ACJC Electromagnetic Induction Lecture Notes

Anglo-Chinese Junior College Lecture Notes Electromagnetic Induction H2 (9478) JC2 Physics 2026 Page 1 of 30 Electromagnetic Induction Guiding Questions Learning Objectives 1. If an electric current produces a magnetic field, can a magnetic field produce an electric current? (a) define magnetic flux as the product of magnetic flux density and the cross-sectional area perpendicular to the direction of the magnetic flux density (b) show an understanding of and use the concept of magnetic flux linkage (c) recall and solve problems using BA = and N NBA = to solve problems, where N is the number of turns 2. Can we achieve perpetual motion by invoking the magic of electromagnetism? (d) infer from appropriate experiments on electromagnetic induction: i. that a changing magnetic flux can induce an e.m.f. ii. that the direction of the induced e.m.f. opposes the change producing it iii. that factors affecting the magnitude of the induced e.m.f. (e) recall and solve problems using Faraday’s law of electromagnetic induction and Lenz’s law (f) explain simple applications of electromagnetic induction (g) show an understanding of the principle of operation of a simple iron-core transformer and recall and solve problems using pss p p s NV NV== I I for an ideal transformer

Anglo-Chinese Junior College Lecture Notes Electromagnetic Induction H2 (9478) JC2 Physics 2026 Page 2 of 30 Nature of Science Moment – Electromagnetism before Faraday (Source: The birth of the electric machines: a commentary on Faraday (1832) ‘Experimental researches in electricity by Jim Al-Khalili’) The early nineteenth century was an exciting time for experimental physics. It was also a time of great confusion about the nature of electricity. The work of two Italians, Luigi Galvani and Alessandro Volta on the characteristics of bioelectricity had led Volta to invent the battery in 1799. His ‘voltaic pile’ suddenly gave natural phil osophers (the term ‘scientists’ was not coined until 18341) a reliable and far more useful source of electricity than the Leyden jars or ever-more sophisticated electrostatic machines and turned the subject from an intellectual curiosity into a proper science. In a real sense, it galvanised science, which is of course the origin of that everyday word. In particular, it was the confusing relationship between electricity and magnetism that so fascinated many scientists. Indeed, it was argued by some that there was no connection at all between the two phenomena, although it had been known since the mid -eighteenth century that, for example, thunderbolts created certain magnetic effects. Then, in 1820, the Danish scientist Hans Christian Ørsted carried out an experiment for which he is credited with the discovery of electromagnetism. On 21 April 1820, he observed, while preparing for a lecture, that when he ran an electric current through a wire, a nearby compass needle was temporarily deflected from its stable position of pointing towards the magnetic north. This happened at the moment the current from a battery was switched on, and then again when it was switched off, thus confirming a di rect relationship between electricity and magnetism, namely that the change in an electric current (from none to flowing, and vice versa) produced a temporary magnetic effect in its vicinity. Ørsted's original interpretation was that the magnetic effects produced by the current through the wire radiated outwards in the same way that heat or light does. But after further experimentation he showed that in fact the produced magnetic field circled around the wire (although of course no one was yet thinking in terms of fields). Within months of Ørsted's discovery, the French physicist and mathematician André-Marie Ampère had shown that two current carrying wires placed in parallel close to each other each generated magnetic lines of force that caused the wires to be attracted or repelled from each other depending on whether the currents were flowing in the same or in opposite directions. Ampère would go on to help found the field of classical electromagnetism and has the SI unit of electric current named after him. Ampère and Ørsted had shown that, somehow, electricity could be converted into magnetism, but they and others had failed to do the reverse: to create electricity from magnetism.

Anglo-Chinese Junior College Lecture Notes Electromagnetic Induction H2 (9478) JC2 Physics 2026 Page 3 of 30 An equally famous (at the time, though less so today) French physicist by the name of François Arago then carried out an experiment that completely baffled most scientists at the time and served as one of the main motivations for Faraday's great work. In 1824, Arago demonstrated that a spinning copper disc caused a magnetic needle suspended above it to rotate. This result was remarkable for two reasons. Firstly, there was no external electric current being applied to the copper disc and, secondly, although copper is a conductor, it is not magnetic. Yet here was a magnetic field seemingly being produced just by the rotation of this disc that was influencing the compass needle. It would take the brilliance and inventiveness of Michael Faraday to describe what was going on and, in a series of carefully and clearly described experiments between August and November 1831, he would change the face of science in ways that have an impact on all our lives to this day. The article describes early practitioner of science as natural philosopher before the term “scientist” was coined in 1834. In fact, as the name suggests, science was philosophical, a way of thinking about and explaining natural phenomena. One of the tenets of the nature of science showed by the article is that science relies heavily, but not entirely on observation, experimental evidence, rational arguments and scepticism. It was the confusing relationship between electricity and magnetism that fascinated scientists. Some argued that there was no connection at all between the two phenomena. Only in 1820, it was Hans Christian Ørsted who observed the movement of a compass needle in the vicinity of a current-carrying wire that led to the discovery of electromagnetism. Science has a tentative character; it is both evolutionary and revolutionary. French physicist, François Arago built on Ørsted and Ampère’s discovery and in 1824 demonstrated that a spinning copper disc caused a magnetic needle suspended above it to rotate . This set the stage for Michael Faraday’s great work on the concept of Electromagnetic Induction.

Anglo-Chinese Junior College Lecture Notes Electromagnetic Induction H2 (9478) JC2 Physics 2026 Page 4 of 30 1. Magnetic flux and weber 1.1 Magnetic Flux BA =  (source: Serway Physics for Scientists & Engineers) If the angle between B and A is  , the magnetic flux,  through the surface area, A is thus cosBA= Magnetic flux,  is a scalar quantity. SI unit of  : weber (Wb) If an electric current produces a magnetic field, can a magnetic field produce an electric current? The magnetic flux,  is defined as the product of magnetic flux density and the cross-sectional area perpendicular to the direction of the magnetic flux density. B B sin  B cos  A 

Anglo-Chinese Junior College Lecture Notes Electromagnetic Induction H2 (9478) JC2 Physics 2026 Page 5 of 30 1.2 The Weber The weber is the flux of a uniform magnetic field of flux density 1 tesla, through a plane surface area 1 m2, placed normal to the magnetic flux density. 1.3 Magnetic Flux Linkage The magnetic flux linkage,  through a whole coil of N turns is the sum of the magneti