Page Experiments with Magnets Introduction In this lab, the student will investigate magnetic field direction and magnitude for a permanent magnet and a long straight wire. The student will be introduced to the concept of magnetic induction and will demonstrate Faraday's Law and Lenz's Law. The student will demonstrate the magnetic field accompanying an electric current in a wire, and look at forces on currents due to magnetic fields. Theory Magnetic Fields; The field of a magnet may be described in terms of magnetic poles. The magnet has a north pole and a south pole at either end. Opposite poles will attract each other while like poles will repel each other. If a magnet were to be cut into pieces, each piece would have both a north and south pole. The magnetic field may be represented by lines, just as with electric fields. The lines go between the north and south magnetic poles and their direction is, by convention, pointing from north to south. Unlike electric fields, magnetic field lines do not point in the direction of the force on a charge. Also unlike electric fields, there is no experimental evidence for the existence of a magnetic monopole. The SI unit for measuring the strength of a magnetic field, B is Tesla (T): T = 10* Gauss. The needle of a compass is a small permanent magnet. If the compass is out of the immediate vicinity of local sources of magnetic fields, its needle will align itself with the Earth's magnetic field pointing to the south of magnetic pole of the Earth (and the geographic North). When electric charge is in motion, a magnetic field is established in the space surrounding it. For a long straight wire carrying a current I, the magnetic field at a distance r from the wire is given by: 2717 (1) where po is the magnetic permeability of free space. To determine the direction of the magnetic field lines surrounding a wire, point the thumb of your right hand in the direction that the current travels. Your fingers curl around the wire in the direction of the magnetic field. The field lines circle the wire. A solenoid consists of multiple (N) loops of wire with current I flowing through it. If the solenoid has length 1, the magnetic field within the solenoid is given by: B _ HoIN (2)The direction of the magnetic flux from a solenoid can also be obtained by the right hand rule. Curl the fingers of your right hand such that they go around the loops of wire in the direction of the current. Your thumb will then point in the direction of the magnetic field. Induced Currents: Magnetic flux () through a surface is a measure of the magnetic field passing through the surface. A magnetic field directed at an angle q to the normal to a surface with area A generates a magnetic flux of: D - BAcos # (3) Thus the flux is maximum when the magnetic field is perpendicular to the surface, and minimum when it is parallel to the surface. There are three ways to change the magnetic flux: to change the magnetic field, to change the area and to change the angle of the field to the surface. We will investigate all three of these in this laboratory. Faraday's Law of Induction states that a changing magnetic flux will induce an emf a in a conductor with N turns of wire, proportional to the time rate of change of the flux, . 2 =-N- (4) If the conductor is part of a closed circuit, a current is induced in the conductor. This current creates its own magnetic field. The direction of the induced current is such as to oppose the cause producing it. This relationship is Lenz's Law. In Parts 4 and 5 of this laboratory, we will study the effects of changing magnetic flux on the voltage induced in a coil There are several demonstrations of Lenz's law in the video clips in Part 6 of this laboratory. These will be described in that section. Magnetic Forces: As discussed above, an electric current generates a magnetic field. Since magnetic fields interact (like poles repel, opposites attract), an electrical current will feel a force on it in a magnetic field. The magnitude of the force,F, on a wire of lenth L, carry a current I in a magnetic field B is given by: F - ILB sin e Here, 0 is the angle between the current and the magnetic field. The direction of this force can be determined by the right hand rule: put the fingers of your right hand along the current direction, and then bend them toward the direction of the applied magnetic field. The force will be in the direction of your thumb. Reversing the direction of current will reverse the direction of the force. Two parallel wires with current flowing in them will also exert a force on each other: currents in the same direction attract, while opposite currents repel-Page 3 Procedure Part 1: Magnetic Field Direction Go to http://www.walter-fendt.de/html5/phen/magneticfieldbar_en.htm (you may need to type this link directly into your browser, as sometimes cutting and pasting from PDF documents doesn't not work) A screen should load that looks as follows: Magnetic Field of a Bar Magnet cution the direction of tu told ines wil reverse The cafe button manas a possible to clear all hale 1. Move the compass around the screen, and watch the way the needle points as you move it. Is the red end of the compass a north pole or a south pole? 2. Every time you release the compass, the app will draw a magnetic field line with arrows showing the direction of the field lines. In what direction do the field lines point? 3. Place the compass directly over one of the field lines on the screen. How does the needle on the compass compare to the direction of the magnetic field lines shown screen? 4. Click "Turn Magnet" in the right hand bar. What happens to the direction of the magnetic field lines? 5. Move the compass around the magnet again. Do your results confirm the results from step 1?Part 2: Magnetic Field Strength from a Long Straight Wire Go to http:/www.thephysicsaviary.com/Physics/Programs/Labs/FieldFrom Wire/index.html, and click "Begin" A page will load that looks like the following: The solid gray bar represents a long straight wire. The gray loops around the wire are the magnetic field lines. Magnetic Field from a Wire Lab Field Strength er MAGNETIC FIELD Location of Field Sensor Direction (Electron Flow) Current Click in the red outlined box labeled "Direction (electron flow)", it will open and you will see a box as shown on the right. Click the red Direction (Electron Flow) arrow, and notice that the direction of the small blue circles reverses direction. What happens to the direction of the magnetic field lines when you reverse the electron flow direction? In the same box, click on the words "change to conventional". Now. rather than showing electrons moving through the wire, the animation will show the direction of positive charge flow. According to the right hand rule, if you place the thumb of your right hand in the direction that the current is flowing, your fingers should curl around the wire in the direction of the magnetic field lines. Check that this is true. Click the arrow to reverse the current flow, and try it again. 3. Click the box labeled "Grid" and the box labeled "Field Strength". The box at the top right should now show the magnitude of the magnetic field at the location labeled + on the magnetic field sensor Inler MAGNETIC FIELD A positive field value means that the magnetic field is out of the plane, and a negative value means that it is into the plane.Page Click the arrow to reverse the current direction, without moving the magnetic field sensor. Does the magnitude of the magnetic field change? What happens to the sign of the field? S. Next we will verify equation 1 for the magnitude of magnetic field as a function of distance from a current carrying wire. On the bottom bar, click the "Location of field sensor" and "Current" boxes to open them. Your screen should look something like this: Magnetic Field from a Wire Lab 3 Flald Labquest @ 1 1 Magnetic Field TIC FIELD Vemier 20.11 AT Distances (cm) Field Strength Location of Field Sensor Direction (Electron Flow) Current 5.5 A You can move the field sensor location around the grid by clicking on the appropriate arrows. You can also increase and decrease the current. Note that the magnetic field is located at the center of the + crosshairs on the magnetic field sensor. a. Move the magnetic field sensor so that it is located at coordinates (1,1). The first number in the brackets indicates the horizontal location (along the wire) while the- second number indicates the vertical location (away from the wire). Note the magnetic field value. Now move the sensor to the left and right, always leaving it I cm away from the wire. Is there any change in field when you do this? What would you expect from equation 1? b. Return the magnetic field sensor to (1,1). Write down the value for B. (We will call this BI) Note that the units for magnetic field on this sensor is micro Tesla rather than Tesla, since one Tesla is a huge magnetic field. c. Pull the field meter one more grid space to up to (1,2), and record the value for B, we will call this B2.Page 16XUJovELU https://www.youtube.com/watch?=N7:Hi71-AjA