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Magnet: what is it?. Rock from Asia. Magnet: what’s it do?. “North”. Magnet: what’s it do?. S. N. North. A compass. S. N. Definition: the North end (pole) of a magnet is the end which wants to point towards the geographic North pole of Earth!. Trouble:. Trouble:.
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Magnet: what is it? Rock from Asia.
Magnet: what’s it do? “North”
Magnet: what’s it do? S N North
A compass S N
Definition: the North end (pole) of a magnet is the end which wants to point towards the geographic North pole of Earth!
Compasses (magnets) interact! The north pole of a magnet seems attracted to the south pole of another, and vice-versa! Scientific: make a little compass, and test it out around a big one! Compass sim (Remember not to close it !!!) http://phet.colorado.edu/en/simulation/magnet-and-compass
Semi-Definition of a magnetic field (produced by some magnet) Pick a point in space. Put a small test compass there. points in the direction of the compass…. (the way the north end points). Strength seems to have something to do with distance (magnitude B comes later) Back to sim!! (don’t close it) Compass sim Show field
Maybe Earth is a big Magnet? But it must be that the North geographic pole is a South magnetic pole: Back to sim!! (don’t close it) Compass sim
Astounding fact: electric currents affect compasses! Wire with no current ( I = 0).
Astounding fact: electric currents affect compasses! current on ( Iup !).
Astounding fact: electric currents affect compasses! current on ( Idown !).
( Iup/out !). DIRECTION?
( Idown/in !). DIRECTION?
( I= 0 ). DIRECTION?
Current Loop: down/in up/out
Back to sim!! (don’t close it) Compass sim
Remember how it was with electric fields: action at a distance:
Remember how it was with electric fields: Field concept: Charge Q1 SOURCE
Remember how it was with electric fields: Field concept: Charge Q2 SOURCE
Same idea for B-fields: Current S N Magnet
Same idea for B-fields: Forces on Magnet Current S N SOURCE
Same idea for B-fields: Force on Current ? Magnet S N SOURCE DEMO to prove it! Hanging wire
To define the magnitude of B we have to do some kind of experiment Why not put a wire with a known current into a uniform B-field? How do we make a uniform B-field?
How do we make a uniform B-field? N S N S Each magnet produces an individual magnetic-field, so ….
How do we make a uniform B-field? N S N S …the total field due to both is just the (vector) sum of each individual field
How do we make a uniform B-field? N S N S Bring them close S N
Now the experiment!! S N Wire with no current ( I = 0).
Now the experiment!! S N Wire with current ( I = out/up).
Now the experiment!! S N Wire with current ( I = in/down).
RESULTS I: Force on the wire is always perpendicular to bothB and the direction of I – in a right-handed kind of way Conventional current!
Right-Hand rule (#2) “I crossed into B gives F” DEMO examples with current swing