Electromagnetism

Whenever a current flows through a conductor, a MAGNETIC FIELD is produced around it.

Field around a straight wire — concentric circles centred on the wire. Direction given by the right-hand grip rule: grip the wire with your right hand, thumb pointing in the direction of CONVENTIONAL current; your fingers then curl in the direction of the field lines.

Field around a flat coil (single loop) — circular field lines that go THROUGH the coil in one direction inside it, loop around the wire and return on the outside. The coil acts like a tiny bar magnet with N and S faces.

Field around a solenoid (long coil with many turns) — strong, uniform field along the AXIS inside the coil; outside, the field looks like a bar magnet's (lines emerge from the N end, loop around and return to the S end). The solenoid's pole at each end is determined by the current direction (looking at the end, anticlockwise current → north pole).

These three field patterns are all required Paper 2 sketches (spec 6.10P).

An electromagnet consists of an insulated coil of wire wound around a soft iron core.

How it works. When current flows in the coil, the magnetic field it produces magnetises the soft iron, GREATLY increasing the overall field strength.

Why SOFT iron? Soft magnetic materials magnetise and DEMAGNETISE easily. Switch the current off, and the soft iron loses its magnetism almost instantly — so the electromagnet only attracts when you want it to.

Strengthening the field (spec 6.9P + 6.10P). Increase the current, increase the number of turns of wire, or use a softer / more permeable core material.

Uses. Electromagnetic cranes (lifting scrap iron in a scrapyard), magnetic door locks, relays (a small current activates an electromagnet which closes a switch handling a large current), electric bells, MRI scanners.

When a current-carrying wire is placed in a magnetic field that is NOT parallel to the current, the wire experiences a FORCE.

Fleming's left-hand rule (spec 6.13). Hold your LEFT hand with thumb, first finger and second finger mutually perpendicular:

• First finger = direction of magnetic Field (N → S).
• SeCond finger = direction of conventional Current (+ to −).
• ThuMb = direction of resulting Motion (force).

For an ELECTRON beam (negative charge), the conventional current is OPPOSITE to the electron flow — reverse the second finger or reverse the final force direction.

Factors affecting the force size (spec 6.14).

• Stronger magnetic field (B) → larger force.
• Larger current (I) → larger force.
• Longer wire in the field (L) → larger force.
• Angle. Force is MAXIMUM when wire and field are perpendicular; ZERO when wire and field are parallel.

Force direction reverses if:

• The current direction reverses, OR
• The field direction reverses,
• But NOT if BOTH reverse (two reversals cancel).

A DC motor converts electrical energy into rotational kinetic energy.

Construction.

• A rectangular coil of wire mounted on an axle, between the poles of two permanent magnets.
• A split-ring commutator mounted on the axle, in two halves with a gap between them.
• Carbon brushes press against the commutator and connect it to an external DC supply.

Operation.

1. Current flows through the coil. On the LEFT side, current direction is (say) into the page; on the RIGHT side, OUT of the page (because it's one continuous loop).
2. By Fleming's left-hand rule, the LEFT side experiences a DOWNWARD force; the RIGHT side experiences an UPWARD force.
3. The pair of opposite forces forms a COUPLE → the coil ROTATES.
4. After a quarter turn, the coil is vertical and would stop (the forces would be along the axle, not turning). The MOMENTUM carries it past this point.
5. As the coil rotates past vertical, the SPLIT-RING COMMUTATOR brings the two halves into contact with the OPPOSITE brushes → the current direction in the coil REVERSES → the force directions also reverse → the coil continues rotating in the SAME direction.

Increasing the motor's speed/turning effect (spec 6.14).

• Increase the current.
• Use stronger magnets.
• Use more turns of wire on the coil.
• Use a coil with a larger area (so longer wire in the field).

Loudspeaker. Uses the same force-on-current principle. A coil of wire is attached to a paper cone and sits between magnetic poles. An alternating audio current in the coil → alternating force → cone vibrates → sound waves at the audio frequency.

When a charged particle moves through a magnetic field with velocity NOT parallel to the field, it experiences a force.

Properties.

• The force is PERPENDICULAR to both the velocity AND the magnetic field (use FLHR; reverse for negative charges).
• The force does NO WORK on the particle — perpendicular force ⊥ motion → no displacement in the direction of the force → no energy transfer.
• The particle's SPEED stays constant; only its DIRECTION changes.

Resulting motion. If the field is uniform and perpendicular to the velocity:

• The force acts as a CENTRIPETAL FORCE.
• The particle moves in a CIRCLE.
• A larger field, slower particle, larger charge or smaller mass gives a SMALLER radius.

If the velocity has a component PARALLEL to the field, that component is unaffected (no force) → the particle spirals (helical path).

If the velocity is exactly PARALLEL to the field, there's NO force at all (spec 6.11P explicitly notes this).

Edexcel applications. Cathode-ray tubes (deflecting electron beams), mass spectrometers, particle accelerators, the aurora (charged particles spiral along Earth's field lines).