7.4 Magnetism, Electromagnetism, Motors, and Generators

7.4 Magnetism, Electromagnetism, Motors, and Generators

Core Principle: Roughly a third of Electronics Information items are not circuit-arithmetic problems at all — they ask about magnetism and the machines built from it. Starters, alternators, relays, solenoids, transformers, speakers, and every electric motor in a military vehicle work because current and magnetism generate each other. At about 40 seconds per item, these questions are pure recognition: you either know that a generator uses relative motion or you do not.

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1. Permanent Magnets and Magnetic Flux

A magnet is a material that produces a magnetic field, and the region of influence around it is described by magnetic flux ($\Phi$), measured in webers (Wb). Flux density ($B$) is measured in teslas (T).

The five flux rules the ASVAB tests:

1. Flux lines exit the north pole and enter the south pole outside the magnet, then continue south to north inside the magnet — every line is a closed loop.
2. Flux lines never cross one another.
3. Flux lines repel each other laterally but behave as though under tension along their length, which is why they try to shorten.
4. Like poles repel; unlike poles attract. The force falls off rapidly with distance.
5. Flux takes the path of least reluctance, which is why an iron core dramatically strengthens an electromagnet — iron has far lower reluctance than air.

Classes of magnetic material

Retentivity is a material's ability to keep its magnetism after the magnetising force is removed. Soft iron has low retentivity — ideal for an electromagnet core that must switch off cleanly. Hardened steel and alnico have high retentivity — ideal for permanent magnets. A relay armature uses soft iron precisely so the contacts release the instant coil current stops.

Curie point: heating a ferromagnetic material past its Curie temperature destroys its magnetism. Hammering or dropping a permanent magnet also degrades it by randomising domain alignment.

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2. Electromagnetism: Current Creates a Magnetic Field

Every current-carrying conductor is surrounded by a circular magnetic field. This is the single most important fact in this section, because everything else is built on it.

        CURRENT IN A STRAIGHT CONDUCTOR
                  |  I (conventional current, into page = X)
        .-------. | .-------.
      /           X           \        Concentric circular flux
      \           |           /        rings surround the wire
        '-------' | '-------'
            Field strength falls off with distance from the conductor

• Left-hand rule for conductors (electron flow): grasp the wire with your left hand, thumb pointing in the direction of electron flow (negative to positive); your fingers curl in the direction of the flux.
• Right-hand rule (conventional current): the same geometry using your right hand and conventional current (positive to negative). Both give the same physical field — they differ only in which current convention you were taught. If an ASVAB item specifies "conventional current," use the right hand.

Building an electromagnet

Wind that conductor into a coil (solenoid) and the individual circular fields add together down the middle of the coil, producing a concentrated field with a definite north and south pole. Insert a soft-iron core and the field strengthens enormously because iron offers far less reluctance than air.

Magnetomotive force is measured in ampere-turns:

$\text{mmf} = N \times I$

where $N$ is the number of turns and $I$ is the current in amperes. The practical consequences are worth memorising because they are exactly what the exam asks:

• Doubling the current doubles the ampere-turns.
• Doubling the number of turns doubles the ampere-turns.
• A 500-turn coil at 2 A (1,000 ampere-turns) and a 1,000-turn coil at 1 A (1,000 ampere-turns) produce the same magnetomotive force.
• Reversing the current reverses the poles. This is why a DC electromagnet's polarity flips when you swap the battery leads.

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3. Induction: Magnetism Creates Current

Faraday's law of induction: a voltage is induced in a conductor whenever there is relative motion between the conductor and a magnetic field — or, equivalently, whenever the flux linking a coil changes. Three factors set the magnitude of the induced voltage:

1. the strength of the magnetic field,
2. the number of turns in the coil,
3. the rate at which the flux changes (faster motion, higher voltage).

The word that matters is change. A stationary conductor sitting inside a steady magnetic field has zero induced voltage no matter how strong the field is. This is the most common EI trap on induction items.

Lenz's law: the induced current always flows in the direction that opposes the change that created it. That opposition is why a generator gets physically harder to turn when you connect a load, and why an inductor resists sudden changes in current.

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4. Motors and Generators: The Same Machine, Run Backwards

Motor action: a current-carrying conductor placed in an external magnetic field experiences a force perpendicular to both the current and the field. Loop that conductor and the two sides feel opposite forces, producing a couple — that is torque.

The commutator problem. In a DC motor the current in the loop must reverse every half-turn or the loop would simply oscillate and stop. Two solutions appear on the exam:

• DC machines use a split-ring commutator and brushes to mechanically reverse the connection twice per revolution.
• AC machines use slip rings (continuous rings, no reversal) because the supply is already alternating.

Recognising the pair in a vehicle. The automotive starter is a motor: battery current in, cranking torque out. The alternator is a generator: belt-driven rotation in, three-phase AC out, then rectified to DC by a diode bridge. Both are wrapped around the same physics.

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5. Electromagnetic Devices You Will Be Asked to Identify

The DC transformer trap

A transformer requires a changing flux, which requires a changing primary current. Connect a transformer primary to a steady DC source and the secondary produces nothing once the initial switch-on transient dies away — and the primary, seeing only its DC resistance, will likely overheat. This is a favourite EI distractor. Automotive ignition coils get around it by interrupting the DC with points or an ignition module, which is what creates the changing flux that fires the plug.