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Electromagnetic Induction Induced EMF (Electromotive Force)

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Electromagnetic Induction Induced EMF (Electromotive Force) Slide 1 / 52 Slide 2 / 52 New Jersey Center for Teaching and Learning Progressive Science Initiative This material is made freely available at www.njctl.org and is intended for the non-commercial use of students and teachers. These materials may not be used for any commercial purpose without the written Electromagnetic permission of the owners. NJCTL maintains its website for the convenience of teachers who wish to Induction make their work available to other teachers, participate in a virtual professional learning community, and/or provide access to course materials to parents, students and others. Click to go to website: www.njctl.org www.njctl.org http:/ / njc.tl/ gu Slide 3 / 52 Slide 4 / 52 How to Use this File Table of Contents Click on the topic to go to that section · Each topic is composed of brief direct instruction · There are formative assessment questions after every topic · Induced EMF (Electromotive Force) denoted by black text and a number in the upper left. · Magnetic Flux > Students work in groups to solve these problems but use student responders to enter their own answers. · Faraday's Law of Induction · Lenz's Law > Designed for SMART Response PE student response systems. · EMF induced in a moving conductor > 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 http:/ / njc.tl/ gu Slide 5 / 52 Slide 6 / 52 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 Induced EMF an historical term that has not gone away. (Electromotive Force) 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 current carrying wire that is not connected to a battery. Return to Table of Contents http:/ / njc.tl/ gu http:/ / njc.tl/ gu Slide 7 / 52 Slide 8 / 52 Induced EMF Induced EMF Michael Faraday connected a battery to a metal coil (to increase Previously, it was shown due to the work of Oersted and Ampere that a current will generate a magnetic field. After this discovery, physicists the magnetic field) and found that a current would be induced in looked to see if the reverse could be true - whether a magnetic field the current loop on the right when the switch on the left side was could generate a current. closed and opened. Michael Faraday was able to make this connection in 1831. 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. There would be zero current on the right side when the current on the left side was steady. http:/ / njc.tl/ gu http:/ / njc.tl/ gu Slide 9 / 52 Slide 10 / 52 Induced EMF Induced EMF This now provided evidence that a magnetic field could generate a current. But, there is a difference. Faraday's Disk Generator - by spinning the metal disk between the poles of the U A steady current will generate a magnetic field. shaped magnet (A), the changing magnetic field will But, a steady magnetic field and a non moving loop of wire will induce an EMF, and hence, a NOT result in a current in the wire. current in the disk (D), which will flow out of the machine via A constant magnetic field and a moving loop of wire will result in a terminals B and B'. current. A changing magnetic field and a stationary loop of wire will result in a current. A bar magnet that moves towards or away from a loop of We need to define Magnetic Flux, and understand it before we can wire will generate an EMF, and fully understand this phenomenon. then a current in the loop. http:/ / njc.tl/ gu http:/ / njc.tl/ gu Slide 11 / 52 Slide 11 (Answer) / 52 1 A bar magnet is moved towards a circular conducting loop. As this 1 A bar magnet is moved towards a circular conducting loop. As this occurs: occurs: A The magnetic field in the loop decreases, and no current A The magnetic field in the loop decreases, and no current flows in the loop. flows in the loop. B The magnetic field in the loop decreases, and a current flows B The magnetic field in the loop decreases, and a current flows in the loop. in the loop. C The magnetic field in the loop increases, and a current flows C The magnetic field in the loop increases, and a current flows in the loop. in the loop. D The magnetic field in the loop increases, and no current D The magnetic field in the loop increases, and no current flows in the loop. flows in the loop. C Answer Answer [This object is a pull tab] http:/ / njc.tl/ gv http:/ / njc.tl/ gv Slide 12 / 52 Slide 12 (Answer) / 52 2 The units of EMF are: 2 The units of EMF are: A Joules A Joules B Volts B Volts C Newtons C Newtons D Coulombs D Coulombs B Answer Answer [This object is a pull tab] http:/ / njc.tl/ gw http:/ / njc.tl/ gw Slide 13 / 52 Slide 14 / 52 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: Magnetic Flux B BA B is the Greek letter "phi" and stands for "flux," or flow. The unit of Magnetic Flux is the weber, Wb, where 1 Wb = 1 Tm2 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. Return to Table Maximum Flux: Field lines perpendicular to surface of Contents Field lines parallel to normal to surface. http:/ / njc.tl/ gx http:/ / njc.tl/ gx Slide 15 / 52 Slide 16 / 52 Magnetic Flux Magnetic Flux The Magnetic Field (blue) is The Magnetic Field (blue) is parallel perpendicular to the plane of the to the plane of the loop of wire loop of wire (orange) and (orange) and perpendicular to its parallel to its normal (red) so the normal (red) so the Magnetic Flux Magnetic Flux is at a maximum is at a minimum and is given by and is given by ΦB = BA. ΦB= 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. http:/ / njc.tl/ gx http:/ / njc.tl/ gx Slide 17 / 52 Slide 18 / 52 Magnetic Flux Magnetic Flux T The Magnetic Flux is proportional to the total number of Magnetic The Magnetic Flux is proportional to the total number of Magnetic Field lines passing through loop. Field lines passing through loop. The Magnetic Flux is at a The black lines are the minimum when the field lines normal lines to the loop. make an angle of zero with the normal. Physically - you can see that no lines go through the loop. Here is a constant Magnetic Field directed to the right with The flux increases as the loop the same loop in three rotates as more field lines pass different positions where through the loop, and reaches is the angle between the a maximum when the field Magnetic Field lines and the lines are parallel with the normal to the surface of the normal. loop. http:/ / njc.tl/ gx http:/ / njc.tl/ gx Slide 19 / 52 Slide 19 (Answer) / 52 3 What is the magnetic flux through a loop of wire of cross sectional 3 What is the magnetic flux through a loop of wire of cross sectional area 5.0 m2 if a magnetic field of 0.40 T is perpendicular to the area 5.0 m2 if a magnetic field of 0.40 T is perpendicular to the area (and parallel to the normal)? area (and parallel to the normal)? Answer Answer [This object is a pull tab] http:/ / njc.tl/ gy http:/ / njc.tl/ gy Slide 20 / 52 Slide 20 (Answer) / 52 4 What is the magnetic flux through a circular loop of wire of radius 4 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 2.0 m if a magnetic field of 0.30 T is perpendicular to the area (and parallel to the normal)? parallel to the normal)? Answer Answer [This object is a pull tab] http:/ / njc.tl/ gz http:/ / njc.tl/ gz Slide 21 / 52 Slide 21 (Answer) / 52 5 What is the magnetic flux through the loop of wire shown below? 5 What is the magnetic flux through the loop of wire shown below? The magnetic field is 1.0 T and the area of the loop is 5.0 m2. The magnetic field is 1.0 T and the area of the loop is 5.0 m2. N N B B N Answer N Answer [This object is a pull tab] http:/ / njc.tl/ h0 http:/ / njc.tl/ h0 Slide 22 / 52 Slide 23 / 52 Faraday’s Law of Induction Michael Faraday and Joseph Henry showed that a changing current will induce an EMF which creates an electric current in a second loop.
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