HR30 - Chapter 30 Induction and Inductance In this chapter...

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Unformatted text preview: Chapter 30 Induction and Inductance In this chapter we will study the following topics:-Faradays law of induction -Lenzs rule -Electric field induced by a changing magnetic field -Inductance and mutual inductance - RL circuits -Energy stored in a magnetic field (30 1) In a series of experiments Michael Faraday in England and Joseph Henry in the US were able to generate electric currents without the use of batteries Below we describe some of the Faraday's experiments se experiments that helped formulate whats is known as "Faraday's law of induction" The circuit shown in the figure consists of a wire loop connected to a sensitive ammeter (known as a "galvanometer"). If we approach the loop with a permanent magnet we see a current being registered by the galvanometer. The results can be summarized as follows: A current appears only if there is between the magnet and the loop Faster motion results in a larger current If we 1. relative motion 2. 3. reverse the direction of motion or the polarity of the magnet, the current reverses sign and flows in the opposite direction. The current generated is known as " "; the emf that appears induced current is known as " "; the whole effect is called " " induced emf induction (30 2) In the figure we show a second type of experiment in which current is induced in loop 2 when the switch S in loop 1 is either closed or opened. When the current in loop 1 is constant no induced current is observed in loop 2. The conclusion is that the magnetic field in an induction experiment can be generated either by a permanent magnet or by an electric current in a coil. loop 1 loop 2 Faraday summarized the results of his experiments in what is known as " " Faraday's law of induction An emf is induced in a loop when the number of magnetic field lines that pass through the loop is changing Faraday's law is not an explanation of induction but merely a description of of what induction is. It is one of the four " " all of which are statements of experim Maxwell's equations of electromagnetism ental results. We have already encountered Gauss' law for the electric field, and Ampere's law (in its incomplete form) (30 3) B r n dA The magnetic flux through a surface that borders a loop is determined as follows: B Magnetic Flux 1 we divide the surface that has the loop as its border into area elements of area . dA . For each element we calculate the magnetic flux through it: cos Here is the angle between the normal and the magnetic field vectors at the position of the element. We integrate a B d BdA n B = 2. 3. r 2 : T m known as the Weber (symbol ll the terms. cos We can express Faraday's law of induction in the folowin W g b) form: B BdA B dA = = SI magnetic flux unit r r B The magnitude of the emf induced in a conductive loop is equal to rate at which the magnetic flux through the loop changes with time E B B dA = r r B d dt = - E (30 4) Michael Faraday 1791-1867 Joseph Henry 1797-1878 B...
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HR30 - Chapter 30 Induction and Inductance In this chapter...

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