Magnetism

1. Magnetism Part 2

2. Review: What elements are magnetic? • Unlike electricity, NOT all metals can be magnetic: • Of the about 100 elements we know about, ONLY iron, nickel, and cobalt can become magnetic (whereas almost all metals are good electrical conductors). • Note aluminum can NOT become magnetic. Don’t be confused by references to Alnico magnets made of aluminum, nickel, and cobalt. Aluminum is only a filler in those magnets!

3. Review: HOW do iron, nickel, & cobalt become magnetic? …. • The “electron-spin” of electrons in atoms is called the “domain”. In all of the elements, the electron-spins are going every which way, and essentially canceling each other out. (See figure a at right.) • In iron, nickel, and cobalt, the electron-spins or domains can become aligned. (See figure b at right.) • When the electron-spins/domains of iron, nickel, or cobalt become aligned, then they become magnetic. • Note – you can NOT say just “electrons” become aligned. It is either “electron-spins” or “domains”. Be very careful about that! • Remember - NOBODY really understands what’s going on! Why do those three elements electron-spins do that? Why only those three? Find out yourself, and get a Nobel prize in Physics!

4. Review: Magnetic Domains aligned & not… “stroking” technique; what’s the other? How could you get domains unaligned? By dropping or heating the magnet!

5. 2 kinds of charge: + and – Can have single charges alone Opposite charges attract Like charges repel Neutrals are ALWAYS attracted through “induction” Electric fields do exist; closer field lines = more powerful/more forceful there; field lines around +, , +&, &, and +&+ you already know 2 kinds of poles: N and S Can NOT have single poles alone (no “monopoles”; if you break a magnet in half, you just get 2 smaller magnets that both have N&S poles.) Opposite poles attract Like poles repel “Neutrals” (meaning in this case samples of iron, nickel, & cobalt that are NOT yet magnetized, or do NOT yet have their electron-spins/domains aligned) are ALWAYS attracted. Magnetic fields do exist; closer field lines more powerful/forceful there; field lines around a single bar magnet are closed loops from N to S; Electricity vs Magnetism:

6. Electricity & Magnetism are “wed”: • *Electricity causes magnetism.... • There are 2ways to make iron, nickel or cobalt domains aligned: • “stroke” a needle/nail over & over again in the same direction with a magnet (see bottom left of slide 4) • but also by wrapping a needle/nail with a wire (several coils all going in the same direction) and attach the wire to a power supply like a battery (an “electromagnet”) • Cst 5f=“Students know magnetic materials AND electric currents (moving electric charges) are sources of magnetic fields and are subject to forces arising from the magnetic fields of other sources.” • “All magnetic fields are actually caused by the motion of _____ particles inside the metal. A spinning electron constitutes a “charged particle in _____.”

7. More on electricity causes magnetism: • Compasses laid out around a current-carrying wire will point in the direction of the magnetic field, which happens to form a circle around the wire. Forms concentric circles with magnetic field tangential to circle at all points

8. Electricity & Magnetism are “wed” • *Magnetism causes electricity.... • Watch us spin the handle to cause a lightbulb to light (but only if the magnet is attached!) • This is an example of a generator. • 5h=“Students know changing magnetic fieldsproduce electric fields, thereby inducing currents in nearby conductors.”

9. Energy conversions: • Generators are a device that converts _____ energy into _____ energy. • Batteries are devices that convert _____ energy into _____ energy. • Motors are a device that converts _____ energy into _____ energy. (this is the new one!)

10. Temporary vs permanent magnets: • Do the domains remain aligned for a long time after you remove the thing that was doing the aligning? (ie: remove the current, and/or remove the other magnet) • If YES – its a “permanent” magnet • If NO – its only a “temporary” magnet • In the picture, the needle/nail hanging from the magnet is only a temporary magnet, as it will lose its magnetism once it is removed from the bar magnet.

11. Earth’s Magnetic Field: • Earth has a magnetic field! • Over the history of the earth – its magnetic poles have reversed multiple times! compass pointing to north pole

12. More on these reversals: • There is evidence in rock samples that the Earth’s magnetic field has diminished to zero in the past and even reversed itself! • More than 20 reversals have taken place in the past 5 million years, with the most recent being 70,000 years ago. The magnetic field was switched off entirely for 10,000-20,000 years when modern humans emerged. • We can not really predict when the next reversal will come. But recent evidence shows a decrease of 5% in the last 100 years, which may mean another magnetic field reversal within 2000 years. • Since the Earth’s magnetic field shields us from most of the harmful radiation coming from outer space, what will happen to humans when it is gone??????

13. The field lines around the Earth look just like those around a bar magnet

14. Earth’s Magnetic Field cont… • Compasses are just floating magnets attracted by the Earth’s magnetic field • Recall: The true “geographic” north pole, or “spin-axis north pole” is NOT the same an magnetic North. They are off by what is called the “magnetic declination” • Geese & other animals have small magnets in their brains to navigate by (“animal magnetism”) • Big steel things like the Eiffel tower, make compasses point right at them!  • Thus, on a big steel ship, you need a “binnacle” so the ship’s compass does NOT align with the bow & stern. (note – steel ships only, not wood boats!) • Other astronomical objects: • planets (except Venus, Mars, Pluto) have magnetic fields. • Gas giants have BIG magnetic fields. • Little rocky planets rarely have enough iron & nickel in their cores, or their cores don’t spin fast enough to align the electron-spins. • The moon does not have a magnetic field; but the sun does! • Our galaxy has a very small magnetic field over-all

15. Northern & Southern Lights: • The earth’s magnetic field extends way into outer space. • The earth’s magnetic field is strongest at the poles • Charged particles coming from the sun are called the “solar wind”. • The charge particles get trapped in our magnetic field, and those “belts” of charged particles floating out there are called the “Van Allen belts” (after the famous American physicist who discovered them!) • Since the earths magnetic field is strongest at the poles, a lot of charged particles get SUCKED in to the poles, and when they collide you get …. • “Aurora Borealis” and “Aurora Australis” (which are actually plasma’s!)

16. END

17. The 2nd Right-hand rule: • Curl fingers of right hand in direction of conventional current • Conventional current is defined to be “opposite direction of electron-flow current” (so from + to - ) • Thumb points towards North pole!

18. Practice 2nd right hand rule • Take pen/pencil, lay it east/west, POINTING west. • Wrap wire around pencil starting at eraser, going CLOCKWISE if looking at eraser. • Pretend to “plug in” eraser end of wire into a POSITIVE lead of a battery. • Wrap the fingers of your right hand in the direction of the conventional current flow. • The North pole is which of these 6 directions: • Up, down, left, right, “into the page”, “out of the page” • Lay the same pencil north/south facing north, and now pretend to plug in the north end of the wire. • Wrap the wire the other way. • Answer the question below:

19. END

20. The 1st Right-hand rule: • Let’s “see” the 3 dimensions (pink hand). • Thumb goes in direction of conventional current. • Palm/compass to thumb/wire: palm must be positioned next to thumb as compass is positioned next to wire • Finger tips point in direction of magnetic field (compass alignment) Forms concentric circles with magnetic field tangential to circle at all points

21. ABOVE paper at ___ • BELOW paper at ___ • To the LEFT/West of wire ___ • To the RIGHT/East of wire ___ • To the north of wire ___ • To the south of wire ___ Practice: a e c f d g b • Possible answers: • Left/west • Right/east • North/up • South/down • “into the paper” • “out of the paper” h i m k j n o l p

22. Conceptual Questions:

23. That’s different! • Oddly enough, when two wires have current going in the SAME direction lie parallel to each other, they attract. • Similarly, when two wires have current going in the OPPOSITE directions lie parallel to each other, they repel.