Transcript Magnetism
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 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?????? The field lines around the Earth look just like those around a bar magnet 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 Northern & Southern Lights: 1. 2. 3. 4. 5. 6. 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!) 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! Practice 2nd right hand rule 1. Take pen/pencil, lay it east/west, POINTING west. 2. Wrap wire around pencil starting at eraser, going CLOCKWISE if looking at eraser. 3. Pretend to “plug in” eraser end of wire into a POSITIVE lead of a battery. 4. Wrap the fingers of your right hand in the direction of the conventional current flow. 5. The North pole is which of these 6 directions: 6. Up, down, left, right, “into the page”, “out of the page” 7. Lay the same pencil north/south facing north, and now pretend to plug in the north end of the wire. 8. Wrap the wire the other way. Practice 2nd right hand rule 1. Do together some questions on class-set of practice dittos. (Don’t do any like #4 – too difficult to read!) 2. Answer the question below… Note this is how it is on the test – YOU have to figure out the direction of the conventional current! 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 1. ABOVE paper at ___ Practice: 2. BELOW paper at ___ 3. To the LEFT/West of wire ___ a 4. To the RIGHT/East of wire ___ e 5. To the north of wire ___ c f d 6. To the south of wire ___ 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 Conceptual Questions: That’s different! A. Oddly enough, when two wires have current going in the SAME direction lie parallel to each other, they attract. B. Similarly, when two wires have current going in the OPPOSITE directions lie parallel to each other, they repel.