Slide 1 / 76 Slide 2 / 76 Electromagnetic Induction www.njctl.org Slide 3 / 76 How to Use this File Each topic is composed of brief direct instruction · There are formative assessment questions after every topic · denoted by black text and a number in the upper left. > Students work in groups to solve these problems but use student responders to enter their own answers. > Designed for SMART Response PE student response systems. > Use only as many questions as necessary for a sufficient number of students to learn a topic. Full information on how to teach with NJCTL courses can be · found at njctl.org/courses/teaching methods
Slide 4 / 76 Table of Contents Click on the topic to go to that section Induced EMF (Electromotive Force) · Magnetic Flux · Faraday's Law of Induction · Lenz's Law · EMF induced in a moving conductor · Electromagnetic Induction Applications · Slide 5 / 76 Induced EMF (Electromotive Force) Return to Table of Contents Slide 6 / 76 Electromotive Force (EMF) Electromotive Force is actually a potential difference between two points that is measured in Volts. It is NOT a force, but it is an historical term that has not gone away. Because it is an unfortunate name, it is frequently just referred to as EMF or . It represents the voltage developed by a battery. This chapter will show a way that a voltage can be developed in a conducting wire that is not connected to a battery.
Slide 7 / 76 Induced EMF In the Magnetism chapter, it was shown, due to the work of Oersted and Ampere, that a current will generate a magnetic field. After this discovery, physicists looked to see if the reverse could be true - whether a magnetic field could generate a current. Michael Faraday was able to make this connection in 1831 - with a modification involving a changing magnetic field or a changing area through which a constant magnetic field operates. In America, Joseph Henry performed a similar experiment at the same time, but did not publish it. This happens a lot in Mathematics and Physics - Newton (in the U.K.) and Leibniz (in Germany) developed related forms of Calculus at the same time, independent of each other. Slide 8 / 76 Induced EMF Michael Faraday connected a battery to a metal coil via insulated wires (the coil increased the magnetic field) and found that a current would be induced in the current loop on the right when the switch on the left side was closed and opened. There is zero current present in the coil at all times. There would be zero current on the These are insulated wires, right side when the current on the and any current present in them is NOT passing through left side was steady. the metal coil. Slide 9 / 76 Induced EMF Faraday's Disk Generator - by spinning the metal disk between the poles of the U shaped magnet (A), the changing magnetic field will induce an EMF, and hence, a current in the disk (D), which will flow out of the machine via terminals B and B'. A bar magnet that moves towards or away from a loop of wire will generate an EMF, and then a current in the loop.
Slide 10 / 76 Induced EMF This now provided evidence that a magnetic field could generate a current. But, there is a difference. A steady current will generate a magnetic field. But, a steady magnetic field and a non moving, constant area loop of wire will NOT result in a current in the wire. A constant magnetic field and a moving loop of wire will result in a current. A changing magnetic field and a stationary loop of wire will result in a current. A constant magnetic field and a changing area of the loop of wire will result in a current. We need to define Magnetic Flux before we can fully understand this phenomenon. Slide 11 / 76 1 A bar magnet is moved towards a circular conducting loop. As this occurs: A The magnetic field in the loop decreases, and no current flows in the loop. B The magnetic field in the loop decreases, and a current flows in the loop. C The magnetic field in the loop increases, and a current flows in the loop. D The magnetic field in the loop increases, and no current flows in the loop. Slide 11 (Answer) / 76 1 A bar magnet is moved towards a circular conducting loop. As this occurs: A The magnetic field in the loop decreases, and no current flows in the loop. B The magnetic field in the loop decreases, and a current flows in the loop. C The magnetic field in the loop increases, and a current flows in the loop. Answer D The magnetic field in the loop increases, and no current C flows in the loop. [This object is a pull tab]
Slide 12 / 76 2 The units of EMF are: A Joules B Volts C Newtons D Coulombs Slide 12 (Answer) / 76 2 The units of EMF are: A Joules B Volts C Newtons D Coulombs Answer B [This object is a pull tab] Slide 13 / 76 3 Which of the following cases will generate an EMF (and a current) in a conducting loop? Select two answers . A A powerful magnet sits outside the loop. B A magnet moves towards a loop. C A magnet is stationary relative to a loop of wire, and the loop expands in area. D A magnet moves to the right, towards a loop, and the loop is also moving to the right at the same velocity.
Slide 13 (Answer) / 76 3 Which of the following cases will generate an EMF (and a current) in a conducting loop? Select two answers . A A powerful magnet sits outside the loop. B A magnet moves towards a loop. Answer B, C C A magnet is stationary relative to a loop of wire, and the loop expands in area. D A magnet moves to the right, towards a loop, and the loop is also moving to the right at the same [This object is a pull tab] velocity. Slide 14 / 76 Magnetic Flux Return to Table of Contents Slide 15 / 76 Magnetic Flux Magnetic Flux describes the quantity of Magnetic Field lines that pass in a perpendicular direction through a given surface area and is represented by: # B # B # A # is the Greek letter "phi" and stands for flux or flow. Adding the subscript "B" makes it Magnetic Flux . The unit of Magnetic Flux is the weber, Wb, where 1 Wb = 1Tm 2 The concept of "normal" is also used here. The normal is a line that is perpendicular to the surface at the point of interest. The Magnetic Flux would be at a maximum at a point on the surface where it is parallel to the normal. Field lines perpendicular to surface Maximum Flux: Field lines parallel to normal to surface.
Slide 16 / 76 Magnetic Flux The unit is named after German Professor and Physicist, Wilhelm Eduard Weber, who stressed the importance of experiments for students learning physics. He also worked and published with Carl Friedrich Gauss and together, they developed the first electromagnetic telegraph . He was dismissed from one of his university teaching positions as he became involved in politics against the King of Hanover. "Wilhelm Eduard Weber II" by Rudolph Hoffmann - Transferred from en.wikipedia. Originally from de.wikipedia. Licensed under Public Domain via Wikimedia Commons - https://commons.wikimedia.org/wiki/ File:Wilhelm_Eduard_Weber_II.jpg#/media/File:Wilhelm_Eduard_Weber_II.jpg Slide 17 / 76 Magnetic Flux The Magnetic Field (blue) is Normal line perpendicular to the plane of the loop of wire (orange) and parallel to its normal (red) so the Magnetic Flux is at a maximum and is given by # B = BA. Normal line Slide 18 / 76 Magnetic Flux The Magnetic Field (blue) is parallel to the plane of the loop of wire (orange) and perpendicular to its normal (red) so the Magnetic Flux is at a minimum and is given by # B = 0. An easy way of looking at this, is if there are no Magnetic Field lines going through the plane of the loop of wire, then there is zero flux.
Slide 19 / 76 Magnetic Flux The Magnetic Flux is proportional to the total number of Magnetic Field lines passing through the loop. The black lines are the normal lines to the loop. Here is a constant Magnetic Field directed to the right with the same loop in three different positions where is the angle between the By convention, all angles are Magnetic Field lines and the measured relative to the Normal. normal to the surface of the loop. Slide 20 / 76 Magnetic Flux The Magnetic Flux is proportional to the total number of Magnetic Field lines passing through the loop. The Magnetic Flux is at a minimum when the field lines make an angle of zero with the normal. Physically - you can see that no lines go through the loop. The flux increases as the loop is turned, as more field lines pass through the loop, and reaches a maximum when the field lines are parallel with the normal. Slide 21 / 76 4 What is the magnetic flux through a loop of wire of cross sectional area 5.0 m 2 if a magnetic field of 0.40 T is perpendicular to the area (and parallel to the normal)?
Slide 21 (Answer) / 76 4 What is the magnetic flux through a loop of wire of cross sectional area 5.0 m 2 if a magnetic field of 0.40 T is perpendicular to the area (and parallel to the normal)? Answer [This object is a pull tab] Slide 22 / 76 5 What is the magnetic flux through a circular loop of wire of radius 2.0 m if a magnetic field of 0.30 T is perpendicular to the area (and parallel to the normal)? Slide 22 (Answer) / 76 5 What is the magnetic flux through a circular loop of wire of radius 2.0 m if a magnetic field of 0.30 T is perpendicular to the area (and parallel to the normal)? Answer [This object is a pull tab]
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