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Welcome to GCSE Edexcel Science revision.

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Unit S P 12: Magnetism and the motor effect.

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A permanent magnet produces its own lasting magnetic field.

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An induced magnet becomes magnetised in another field and can lose much of that magnetism when the field is removed.

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Magnets have north-seeking and south-seeking poles.

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Like poles repel; unlike poles attract.

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An isolated ordinary north pole cannot be obtained by simply cutting a magnet in half: each piece has both poles.

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The arrows show forces on the magnets; like poles repel.

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Iron, nickel, cobalt and many alloys containing them can be magnetic.

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Steel can retain magnetism, making it suitable for permanent magnets; soft iron magnetises and demagnetises readily, suiting temporary electromagnet cores.

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An initially unmagnetised magnetic material is attracted towards a magnet through induced magnetism.

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Aluminium, copper and plastic are not the usual magnetic materials in this GCSE model.

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Uses include magnetic catches or locks, sorting iron-containing materials, motors and speakers.

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The choice of permanent or switchable magnet depends on the intended task.

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A magnetic field is a region where magnets or magnetic materials experience magnetic forces.

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Field lines show the direction a north-seeking test pole would move.

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Outside a bar magnet, field lines go from north to south.

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Lines form loops, continuing inside from south to north, and do not cross.

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Field lines form loops and do not cross.

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Closer field-line spacing represents a stronger field.

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Around a bar magnet, the field is strongest near the poles.

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A uniform field is represented by parallel, equally spaced lines in one direction.

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It can be approximated in the gap between broad opposing poles.

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Use a plotting compass to map a field: mark the direction of its north-seeking end at successive positions, then join marks with arrows.

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Move the compass systematically and keep the magnet fixed.

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Iron filings show the field's pattern but not its direction by themselves.

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A compass provides directional information.

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A compass turns to align with Earth’s field.

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Near geographic north, Earth has the magnetic character of a south pole, attracting a compass's north-seeking end.

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Earth's field is approximately dipole-shaped and provides evidence of a magnetic interior;

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its origin is moving conducting material in the core,

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not a permanent solid bar magnet at the centre.

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A current in a wire produces a magnetic field.

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A nearby compass deflects when the current is switched on; reversing current reverses the deflection.

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Around a long straight wire, field lines are concentric circles centred on the wire.

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The field becomes weaker further from the wire and stronger for a larger current.

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Use the right-hand grip rule: thumb points in conventional-current direction and curled fingers show the field direction.

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A dot means out of the page; a cross means into the page.

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For current out of the page, the circular field is anticlockwise as viewed.

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For current into the page, the field is clockwise.

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Right-hand thumb points towards you; fingers curl anticlockwise.

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A solenoid is a coil of wire.

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Its individual turns combine to produce a strong nearly uniform field along its interior, with a weaker external pattern resembling a bar magnet.

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Reversing current reverses the poles and field direction.

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Increase solenoid field using greater current, more turns per unit length or a soft-iron core.

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The field can be switched on or off with current; reversing current reverses the poles.

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Use low-voltage supplies, limit current and switch off when not measuring to avoid wire overheating.

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Compare electromagnets with a repeatable measure, such as the mass they can lift.

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A current-carrying conductor in a magnetic field can experience a force due to interaction of the fields.

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The magnet experiences an equal and opposite force; these act on different objects.

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The motor effect is this force on a current-carrying conductor.

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It can produce movement; reversing current or field reverses the force direction.

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The force is greatest when current is perpendicular to the magnetic field and zero when parallel.

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Reversing both current and field leaves the force direction unchanged.

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Fleming’s left-hand rule uses the thumb, first finger and second finger at right angles to each other.

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Thumb indicates force or motion; first finger indicates magnetic field from north to south; second finger indicates conventional current.

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If field points right and current is out of the page, force is upwards.

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If current is into the page instead, force is downwards.

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Fleming’s left-hand rule gives these three perpendicular directions.

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Do not confuse the two rules: right-hand grip finds a wire's own field; Fleming’s left hand finds the motor-effect force when current and external field are known.

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For a straight conductor at right angles to a uniform magnetic field, F equals B times I times length L.

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Force F is in newtons, flux density B in tesla, current I in amperes and length L in metres.

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Use only the length of wire actually inside the field.

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Convert centimetres to metres; the equation as written assumes perpendicular current and field.

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For B equals zero point five tesla, I equals two amperes and length L equals zero point one metres, F equals zero point one newtons.

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Increasing any one factor increases force proportionally if the others stay fixed.

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Length L is the length within the magnetic field.

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Rearrange to B equals F divided by the product I times length L; I equals F divided by the product B times length L; or length L equals F divided by the product B times I.

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Magnetic flux density B measures how strong the magnetic field is in this relationship.

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It is measured in tesla, not newtons or joules.

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(Higher tier) A current-carrying coil in a magnetic field has forces on opposite sides in opposite directions because the currents there run opposite ways.

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These separated forces create a turning effect.

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(Higher tier) Use Fleming’s left-hand rule to link field, conventional current and force: first finger for field, second finger for current and thumb for force.

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The rule does not use electron-flow direction.

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(Higher tier) In a simple direct current motor, a split-ring commutator reverses the current every half-turn so the turning effect continues in the same rotational direction.

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Brushes maintain electrical contact with the rotating commutator.

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Current reversal every half-turn sustains the rotational direction.

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(Higher tier) Increasing current, magnetic-field strength or suitable coil turns can increase the turning effect.

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Motor operation transfers electrical energy to kinetic energy with some dissipation; a motor is not a generator operating without input.

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(Higher tier) Reversing only the current or only the field reverses rotation; reversing both leaves the force directions and rotation unchanged.

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At some coil orientations the turning effect is zero, and momentum carries a spinning coil through.

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That completes Magnetism and the motor effect.

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Revisit the notes and test yourself on the revision website.