Laws of electromagnetic induction

Born on 22nd September, 1791 to an English blacksmith, Michael Faraday is legendary for his many contributions to the field of science. Electro- magnetic induction, electro- magnetic rotations, magneto- optical effects, diamagnetism and filed theory are few of his many discoveries. It may be difficult to believe, but the discoverer of all these phenomenon had little formal education. At the tender age of fourteen years, he was apprenticed to a bookseller and bookbinder.

But this apprenticeship turned out to be a blessing in disguise as Faraday took to reading all the books brought to him for binding. He also attended lectures by Humphrey Davy, the great scientist and this turned out to be turning point in his life. In 1812, he wrote to Davy enclosing his extensive notes that he had taken during the lectures. Impressed by the quality of the notes, the willingness to learn and also due to a conjecture of circumstances, Davy appointed Faraday as an assistant in research and lectures. Thus Davy became a mentor for the young Faraday and showed him the line that Faraday was to take up later in life.

By 1820, Faraday was doing research on his own and his papers on chemistry were accepted by the Royal Society. Following this breakthrough, Faraday went on to show that magnets exert force on current carrying conductors and also discovered benzene- an important organic compound used frequently in industry. However, his growing fame soured his relationship with Davy who felt let down by the fact that he was not given any credit by Faraday. The last nail in the coffin of their relationship came when Faraday was elected to the Royal Society in 1824.

His epic experiments on electromagnetic induction happened in 1831 when he showed that a changing magnetic field can induce an EMF whose magnitude is equal to the rate of change of the magnetic field. This discovery won him worldwide acclaim. His ingenious invention of the concept of magnetic field lines to better understand the nature of magnetic field was a wonderful concept which revolutionized the entire research on magnetism. Faraday was never a mathematician, he developed simpler ways of understanding physics and his laws on electro- magnetic induction were put into mathematical form later by Maxwell.

His numerous other contributions to science include the first electrical generator, the correct and accurate laws of electrochemistry, behavior of light in magnetic field and the discovery of benzene which has been mentioned earlier. Despite all his achievements and unprecedented fame, Faraday remained humble, modest and simple. He declined knighthood, honorary degrees and only reluctantly agreed to receive a pension in his later years after retirement. He died of ill health on 25th August 1867 and was buried in the Sandemanian plot in the Highgate Cemetery.

Laws of Electromagnetic Induction:-

Faraday performed certain experiments which led to the discovery of the laws of electromagnetic induction. To understand the laws properly, we must first look through these experiments and then move on to his deductions and observation which form these laws.

Experiment 1:-

In the shown figure, the conducting coil is attached to a galvanometer. When the bar magnet is pushed towards the coil, the galvanometer shows a deflection thus indicating the flow of current through the coil. This observation tells us that current has been set up in the coil even in the absence of a battery. When the magnet stops moving with respect to the coil, the current stops flowing and deflection stops.

A similar phenomenon is observed when the magnet is moved away from the coil but now the deflection of the galvanometer needle is in the opposite direction. The experiment works similarly when the coil is moved back and forth while keeping the magnet stationary. Thus, we can conclude that what causes current to be set up in the coil is the relative motion between the coil and the magnet.

Experiment 2:-

The permanent magnet in the above experiment can be replaced by an electromagnet and the experiment will work just as well. Also, Faraday showed that the relative motion is also not an absolute requirement. The emf will be induced in the coil without battery even when it is kept near a current carrying coil in which the current is varying. In the setup shown in the figure 1.2, the two coils are independent of each other. When the key S is closed, Galvanometer G shows a momentary deflection.

The plausible explanation for this is that when the key is closed, the current in coil 2 takes a finite time to reach its maximum value and in this small time interval the magnetic field associated with coil 2 also changes.

This change causes the momentary current and emf to be induced in coil 1. Similarly, when the key is opened after remaining closed for some time, there is again a momentary deflection in the galvanometer, albeit in the opposite direction. The explanation is similar. The current in coil 2 reduces to zero gradually and this decreasing current causes a variable magnetic field to be associated with coil 2 and this variable magnetic field induces emf in the coil 1.

Faraday deduced that the common factor in the experiments described above is the changing or varying magnetic field (flux). The findings and observations made by Faraday were put into the form of the famous laws of electromagnetic induction, more commonly called Faraday’s Laws. The following are the statements of the two laws.

Law 1:-

            Faraday’s first law states that ‘Whenever there is a change of magnetic flux through a circuit, there will be an induced emf and this induced emf will last as long as the change persists’.

Law 2:-

            The second law states that ‘The magnitude of the induced emf is equal to the time rate of change of the magnetic flux’. Mathematically, it can be expressed as :-

                                               e = - dQ

                                                         dt

            where e = the induced emf

                      Q=the magnetic flux

The negative sign indicates the direction of e and hence that of the current in a closed loop. It was shown by German Physicist Heinrich Lenz that ‘the polarity of the induced emf is such that it tends to produce a current which opposes the change that produces it’.

Thus, the laws of electromagnetic induction, as put forward by Faraday, are very simple to understand and explain the very important correlation between electricity and magnetism. The phenomenon of induction is not merely of academic or theoretical interest. These phenomenon are the basic principle on which modern generators and transformers are built. Civilization as it exists today, owes a lot to Michael Faraday for his pioneering work on electromagnetic induction.

WORKS CITED

1.      Brief Biography  http://www.rigb.org/rimain/heritage/faradaypage.jsp

2.      Michael Faraday http://www.slcc.edu/schools/hum_sci/physics/whatis/biography/faraday.html

3.      Fundamentals of Physics, David Halliday, Robert Resnick and Jearl Walker, John Wily, 1997