PHYSICS UNIT 8: MAGNETISM. MAGNETISM Magnetic Fields are always dipolar: N and S poles law of magnetism: like poles repel, unlike poles attract magnetic

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PHYSICS UNIT 8: MAGNETISM Slide 2 MAGNETISM Magnetic Fields are always dipolar: N and S poles law of magnetism: like poles repel, unlike poles attract magnetic force is directly proportional to the magnets strength, inversely proportional to the square of the distance between the magnets only certain materials can be magnetized ferromagnetic materials (Fe,Co,Ni ores/alloys) Slide 3 MAGNETISM Magnets are always dipoles: north- seeking N pole, south- seeking S pole Slide 4 MAGNETISM magnetic field: region of magnetic force detect using a compass (needle points in field direction) field runs N to S outside magnet, S to N inside magnet Slide 5 MAGNETISM magnetic field strength, B: strength of the magnetic field, in Tesla, T earths magnetic field at the surface = 0.5 x 10 -4 T field directions: left right up down out toward you in away from you Slide 6 MAGNETISM Earth is a magnet north geographi c pole is an S pole field like a bar magnet Slide 7 MAGNETISM Source of Magnetism electrons are magnetic due to spin paired electrons: opposite spin, cancel out occurs in most materials unpaired electrons magnetic atoms occurs in ferromagnetic materials Slide 8 MAGNETISM magnetic domains: regions of aligned magnetic atoms magnetic object: aligned magnetic domains Slide 9 ELECTROMAGNETISM Field Around A Wire: a current (moving charge) produces a magnetic field encircling the wire right hand rule: grasp conductor in right hand, with thumb pointing in I direction; fingers will circle wire in B direction Slide 10 ELECTROMAGNETISM magnetic field strength around a wire depends on amount of current in wire, & distance from wire B= 0 I/ 2 r B: magnetic field strength (T) I : current in wire (A) r: distance from wire (m) 0 =4 10 -7 Tm/A Slide 11 ELECTROMAGNETISM Field Around A Solenoid (coil) like bar magnet when carrying current add iron core: electromagnet (much stronger field) solenoid field strength, B= 0 n I n = #loops/length of coil Slide 12 ELECTROMAGNETISM right hand rule for solenoids: grasp coil in right hand, with fingers circling coil in I direction; thumb will point to N pole core is pulled into solenoid when turned on (striker, switch) Slide 13 ELECTROMAGNETISM parallel wires exert forces on each other due to their magnetic fields currents in same direction wires attract currents in opposite direction wires repel Slide 14 PHYSICS UNIT 8: MAGNETISM Slide 15 MAGNETIC FORCES cathod e ray tube (TV & monitor picture tube) Slide 16 MAGNETIC FORCES cathod e ray tube (TV & monitor picture tube) Slide 17 MAGNETIC FORCES Magnetic fields exert force on moving charges magnetic force on a moving charge, F = qv x B (vector cross product) F: force (N) B: field strength (T) q: particle charge (C) v: speed (m/s) Slide 18 MAGNETIC FORCES F is perpendicular to v and B can only make the particle turn F is a centripetal force Bqv = mv 2 /r mass spectrometer: shoot particles into magnetic field to find their mass m = Bqr/v Slide 19 MAGNETIC FORCES right hand rule for + particles, left hand rule for particles: point thumb in v direction, index finger in B direction (N to S); other fingers will bend in F direction N S proton: v S N electron: v Slide 20 MAGNETIC FORCES Magnetic fields exert force on current- bearing wires magnetic force on a wire, F = B I L B: magnetic field strength (T) I : current (A) L: length of wire in field (m) for a coil, L = length of wire in field # of loops Slide 21 MAGNETIC FORCES right hand rule for wires: point thumb in I direction, index finger in B direction; other fingers will bend in F direction I I Slide 22 PHYSICS UNIT 8: MAGNETISM Slide 23 ELECTROMAGNETIC INDUCTION Electromagnetic Induction: a voltage (and a current) is induced in a wire when it moves in a magnetic field induced voltage V = BLv V: potential difference between ends of wire (V) L: wire length inside field (m) v: wire speed (m/s) induced current I = V/R Slide 24 ELECTROMAGNETIC INDUCTION either the wire or the field can move wire must cut across field lines to induce a voltage Slide 25 ELECTROMAGNETIC INDUCTION right hand rule for electromagnetic induction: point thumb in v direction, index finger in B direction; other fingers will bend to point I direction v v Slide 26 ELECTROMAGNETIC INDUCTION Generators: spinning a coil in a B field induces current in the coil Green wire voltage Slide 27 ELECTROMAGNETIC INDUCTION generator current is alternating current (AC) frequency: current cycles/sec (Hz) = coil rotation rate DC generator: coil connects to commutator, which switches external connection when current switches Slide 28 ELECTROMAGNETIC INDUCTION Lenzs Law: an induced current generates a magnetic field which opposes the change that induced it mechanical energy must go in to get electrical energy out ex: The Drop Zone - magnets on cars move past metal fins; current is induced in fins which takes energy from the cars, slowing them down Slide 29 PHYSICS UNIT 8: MAGNETISM Slide 30 UNIT 8 REVIEW 0 = 4 10 -7 Tm/A B = 0 I /2 r B = 0 n I n = #loops/length of coil F = Bqv m = Bqr/v F = B I L V = BLvV = IR e = +/- 1.60x10 -19 C right hand rule for field around a wire right hand rule for field around a solenoid right (or left) hand rule for force on particles right hand rule for force on wires right hand rule for induced voltage

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