SP12 · Magnetism and the motor effect · Topic 12 — Magnetism and the motor effect | GCSE Edexcel Separate Sciences

Revise the key ideas

A permanent magnet produces its own lasting magnetic field. An induced magnet becomes magnetised in another field and can lose much of that magnetism when the field is removed.
Magnets have north-seeking and south-seeking poles. Like poles repel; unlike poles attract. An isolated ordinary north pole cannot be obtained by simply cutting a magnet in half: each piece has both poles.
The arrows show forces on the magnets; like poles repel.
Iron, nickel, cobalt and many alloys containing them can be magnetic. Steel can retain magnetism, making it suitable for permanent magnets; soft iron magnetises and demagnetises readily, suiting temporary electromagnet cores.
An initially unmagnetised magnetic material is attracted towards a magnet through induced magnetism. Aluminium, copper and plastic are not the usual magnetic materials in this GCSE model.
Uses include magnetic catches/locks, sorting iron-containing materials, motors and speakers. The choice of permanent or switchable magnet depends on the intended task.

A magnetic field is a region where magnets or magnetic materials experience magnetic forces. Field lines show the direction a north-seeking test pole would move.
Outside a bar magnet, field lines go from north to south. Lines form loops, continuing inside from south to north, and do not cross.
Field lines form loops and do not cross.
Closer field-line spacing represents a stronger field. Around a bar magnet, the field is strongest near the poles.
A uniform field is represented by parallel, equally spaced lines in one direction. It can be approximated in the gap between broad opposing poles.
Use a plotting compass to map a field: mark the direction of its north-seeking end at successive positions, then join marks with arrows. Move the compass systematically and keep the magnet fixed.
Iron filings show the field's pattern but not its direction by themselves. A compass provides directional information.
A compass turns to align with Earth’s field. Near geographic north, Earth has the magnetic character of a south pole, attracting a compass's north-seeking end.
Earth's field is approximately dipole-shaped and provides evidence of a magnetic interior; its origin is moving conducting material in the core, not a permanent solid bar magnet at the centre.

A current in a wire produces a magnetic field. A nearby compass deflects when the current is switched on; reversing current reverses the deflection.
Around a long straight wire, field lines are concentric circles centred on the wire. The field becomes weaker further from the wire and stronger for a larger current.
Use the right-hand grip rule: thumb points in conventional-current direction and curled fingers show the field direction. A dot means out of the page; a cross means into the page.
For current out of the page, the circular field is anticlockwise as viewed. For current into the page, the field is clockwise.
Right-hand thumb points towards you; fingers curl anticlockwise.
A solenoid is a coil of wire. Its individual turns combine to produce a strong nearly uniform field along its interior, with a weaker external pattern resembling a bar magnet.
Reversing current reverses the poles and field direction.
Increase solenoid field using greater current, more turns per unit length or a soft-iron core. The field can be switched on/off with current; reversing current reverses the poles.
Use low-voltage supplies, limit current and switch off when not measuring to avoid wire overheating. Compare electromagnets with a repeatable measure, such as the mass they can lift.

A current-carrying conductor in a magnetic field can experience a force due to interaction of the fields. The magnet experiences an equal and opposite force; these act on different objects.
The motor effect is this force on a current-carrying conductor. It can produce movement; reversing current or field reverses the force direction.
The force is greatest when current is perpendicular to the magnetic field and zero when parallel. Reversing both current and field leaves the force direction unchanged.
Fleming’s left-hand rule uses the thumb, first finger and second finger at right angles to each other. Thumb = force/motion; first finger = magnetic field from N to S; second finger = conventional current.
If field points right and current is out of the page, force is upwards. If current is into the page instead, force is downwards.
Fleming’s left-hand rule gives these three perpendicular directions.
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.

For a straight conductor at right angles to a uniform magnetic field, F = BIl. Force F is in N, flux density B in tesla (T), current I in A and length l in m.
Use only the length of wire actually inside the field. Convert centimetres to metres; the equation as written assumes perpendicular current and field.
For B = 0.5 T, I = 2 A and l = 0.1 m, F = 0.1 N. Increasing any one factor increases force proportionally if the others stay fixed.
l is the length within the magnetic field.
Rearrange to B = F/(Il), I = F/(Bl) or l = F/(BI). Magnetic flux density B measures how strong the magnetic field is in this relationship. It is measured in tesla, not newtons or joules.

(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. These separated forces create a turning effect.
(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. The rule does not use electron-flow direction.
(Higher tier) In a simple d.c. motor, a split-ring commutator reverses the current every half-turn so the turning effect continues in the same rotational direction. Brushes maintain electrical contact with the rotating commutator.
Current reversal every half-turn sustains the rotational direction.
(Higher tier) Increasing current, magnetic-field strength or suitable coil turns can increase the turning effect. Motor operation transfers electrical energy to kinetic energy with some dissipation; a motor is not a generator operating without input.
(Higher tier) Reversing only the current or only the field reverses rotation; reversing both leaves the force directions and rotation unchanged. At some coil orientations the turning effect is zero, and momentum carries a spinning coil through.