Magnetism and Electromagnetism: Practice Questions & Explanations

Electromagnetic induction (discovered by Michael Faraday, 1831): a changing magnetic field through or near a conductor INDUCES a voltage in the conductor. If the conductor is part of a circuit, this induced voltage drives a current. Faraday's Law: EMF = -N × dΦ/dt, where N is number of turns, Φ is magnetic flux, and t is time. Negative sign reflects Lenz's Law: induced current opposes the change causing it. Key principle: the magnetic field must be CHANGING (relative to the conductor) for induction. Three ways to create a changing field: (1) Move the conductor through a stationary field; (2) Keep the conductor still, move the field source; (3) Change the field strength while conductor is stationary (e.g., AC current in a nearby coil). Applications: (1) GENERATORS — rotate a coil through a magnetic field (or rotate a magnet near a coil); produces AC; the foundation of nearly all electricity generation (coal, gas, hydro, nuclear, wind — they all spin a generator); (2) TRANSFORMERS — AC in primary creates changing field; secondary coil sees changing field; voltage induced; (3) ELECTRIC MOTORS — opposite of generators (apply electrical energy to produce rotation); current in coil within magnetic field experiences force; (4) WIRELESS CHARGING — varying magnetic field induces voltage in coil of receiving device; (5) GUITAR PICKUPS — vibrating steel guitar string in magnetic field of pickup coil induces signal in coil; (6) METAL DETECTORS — coils create field; metal disturbs field; second coil detects disturbance; (7) INDUCTION COOKTOPS — varying magnetic field induces current in iron/steel pan, heating it directly (the pan is the conductor); (8) MAGNETIC TAPE/HARD DRIVE READING — moving magnetized surface past a read head induces signals. Magnetic flux is measured in Webers (Wb); flux density in Tesla (T). Magnetic field around a current-carrying wire: right-hand rule predicts direction (thumb in direction of current, fingers curl in direction of magnetic field).

A coil of wire carrying current behaves like a bar magnet — this device is called an ELECTROMAGNET or SOLENOID. The magnetic field strength depends on: (1) CURRENT — more current = stronger field (proportional); (2) NUMBER OF TURNS — more turns concentrate the field; (3) CORE MATERIAL — iron core dramatically increases field strength (typically 1000x) due to high magnetic permeability; (4) COIL GEOMETRY — tighter, denser coils make stronger fields. Right-hand rule for solenoid: curl right-hand fingers in the direction of current flow; thumb points to the NORTH pole of the resulting magnet. Applications: (1) ELECTROMAGNETS — large lifting magnets in scrapyards; door locks; magnetic levitation; (2) RELAYS — small current activates electromagnet, which mechanically closes a switch controlling a larger circuit; (3) SOLENOIDS — coil with a movable iron core; energizing the coil pulls the core in; used in solenoid valves (control flow of fluids/gas), starter motors (engaging starter to flywheel), pinball machines, door locks; (4) MOTORS — coils in rotor and/or stator create magnetic fields; rotational force from interaction of fields; (5) GENERATORS — reverse of motors; (6) LOUDSPEAKERS — varying current in coil within magnetic field moves the coil and attached cone, producing sound; (7) MRI MACHINES — extremely powerful superconducting electromagnets create magnetic fields for imaging. Permanent magnets and electromagnets share the property of magnetic field but: permanent magnets have constant field strength (set by material); electromagnets have variable field controlled by current — turn off the current, field disappears. Magnetic poles: north and south, attract opposite poles, repel like poles. Earth has a magnetic field that compass needles align with — Earth's magnetic NORTH POLE is actually a SOUTH magnetic pole (which attracts compass north needles).

Electricity and magnetism are unified phenomena — electromagnetism. Key relationships: (1) ELECTRIC CURRENT creates a MAGNETIC FIELD around the wire (Oersted's discovery, 1820). Direction follows the right-hand rule: thumb in direction of current, fingers curl in direction of magnetic field. Field strength = μ₀I/(2πr) for a straight wire, where r is distance. Coiling the wire (solenoid) concentrates the field — basis for electromagnets. (2) CHANGING MAGNETIC FIELD creates an ELECTRIC FIELD that drives current in conductors (Faraday's discovery, 1831). Basis for generators, transformers, inductors, motors. (3) UNIFICATION (Maxwell's equations, 1860s): four equations describe all classical electromagnetism — Gauss's law (electric), Gauss's law for magnetism, Faraday's law of induction, Ampère's law with Maxwell's correction. They predicted electromagnetic waves traveling at the speed of light, ultimately unifying optics with electromagnetism. Light is an electromagnetic wave. Modern physics treats electricity and magnetism as different aspects of the same field — they appear differently depending on reference frame. Practical implications: (1) ELECTRIC MOTORS — current in a wire within a magnetic field experiences a force (F = IL × B); causes rotation in motor coils; converts electrical to mechanical energy; (2) GENERATORS — rotating coil in magnetic field induces voltage; converts mechanical to electrical; (3) TRANSFORMERS — AC in primary creates changing magnetic field; secondary coil sees changing field; voltage induced; (4) RADIO/COMMUNICATIONS — accelerating electrons (in antennas) create electromagnetic waves; (5) MRI — strong magnetic fields align nuclear spins; radio pulses then perturb them; signals back from the relaxing nuclei produce images; (6) MAGNETIC LEVITATION TRAINS (maglev) — magnetic forces support and propel trains; (7) PARTICLE ACCELERATORS — magnetic fields steer charged particles in beam; (8) HARD DRIVES — magnetic regions on disk encode data; read/write heads detect/change magnetization. Lorentz force: F = qE + qv × B (charged particle in electric and magnetic fields experiences force). Underlies operation of CRTs, particle accelerators, mass spectrometers, plasma physics. ASVAB EI may test basic understanding that current causes magnetic fields and that changing fields induce currents — both crucial to electrical engineering.

When electric current flows through a coil of wire, it produces a magnetic field, creating an electromagnet. The strength of the magnetic field can be increased by adding more turns of wire, increasing the current, or placing an iron core inside the coil. Unlike a permanent magnet, an electromagnet's field can be turned on and off by controlling the current, which makes electromagnets useful in motors, relays, speakers, and many devices. This relationship between electricity and magnetism (electromagnetism) is a key topic on the Electronics subtest.