# Name: Date: Regents Physics Mr. Morgante UNIT 4B Magnetism

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1 Name: Regents Physics Date: Mr. Morgante UNIT 4B Magnetism

2 Magnetism -Magnetic Force exists b/w charges in motion. -Similar to electric fields, an X stands for a magnetic field line going into the page, and a ٠ represents a magnetic field line coming out of the page. -Magnetic properties are due to the electrons orbiting the nucleus of an atom. -The electrons generate magnetic fields by spinning on their axis. Magnetic fields are similar to electric fields. The only difference is that electric fields dealt with negative and positive terminals, and magnetic fields deal with north and south poles. Flux Density- measurement of the field intensity per area. The more field lines (as shown above, the more flux density. A magnetic field is defined as a region where magnetic force may be detected. If you take a compass or some other similar device, you should have movement of the needle in a magnetic field.

3 Direction of a magnetic field is the direction in which the N-pole of a compass would point in the field. Magnetic field around a bar magnet is mapped by drawing magnetic flux lines (lines of magnetic force) The flux lines always form closed paths and they never cross, just the same as you learned with electric fields lines. Direction of magnetic fields are tangent to the field line at any point. 3

4 Tangent Line Tangent Line Tangent Line Take a look at this website, It has great diagrams of magnetic fields. -Remember, the field lines bunched together are called the flux, and the amount of flux lines in a specific area is called flux density. SI unit for flux is the Weber and Weber/m 2 for flux density. -Field is strongest near the poles, where the lines are closest together. This statement corresponds to the flux density definition given earlier. -If a magnet were broken into pieces, each piece would have a north and south pole. Moving electric charges produce magnetic fields! If you have current moving through a wire, a magnetic field will be generated. CURRENTS ACT ON MAGNETS, AND VICE VERSA! Drawing Magnetic Field Lines Anytime we need to draw vectors pointing into the page (away from you) or out of the page (towards you) we will use the following symbols

5 Into Page Out of Page 1. The into page is supposed to look like the feathers on the end of an arrow or dart that is flying away from you. 2. The out of page is supposed to look like the tip of an arrow coming at you. 3. This leads to our third hand rule. The following right hand rules help determine the direction of magnetic fields, electric currents and the magnetic forces that develop between the two. Right Hand Rule #1- Step 1- Point thumb in dir of electric flow Step 2- Grasp wire w/ right hand Step 3- The four fingers wrapped around the wire point in the direction of the magnetic field! "When a current-carrying wire is grasped in the right hand, the thumb pointing in the direction of the electron current, from negative (-) to positive (+), the fingers will point in the direction of the magnetic induction field." Right hand rule #2- Use this rule for a coil shaped current carrying wire, also called a solenoid. 5

6 With right hand rule #2, fingers now are direction of e- flow & thumb is direction of magnetic field. "When a current-carrying coil of wire is grasped in the right hand, the fingers curled around the coil in the direction of electron flow, from negative (-) to positive (+), the thumb will point toward the North pole of the electromagnet." Right Hand Rule #3 What happens when a wire carrying current is within a magnetic field? This is the Right Hand Rule #3 (for motors also, as will be discussed later). The middle finger points in the direction of the magnetic field (first - field), which goes from the North pole to the South pole. The index finger points in the direction of the current in the wire (second - current). The thumb then points in the direction the wire is thrust or pushed while in the magnetic field (thumb - torque or thrust).

7 So, when a wire carrying current sits perpendicular to a magnetic field, a force is created on the wire causing it to move perpendicular to the field and direction of current. The greater the current in the wire, or the greater the magnetic field, the faster the wire moves because of the greater force created. If the wire sits parallel with the magnetic field, there will be no force on the wire. In between there will be some force, but a maximum force is only achieved in the perpendicular. This is what Tesla exploited to make AC motors. Magnetic Fields Exert Forces on Current Carrying Wires -When a current carrying wire is placed in a magnetic field, a force is exerted on the conductor. -The force is perpendicular to both the field & the current. I= Current B= Mag. Field F= Force This is determined by right hand rule #3 where: 7

8 B= Index Finger = Mag field dir. I= Middle Finger = Current dir. F= Thumb = Mag field dir. This diagram illustrates right hand rule #3 in two dimensions. Current Carrying Wires Exert Forces on Each Other Each current carrying wire exerts a force through the magnetic field that each creates. This concept is the basis for magnetic levitating trains (MAGLEV)! Circular magnetic fields are generated around current carrying wires. The strength of these fields varies directly with the size of the current flowing through the wire and inversely to the distance from the wire. In this diagram, the solid purple circle in the center represents a cross-section of a currentcarrying wire in which the current is coming out of the plane of the paper. The concentric circles surrounding the wire's cross-section represent magnetic field lines.

9 If currents pass along two parallel wires, each wire will set up a magnetic field and the fields will interact according to the rules described in the previous topic. Two parallel conductors which each carry a current in the same direction will attract one another, while two such conductors which carry currents in an opposite direction will repel each other. Why do parallel current-carrying conductors attract one another? Conductor B produces a magnetic field, which is perpendicular to the current flow in A. The force acting on A will be at right angles to the current flow and at right angles to the magnetic field. Now apply the right hand rule: point the first finger in the direction of the field, the second finger in the direction of the current, and then your thumb will point in the direction of the direction of the force experienced by the conductor A. A similar argument will show that the field due to conductor A will cause a force on B towards A. Hence the conductors are attracted to each other. 9

10 Electromagnetism Electromagnetic Induction - A straight conductor is moved across a magnetic field. The charge (e-) in the conductor will be acted on by the magnetic force. The magnetic force will move the charge along the conductor from one end to another. This will cause a buildup of negative charge at one end and a deficiency of charge at the other. This produces a potential difference. The potential difference at one end then develops at the other end of the conductor (high to low concentration concept). This process keeps on occurring at the opposite ends of the conductor which when connected to a mechanical device such as an axle creates the basis for a motor or generator which we will discuss next. The same thing happens if you move the magnetic field through a stationary wire as in right hand rule #3. By wrapping an insulated strand of wire around a piece of iron or steel and running electrical current through the wire, the metal can become magnetized. This is called an electromagnet. A magnetic field is created around a wire when it has electrical current running through it, and creating a coil of wire concentrates the field. Wrapping the wire around an iron core greatly increases the strength of the magnetic field. Questions you may have include: How can you make an electromagnet? What factors are involved in electromagnetism? How does an iron core affect the strength? Making an electromagnet If you wrap a wire around an iron core, such as a nail, and you send electrical current through the wire, the nail will become highly magnetized. You can verify that by picking up small objects or by showing its effect on a compass. This is called the electromagnet. Creating a simple electromagnet using a nail

11 Insulated wire Note that the wire must be an insulated wire. A bare wire would short out through the nail or metal core. In some electromagnets, like in an electric motor, the wire will look like bare copper. In reality, it is insulated with a thin coating of a clear material. Also, if the wire is thin, it may get warm from the resistance to the electricity passing through it. Turn on and off The most interesting feature of the electromagnet is that when the electrical current is turned off, the magnetism is also turned off. This is especially true if the core is made of soft iron, which won't become a permanent magnet. Being able to turn the magnetism on and off has lead to many amazing inventions and applications. How electromagnetism works When electricity passed through a wire, a magnetic field is created around the wire. Looping the wire increases the magnetic field. Adding an iron core greatly increases the effect and creates an electromagnet. Magnetic field When DC electricity is passed through a wire, a magnetic field rotates around the wire in a specific direction. Magnetic field rotating around wire Compass can show field 11

12 This can be demonstrated by connecting a wire to a battery and placing a compass near the wire. When the current is turned on, the compass needle with move. If you reverse the direction of the current, the needle will move in the opposite direction. Right hand rule To find the direction the magnetic field is going, you can use the "right-hand rule" to determine it. If you take your right hand and wrap it around the wire, with your thumb pointing in the direction of the electrical current (negative to positive), then your fingers are pointing in the direction of the magnetic field around the wire. Try it with the picture above. Wire in a coil Wrapping the wire in a coil concentrates and increases the magnetic field, because the additive effect of each turn of the wire. Coiled wire increases magnetic field A coil of wire used to create a magnetic field is called a solenoid. Iron core Wrapping the wire around an iron core greatly increases the magnetic field. If you put a nail in the coil in the drawing above, it would result in an electromagnet at the north seeking pole on the "N" side. Using AC electricity If AC electricity is used, the electromagnet has the same properties of a magnet, except that the polarity reverses with the AC cycle. Note that it is not a good idea to try to make an AC electromagnet. This is because of the high voltage in house current. Using a wire around a nail would result in a blown fuse in the AC circuit box. There is also the potential of an electric shock. Strength of electromagnetic field The strength of the electromagnetic field is determined by the amount of current, number of coils of wire, and the distance from the wire. Unit The unit of magnetic force is called the tesla (T). Near a strong magnet the force is 1-T. Another unit used is the gauss, where 10 4 gauss (10,000) equals 1 tesla. Current

13 The strength of the magnetic field is proportional to the current in the wire. If you double the current, the magnetic force is doubled. Since Voltage = Current x Resistance, you can double the current in a wire by doubling the voltage of the source of electricity. Turns of coil If you wrap the wire into a coil, you increase the magnetic force inside the coil, proportional to the number of turns. In other words, a coil consisting of 10 loops has 10 times the magnetic force as a single wire with the same current flowing through it. Likewise, a coil of 20 loops has 2 times the magnetic force than one with 10 loops. Distance The greater the distance from the wire, the less the force, inversely proportional to the square of the distance. For example the force at 2 cm. from a wire is 1/4 that of at 1 cm., and the force at 3 cm. is 1/9 the force at 1 cm. Effect of iron core When the coil is wrapped around an iron core, the strength of the electromagnetic field is much greater than the same coil without the iron core. This is because the atoms in the iron line up to amplify the magnetic effect. The orientation of the atoms in the iron is called its domain. Current When you increase the current, the magnetic strength increases, but it is not exactly linear as it is with the coil by itself. The characteristics of the core cause the curve of magnetic strength versus current to be an s-shaped hysteresis curve. The shape of this curve depends on how well the material in the core becomes magnetized and how long it remains magnetized. Soft iron loses its magnetism readily, while hard steel tends to retain its magnetism. In conclusion By wrapping a wire around an iron core and applying an electric current through the wire, you create an electromagnet. This device is magnetic only when the current is flowing. The iron core greatly increases the magnetic strength. Other main ideas include: - A conductor that cuts the lines of flux creates a potential difference in the 13

14 conductor. -If the conductor is connected to a circuit, a current will flow. - The direction of the current that is induced is such that its magnetic field opposes that of the field that induced the current! The Generator Principle A conducting loop rotating in a uniform magnetic field experiences a continual change in the # of flux lines crossing the loop. This change induces a potential across the ends of the loop, which alternates in direction & varies in magnitude (b/w none and maximum). When the plane of the loop is perpendicular to the field, there is no induced potential. When the plane of the loop is parallel to the field, the involved potential is maximum. -Magnitude of induced potential is proportional to: 1) Velocity perpendicular to the field. 2) Intensity of magnetic field. - A current will be created in a circuit as a result of the induced potential. -Since the induced potential alternates b/w both ends, the current is an alternating current. -Transformer= A device used to increase or decrease voltage. Does not affect power generated. Loop is spun If the wire is made into a loop that is then spun or rotated in the magnetic field, you can have continuous current. Since each side of the loop is going in a different direction in the magnetic field, the current flows around the loop, depending on which direction it is rotated. Transfer current There must also be some way to transfer the current to the rest of the circuit. In an AC generator, this is done by having a ring on each end of the wire. A metal contact or brush rubs or slides against each ring, allowing the electricity to flow through the circuit. In a DC generator, this is done using one split-ring called a commutator. An AC generator uses two slip rings.

15 Comparison of DC and AC loops and rings Generator in action The following picture shows an AC generator. As one side of the loop moves to the other pole of the magnetic field, the current in it changes direction. The two slip rings of the AC generator allow the current to change directions and become alternating current. Simple AC generator (Image from the PBS American Experience series: Inside the AC Generator) 15

16 In a DC generator, the split-ring commutator accommodates for the change in direction of the current in the loop, thus creating DC current going through the brushes and out to the circuit. Note that the DC current is not a steady value. Rather, it is a "bumpy" signal, with zero voltage at the break in the ring. The power from the current could be mathematically described as a sine wave squared. Since most DC generators have many more than one loop, the "bumps" even out and are not noticed. The faster the wire passes through the magnetic field, the greater the current. Full-sized generators Instead of having a single loop, generators used to supply electricity to homes and businesses have multiple magnets and loops consisting of wires wound around an iron core, similar to an electromagnet. The more loops of wire passing through the magnetic field, the higher the voltage that is created. Large generator with multiple windings Generators used to provide electricity to the community are huge. The rotor can be well over 10 feet in diameter. Can be used as motor Note that when a generator has its wire wound around a iron core, it can also be used as an electric motor. Instead of rotating the loops through a magnetic field to create electricity, a current is sent through the wires, creating electromagnets. They will then be repelled by the outer magnets, thus rotating the shaft as an electric motor. If the current is DC, the split-ring commutators are required to create a DC motor. If the current is AC, the two slip rings are required to create and AC motor.

17 Examine an unplugged electric motor to see how both a motor and generator looks inside. In conclusion Electrical current is generated by moving wire through a magnetic field. Electrical generators rotate a coil of wires through a magnetic field. The difference between an AC and a DC generator is that the AC generator uses slip rings to transfer the current to the electrical circuit, while the DC generator uses a split-ring commutator. Very large generators create electricity for the community. An electric motor is very similar to a generator, except that power is provided to turn the rotors. 17

18 NAME DATE Regents Physics Mr. Morgante Magnetism Notesheet Raise your hand if you are tired of doing my Regents Physics Notesheets- Good! Do your own for a change! Directions: Read Review Book pages on Magnetism. Create some sort of outline that encompasses bold print, sketches, units, vectors, + / - charges, etc. Be ready to explain your outline in an interview.

19 Name: Regents Physics Date: Mr. Morgante Magnetism Problems 1. A wire conductor is moved at constant speed perpendicularly to a uniform magnetic field. If the strength of the magnetic field is increased, the induced potential across the ends of the conductor a) remains the same c) can't be determined b) decreases d) increases 2. Which diagram below best represents the magnetic field near a bar magnet? a) b) c) d) 3. The diagram below shows a bar magnet. Which arrow best represents the direction of the needle of a compass placed at point A? a) b) c) d) 4. In order to produce a magnetic field, an electric charge must be a) stationary b) negative c) positive d) moving 19

20 5. The diagram below represents a 0.5-kilogram bar magnet and a 0.7-kilogram bar magnet with a distance of 0.2 meter between their centers. Which statement best describes the forces between the bar magnets? a) Gravitational force and magnetic force are both attractive. b) Gravitational force and magnetic force are both repulsive. c) Gravitational force is attractive and magnetic force is repulsive. d) Gravitational force is repulsive and magnetic force is attractive. 6. The diagram below shows an electron, e, located in a magnetic field. There is no magnetic force on the electron when it moves a) toward the right side of the page c) toward the top of the page b) into the page d) out of the page 7. The diagram below represents the magnetic lines of force around a bar magnet. At which point is the magnitude of the magnetic field strength of the bar magnet the greatest? a) A b) D c) B d) C

21 8. An electromagnet with an air core is located within the magnetic field between two permanent magnets. The air core of the electromagnet is replaced with an iron core. Compared to the strength of the magnetic field in the air core, the strength of the magnetic field in the iron core is a) the same b) can't be determined c) greater d) less 9. The two ends of a wire are connected to a galvanometer, forming a complete electric circuit. The wire is then moved through a magnetic field, as shown in the diagram below. The galvanometer is being used to measure a) potential difference b) temperature change c) resistance d) current 10. The diagram below shows a proton moving with velocity v about to enter a uniform magnetic field directed into the page. As the proton moves in the magnetic field, the magnitude of the magnetic force on the proton is F. If the proton were replaced by an alpha particle under the same conditions, the magnitude of the magnetic force on the alpha particle would be a) F/2 b) F c) 4F d) 2F 21

22 11. The diagram below represents the magnetic field near point P. If a compass is placed at point P in the same plane as the magnetic field, which arrow represents the direction the north end of the compass needle will point? a) b) c) d) 12. The diagram below shows a wire moving to the right at speed v through a uniform magnetic field that is directed into the page. As the speed of the wire is increased, the induced potential difference will a) decrease b) remain the same c) increase d) can't be determined 13. Which type of field is present near a moving electric charge? a) both an electric field and a magnetic field c) an electric field, only b) a magnetic field, only d) neither an electric field nor a magnetic field 14. The diagram below shows the lines of magnetic force between two north magnetic poles. At which point is the magnetic field strength greatest? a) C b) D c) B d) A

23 15. A charged particle is moving with a constant velocity. On entering a uniform magnetic field, the particle a) must change the magnitude of its momentum c) may change its direction of motion b) may increase in kinetic energy d) must decrease in speed 16. Which diagram correctly shows a magnetic field configurations? a) b) c) d) 17. The diagram below shows a compass placed near the north pole, N, of a bar magnet. Which diagram best represents the position of the needle of the compass as it responds to the magnetic field of the bar magnet? a) b) c) d) 18. The diagram below shows a coil of wire (solenoid) connected to a battery. The north pole of a compass placed at point P would be directed toward point a) C b) B c) D d) A 23

24 19. Two bar magnets of equal strength are positioned as shown. At which point is the magnetic flux density due to the two magnets greatest? a) C b) A c) D d) B 20. In the diagram below, steel paper clips A and B are attached to a string, which is attached to a table. The clips remain suspended beneath a magnet. Which diagram best represents the induced polarity of the paper clips? a) b) c) d) 21. A magnetic force acts on a charged particle moving at a constant speed perpendicular to a uniform magnetic field. The magnitude of the magnetic force on the particle will increase if the a) magnitude of the charge decreases c) speed of the charge decreases b) time of travel of the charge increases d) flux density of the magnetic field increases 22. Which statement best describes the torque experienced by a current-carrying loop of wire in an external magnetic field? a) It is due to the interaction of the external magnetic c) It is inversely proportional to the length of the field and the magnetic field produced by current in the loop. conducting loop in the magnetic field. b) It is due to the current in the loop of wire, only. d) It is inversely proportional to the strength of the permanent magnetic field.

25 23. The diagram below represents magnetic lines of force within a region of space. The magnetic field is strongest at point a) A c) D b) C d) B 24. Which diagram best represents magnetic flux lines around a bar magnet? a) c) b) d) 25. A student sprinkled iron filings around a bar magnet and observed that the filings formed the pattern shown below. The magnetic field is strongest at point a) A b) D c) C d) B 25

26 26. Two solenoids are wound on soft iron cores and connected to batteries, as shown in the diagram below. When switches S1 and S2 are closed, the solenoids a) neither attract nor repel c) repel because of adjacent south poles b) repel because of adjacent north poles d) attract because of adjacent north and south poles 27. Which diagram below best represents the magnetic field near a bar magnet? a) b) c) d) 28. The diagram below represents lines of magnetic flux within a region of space. The magnetic field strength is greatest at point a) C b) B c) A d) D Z:\Physics\Regents Physics\Class Material\Unit 4B Magnetism doc

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