Electromagnet Motor Generator

Transcription

1 Magnetism and Electromagnetic Induction Study Guide Chapter 36 & 37 Key Terms: Magnetic Pole Magnetic Field Magnetic Domain Electromagnet Motor Generator Electromagnetic Induction Faraday s Law Transformer Key Ideas: Chapter : Magnetic Poles o Know the magnetic poles and how they interact with each other (repel VS attraction) o Know how magnetic poles are similar and different to electric charges (What do they produce, can they be isolated?) o Know the direction of magnetic field lines around a bar magnet, as well as between attracting and repelling magnets o Be able to describe what causes a material to be magnetic or not be magnetic o Know the difference between a permanent magnet and a solid piece of iron Chapter : Electric Currents and Magnetic Fields o Know that an electric current produces a magnetic field know the shape of the magnetic field around a current carrying wire o Know what an electromagnet is and what happens when the loops on it are increased o Know that the magnetic force experienced by a moving charged particle is perpendicular to both the motion of the charge as well as the direction of the magnetic field be able to apply this same idea to a current carrying wire o Be able to calculate the Magnetic Force on a moving charge as well as a current carrying wire o FB = Bqv = ILB Chapter : Meters, Motors, and Earth s Magnetic Field o Know how the basic galvanometer and motor operate using induction of a magnetic field. o Identify the input energy and output energy of a motor o Describe how Earth is a large magnet that s magnetic field is changing over time, explain where evidence for this changing magnetic field is found Chapter : Electromagnetic Induction and Faraday s Law o Understand that an electric current can also be induced by a changing magnetic field o Be able to describe electromagnetic induction in terms of how the current is produced in the coil both when the magnet is brought into the coil and pulled out of the coil o Know what would happen if the magnet were left inside of the coil instead of moved in or out. o Be able to calculate the magnetic flux Φ= AB o Use Faraday s Law to describe the potential difference that is produced by the changing magnetic field V = NΔΦ/t Chapter : Generators and Alternating Current o Know how a simple generator works in terms of a magnetic field inducing electricity. Be able to talk about the input energy and the output energy o Know how magnetic flux applies to the changing magnetic field in a generator o Describe the type of current produced by a generator o Be able to compare and contrast a motor and a generator

2 Chapter : Transformer s and Power Transmission o Know the basic structure of a transformer and describe what the functions of a transformer are. o Describe how a transformer Steps up and steps down voltage o Describe what must happen to the current in order for Energy to be conserved in the stepping up or down of voltage o Be able to use the ratios of the number of coils the primary and secondary coils to determine the voltage steps Vp/Np = Vs/Ns o Use the conservation of power to determine the current in either the primary or secondary coils. o Explain why it is best to carry alternating current through power lines and the role of transformers in bringing current from the power companies to the 120 volts we have access to in our homes Chapter : Induction of Electric and Magnetic fields & Electromagnetic Waves o Know that any changing electric field induces a magnetic field and any magnetic field induces an electric field in response o Be able to describe how electromagnetic waves are composed of oscillating electric and magnetic fields that produce each other o Describe that in order for an electromagnetic wave to continuously produce itself, the speed of light is at 3.0x10 8 m/s. Know that any speed less or greater than that would not lead to the same result o Understand that the visual spectrum of light corresponds to frequencies within the range of 4.3x10 14 to 7.0x10 14 vibrations per second. Conceptual Based Questions 1. What is the source of a magnetic field? *The source of a magnetic field is a changing electric field, essentially any moving charge will produce an magnetic field. 2. If a bar magnet were hung from a string, what pole of the magnet would eventually point to the geometric North Pole? The North side of the magnet would point to the Geometric North pole because North really means North Seeking 3. Look at the bar magnet below. Draw the field lines in the correct direction. The lines should travel away from the north and into the south, some appear to go straight out, others curve around the magnet 4. What is the cause for a material like iron to have magnetic properties? What is the difference between a chunk of iron and a permanent iron magnet? All atoms have electrons that spin, these spinning electrons produce mini magnetic fields. In most atoms, the spins are canceled out, but in Iron they are not. These mini magnetic fields align with each other to form magnetic domains. In a regular chunk of iron, the magnetic domains are randomly oriented and in a permanent magnet all of the magnetic domains are aligned with each other.

3 5. If the bar magnet show below were cut in half what would the two halves of the magnet look like in terms of magnetic poles? (i.e. Can you isolate a magnetic pole?) You cannot isolate a magnetic pole, each half of the magnet would have its own North and South Pole in the same orientation as the magnet above it. 6. Look at the current carrying wire draw the shape of the induced magnetic field. Use the Right Hand Rule to determine the direction of the magnetic field. Point thumb in the direction of the current, fingers will curl in the direction of the magnetic field, the lines are concentric circles around the wire. 7. Would these two electromagnets be attracted to each other or repelled by each other? Indicate the N and S pole on each magnet based on the direction of the current. Wrap your right hand in the direction of the current flow, your thumb will point in the direction of the north pole of each electromagnet. If like sides are near eachother they will repell, if opposite sides are near each other they will attract 8. Compare the input and output of a motor and a generator. Motor Input = Electrical, Output = Mechanical Generator Input = Mechanical, output = Electrical 9. How does the number of loops in a coil affect the amount of voltage that is induced when a magnet is moved through the coil? The more loops that a coil has, the greater the amount of induced voltage 10. Compare the induced voltage and current in a coil of Copper wire VS a coil of rubber that contains the same number of turns. The induced voltage across each coil will be the same; however, the current will depend on how conductive the material is (Essentially the materials resistance) The copper is conductive therefore will have a greater current than the rubber which is an insulator. 11. Explain why the current produced by a generator is alternating and not direct current. As the coil is moved through the magnetic field, the flux is both increasing and decreases, this increase and decrease of magnetic flux cause the induced current to switch directions and produce alternating current rather than direct current like a battery. 12. What is the purpose of a transformer? The purpose of a transformer is to step up or down voltage to run whatever device is currently being run. For Example, Voltage from our outlets is 120 V, but often times a transformer is used to step down the voltage to 12 Volts or 60 Volts for devices that would breakdown with more than that amount of voltage

4 13. How do the speeds of high frequency and low frequency electromagnetic waves compare to each other? *NOT ON TEST 14. How is an electromagnetic wave produced? An electromagnetic wave is produced whenever there is a changing electric field. This changing electric field than causes a magnetic field to be induced, this induced magnetic field than induces an electric field, which then produces another magnetic field. The pattern continues on creating a wave of oscillating energy through space. The electric field and magnetic field travel perpendicular to each other. 15. If the magnet shown below is being brought OUT of the coil shown, what direction will the current be induced in the coil? In the picture, the South side of the magnet is closet to the coil When a magnet is drawn out of the coil, the coil wants to oppose that motion. In this case because the magnet is being pulled away, the induced magnetic field will attract the magnet to it, to cause resistance to the change in magnetic field. In order for the magnetic field to attract the magnet, the North side of the induced magnetic field will be on the Left, closest to the actual magnet. In order for this to occur, the current will be going into the page going away from us. Take thumb and point it in direction of North (to the left) curl fingers this is the direction of the current in the coil. 16. Describe the process of how a transformer works and explain why alternating current is a better source of current than direct current when using a transformer. You have two coils, a primary and a secondary coil. The primary coil is connected to some voltage source, and the secondary coil is near the primary coil. In general, a magnetic core is used to increase the strength of the induced magnetic field. When a current runs through the primary coil a magnetic field is induced. This field then enters the secondary coil and because the magnetic field is changing the secondary coil experiences an induced current. Alternating current is a better source because the current changes directions because of this the induced magnetic field is also constantly changing, which means the current in the secondary current will be continuous as well. Problem Solving: 1. A 0.25 m long wire in a circuit carries a current of 0.5 Amps. It is placed in a magnetic field an experiences a deflection force of 5x10 3 N. What is the field strength of the magnetic field? L = 0.25 m B = (FB/LI) I = 0.5 Amps =.04 T FB = 5x10 3 L N I F B

5 2. A proton experiences a deflection of 3x10 10 N when traveling through perpendicular to a magnetic field of 58 T. What speed was the proton traveling at in order to experience this deflection? (q = 1.6x10 19 C) q = 1.6x10^ 19 C F = Bqv F = 3.0x10^ 10 B = 58 T F/Bq = v 3.0x10^ 10/(1.6x10^ 19x58T) =3.2x10^7 m/s 3. A circular coil contains 150 turns and has a radius of 0.25 cm. The coil, which starts out perpendicular to the magnetic field is rotated 90 degrees every 3 seconds through a magnetic field of 6.0 Teslas. What is the induced voltage across the coil? N = 150 A = pir^2 = pix.0025^2 = m^2 r = 0.25 cm Flux = AB = m^2x6.0T = Wb B = 6.0 T V = NxChange in Flux/time T = 3.0 s V = 150x wb/3.0s =0.006 Volts 4. You need a transformer to step down the 120 volts from your outlet to a useable 6 volts for your computer. Your primary coil contains 100 coils how many coils would your secondary coil require in order for the correct voltage step to occur? Vp = 120 volts Vp/Np = Vs/Ns Vs = 6 volts Ns = VsNp/Vp Np = 100 = 5 loops