SPH3U UNIVERSITY PHYSICS

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1 SPH3U UNIVERSITY PHYSICS ELECTRICITY & MAGNETISM L Faraday s Discovery (P ) Faraday s Discovery In 1819, when Oersted demonstrated the ability of a steady current to produce a steady magnetic field, scientists assumed that a steady magnetic field would produce a steady current. It seems logical and yet it doesn t work. October 20, U4 - Faraday's Discovery 1 Faraday s Discovery It wasn t until 1831 that Michael Faraday discovered the basic principle of electromagnetic induction. Faraday made his discovery by experimenting with conductors in the vicinity of magnetic fields. His investigations involved three scenarios. October 20, U4 - Faraday's Discovery 2 1

2 Faraday s Discovery SCENARIO#1 Moving a wire through the jaws of a horseshoe magnet. In this case, Faraday found that electric current only flowed while the conductor was cutting across the magnetic field (direction C). October 20, U4 - Faraday's Discovery 3 Faraday s Discovery SCENARIO#2 Plunging a bar magnet into and out of the core of a coil. In this case, Faraday found that electric current only began to flow when the bar magnet was moving into or out of the coil. October 20, U4 - Faraday's Discovery 4 Faraday s Discovery SCENARIO#3 Touching the iron core of a coil with a bar magnet. In this case, Faraday found that electric current was only observed in the coil when the iron core was being magnetized or demagnetized. October 20, U4 - Faraday's Discovery 5 2

3 Faraday was able to combine these three observations into one general statement, now known as the law of electromagnetic induction. Any change in the magnetic field near a conductor induces a voltage in the conductor which induces an electric current in the conductor. October 20, U4 - Faraday's Discovery 6 LAW OF ELECTROMAGNETIC INDUCTION any change in the magnetic field near a conductor induces a voltage in the conductor which induces an electric current in the conductor NOTE! We call it an induced current because it is not an already existing current; it is formed by the action of the changing magnetic field. October 20, U4 - Faraday's Discovery 7 PRACTICE 1. The following diagram shows a loop of wire inside a uniform magnetic field. Which of the following will produce an induced current in the loop? Explain. (a) the wire is moved up or down (a) no the magnetic field is not changing October 20, U4 - Faraday's Discovery 8 3

4 PRACTICE 1. The following diagram shows a loop of wire inside a uniform magnetic field. Which of the following will produce an induced current in the loop? Explain. (b) the wire is moved horizontally out of the magnetic field (b) yes the magnetic field is changing October 20, U4 - Faraday's Discovery 9 PRACTICE 1. The following diagram shows a loop of wire inside a uniform magnetic field. Which of the following will produce an induced current in the loop? Explain. (c) the loop is rotated in the magnetic field (c) yes the magnetic field is changing October 20, U4 - Faraday's Discovery 10 PRACTICE 1. The following diagram shows a loop of wire inside a uniform magnetic field. Which of the following will produce an induced current in the loop? Explain. (d) the loop is compressed, decreasing its area (d) yes the magnetic field is changing October 20, U4 - Faraday's Discovery 11 4

5 Faraday s Iron Ring Induction can be demonstrated with a device that Faraday constructed and used himself in his early studies of the induction effect. Known as Faraday s iron ring, it consists of a doughnut-shaped ring of soft iron with two separate coils of wire wound around it. October 20, U4 - Faraday's Discovery 12 Faraday s Iron Ring The primary coil is connected through a switch to a voltage source. The secondary coil is connected directly to a galvanometer. The soft-iron ring enhances the strength of the magnetic field, and the ring itself becomes magnetized. October 20, U4 - Faraday's Discovery 13 Faraday s Iron Ring FARADAY S IRON RING consists of a doughnut-shaped ring of soft iron (enhances the magnetic field) with two separate coils of wire wound around it primary coil is connected to a voltage source secondary coil is connected to a galvanometer October 20, U4 - Faraday's Discovery 14 5

6 Electromagnetic Induction When the switch is closed the current in the primary circuit causes the entire ring to become magnetized. This sudden increase in magnetic field strength (from zero to some value) induces a current in the secondary coil. October 20, U4 - Faraday's Discovery 15 Electromagnetic Induction However, once the magnetic field in the iron ring is stable and no longer changing, the induced current no longer exists. Remember that you need a changing magnetic field to induce an electric current. October 20, U4 - Faraday's Discovery 16 Electromagnetic Induction When the switch is opened, the electric current stops flowing and the magnetic field in the soft-iron ring collapses from maximum strength to zero. This change in the magnetic field causes an induced electric current in the opposite direction in the secondary coil. October 20, U4 - Faraday's Discovery 17 6

7 Electromagnetic Induction Once the magnetic field in the iron ring is zero, the induced current no longer exists. October 20, U4 - Faraday's Discovery 18 Electromagnetic Induction NOTE! Direct currents only produce electromagnetic induction for brief instants when the primary circuit is switched on or off. October 20, U4 - Faraday's Discovery 19 Factors Affecting Electromagnetic Induction By observing what happens when a bar magnet is plunged into the core of a coil that is connected to a galvanometer, it is possible to conclude that the factors affecting the magnitude of the induced current are: 1. the number of turns on the induction coil, 2. the rate of change of the inducing magnetic field, and 3. the strength of the inducing magnetic field. October 20, U4 - Faraday's Discovery 20 7

8 Factors Affecting Electromagnetic Induction ELECTROMAGNETIC INDUCTION FACTORS factors that affect the amount of induced current are: the number of turns on the induction coil the rate of change of the inducing magnetic field the strength of the inducing magnetic field. NOTE! DC only produces electromagnetic induction when the primary circuit is switched on or off. October 20, U4 - Faraday's Discovery 21 Applications of Induction Cooking Cooking food involves the transfer of thermal energy. In an electric stove, the efficiency of this process is low because the stove element has to get hot, the pot has to get hot, and finally the food is heated. In the process, much thermal energy is lost to the environment. October 20, U4 - Faraday's Discovery 22 Applications of Induction Cooking Cooking using an induction cooker involves a rapidly changing magnetic field in the stove element, which induces an electric current in the pot. The electric current heats the pot because of the electrical resistance of the pot. October 20, U4 - Faraday's Discovery 23 8

9 Applications of Induction Cooking NOTE! Iron pots work better than copper or aluminum due to their higher electrical resistance. Insulating materials like glass will not work on an induction cooker. October 20, U4 - Faraday's Discovery 24 Applications of Induction Cooking Cooking with an induction cooker is more efficient because it is a more direct transfer of thermal energy to the food. Another benefit is that the induction cooking surface does not get hot, so food that spills onto the cooking surface does not burn. Since the induction cooking surface is not hot, the food immediately stops heating up once the induction cooker is turned off. October 20, U4 - Faraday's Discovery 25 Applications of Metal Detectors Electromagnetic induction is also used to detect metals. Metals detectors use a coil that generates a rapidly changing magnetic field. This magnetic field induces a current in any metal near it. The induced electric current in the detected metal also produces an induced magnetic field of its own. Sensitive measurements of the magnetic field near a metal detector are used to detect the induced magnetic field. October 20, U4 - Faraday's Discovery 26 9

10 Applications of Metal Detectors Metal detectors have many uses for humanitarian purposes to help locate land mines, for security reasons, by hobbyists searching for buried treasure, by loggers,... October 20, U4 - Faraday's Discovery 27 Applications of Induction Chargers Electromagnetic induction can be used to charge low-power electronic devices such as electric toothbrushes, or even cellphones. The charger is plugged into a wall outlet. Both the charger and the device to be charged contain a wire coil. When the device is placed on the charger an electric current is induced in the coil inside the device. This induced current charges the internal battery of the device October 20, U4 - Faraday's Discovery 28 Applications of Induction Chargers The benefit of this type of charging is: you do not need wires to plug directly into the device it can be used in a wet location you can use a single charging station to charge several devices at once October 20, U4 - Faraday's Discovery 29 10

11 Applications of Electromagnetic Induction APPLICATIONS OF... it is used in many technologies including: generators transformers induction cooking metal detectors induction chargers October 20, U4 - Faraday's Discovery 30 U Check Your Learning TEXTBOOK P.591 Q.1,3 October 20, U4 - Faraday's Discovery 31 11