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Magnetism and electromagnetism

Key idea: Magnet properties, fields from currents, the motor effect, Fleming’s left-hand rule and the turning effect on a coil.

  • SEC G3 Combined Science Physics component 2027
Syllabus and review details

For this course, learn why a current-carrying coil turns in a magnetic field. You do not need to memorise the structure or component functions of an electric motor.

Work through the four stages in order. For each check, draw or write your decision and its physical reason before opening the feedback. Keep field direction, current direction and force direction separate.

Use repulsion as the sure test for a magnet

Every magnet has north and south poles. Unlike poles attract; like poles repel. Attraction alone is not proof that an object is a magnet because an unmagnetised magnetic material can also be attracted. Repulsion between known and unknown poles proves that both objects are magnets.

Magnetic interactions and the sure testThree comparisons show unlike poles attracting, like poles repelling, and a magnet attracting an unmagnetised iron bar. A final panel shows that cutting a bar magnet produces two smaller magnets, each with north and south poles.Read the facing ends firstUnlike polesNSattractForce is towards the gap.Like polesNNrepelForce is away from the gap.Magnet + ironNironalso attractsInduced magnetism makes thenear end an opposite pole.Cutting does not isolate one poleNScutN SN Seach piece has both polesSure-test conclusionUse the same known pole atboth ends of the unknown bar.Repulsion at either end:unknown is a magnetAttraction at both ends:could be unmagnetised iron
Scroll diagram horizontally to read all labels.
Unlike poles attract and like poles repel. Attraction is inconclusive because an unmagnetised iron bar is also attracted; observed repulsion with a known magnet proves the unknown is a magnet.

Worked inference: attraction leaves two explanations

A known north pole attracts end A of an unknown bar. A could be a south pole of a permanent magnet, or an opposite pole induced in initially unmagnetised magnetic material. Attraction does not choose between those explanations. Repulsion from a known pole would establish that the tested end is itself a like magnetic pole.

Now decide: the same known north pole repels end B. What pole is B, and what has this extra observation established? Explain before checking.

Check the repulsion inference

B is north. Like poles repel, so this observation establishes that the unknown bar is a magnet. If you chose south because “a magnet attracts”, separate the two possible interactions: unlike poles attract, while like poles repel.

Recall
Breaking a bar magnet does not isolate one pole. Each piece becomes a smaller magnet with both a north and a south pole.

A magnetic field can induce poles in a material

Put iron near a strong magnet or inside a current-carrying solenoid. The field aligns magnetic regions in the iron, and the end nearest a magnet pole becomes the opposite pole. This explains attraction before the iron was a permanent magnet.

Magnetically soft material: soft iron

Magnetises readily and loses most magnetism when the field is removed. Use it where magnetism must switch on and off.

Magnetically hard material: suitable steel

Is harder to magnetise but retains magnetism. Use it for a compass needle or permanent bar magnet.

“Soft” and “hard” here describe how readily a material gains and retains magnetisation, not whether it feels soft or resists scratching. For an electromagnet that should release its load when switched off, low retained magnetisation is useful. For a compass needle, retaining magnetisation is useful.

Independently consider an initially unmagnetised soft-iron bar placed near a permanent magnet's south pole. Name its nearer induced pole and explain the attraction. Then choose between soft iron and a magnetically hard material for a needle that must retain its magnetism after the magnetising field is removed.

Check induction and material choice

The nearer end becomes north, so it attracts the known south pole. Induction does not establish that the bar will remain a permanent magnet after the field is removed. Choose the magnetically hard material for the needle because it retains magnetisation; the soft-iron bar is suited to magnetism that must switch off readily.

Field arrows show the direction a north pole would move

Bar-magnet field direction and plotting compassField lines curve from the north pole to the south pole outside a bar magnet. A compass on one field line is aligned tangentially, and a three-step plotting method is shown.Field direction around a bar magnetNSOutside: N → SArrows show direction.lines are densernear the polesNN end followsthe tangentPlot one line:1. mark both needle ends2. move S end to the previous N mark3. repeat and join smoothlyarrow follows the north-seeking end
Scroll diagram horizontally to read all labels.
Outside a bar magnet, field lines run from N to S. A plotting compass is tangent to the local field line, with its north-seeking end pointing along the field.

Outside a bar magnet, field lines point from north to south. A plotting compass aligns with the local field: mark the direction of its north-seeking end, move it a short distance and repeat. Join the marks into smooth directed lines.

  • Lines are closest where the field is strongest, normally near the poles.
  • Lines never cross because the field has only one direction at a point.
  • Between unlike poles, lines join across the gap; between like poles, they bend apart.
Magnetic fields between unlike and like polesThe left panel shows directed field lines joining a north pole to a facing south pole. The right panel shows field lines from two facing north poles bending away from the centre, leaving a weak-field point midway between equal magnets.A. Unlike poles attractSNSNLines join across the central gap.B. Like poles repelSNNSweak pointLines bend apart; they do not cross.
Scroll diagram horizontally to read all labels.
Between unlike poles, field lines join across the gap from N to S. Between like poles, lines bend away and never cross. Arrowheads always show the direction a north pole would move.

Field lines are a model of direction and relative strength, not physical wires in space. Compare their spacing within a consistently drawn pattern; simply adding more lines to a sketch does not change the physical field.

Read a direction, then repair a pattern

In the single-bar figure, N is on the left and S on the right. Following an outside line across the top therefore takes you from left to right. A compass's north-seeking end follows the local tangent, not a straight arrow aimed at a pole.

Copy the lower outside arc of that pattern but leave its arrowhead blank. Add the missing direction. A second copy shows two lines crossing: explain why that is not a valid field pattern before checking.

Check the missing arrow and crossing

The lower arc also runs from N to S, so its arrow points left to right near the midpoint. Crossing would assign two different field directions to the same point. Redraw smooth non-crossing lines; do not treat the crossing as a wire junction.

For independent practice, use the facing-north-poles panel. At the exact midpoint between equal magnets, explain why no unique compass direction is established by the two equal opposing fields. Then sketch how nearby lines bend away from that point, with arrows leaving both north poles.

Check the midpoint and nearby pattern

At that ideal midpoint the two contributions cancel, so the resultant field is zero and has no defined direction. Nearby lines bend away without crossing. A compass at the midpoint could respond to other fields; the two magnets alone do not give it a unique direction there.

Drawing guidance
Draw several smooth lines, add arrowheads from north to south outside each magnet, and use line spacing to show relative field strength. Field lines must never cross.

Current produces a magnetic field

Current-produced fields and the motor effectThree panels show an anticlockwise field around current out of the page; a longitudinal section through a solenoid, with current out of the page in the upper parts of every turn and into the page in the lower parts, giving internal field to the right and a north pole at the right end; and the motor-effect directions for field right, current out of the page and force up.Straight wireSolenoidMotor effectcurrent out of pagethumb points out; fingers curlanticlockwise around the wireLarger current → stronger field.Further away → weaker field.Section through the coil axisupper parts: I out of pageSNlower parts: I into pageinside field: S → NEach upper/lower pair belongsto the same coil turn.Curl fingers with current;thumb points right, to N.Return field outside not shown.B: N → SBIout of pageFupFleming’s left handfirst finger → field Bsecond finger → current Ithumb → force FReverse B or I → F reverses.Reverse both → F is unchanged.
Scroll diagram horizontally to read all labels.
Use the right-hand grip rule for fields made by currents; use Fleming’s left-hand rule for the force on a current in an external magnetic field.

Straight wire

Field lines are concentric circles centred on the wire. Use a right-hand grip: thumb in conventional-current direction, curled fingers in field direction.

Solenoid

The field resembles a bar magnet, with a strong, nearly uniform field inside. Curl fingers with conventional current around the turns; the thumb points to the solenoid’s north pole.

Increasing current strengthens either field. Reversing current reverses the circular field direction and swaps a solenoid’s north and south poles.

A dot represents conventional current towards you, like an approaching arrow tip; a cross represents current away from you, like its tail. In the solenoid panel, you are looking at a section through the coil axis. Upper and lower current symbols belong to the same turns, so the winding direction is specified.

Apply the grip rule to a changed current

Worked decision: point your right thumb out of the page for the straight-wire dot. Your fingers curl anticlockwise. Now redraw the wire with a cross instead of a dot. Predict its field direction, then explain what doubling the current magnitude changes while keeping the direction fixed.

Check direction and strength separately

With current into the page, the field is clockwise. Doubling current magnitude strengthens the field at the same position; it does not reverse the arrows. Do not confuse a direction reversal with a change in strength.

Independently reverse every current symbol in the solenoid section while retaining the viewing direction. Mark both new poles and the new internal field direction before checking.

Check the reversed solenoid

The right-hand grip now points left: the left end is N, the right end S, and the internal field points left. Reversing equal-magnitude current changes polarity, not the field-strength comparison for otherwise unchanged conditions.

Compare one change at a time

The following fictional controlled records use coils of the same length and diameter. The sensor stays at the same position and orientation; the listed current is maintained in every trial. The sensor reading is on one common relative scale.

Supplied electromagnet comparison records
TrialCurrent / ATurnsCoreRelative reading
A1.0100Air2
B2.0100Air4
C1.0200Air4
D1.0100Soft iron10

A and B isolate the effect of current: turns and core are unchanged. Which comparisons isolate turns and core? Explain why comparing B with D cannot identify the effect of just one change.

Check the controlled comparisons

A–C isolates turns; A–D isolates core. Each produces a greater reading in these records. B–D changes both current and core, so its difference cannot be assigned to either one alone. These supplied readings are not a universal numerical rule for every electromagnet.

Test the force on a current-carrying conductor

  1. Place a straight conductor at right angles to a uniform magnetic field.
  2. Pass current through it and observe the sideways motion or force.
  3. Reverse only the current: the force reverses.
  4. Restore the current and reverse only the field: the force reverses again.

Current and field interact to produce the motor-effect force. If both directions are reversed together, the force direction stays unchanged. The conductor must run across the field for this force to occur: a straight current-carrying section parallel to the field has no magnetic force from it. Increasing current cannot fix that parallel arrangement; turn the test section across the field, with the return leads outside the field.

Use Fleming’s left-hand rule with conventional current

Hold the left thumb, first finger and second finger mutually perpendicular. First finger points along the magnetic field from north to south; second finger points along conventional current; thumb gives theforce.

Important
Use conventional current, not electron flow. The rule applies when field and current are perpendicular; reverse either one to reverse the force.

Worked direction check

A field points left to right and conventional current points upward. Set the first finger right and second finger up: the thumb points into the page, so the conductor’s force is into the page.

Guided check: keep that rightward field, but put conventional current into the page instead of upward. Determine the force, then reverse only the field. Write both directions before opening the answer.

Check the perpendicular directions

Into-page current in a rightward field gives a downward force. Reversing only the field makes the force upward. If you drew force along the field, reset your three mutually perpendicular fingers: field, current and force are different directions.

Opposite forces on a coil form a turning effect

Opposite forces turn a current-carrying coilEnd view along the rotation axis of a rectangular coil, between north on the left and south on the right. The active sides run perpendicular to the page and appear as two circular cross-sections. Current is out of the page on the left and into the page on the right. The field is rightward, so the left force is upward and the right force downward, producing clockwise torque in this view. The two end connections overlap as one horizontal line in projection.End view along the coil’s rotation axisActive sides are perpendicular to the page; dot = current out, cross = current in.NSfield B →rotation axisF upF downclockwiseI out of pageI into pageReverse current OR field: both forces and the turning direction reverse.Reverse BOTH: directions stay unchanged for the same magnitudes.
Scroll diagram horizontally to read all labels.
At the orientation shown, the separated active sides carry opposite currents and experience opposite forces. Viewed along the rotation axis, they form a clockwise couple. The connecting ends lie at different depths and overlap in this end view; the horizontal line is their projection.

Current runs in opposite directions along the two active sides of a coil. In the same magnetic field, Fleming’s rule gives opposite forces on those sides. The separated forces form a couple, so the coil turns.

Note
Focus on the cause-and-effect chain: opposite currents in one magnetic field give opposite forces on separated sides, so the coil experiences a turning effect.

Model a complete explanation

“The current is in opposite directions on the two active sides of the coil. In the same magnetic field, the sides experience forces in opposite directions. Because the forces act on different sides of the coil, they form a couple and turn the coil.”

Independently use the end view in the figure. Keep the current directions and reverse only the external field. Draw both forces and give the turning direction in this same view. Then reverse both current and field instead.

Check both sides before naming the turn

Reversing only the field makes the left force downward and the right force upward, giving anticlockwise turning in this end view. Reversing both current and field restores the original left-up/right-down forces and clockwise turning. There is no extra “motor force”: the two current–field forces already explain the couple.

Common mistakes

“Attraction proves it is a magnet”

Remember: induced magnetism also attracts; repulsion is the sure test.

“Field lines can cross”

Remember: crossing would assign two field directions to one point.

“Reversing current weakens the field”

Remember: reversal changes direction; magnitude controls strength.

“The force follows the field”

Remember: motor-effect force is perpendicular to field and current.

Check your understanding

Guided direction check

In the motor-effect panel above, the field points left to right and conventional current is changed to point into the page. State the force direction. Then reverse only the field.

Check the reasoning

Fleming’s left-hand rule gives a downward force at first. Reversing only the field reverses the force, so it becomes upward. Its magnitude is unchanged if current magnitude and field strength are unchanged.

Practise this independently

  1. An unknown bar attracts the north pole of a known magnet. Explain why this is not proof that the bar is a magnet, and state the observation that would prove it.
  2. Sketch the field around one bar magnet and between two facing north poles. Add arrowheads and explain what the line spacing means.
  3. The current in a solenoid is reversed without changing its size. State what happens to its poles, field direction and field strength.
  4. A wire in a magnetic field experiences an upward force. Predict what happens when both the current and field are reversed together.
  5. Explain fully why a current-carrying coil turns in a magnetic field.
Compare with the model answers
  1. Attraction can occur because the known magnet induces an opposite pole in an unmagnetised magnetic material. Repulsion between the same known pole and one end of the unknown bar would prove that the unknown is a magnet.
  2. Outside one magnet, lines curve from N to S. Between facing north poles, they bend away from the centre and do not cross. Closer lines represent a stronger field.
  3. The north and south poles swap and every field arrow reverses. The field strength stays the same because the current magnitude is unchanged.
  4. The force stays upward. Reversing either current or field reverses the force; reversing both reverses it twice.
  5. Current flows in opposite directions on the two active sides. In the same field, these sides experience opposite forces. The separated forces form a couple, producing a turning effect on the coil.

Try this next

Close the answers and redraw the three field patterns from memory. For direction questions, label field, conventional current and force before applying a hand rule; then change only the quantity named in the question.

Practise this topic

The topic check covers magnets, fields from currents, the motor effect and coil turning. Use the feedback to revisit the matching explanation before trying again later.

Magnetism and Electromagnetism check

Use the check to guide revision; also practise drawing field patterns and writing complete explanations.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Combined Science Physics component
Edition
SEC G3 Combined Science Physics component 2027