(c) Electromagnetism
- Syllabus
- 2024
- Topic
- —
- Level
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Learning objectives
6.8Current and magnetic fieldsKnow that an electric current in a conductor produces a magnetic field around it69P ElectromagnetsDescribe the construction of electromagnets610P Magnetic field patternsDraw magnetic field patterns for a straight wire, a flat circular coil and a solenoid when each is carrying a current611P Charged particles in magnetic fieldsKnow that there is a force on a charged particle when it moves in a magnetic field as long as its motion is not parallel to the field6.12Motor effect applicationsUnderstand why a force is exerted on a current-carrying wire in a magnetic field and how this effect is applied in simple d.c. electric motors and loudspeakers6.13Left-hand ruleUse the left-hand rule to predict the direction of the resulting force when a wire carries a current perpendicular to a magnetic field6.14Changing magnetic forceDescribe how the force on a current-carrying conductor in a magnetic field changes with the magnitude and direction of the field and current
Connect electric current to a magnetic field
An electric current in a conductor produces a magnetic field around the conductor. When the current stops, this current-produced field disappears.
A plotting compass placed near a wire changes direction when the wire is switched on because the compass needle aligns with the new magnetic field. This provides evidence for the field even though the field itself is not visible.
Reversing the current reverses the direction of the magnetic field. Increasing the current makes the field stronger, while the field becomes weaker farther from the conductor. The exact line patterns for different conductor shapes are developed next.
The wire does not need to be made from a magnetic material: moving charge—the current—creates the field. Do not confuse this effect with electromagnetic induction, where a changing magnetic field produces a voltage.
Construct a simple electromagnet
A simple electromagnet is a coil of insulated wire wound around a magnetically soft iron core and connected so that a direct current flows through the coil.
Construction: 1. Wind many turns of insulated wire in one direction around an iron rod. 2. Connect the wire ends to a d.c. supply, with a switch in series. 3. Close the switch so current flows through the coil and creates a magnetic field that magnetises the core.
Opening the switch stops the current, so the coil's field disappears and the soft iron loses most of its magnetism. This makes the device a controllable magnet rather than a permanently magnetised bar.
Use soft iron, not steel, when the magnet must switch off readily: steel is magnetically hard and retains magnetism. The iron core is not itself a permanent bar magnet, and a complete iron loop is not the simple core arrangement required here.
Draw fields around wires, coils and solenoids
Current shape controls magnetic-field shape. Draw continuous field lines with consistent arrows; the lines do not cross or begin and end on the wire.
| Current-carrying conductor | Magnetic-field pattern | Direction rule |
|---|---|---|
| straight wire | concentric circles centred on the wire, in planes perpendicular to it | point the right thumb along conventional current; curled fingers show field direction |
| flat circular coil | field lines from all parts combine into a nearly straight field through the centre, perpendicular to the coil's plane, and loop back outside | curl right-hand fingers with the current; thumb gives the central field direction and the coil's north face |
| solenoid | nearly straight, parallel, equally spaced lines inside; curved loops outside like a bar magnet | curl right-hand fingers with the coil current; thumb points to the north end and along the internal field |
Outside a coil or solenoid, field arrows leave its north end and return to its south end. Reversing the current reverses every field arrow and swaps the north and south ends, but the basic field-line shape remains the same.
A dot represents current coming out of the page and a cross represents current going into it. Apply the direction rule to the stated conventional current; do not use Fleming's left-hand rule here, because that predicts force rather than the field made by a current.
Recognise force on a moving charged particle
A charged particle moving through a magnetic field experiences a magnetic force whenever its motion is not parallel to the field.
| Particle motion relative to field | Magnetic force |
|---|---|
| parallel or antiparallel | zero |
| at an angle | non-zero |
| perpendicular | maximum for the same charge, speed and field strength |
The force acts at right angles to both the particle's velocity and the magnetic field. It therefore changes the direction of motion rather than directly changing the speed; for perpendicular motion in a uniform field, the continually turning force can produce a circular path.
The magnetic force does not start the particle moving: the charge must already be moving. A stationary charge has no magnetic force, and motion exactly along a field line also gives no magnetic force. Direction prediction is handled with the left-hand rule in the next steps.
Explain the motor effect in motors and loudspeakers
A current-carrying wire in an external magnetic field experiences a force because the wire's magnetic field interacts with the external field. This is the motor effect.
| Application | How the motor effect produces motion | How motion continues or changes |
|---|---|---|
| simple d.c. motor | opposite sides of a current-carrying coil experience forces in opposite directions, creating a turning effect | brushes maintain electrical contact; the split-ring commutator reverses current every half-turn so the turning effect stays in the same rotational direction |
| loudspeaker | current in a coil within a permanent magnet's field produces a force on the coil and attached cone | an a.c. signal reverses current and force repeatedly, so the cone vibrates and makes the surrounding air vibrate as sound |
The reusable chain is: current creates a field → the two fields interact → the conductor experiences a force → the attached component moves. In a motor the component rotates; in a loudspeaker it oscillates.
The force is not electromagnetic induction and is not explained as attraction between fixed north and south poles on the wire. In a d.c. motor, the commutator must reverse current in the coil every half-turn; without that reversal, the turning effect would not remain in one rotational direction.
Use Fleming's left-hand rule
Fleming's left-hand rule predicts the force direction when conventional current is perpendicular to a magnetic field. Hold the thumb, first finger and second finger of the left hand mutually perpendicular.
| Left-hand direction | Represents | Direction to use |
|---|---|---|
| first finger | magnetic field | from north to south |
| second finger | conventional current | from positive to negative through the conductor |
| thumb | force or motion | the direction the conductor is pushed |
Method: 1. Align the first finger with the field. 2. Rotate the hand while keeping that alignment until the second finger follows the current. 3. Read the thumb for the force. A dot means a direction out of the page; a cross means into the page.
For a positive moving particle, use its direction of motion as conventional current. For a negative particle, conventional current is opposite to the particle's motion. Do not swap in the right-hand grip rule: that rule gives the field around a current, not the motor-effect force.
Predict how magnetic force changes
For a conductor at right angles to a magnetic field, a larger current or a stronger magnetic field produces a larger magnetic force.
| Change made | Effect on force |
|---|---|
| increase current, same field | magnitude increases |
| increase field strength, same current | magnitude increases |
| reverse current only | direction reverses; magnitude is unchanged if current size is unchanged |
| reverse magnetic field only | direction reverses; magnitude is unchanged if field strength is unchanged |
| reverse both current and field | direction is unchanged because both directional inputs reverse |
Orientation also matters: the force is greatest when current and field are perpendicular and zero when they are parallel. Moving stronger magnet poles closer can increase field strength; increasing the supply voltage may increase current if the circuit resistance is unchanged.
Direction depends on directions, not magnitudes. A smaller current weakens the force but does not reverse it. Similarly, a physically larger magnet is not necessarily stronger; the relevant change is magnetic field strength at the conductor.