Electromagnetic Induction

6.1 INTRODUCTION

Electricity and magnetism were long considered separate and unrelated phenomena until the early 19th century. The pioneering experiments by scientists such as Oersted and Ampere established that moving electric charges (electric currents) produce magnetic fields, evidenced by the deflection of a magnetic compass needle placed near a current-carrying wire. This discovery raised fundamental questions about the converse effect: can moving magnets produce electric currents? The answer, demonstrated conclusively by Michael Faraday in England and Joseph Henry in the USA around 1830, is yes. They showed that electric currents can be induced in closed coils when subjected to changing magnetic fields. This phenomenon, where electric current is generated by varying magnetic fields, is called electromagnetic induction. Faraday's discovery was not only of theoretical importance but also of immense practical utility, leading to the development of modern generators and transformers, which are the backbone of today's electrical civilization. Faraday famously responded to skepticism about the utility of his discovery by comparing it to a newborn baby, implying that its potential was vast and yet to be realized.

• Electricity and magnetism were initially thought to be unrelated phenomena.
• Oersted and Ampere demonstrated that electric currents produce magnetic fields.
• Faraday and Henry showed that changing magnetic fields induce electric currents.
• Electromagnetic induction is the generation of current by varying magnetic fields.
• This discovery led to practical devices like generators and transformers.
• Faraday's work laid the foundation for modern electrical technology.
• 📌 Electromagnetic induction: The phenomenon of inducing electric current by changing magnetic fields.
• 📌 Magnetic field: A field produced by moving electric charges that exerts force on other moving charges or magnets.

6.2 THE EXPERIMENTS OF FARADAY AND HENRY

The discovery of electromagnetic induction is based on a series of experiments by Michael Faraday and Joseph Henry. These experiments demonstrate that an electric current is induced in a coil when there is a change in magnetic flux linked with it. In Experiment 6.1, a coil C1 connected to a galvanometer is used. When the North pole of a bar magnet is pushed towards the coil, the galvanometer needle deflects, indicating the presence of an induced current. The deflection lasts only while the magnet is moving; when held stationary, no deflection is observed. Pulling the magnet away causes the galvanometer to deflect in the opposite direction, indicating a reversal of current. Similar effects are observed when the South pole is moved towards or away from the coil, with the direction of deflection reversed compared to the North pole. The magnitude of the deflection increases with the speed of the magnet's motion. Moving the coil instead of the magnet produces the same effects, showing that relative motion between magnet and coil is responsible for induction. In Experiment 6.2, the bar magnet is replaced by a second coil C2 connected to a battery producing a steady current and thus a steady magnetic field. Moving coil C2 towards or away from coil C1 induces a current in C1, detected by the galvanometer. The deflection reverses when the direction of motion reverses. Holding either coil fixed and moving the other produces the same results, again emphasizing relative motion. Experiment 6.3 shows that relative motion is not an absolute requirement. Two coils C1 and C2 are held stationary; C1 is connected to a galvanometer and C2 to a battery through a key. When the key is pressed, the galvanometer shows a momentary deflection as the current in C2 rises, changing the magnetic flux through C1. Holding the key pressed produces no deflection, as the current and flux are steady. Releasing the key causes a deflection in the opposite direction. Inserting an iron rod inside the coils increases the deflection, showing the effect of magnetic permeability. These experiments collectively demonstrate that a changing magnetic flux through a coil induces an emf and hence a current.

• Induced current appears only when there is relative motion or changing magnetic flux.
• Direction of induced current depends on the direction of motion and pole of the magnet.
• Induced emf is observed when current in a nearby coil changes, even without motion.
• Inserting an iron rod increases the induced emf due to increased magnetic flux.
• Galvanometer deflection indicates presence and direction of induced current.
• Relative motion between magnet and coil or between two coils is key to induction.
• 📌 Galvanometer: An instrument for detecting and measuring small electric currents.
• 📌 Magnetic flux: The measure of magnetic field passing through a given area.
• 📌 Induced emf: Electromotive force generated due to change in magnetic flux.