Magnetism Review

1. Magnetism Review

2. Magnetism Review • Lodestone- are magnetic rocks that are rich in (3.)magnetite, a iron oxide mineral • William Gilbert- made observations and proved in the 1600s that the earth is a great magnet or giant lodestone • Andre Marie Ampere- proposed the theory of magnetism in Fe to explain electromagnets. It is a result of electric current through its loops. The force between 2 long straight parallel currents was proportional to the distance between them and proportional the intensity of the current flowing in each.

3. Magnetism Review • Magnet Domains- each electron in an atom acts like a tiny electromagnet. When the magnetic fields of the electrons align in the same direction, the group is called a magnetic field. Draw Picture • Magnetic Field- (symbol B) (unit T(tesla) or G(gauss))- are the forces around magnets, they are vector quantities. When drawing the magnetic field lines they leave the North pole and enter the South Pole Draw Picture • Electromagnet- a magnet created when current flows through a wire coil.

4. Magnetism Review • Hans-Christian Oersted- in 1820 laid a wire across the top of a small compass and connected the ends of the wire to an electrical circuit. He expected needle to point in the same direction as current wire but it pointed perpendicular. ***So the electromagnetic force is perpendicular to the direction of current. *** Show Phet

5. Magnetism Review 9. Michael Faraday- given credit for creating the first motor, generator, transformer, and capacitor. Mainly in this topic we discuss the 3 right-hand rules that he came up with about Magnetic Fields.

6. 1st Right-Hand Rule • It is a method used to determine the direction of a magnetic field relative to the current (+ current)(from the positive side of the terminal) • Your thumb is the current in the wire and your folded fingers are the magnetic field. Draw picture.

7. 2nd Right-Hand Rule • Used to determine the direction of the magnetic field produced by an electromagnet relative to the current. • The thumb is the north pole. Draw a picture

8. 3rd Right Hand Rule • Used to determine the direction of the magnetic force on a current- carrying wire in a magnetic field. • Draw a picture • Formulas: • F = ILB • F = qvB • Faraday’s Law: • Emf(electromotive force)= • (anything that causes voltage) = N(∆BA)/t • ΔBA stands for flux

9. Magnetism Review • Changing Magnetic Field- Can cause current in a wire. • Electromagnetic Induction – the process of generating a current through a circuit due to the relative motion between a wire and a magnetic field when the wire is moved through the magnetic field or the magnetic field moves past the wire. Draw a picture. Use Phet.(use ac current)

10. Magnetic Review • James Maxwell- postulated that a changing electric field produces a changing magnetic field, which was later proven by Hertz. This lead to the discovery of electromagnetic waves(19)- which are a combination of electric and magnetic fields that travel through space. • Generator and motors- converts mechanical energy to electrical energy . It has a number of wire loops that placed in a strong magnetic field. The wire is wound around an iron core to increase the strength of the magnetic field. The wires and the iron are called the armature, which is similar to that of an electric motor. http://physci.kennesaw.edu/mzoughi/Simulations.shtm

11. Magnetism Review 13. The armature is mounted so that it can rotate freely in the magnetic field. As the armature turns, the wire loops cut through the magnetic field lines and induce an EMF. Commonly called voltage, the EMF developed by the generator depends on the length of the wire rotating in the field. Only the portion within the magnetic field induces an EMF. In a closed circuit an electric current is created. Phet.

12. To keep the torque on a DC motor from reversing every time the coil moves through the plane perpendicular to the magnetic field, a (16)split-ring commutatoris used to reverse the current at that point. The electrical contacts to the rotating ring are called "brushes" since copper brush contacts were used in early motors. Modern motors normally use spring-loaded carbon contacts, but the historical name for the contacts has persisted.

13. (17) Slip ring commutator-are electrical connections that are used to transfer power to and from the rotor of an AC generator . The slip ring consists of a circular conducting material that is connected to the rotor windings and insulated from the shaft.   Brushes ride on the slip ring as the rotor rotates.   The electrical connection to the rotor is made by connections to the brushes. Slip rings are used in AC generators because the desired output of the generator is a sine wave. In a DC generator, a commutator was used to provide an output whose current always flowed in  the  positive  direction.   This  is  not  necessary  for  an  AC  generator. Therefore, an AC generator may use slip rings, which will allow the output current and voltage to oscillate through positive and negative values.

14. Magnetism Review 14. Transformer- are used to increase or decrease AC voltage. Uses of transformers is common because they change voltage with relatively little loss of energy. Many printers, game systems, and computers have transformers inside their casings or part of their cords.

15. Magnetism Review • 14.transformer- How they work: A transformer has 2 coils, electrically insulated from each other, but wound around the same iron core. One coil is called the primary coil, the other coil is called the secondary coil. When the primary coil is connected to a source of AC voltage, the changing current creates a changing magnetic , which is carried through the core to the secondary field. This changing field induces a varying EMF.

16. Transformers • Formula : Vs/Vp= Ns/Np= Is/Ip V stands for voltage N stands for number of turns of coil Draw picture • Voltage increase corresponds to current decrease

17. Draw Magnetic Fields coming in and out of board

18. Magnetism Review • Mass spectrometer- an instruments that can distinguish between isotopes. It can measure charge-mass ratios of positive ions. A magnet causes the positive ions in a vacuum chamber to be deflected according to their mass and it is recorded on a photographic piece of paper. Draw picture.

19. Magnetism Random Facts • Cathode Ray tube computer minitorsand TV screens use magnets to focus and direct particles on a screen which causes the colors you see. • Electromagnets are in the heads of audiocassette and video recorders. The recorder creates the electrical signal and the recorder head creates the magnetic field. The recording tape has tiny bits of magnetic material bonded to thin plastic . This tape passes over the recording head and the domains align according to the direction of the current. These stay until they are erased and the domains become scattered again.