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NCERT's Class 12 chapter on alternating current calls the rotating magnetic field the basis of practically all alternating current machinery. Nikola Tesla patented a motor built on it in 1888: a magnetic field that turns by itself, made from alternating currents in coils that do not move. NCERT mentions him only in a side box, but the physics behind his motor uses ideas you already know, vectors, Faraday's law and Lenz's law. This post gives the verified story, explains the rotating field with a short derivation, and shows why alternating current won for power lines.
Who was Nikola Tesla?
The IEEE's Engineering and Technology History Wiki gives his birth date as 10 July 1856, in what is now Croatia, to Serbian parents, and his death date as 7 January 1943. He studied at the Graz Polytechnic Institute in Austria, where, according to the same biography, he began to think about the possibilities of alternating current. He later worked in Budapest and for Edison's company in Paris and Strasbourg, and in 1884 went to New York, where he took a job with Edison.
In 1886 he founded the Tesla Electric Company. He returned to his AC experiments and, within two years, had applied for more than thirty patents on his system. He then signed a contract that turned over AC development and his patents to the Westinghouse company. In 1916 the American Institute of Electrical Engineers gave him its Edison Medal for "meritorious achievement in his early original work in polyphase and high-frequency electrical currents." The unit of magnetic flux density, the tesla, was named in his honour in 1960.
What did Tesla's 1888 patent describe?
His US patent 381,968, "Electro-magnetic motor", was filed on 12 October 1887 and granted on 1 May 1888. It describes a motor with "two or more independent circuits through which alternate currents are passed at proper intervals", producing "a progressive shifting of the magnetism or of the lines of force", which drives the motor. The same year he presented A New System of Alternate Current Motors and Transformers in the Transactions of the American Institute of Electrical Engineers.
NCERT's chapter on alternating current sums up the importance: Tesla conceived the idea of the rotating magnetic field, the basis of practically all alternating current machinery, and invented the induction motor and the polyphase system of AC power, among other things.
How do two alternating currents make a rotating field?
Take two identical coils at right angles, one along x and one along y. Feed them alternating currents of the same frequency, but a quarter of a cycle apart. Their fields at the centre are:
Bx = B0 cos ωt
By = B0 sin ωt
Add them as vectors.
Magnitude: B = √(Bx² + By²) = B0 √(cos² ωt + sin² ωt) = B0. The field strength never changes.
Direction: tan θ = By / Bx = tan ωt, so θ = ωt. The direction turns steadily.
Speed: the field makes one full turn per cycle. On a 50 Hz supply, the frequency NCERT uses in its examples, that is 50 turns a second, or 3000 a minute, for this simple two-coil arrangement.
Neither coil moves, yet the field rotates. That is the "progressive shifting" in Tesla's patent.
Why does an induction motor turn?
Put a closed metal rotor in the rotating field. From the rotor's point of view the flux through it keeps changing, so by Faraday's law currents are induced in it. By Lenz's law those currents oppose the change that causes them, and the way to reduce the relative motion is for the rotor to follow the field. So it turns in the same direction.
Worked numbers: if the field turns at 3000 revolutions per minute and the rotor at 2880, the rotor falls behind by 120 per minute, or 120 ÷ 3000 = 4 per cent. Engineers call this lag the slip. With a heavier load, the rotor slows a little more, the relative motion grows, larger currents are induced and the torque rises to match. Our post on Lenz's law problems gives more practice with the direction of induced currents.
Induction motors are not a named topic in the CBSE Class 12 physics curriculum, which lists AC generators and transformers. The reasoning above, however, uses only vectors and Faraday's and Lenz's laws, which are in it.
Why did alternating current win for power lines?
Alternating current can be stepped up and down with transformers. Here is why that matters. Suppose a line with 2 Ω resistance must deliver 10 kW.
At 200 V: current I = P/V = 10,000 / 200 = 50 A. Power lost in the line = I²R = 50² × 2 = 5000 W, half the power sent.
At 10,000 V: I = 10,000 / 10,000 = 1 A. Loss = 1² × 2 = 2 W.
Raising the voltage by a factor of 50 cuts the current by 50 and the loss by 2500. NCERT's AC chapter also explains why mains is quoted as an rms value: 220 V rms corresponds to a peak of √2 × 220 ≈ 311 V. Our post on transformers and AC generators works through turns ratios.
What can a student take from Tesla's way of working?
Return to a hard idea. By the biography's account, Tesla started thinking about alternating current as a student in Graz and patented his motor about a decade later.
Think in systems. His patent claims the generator, the circuits and the motor together, not just one machine.
Check the idea with mathematics. The rotating field is two lines of trigonometry. Writing it down turns a vague picture into something you can test.
A rotating field is much easier to understand when you watch the two components add. The Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read, and a field built from two changing components is one of those ideas.
Draw Bx and By at four moments, ωt = 0, 90, 180 and 270 degrees, and mark the direction of the total field each time.
