent magnetic force, not gravity or support forces. Parallel current and field give no magnetic force in the model.
The coil's turning effect
Viewed along the axle with field right, the left active side carrying current out of the page is forced up; the right side carrying current into the page is forced down. Their separated lines of action give clockwise turning. Equal opposite forces can have zero resultant force and a nonzero combined moment.
Identify both force directions and their distances from the axle. If their lines pass through the axle, their moments about it are zero.
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