Magnetic Effects of Current and Magnetism

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In 30 seconds

Oersted: Discovered current produces magnetic field (1820). Foundation of electromagnetism.
Right hand rule: Thumb = current direction; fingers curl = magnetic field direction.
Faraday's law: Changing magnetic flux induces EMF. Basis of transformers, generators, and induction.

Electromagnetism unifies electricity and magnetism. NDA tests Oersted's discovery, right-hand rule, magnetic effects of solenoids, transformers, motors, generators, and Faraday's law.

Oersted's Discovery

1820: Hans Christian Oersted noticed a compass needle deflected near a current-carrying wire.
Demonstrated that electric currents produce magnetic fields.
Marked the beginning of electromagnetism as a unified field.
Followed by Ampere, Faraday, Maxwell — culminating in Maxwell's equations (1865).

Field Direction Rules

Right-hand thumb rule (for straight wire): Point thumb in direction of current; curled fingers show direction of magnetic field.
Right-hand thumb rule (for solenoid): Curl fingers in direction of current flow; thumb shows direction of magnetic field (north pole).
Fleming's Left-Hand Rule (motor rule): Forefinger = magnetic field, Middle finger = current, Thumb = force on conductor.
Fleming's Right-Hand Rule (generator rule): Forefinger = field, Thumb = motion, Middle finger = induced current.

Solenoid and Electromagnet

Solenoid: Coil of wire. When current flows, generates magnetic field like a bar magnet — one end N, other S.
Field inside solenoid is strong, uniform, parallel to axis.
Electromagnet: Solenoid wound around a soft iron core. Much stronger field. Magnetism only while current flows — can be switched on/off.
Applications: Cranes for lifting iron scrap, electric bells, MRI machines, particle accelerators.

Electric Motor and Generator

DC motor: Converts electrical → mechanical energy. Current-carrying coil in magnetic field experiences force (Fleming's left-hand rule), causing rotation. Commutator and brushes reverse current to maintain rotation.
AC/DC generator: Converts mechanical → electrical energy. Coil rotated in magnetic field induces EMF (Faraday's law).
AC generator: produces sinusoidal alternating current. DC generator: uses commutator to convert to one-direction current.

Electromagnetic Induction

Faraday's Law: A change in magnetic flux through a coil induces an EMF.
EMF = -N × dΦ/dt (Φ = magnetic flux, N = number of turns).
Lenz's Law: Induced current opposes the change that produced it (energy conservation).
Applications: Transformers, generators, induction cookers, wireless charging.

Transformer

Used to step up or step down AC voltage.
Two coils (primary and secondary) on a common iron core.
V_p/V_s = N_p/N_s (voltage ratio = turns ratio).
Power conservation (ideal): V_p × I_p = V_s × I_s. Step-up voltage → step-down current.
Step-up transformer at power plants (e.g., 11 kV → 220 kV) for transmission.
Step-down transformer at substations and household level (e.g., 11 kV → 230 V).
Does NOT work with DC (no changing flux).

NDA PYQ Examples

Q: Who discovered that current produces magnetic field?

(a) Faraday (b) Oersted (c) Maxwell (d) Ampere

Answer: (b) Oersted — 1820.

Q: A transformer works on:

(a) Direct current (b) Alternating current (c) Both AC and DC (d) High voltage DC only

Answer: (b) Alternating current — needs changing flux.

Q: Fleming's right-hand rule applies to:

(a) Electric motor (b) Electric generator (c) Transformer (d) Magnet

Answer: (b) Generator — gives direction of induced current.

Q: In a step-down transformer, secondary coil has:

(a) More turns than primary (b) Fewer turns than primary (c) Same turns (d) No turns

Answer: (b) Fewer turns than primary.

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NDA-pattern items on Magnetic Effects of Current and Magnetism with answer keys and explanations.

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Frequently Asked Questions

How does an MRI scanner work?

Uses powerful electromagnets to create a strong magnetic field. Aligns protons in the body's water molecules. RF pulses tip them; their return to alignment emits signals that computers reconstruct into an image. No ionising radiation — safer than X-ray for soft tissues.

Why don't transformers work with DC?

Transformers need a changing magnetic flux to induce EMF. DC produces a steady magnetic field — no change, no induced EMF in secondary. Only AC's continuously changing current/field can transform voltage.

How do induction cookers work?

An AC current in a coil under the cooktop creates a changing magnetic field. This induces eddy currents in the iron/steel cookware placed above. The eddy currents heat the cookware directly (not the cooktop). Efficient and safe.

What is electromagnetic induction?

The phenomenon where a changing magnetic flux induces an EMF in a conductor. Discovered by Faraday (1831) and Henry independently. Basis of all electric generators, transformers, and inductive charging.

How do two parallel wires with current interact?

If currents flow in the same direction — wires attract. Opposite directions — wires repel. Magnetic field from one wire exerts force on the moving charges in the other (Ampere's law). Force per unit length F/L = μ₀I₁I₂/(2πd).