9.2 Electromagnets & Hand Rules

9.2 Electromagnets & Hand Rules

Quick Answer: An electromagnet is a coil (often on a soft-iron core) energised by current. Field strength ≈ ampere-turns × core effect. Right-hand clasp rule: for a straight wire, thumb = conventional current, fingers = field circles; for a solenoid, fingers = current in turns, thumb = N pole. Reverse current → reverse poles. Fleming rules preview motor (left) and generator (right) force/EMF directions.

Topic 3.10 moves from permanent magnetism to electromagnetism: magnetism produced by electric current. Almost every aircraft electrical device that must pull, hold, or switch with controllable force — relays, solenoids, contactors, clutches, some valves — uses electromagnet principles.

Electromagnet Construction

A basic electromagnet has three ingredients:

Soft core preference. Soft iron or similar soft magnetic material magnetises strongly when current flows and largely demagnetises when current stops — ideal for relays and contactors that must release. A hard core would retain magnetism and might stick (residual hold).

Operation

1. Close the circuit → current I flows in N turns.
2. Magnetomotive force (ampere-turns, detailed in §9.3) establishes field strength H in the magnetic circuit.
3. The core’s high permeability produces large flux density B for a given H (until saturation).
4. External poles appear at the core ends (or armature air-gap faces).
5. Open the circuit → current falls to zero → soft core flux collapses → armature releases (ideally).

Factors affecting strength (qualitative Module 3 list):

Worked qualitative example. A relay coil rated for 28 V DC draws a fixed current set by coil resistance. Doubling turns while keeping the same wire size and voltage is a design change (resistance also rises); conceptually Module 3 wants: more ampere-turns → stronger magnet, and soft core → releases when de-energised.

Polarity. The end that behaves as N or S depends on the sense of current around the coil. Reverse battery polarity → poles swap. That is why DC coil markings and diode suppression orientation matter in practice, and why hand rules matter on the exam.

Magnetic Field Around a Current-Carrying Conductor

A straight conductor carrying current is surrounded by circular magnetic field lines centred on the wire.

Right-hand clasp rule — straight conductor

1. Point the thumb of the right hand in the direction of conventional current.
2. Fingers curl around the wire in the direction of the magnetic field.

End-view memory aids often drawn in textbooks:

• Current out of the page (dot): field circles anticlockwise (right-hand rule).
• Current into the page (cross): field circles clockwise.

Worked example. A vertical wire carries conventional current upward. Standing so you look with current going away from you into the distance is awkward — instead apply the rule directly: thumb up, fingers curl — that curl is the field direction around the wire. If the exam shows a cross-section with a cross (into page), field is clockwise.

Solenoid / Coil Hand Rule

A solenoid is a helical coil. Along its axis the fields of many turns add, producing a strong interior field similar to a bar magnet when energised.

Right-hand clasp rule — solenoid

1. Curl the fingers of the right hand in the direction of conventional current around the turns.
2. The thumb then points toward the north pole end of the solenoid.

Worked polarity example. Viewing a coil end, current runs clockwise around that face → that face is a south pole (because fingers clockwise imply thumb away from you into the coil — north at the far end, south nearest you). Counter-clockwise current on the near face → that face is north. Many CAAS-style stems show a coil sketch and ask which end is N.

Electromagnets in Aircraft Electrical Practice

Failure modes linked to this section’s theory: open coil (no magnetism), shorted turns (weak pull / overheating), residual magnetism in a “soft” assembly that is actually contaminated or wrong material (sticky relay), excessive air gap (weak pull).

Fleming Context Preview (Motor & Generator)

Module 3 later formalises interaction of current, field, and motion. Two classic Fleming rules use the left or right hand with thumb, first finger, and second finger mutually perpendicular:

Memory bridge from clasp rules to Fleming:

• Clasp / grip rules (this section) → direction of B around I, or poles of a coil.
• Fleming left → which way a current-carrying conductor is pushed in a field (motor action).
• Fleming right → which way induced current/EMF goes when a conductor moves in a field (generator action).

Do not confuse the right-hand clasp (field around a wire / solenoid north) with Fleming’s right-hand generator rule — same hand, different job. Exam stems that say “grip rule” or “clasp rule” want circular-field or solenoid-pole answers; stems that say “Fleming” want the three-finger motor/generator set.

Preview worked thought. Conductor in a uniform B-field carries current; left-hand rule gives force direction — that force is why DC motors rotate (Chapter on DC machines). Move a conductor across B; right-hand rule gives induced current sense — that is generator / Faraday territory (Inductance chapter).

Comparison Table: Permanent Magnet vs Electromagnet

Exam Focus Checklist

1. Name construction: coil + soft core + controllable current.
2. Apply right-hand clasp to a straight wire (thumb = I, fingers = B).
3. Apply right-hand clasp to a solenoid (fingers = I in turns, thumb = N).
4. State that reversing current reverses poles / field direction.
5. Distinguish clasp rules from Fleming left (motor) and right (generator) previews.

With construction and hand rules secure, §9.3 attaches the quantitative magnetic circuit language — MMF, H, B, μ, hysteresis, reluctance, saturation, and eddy currents — that Module 3 uses in calculations and loss explanations.