Lecture 19 Chapter 30 Faraday s Law Who cares that Faraday s Law is used here? Course website: http://faculty.uml.edu/andriy_danylov/teaching/physicsii
Today we are going to discuss: Chapter 30: Section 30.3-5
Lenz s rule The minus sign gives the direction of the induced emf. To avoid dealing with this minus, we will calculate EMF in two steps: 1) 2) Apply Lenz s rule i.e. Any system doesn t like changes It opposes to a growing flux And Supports a dying flux
Lenz s Law (example) Pushing the bar magnet into the loop causes the magnetic flux to increase in the upward direction. To oppose the change in flux, which is what Lenz s law requires, the loop itself needs to generate an downward-pointing magnetic field. The induced current ceases as soon as the magnet stops moving. has CW direction
Example (Lenz s Law) The current in the straight wire is decreasing. I has CW direction
ConcepTest The current in the straight wire is increasing. Which is true? Lenz s law A) There is a clockwise induced current in the loop. B) There is a counterclockwise induced current in the loop C) There is no induced current in the loop. 1. The wire s B field is into the screen and increasing. 2. To oppose the increase in flux, the field of the induced current must point out of the screen. 3. From the right-hand rule, an inward field needs a ccw current. has CCW direction
ConcepTest Loop and Wire II What is the induced current if the wire loop moves in the direction of the yellow arrow? 1) clockwise 2) counterclockwise 3) no induced current The magnetic flux through the loop is not changing as it moves parallel to the wire. Therefore, there is no induced current. I = const
Faraday s Law for a U-shaped rail/rod system Let s apply Faraday s law for a conducting rod sliding on a U-shaped conducting rail. B is perpendicular to the plane of the rail. We can find the induced emf and current by using Faraday s law and Ohm s law: The EMF induced in the loop is: The induced current flows through the loop: Direction of the induced current: has CCW direction
Applications of Faraday s Law
Applications (Faraday s Law) How Electric guitar works? How Microphone works? Presence of a permanent magnet makes the guitar wire a magnet. When the magnetic guitar wire vibrates, it changes the magnetic flux in the coil. This flux induces a current in the coil which exactly follow the vibrations of the guitar wire. This current is fed to the amplifier and we can hear the sound of vibration of the guitar wire. When a sound wave strikes the diaphragm, a coil fixed to the diaphragm vibrates over a stationary bar magnet, changing the flux in the coil and induced an emf in the coil which is then amplified and sent to speakers.
Applications (An Alternating-Current Generator) Φ cos cos sin
Generators A generator is a device that transforms mechanical energy into electric energy. A generator inside a hydroelectric dam uses electromagnetic induction to convert the mechanical energy of a spinning turbine into electric energy. Hydroelectric Power Plant Working Animation (start at 1 min)
Jumping Ring Demonstration I increases With a metal ring in place, turn on the switch. The solid ring will jump. Why? This components cancel each other has CW direction Let s look at a force on the ring: Vertical components add each other Thus, the net force is up and the ring jumps This principle is used in a railgun or coil gun
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