1 / 36

Magnetism and Electromagnetic Induction

Magnetism and Electromagnetic Induction. Chapters 24 & 25. General Properties of Magnets. What did you notice about a magnet suspended from a string?. S. N. Microscopic Picture of Magnetic Materials. Electrons moving cause small magnetic fields around the atoms in a substance.

lovie
Download Presentation

Magnetism and Electromagnetic Induction

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Magnetism andElectromagnetic Induction Chapters 24 & 25

  2. General Properties of Magnets • What did you notice about a magnet suspended from a string? S N

  3. Microscopic Picture of Magnetic Materials • Electrons moving cause small magnetic fields around the atoms in a substance. • Each electron acts like a tiny electromagnet. • Magnetic fields in groups of neighboring atoms create a “domain”. • Domains are naturally oriented in different directions and cancel each other out. • When an external magnet is applied, the domains align and the object becomes magnetized (like the nail).

  4. Microscopic Picture of Magnetic Materials – con’t • In permanent magnets, the iron has been alloyed with substances that keep the domains aligned after external magnet is removed. Before the application of an external magnetic field. After the application of an external magnetic field.

  5. N & S Poles of Magnet vs. N & S Poles of Earth • North poles of a magnet are “north-seeking” and point towards Earth’s north geographic pole. • North geographic pole of earth is actually the south pole of a magnet.

  6. How Does a Compass Work? • Small magnet on pivot.

  7. Polarization of a Nail What’s going on? Why do the tacks Stick to each other? Think about electricity…

  8. Magnetic Fields Around Permanent Magnets • Magnetic Fields • Space around magnet where magnetic force exists.

  9. Magnetic Fields Around Permanent Magnets – con’t • Magnetic fields are similar to electric fields (can attract or repel). • Iron filings in lab arranged themselves along field lines (as in picture). • Concentration of field lines is greatest at the end of the magnet (largest amount of flux here).

  10. Magnetic Flux • Number of field lines passing through a surface. • Measures the strength of the field • Strength shown by how close together the lines are. • Flux per unit area is proportional to the strength of the magnet.

  11. How are Poles Oriented in a Disk Magnet? • North and South are up and down (if the disk is laying on the desk). North South

  12. Oersted’s Experiment • Oersted sent an electric current through a wire. • Iron filings and compass needles surrounded wire. • Found that when current was flowing the filings and compass needles arranged themselves in circles around the wire. • Reverse current and needles reversed their direction.

  13. Oersted’s Experiment – con’t

  14. First Right Hand Rule How to find the direction of a magnetic field around a current carrying wire: Grasp the wire with your right hand, keeping your thumb pointed in the direction of the positive current flow. Fingers point in the direction of the magnetic field.

  15. Right Hand Rule Summary • The RHR says if you point your thumb in the direction of positive current flow, fingers will point in the direction of the magnetic field (north to south).

  16. Electromagnet • Put a “core” in the coil and it becomes magnetized by induction. • This is an electromagnet. • Strength of field is proportional to

  17. Forces on Current-Carrying Wires • Put a current-carrying wire in a magnetic field and a force will act on it. • Force will be at right angles to both direction of magnetic field and the direction of the current.

  18. Designating Direction of Current • Think of an arrow…

  19. Magnitude of Force F = ILB(sinθ)

  20. F = ILB(sinθ) • What happens when the current and magnetic field are parallel to each other (Ѳ = 0◦)? • At what angle is the force the greatest?

  21. Use RHR to determine the direction of force when I and B are known. • Find the direction of the force on the wire. • Fingers point in direction of magnetic field (B) • Thumb points in direction of conventional current (I) • Palm of hand shows direction of force(F)

  22. Galvanometers • Used to measure very small currents. • One side of loop is forced down, the other is forced up. The torque on the loop is proportional to the current. N S This side forced down This side forced up

  23. Force on a Single Charged Particle • Charges don’t need to be confined to a wire. • Can deflect a single charge with a magnetic field • Cathode Ray Tube is an example

  24. Magnet electronbeam Magnet Anode Ring Cathode Force on a Single Charge Particle – cathode ray tube Phosphorescent screen

  25. Cathode Ray Tube Example – con’t • Can deflect a single charge with a magnetic field. • Phosphorescent particles on screen light up when hit with electron beam. • Example: TV, computer monitor. • Why should be careful not to bring a magnet too close to your TV or monitor?

  26. Force on a Single Charged Particle…the math • Remember that F = ILB • …look in your book to see the derivation … • F = qvB(sinѲ)

  27. Force on a Single Charged Particle – con’t • To make matters worse: An electron has a negative charge. • Remember that conventional current has a positive charge so the force is opposite that predicted by the RHR.

  28. Magnetic Fields Near a Coil • Imagine what the magnetic field around a loop of wire would be. • Electric current flows through loop of wire. • Magnetic fields are always in the same direction inside the loop and the opposite direction outside the loop. • Use RHR to figure out direction of magnetic field. • What would happen if there were lots of loops together?

  29. Creating Electric Current from Changing Magnetic Fields • Oerstad – Discovered that electric current produces magnetic fields…what about vice versa? • Faraday & Henry – Found that when a wire cuts through magnetic field lines, an electric current is generated. • Electromagnetic Induction – the process of generating a current by moving a wire through a magnetic field.

  30. Electromagnetic Induction • In circuits we know a charge pump is required to keep the current flowing. • Can create a voltage potential that causes current to flow in a wire moving through a magnetic field. Called Electromotive Force (EMF).

  31. Electromotive Force (EMF) • Not really a force, but an increase in electric potential (voltage). • Determined by B, L and velocity of wire in field.

  32. EMF (con’t) • Example: A microphone. • Sound waves vibrate a diaphragm attached to a coil of wire that moves in a magnetic field. • Movement of coil induces EMF and voltage is generated.

  33. Electric Motors • Galvanometers can rotate no more than 180° because opposing forces prevent motion. • For the loop to rotate 360° the current must reverse direction just as the loop reaches vertical position. • Brushes are used to reverse the current in the loop.

  34. Electric Motors

  35. Electric Generators • Convert mechanical energy into electrical energy. • Genecon is a good example. • Armature rotates freely in a magnetic field. • EMF is induced. • Example: Hoover Dam

  36. Hoover Dam • Hydroelectric Power • Movement of water through turbines creates EMF by moving magnets. • Built during the depression in less than five years. • Over 700 feet tall. • Has produced over 4 billion kW-hr of energy.

More Related