Electromagnetism
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electromagnets and generators, earth's magnetic field
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Electromagnetism
- 2. In your lab notebook, please answer as best you can: 1. Which elements are ferromagnetic? • Iron(Fe), Nickel(Ni), Cobalt(Co) & Rare Earth metals (Nd, Sm, Gd)Iron(Fe), Nickel(Ni), Cobalt(Co) & Rare Earth metals (Nd, Sm, Gd) 1. How are magnets made? • Placing a ferromagnetic material in a strong magnetic fieldPlacing a ferromagnetic material in a strong magnetic field • Repeatedly striking a magnetic material with a hammer while it is in a north-southRepeatedly striking a magnetic material with a hammer while it is in a north-south direction to jar the domains into alignmentdirection to jar the domains into alignment • Passing an electrical current through a ferromagnetic materialPassing an electrical current through a ferromagnetic material 3. What are magnetic domains? • Regions within a magnetic material the spin of valence electrons are alignedRegions within a magnetic material the spin of valence electrons are aligned with one anotherwith one another 4. Explain magnetic permeability. • ……the ability of a material to become and/or stay magnetized.the ability of a material to become and/or stay magnetized. 4. What happens when different magnetic field lines meet? • Magnetic field lines (a.k.a. lines of flux) never crossMagnetic field lines (a.k.a. lines of flux) never cross • They combine when opposite (N-S) and push against each other when theThey combine when opposite (N-S) and push against each other when the same (N-N or S-S)same (N-N or S-S) Review Quiz 10
- 3. Experiment 9 Magnetic Fields • Observation • A magnet is any material that produces a magnetic force • Iron, nickel & cobalt are naturally magnetic (ferromagnetic) • Some rare earth metals form strong magnets • Flexible magnets are plastic with tiny iron bits mixed in • Opposite poles attract, like poles repel • Magnetic fields extend all around a magnet – Different fields interact/combine – More/closer field lines = stronger force – QUESTION: • Which magnet has the strongest field? • Where is the field strongest in each magnet? • Hypothesis – What do you think will happen? (Answer your questions) • • Experiment – How can we test our hypothesis? • Which side picks up staples? • How close to staples can magnet go? – What data should be recorded? • Conclusion – Was your hypothesis correct? – How was/should the experiment be modified?
- 4. Magnetic Domains • The spin of electrons create tiny magnetic regions called domains –In some atoms, these domains cancel out –In magnets, domains are all lined up in same direction • Whenever all electrons spin in the same direction, a magnetic field is produced
- 5. • Domains stay aligned within the magnet – dipoles present in each new piece • What would happen to the poles if we cut this magnet in half? • Does size determine magnetic strength? - In general, yes. • But it’s not the only factor.
- 6. Experiment 9 Magnetic Fields • Observation • A magnet is any material that produces a magnetic force • Iron, nickel & cobalt are naturally magnetic (ferromagnetic) • Some rare earth metals form strong magnets • Flexible magnets are plastic with tiny iron bits mixed in • Opposite poles attract, like poles repel • Magnetic fields extend all around a magnet – Different fields interact/combine – More/closer field lines = stronger force – QUESTION: • Which magnet has the strongest field? • Where is the field strongest in each magnet? • Hypothesis • I think the largest magnets will be the strongest because they have the most “pulling” material. • The magnetic field will be strongest at the North Pole. • Experiment – How can we test our hypothesis? • What variables will be tested? • What should be controlled? • What data should be recorded? • Conclusion – Was your hypothesis correct? – How was/should the experiment be modified? Magnet Type (Shape, material, description) Magnetic Field Drawing Distance from or # of Staples Ceramic bar Neodymium disc Oval hematite
- 7. Experiment 9 Magnetic Fields • Observation • A magnet is any material that produces a magnetic force • Iron, nickel & cobalt are naturally magnetic (ferromagnetic) • Some rare earth metals form strong magnets • Flexible magnets are plastic with tiny iron bits mixed in • Opposite poles attract, like poles repel • Magnetic fields extend all around a magnet – Different fields interact/combine – More/closer field lines = stronger force – QUESTION: • Which magnet has the strongest field? • Where is the field strongest in each magnet? • Hypothesis • I think the largest magnets will be the strongest because they have the most “pulling” material. • The magnetic field will be strongest at the North Pole. • Experiment – Procedures: • Describe each magnet • Draw field using iron filings & compass • Record # of staples lifted off table. – Touch pile for 2 seconds, then lift slowly – Test N, S, and middle of each magnet • Conclusion – Summarize results – Was your hypothesis correct? – How was/should the experiment be modified? Magnet Type Magnetic Field # of Staples Ceramic bar N:15 S: 16 Mid: 7 Neodymium disc N: 43 S: 45 Mid: 15 Oval hematite N: 27 S: 18 End: 11
- 8. Permanent Magnets • Field can be lost or removed – Heating to the Curie Point – Sending an Electric Current through • Degaussing • Magnetic storage media erased – Banging It • dislodges aligned domains Substance Curie temp °C Iron (Fe) 770 Cobalt (Co) 1130 Nickel (Ni) 358 Iron Oxide (Fe2 O3 ) 622 • Electron spins (within all or most domains) are aligned • Retains its own magnetic field • Examples: Refrigerator Magnets, Bar Magnets, Lodestone, Horseshoe Magnets, Hematite, etc…
- 9. Temporary Magnets • Domains temporarily line up • Will keep magnetic field only until tampered with –Examples: • Paperclips, scissors, staples, thumb tacks, pins, screwdrivers, refrigerator door, car door locks, etc… • Anything that is magnetic, but will not keep its field
- 10. Making a Temporary Magnet • Magnetic Induction – domains are aligned by touching or bringing it near a magnet – only in some materials • Strength of magnet determines: – strength of induced field – how long induced magnetism lasts • Object loses its magnetism once magnet is removed – Temporary magnets can also be made using electric currents
- 11. Magnetism From Electricity • Electricity is the movement of electrons in a single direction through a conductor – Aligned flow of electrons creates a magnetic field • Similar to electron domains lining up in a permanent magnet • Every wire carrying electricity has a weak magnetic field around it • Ever wonder why dust bunnies collect around power cords?
- 12. Right-hand Rule • The direction of electric current determines the polarity of a magnetic field • Reversing the direction of current changes the polarity of the magnetic field • Stopping the current removes the magnetic field (turns the magnet off)
- 13. Solenoids • Looping a wire strengthens the magnetic field – several loops (coil) form a solenoid – magnetic forces of loops are combine – more loops = stronger field
- 14. Electromagnets • An electromagnet is created by running electric current around a magnetic material – usually a copper solenoid with an iron core • magnetic field disappears when electric flow is removed • poles are reversed if terminals are swapped • More current = stronger field
- 15. Adjustable Magnets Strength controlled by: –Neatness of coiling –Number of loops –Wire gauge –Battery strength –Magnetic permeability of the core material –Temperature
- 16. Examples of Electromagnets • Car brakes and clutches • Electric motors – anything that moves • Automatic car door locks • Hospital equipment (MRI) • Electric generators • Loudspeakers • Television receivers • Atomic particle accelerators • Computers • Junkyard cranes
- 17. Earth as a Magnet • Earth's layers: – Crust – Semi-solid mantle – Liquid iron outer core – Solid iron inner core • No permanent magnet – TOO HOT! • Constantly-moving outer core: – creates electric current – current induces magnetic field around solid iron core (electromagnet) – called the dynamo effect
- 18. Geographic vs. Magnetic Poles • Magnetic north is near but not the same as the geographic south pole • A compass points to magnetic south – not perfectly aligned with Earth's rotational axis • Declination – the angle between magnetic & geographic north
- 19. Earth's Shifting Magnetic Poles • Magma (molten iron) solidifies with domains aligned to earth's magnetic field – evidence that Earth's poles have reversed • Strength of Earth's magnetic field also fluctuates • Declination changes by about a fifth of a degree per year.
- 20. Earth's Magnetosphere • extends 36,000 miles into space – extends 20 times farther on side away from sun • protects our planet from “magnetic storms” – Caused by solar winds (plasma ejected from the sun) • super-hot charged particles • most are deflected by magnetosphere – solar winds change frequently ("space weather") • coronal mass ejections increase solar winds • magnetic storms can heat and distort Earth's atmosphere – radio communications disrupted – GPS function altered – satellite electronics can be damaged – electric power grid surges cause blackouts
- 22. Auroras • Solar wind particles not deflected by the magnetosphere are attracted to Earth's magnetic poles – particles collide with oxygen & nitrogen atoms in the thermosphere, releasing light – observable in high latitudes (near N & S pole)
Editor's Notes
- 1 Coulomb = 6.25 x 1018 electrons
- Write on Dry Erase board what you want them to write in lab book. Experiment Procedure: Write a brief description of a magnet in a table in your lab notebook. (1 inch ceramic bar, 5mm neodymium disc, etc.) Using iron filings and/or a compass, draw the magnetic field surrounding the magnet. Record the # of staples the magnet can pick up. Which pole will be used in each magnet & how will that be determined? How close can the magnet come to the pile of staples? Any other variables to control?
- Write on Dry Erase board what you want them to write in lab book. Experiment Procedure: Write a brief description of a magnet in a table in your lab notebook. (1 inch ceramic bar, 5mm neodymium disc, etc.) Using iron filings and/or a compass, draw the magnetic field surrounding the magnet. Record the # of staples or paper clips the magnet can pick up. OR record distance first attraction is noted Which pole will be used in each magnet & how will that be determined? How close can the magnet come to the pile of staples? Any other variables to control? (time/distance held, person holding/recording)
- Write on Dry Erase board what you want them to write in lab book. Experiment Procedure: Write a brief description of a magnet in a table in your lab notebook. (1 inch ceramic bar, 5mm neodymium disc, etc.) Using iron filings and/or a compass, draw the magnetic field surrounding the magnet. Record the # of staples the magnet can pick up. Which pole will be used in each magnet & how will that be determined? How close can the magnet come to the pile of staples? Any other variables to control?
- The term was first used by (then) Cmdr Charles F. Goodeve, RCNVR, during World War II while trying to counter the German magnetic mines that were playing havoc with the British fleet. The mines detected the increase in magnetic field when the steel in a ship concentrated the Earth's magnetic field over it. Admiralty scientists, including Goodeve, developed a number of systems to induce a small "N-pole up" field into the ship to offset this effect, meaning that the net field was the same as background. Since the Germans used the Gauss as the unit of the strength of the magnetic field in their mines' triggers (this was not yet a standard measure), Goodeve referred to the various processes to counter the mines as degaussing. The term became a common word. Until recently, the most common use of degaussing was in CRT-based TV sets and computer monitors. For example, many monitors use a metal plate near the front of the tube to focus the electron beams from the back. This plate, the shadow mask, can pick up strong external fields and from that point produce discoloration on the display.
- Demo/Lab: U
- Electric motors: car, hairdryer, vacuum cleaner, wrist watch, ship's propulsion system, air compressor, water pump, fan, washing machine
- This is explained, in general terms, in the Introduction to Geomagnetism page given on this website. Briefly, then, as the result of radioactive heating and chemical differentiation, the outer core is in a state of turbulent convection. This sets up a process that is a bit like a naturally occurring electrical generator, where the convective kinetic energy is converted to electrical and magnetic energy. Basically, the motion of the electrically conducting iron in the presence of the Earth's magnetic field induces electric currents. Those electric currents generate their own magnetic field, and, as the result of this internal feedback, the process is self-sustaining, so long as there is an energy source sufficient to maintain convection. The depiction of the geodynamo shown here is only schematic; in fact, the fluid motion and the form of the magnetic field inside the core are still the subject of intensive research.
- At most places on the Earth's surface, the compass doesn’t point exactly toward geographic north. The deviation of the compass from true north is an angle called 'declination'. It is a quantity that has been a nuisance to navigators for centuries, especially since it varies with both geographic location and time. It might surprise you to know that at very high latitudes the compass can even point south! Declination is simply a manifestation of the complexity of the geomagnetic field. The field is not perfectly symmetrical, it has non-dipolar ‘ingredients’, and the dipole itself is not perfectly aligned with the rotational axis of the Earth. Interestingly, if you were to stand at the north geomagnetic pole, your compass, held horizontally as usual, would not have a preference to point in any particular direction, and the same would be true if you were standing at the south geomagnetic pole. Moreover, if you were to hold your compass on its side the north-pointing end of the compass would point down at the north geomagnetic pole, and it would point up at the south geomagnetic pole.
- Models and charts of the magnetic field at the Earth’s surface need to be periodically updated because the field is constantly changing in time. The same fluid motion in the Earth’s core that sustains the main part of the magnetic field also causes the field to slowly change in spatial form, a time-dependence known as ‘secular variation’. This variation can be seen in all vectorial parts of the magnetic field, but it was first noticed in declination several hundred years ago, since it is that quantity that is so important for navigation. In fact, the demands of navigators helped to motivate, centuries ago, some of the original studies of the Earth's magnetic field. On average the declination at the Earth’s surface changes by about a fifth of a degree per year.
- The infrastructure and activities of our modern technologically-based society can be adversely affected by rapid magnetic-field variations generated by electric currents in the near-Earth space environment, particularly in the ionosphere and magnetosphere. This is especially true during so-called ‘magnetic storms’. Because the ionosphere is heated and distorted during storms, long-range radio communication, which relies on sub-ionospheric reflection, can be difficult or impossible and global-positioning systems (GPS), which relies on radio transmission through the ionosphere, can be degraded. Ionospheric expansion can enhance satellite drag and thereby make their orbits difficult to control. During magnetic storms, satellite electronics can be damaged through the build up and subsequent discharge of static-electric charges, and astronaut and high-altitude pilots can be subjected to increased levels of radiation. There can even be deleterious effects on the ground: pipe-line corrosion can be enhanced, and electric-power grids can experience voltage surges that cause blackouts. Electric currents in one place can induce electric currents in another place, this action at a distance is accomplished via a magnetic field. So, even though rapid magnetic-field variations are generated by currents in space, very real effects, such as unwanted electric currents induced in electric-power grids, can result down here on the Earth’s surface.
- The Bow Shock in front of a magnetosphere is very similar to the wave which appears in front of a boat as the ship passes through the water. It is similar to the wave in front of a rock in a stream. A plane traveling at supersonic speeds also produces a bow shock in the air in front of the plane, while it is flying.
- Aurorae are a luminous glow of the upper atmosphere caused by energetic particles descending from the Earth’s magnetosphere or coming directly from the Sun. These energetic particles are mostly electrons, but protons can also be involved, and their energetic rain into the atmosphere is greatest during magnetic storms. As the particles descend, they collide with molecules in the atmosphere, causing an excitation of the oxygen and nitrogen molecular electrons. The molecules can return to their original, unexcited state by emitting a bit of light, a photon. This light is the aurora that we see. Since electrically-charged particles tend to follow magnetic-field lines, and since magnetic-field lines are oriented in and out of the Earth and its atmosphere, near the magnetic poles, aurorae tend to be seen at high latitudes.