Magnetism & EM Induction
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1 Physics Traditional 1314 Williams Magnetism & EM Induction Chapters 19,20
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3 Magnetism Notes Moving charges are the source of all magnetism. Since the smallest charge which can eist is an electron, and electrons move, electrons are the world s smallest magnets. The world s largest magnet would be. the world, since the Earth itself has a magnetic field. A magnet always has two poles: north and south. Opposite poles attract and similar poles repel. This is very much like electrostatics, but please remember that magnets have poles, not charges! The direction of a magnetic field is the way a compass would point if in that field. Thus, the Earth s magnetic field points north. A compass is just a tiny magnet whose north magnetic pole is attracted to the top of the Earth. But since a north pole (compass) is attracted to the north geographic pole of the Earth, there must be a magnetic south pole present there. The Earth acts as though it it, but this is just an illusion. currents movement-- within core are what create the magnetic poles are not eactly south pole is in Canada; the south Pacific. This means your the north or south geographic has a large bar magnet inside Instead, the convection in the liquid nickel and iron magnetic field. The earth s on its ais. The magnetic magnetic north pole is in the compass will not lead you to poles. An atom is magnetic if it has more electrons spinning one direction than another. Most atoms are not magnetic because the electrons are symmetrically placed. If you have more atomic magnets pointing one way than another, the entire material shows magnetism. If you cut it in half, the material will have fewer atomic magnets aligned, so it will have weaker magnetism. Similarly, if you hit it with a hammer or melt it, atomic magnets tend to become randomized. 3
4 On the other hand, if you heat a magnet and let it cool in the presence of a strong magnetic field you can create a stronger magnet. Even putting a solid magnet in a strong magnetic field causes some of the atomic magnets to align a little bit more, strengthening the magnet. Three common elements have strongly magnetic atoms: iron, nickel, and cobalt. One common alloy, steel, is magnetic. It is a combination of iron and carbon. These materials form permanent magnets. However, any material which will carry a current can be made into a special, temporary form of magnet called an electromagnet. To create one, coil up a wire into many loops by wrapping it around a nail. Connect the ends to a battery so current flows through the wire. Each loop of current creates its own magnetic field, and the coiled wire reinforces each loop s field. The strongest magnets in the world are electromagnets. They are used in places such as junkyards where heavy metal objects must be lifted and moved and then released again. When a charged object moves through a magnetic field, it eperiences a force which tries to push it off course. The force, F, the object feels is: F = q v B where: q = the test charge which is moving (C) v = the speed of the test charge (m/s) B = the magnetic field (in Teslas) If instead of a charged object, a wire with a current is in a magnetic field, the equation is as follows: F = I L B where: I = current in the wire (amps) L = length of wire (meters) B = magnetic field (Teslas) Since force, velocity (or current direction), and magnetic field are vectors, they must have directions involved. These directions are all mutually perpendicular to one another. Read the Right Hand Rules notes pages to learn how to determine the directions of these items. 4
5 Right Hand Rules The Formulas: To determine how hard a moving charge or current carrying wire will be pushed when it is in an eternal magnetic field, use the appropriate formula: F = q v B F = I L B Rule #1: To determine the force on a POSITIVE charge moving in an eternal magnetic field: Make a gun out of your right thumb and inde fingers and aim it towards away from you. Hold your remaining fingers together and bend them to point to the left. Thumb = F = direction of the force eperienced by the moving POSITIVE charge(s) Inde finger = qv OR IL = the direction of motion of conventional current. To be consistent with Middle finger = B = the direction of the eternal magnetic field that the moving charges are eperiencing (This is NOT the field caused by the charges themselves!) *** This rule may be used for electron current if done with the left hand. Rule #2: For wires: Grasp the wire in your right hand with your thumb pointing in the direction of the conventional current. Your fingers will curl in the direction of the magnetic field that the current creates around the wire. I o o o o o o o B 5
6 Notes Assignment: Use the notes pages to answer the following questions: 1. Why isn't the following chunk of iron magnetic? 2. What is the world s smallest magnet? 3. What is the world s largest magnet? 4. What are the three elements which have strong magnetism at the molecular level? 5. What is the common alloy which also has strong magnetism at the molecular level? 6. Name two ways of destroying a permanent magnet. 7. Eplain how to create an electromagnet. 8. List at least one pro and one con of using an electromagnet. 9. You are sailing to visit the Special Winter Day Religious Figure of Your Choice, who lives at the north pole (i.e., the eact geographical top of the earth). Will your compass get you there? Eplain. 10. Which formula would determine how hard a proton is pushed if moving north in a magnetic field which is aimed downward? 11. Which formula would eplain why a vertical current carrying wire a feels a push when it is in a magnetic field aimed to the east? 6
7 12. A proton is falling in the Earth s magnetic field. Which way will it be pushed? 13. An electron is falling in the Earth s magnetic field. Which way will it be pushed? 14. A positively charged object is launched to the east in the Earth s magnetic field. Which way will it be pushed? 15. Current is moving to the west in a wire which lies in a magnetic field aimed south. What is the direction of the push on the wire? 7
8 Phun Magnetism Minilab WARNING: Be cautious with electronics you may have on your person around strong magnets! 1. Investigate which materials show magnetism and which do not using the strong magnet provided. Make a table below summarizing your results. 2. Use the magnet with the North and South poles provided to prove earth s geographic north has a south magnetic pole. 3. How does magnetism make the spinner rotate for so long? 4. Please DO NOT remove any magnetic rings. They are brittle. Look at the levitating magnetic rings. Using the magnet with a N and S pole shown and your logic, figure the direction of each pole on the ring. Draw a picture showing your final results. 5. Notice the magnetic field lines. Were they put in that position to illustrate a magnetic field, or do they naturally go in that direction? Do magnets line up parallel or perpendicular to magnetic field lines? 8
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14 Faraday s Law and Lenz Law We already know that a current causes magnetism. In fact, we know that around a current-carrying wire, there are concentric circles of magnetic field lines (B). We can predict whether those lines will be clockwise or counterclockwise in direction by using our Second Right Hand Rule. Faraday s Law shows that a closed loop must eperience a changing magnetic field in order to have current flow in it. So it s not quite symmetric. Current causes magnetism; changing magnetism causes a current. Here is Faraday s Law: = - N (BA) t Where is the emf or induced voltage. It s like a battery voltage only without the battery! This means you can use the induced voltage to calculate current by following Ohm s Law: V=IR. Either the magnetic field B or the area of the loop A must change over time. A compass shows the direction a magnetic field points; we now are considering how strong the field is. You could move a magnet closer or farther from the loop, move the loop closer or farther from a magnet, or increase the current in a wire (since it has a magnetic field around it) which is near a loop in order to make B change. To make A change, you may stretch the loop out, close it up, or turn in so more or fewer magnetic field lines are enclosed in the loop. N represents the number of loops which are in the changing field; often there is just one. Notice the negative sign in Faraday s Law? That represents Lenz Law. When the changing magnetic field causes a voltage in the loop, a current will flow through it. (Ohm s Law) But which way does the current flow? Lenz Law says the induced current will flow so that its magnetic field will oppose the change which caused it. So if the loop is in a magnetic (B) field of increasing s, it will try to add some dots. If the loop is in a magnetic field of decreasing X s, it will try to add more s. Think of it as magnetic inertia. Nature wants to keep the magnetism constant in an area. If the magnetic field is increasing, nature will try to diminish it. If it is decreasing, nature will try to augment it. Lenz Law problems: Determine the direction of the current induced in each loop. I incr I incr I decr I decr I const I incr 14
15 Eample: Suppose we have a wire with increasing current up the page, and there is a loop on the right side of the wire. The loop is in the wire s magnetic field. First, find out the direction of the wire s magnetic field where the loop is. I incr Using your Second Right Hand Rule, your thumb points up the page (following current direction) and your fingers curl into the page to the right of the wire. So you should draw s to the right of the wire. I incr X X X X X X X X X X X X The X s will get stronger as the wire s current increases. Nature doesn t like this. Nature will try to add a few dots to keep the magnetic field constant. How does it do this? Remember that the loop s induced current can only be either clockwise or counterclockwise. X XX X XX X XX If the current is clockwise (left picture) then according to your Second Right Hand Rule, its magnetic field will add X s inside the loop. Check this by pointing your thumb along the current on the left side of the loop; your fingers will curl into the page (X s) inside the loop. But this would make more X s, and we re trying to reduce them. Now try the same thing with the counterclockwise loop. Your fingers will curl out of the page (dots). So in this eample, the current induced in the loop will flow counterclockwise. X X X X X X I incr X X X X X X 15
16 E&M Unit 4 - Magnetism: Worksheet 4 Magnetic flu and induced V The following diagrams show a loop connected to a resistor and a bar magnet. In each situation, determine the direction of the induced current in the loop and which side of the resistor has a higher potential. 1.A magnet moves nearer a loop perpendicular to the plane of the paper Name Date 2.A magnet moves away from a loop perpendicular to the plane of the paper. Pd N S N S b b a 3. A magnet remains in place while the top of the loop is pulled upward, bringing the sides of the loop together. a 4. A magnet is brought near a loop in the plane of the paper. N S N S b a b a 5. A magnet remains in place while the loop, originally perpendicular to the plane of the paper is rotated CW into the plane of the paper. 6. A magnet remains in place while the loop, originally in the plane of the paper is rotated CW out of the plane of the paper. N S N S b a b a 16
17 E&M Unit 4 - Magnetism: Worksheet 2 Part 1 - Magnetic Force Direction The following diagrams show a charged particle or a current carrying wire in a magnetic field. For each diagram use the right-hand rule to draw an arrow on the object that shows the direction of the magnetic force. Remember that a means the direction is into the page and a means the direction is out of the page towards you. 1. B 2. Name B Date Pd B B I vinto paper B out of paper B into paper + I B B out of paper v I out of paper - 17
18 A rigid wire loop is placed in a uniform magnetic field as shown below. Determine the direction of the current in each segment of the loop, then the direction of the force on each segment. 9.Direction of magnetic force on segment 1 a) to the left b) to the right c) out of plane of page d) into plane of page e) none 10.Direction of magnetic force on segment 2 a) to the left b) to the right c) out of plane of page d) into plane of page e) none 11. Direction of magnetic force on segment 3 a) to the left b) to the right c) out of plane of page d) into plane of page e) none 12. Direction of magnetic force on segment 4 a) to the left b) to the right c) out of plane of page d) into plane of page e) none 13. Will the loop tend to rotate? If so, in which direction? Eplain. 18
19 Unit 10 Vocabulary and Equations Magnetism & Induction Vocabulary: previous vocabulary magnetism electromagnet permanent magnet, natural magnet lodestone ferromagnetism magnetic field (B), vector, magnetic field lines field force, magnetic field strength domain theory magnetic North Pole, geographic North Pole induction motor, generator primary coil, secondary coil magnetic south pole, dipole, geographic north pole right-hand rule Tesla (T), Coulomb (C), Amp (A) Lenz's law, Faraday's law of induction Symbols: N, V, B, A, q, v, ε, Δt Equations & constants: You get these on test: N 1 N 2 Transformer equation: = V 1 V 2 N Δ(BA) F = q v B F = i L B ε = Δt Unit Objectives - Williams 1. Cause of magnetism and domain theory 2. F = qvb and F = ILB: The force on a current carrying wire or charge moving through an eternal magnetic field 3. Magnetic fields about wires and permanent magnets 4. Magnetic induction: Faraday s Law and Lenz Law DuPage ROE Objectives 701. I can recognize and eplain what causes magnetic fields I can identify the direction of magnetic fields I can distinguish between magnetic fields and electric fields I can understand the relationship between magnetic and electric fields I can recognize that an object must be charged and moving in a magnetic field in order to eperience a magnetic force. 19
20 Physics Calendar - Magnetism & Induction: (Williams) - Chapters (11 days) Bold and underlined means put in journal notes (for any problems: Show your work!) GOALS: Magnetism overview & introduction Overview and & to slide 20 (Faraday's law) Galvanometer demo (derive/prove Faraday's law) R10-01: Simple right 1 We:02/26/14 Show and o symbols for B direction hand rule problems GOALS: Transformers & mied review INDUCTION! (10-03) p 686: 1-3; p 2 Th:02/27/14 Finish notes, demo transformer, simple transformer math 689: 1-3 Fr:02/28/14 Teacher's 3 Institute No school for students GOALS: Review (3 days off!) R10-02 (mied 4 Mo:03/03/14 Show class review and then group time with mied probs problems) GOALS: Hard right hand rule! F = ilb & F = qvb Go over notes; make and/or use FIB's FIB practice as a class (clickers might be good here) 5 Tu:03/04/14 FIB group practice and/or start HW (including math) (10-06) p 692: 1-4; GOALS: Review & Group time (10-07) p 695+: 6L We:03/05/14 Review from yesterday, then HW time 2,5,6,22,26,35,40b GOALS: Lenz's law & Magnet mini lab 10-05: Show Lenz's law & loop problems (packet) 7 Th:03/06/14 Magnet minilab, then start HW R10-03 GOALS: Go over HW & group activity Go over HW, take any Q's, review loop problems (10-08) p 714: 1,3; p 8 Fr:03/07/14 Start HW as time permits 722: 1 GOALS: Generators Show computer lab simulation of generator (demo) If time permits, do computer lab sim, or more review time depending on class needs (10-09) p 729: 1,3,5; p 9 Mo:03/10/14 Start HW if time 744+: 2,3,8 GOALS: Tie up any loose ends Quick review of unit (hi-lights) (10-10) p 739+: 10 Tu:03/11/14 Group problems 12,34,36,37 GOALS: review day 11 We:03/12/14 Test TOMORROW! 12 Th:03/13/14 Magnetism & Induction Eam 20
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