Magnetism Basics. Magnetic Domains: atomic regions of aligned magnetic poles Random Alignment Ferromagnetic Alignment. Net Effect = Zero!

Transcription

1 Magnetism Basics Source: electric currents Magnetic Domains: atomic regions of aligned magnetic poles Random Alignment Ferromagnetic Alignment Net Effect = Zero! Net Effect = Additive! Bipolar: all magnets have a North Pole & South Pole natural monopoles do not exist! Splitting a magnet results in bipolar mini-magnets! Magnetic Forces: like electric forces Likes Repel Opposites Attract

2 The Earth: A Natural Magnet Earth s Magnetic Poles shift with time even reversing direction! Magnetic Field is 3-Dimensional and deflects downwards at the poles a compass would be totally useless!

3 Magnetic Fields B v Magnetic Fields (B): Force Field acting on magnetic domains like Electric Fields acting on electric charges Direction: points from North to South Poles and flows through interior of magnet or coil Strength: like Gravity, obeys Inverse Square Law: Force ~ 1/d 2. Unlike gravity, a medium (i.e. iron) can alter strength. Density of B field strongest at poles.

4 Astronomical Effects Aurora Borealis: (Northern Lights) Due to shielding effect, cosmic radiation interacts with the atmosphere lighting up the night poles. Magnetic Shielding: Earth s B field naturally shields and bottles highenergy cosmic radiation from the sun towards the poles Solar Flares: ~11 years the Sun experiences large-scale fluctuations in it s B field- -kicking out gases and intense radiation enough to disrupt radio communications on Earth!

5 Motors The motor effect: the magnetic force pushes On the charges perpendicular to the wire so the entire wire moves instead of just the charges down the wire--- see p. 583 in book! N F I S

6 Motor/generator parts Label each: Stator (magnet) Brushings (contacts) Current (wire leads)

7 Summary-chap.28 * The source of magnetism is moving charge * 3 types of magnets 1. permanent magnets- Fe,Co,Ni 2. current in wires ( magnetic field wraps around wire) 3. electromagnetic- battery, wire, iron (iron focuses magnetic field of current) * magnetic domain- microscopic piece of iron in which atomic magnets are aligned * galvanometer is a wire need a magnet (when current flows in wire, it deflects so you can measure very small currents)

8 Summary- video &lab guide Lodestone is a type of magnetic iron oxide To weaken an iron magnet, make the atomic magnets point in random directions by a. Heating b. dropping c. add impurites Your compass measures the total magnetic field of the earth & nearby magnets Magnetic forces enable speakers, mri s, computers, credit cards, etc to work Light is an electro-magnetic wave electro-magnetism is one force! Moving charge creates magnetism, Moving magnets induces current (or voltage)

9 AC vs. DC motors AC motors: uses ac current so the current changes direction and the force changes direction to keep the motor turning- no special contacts Ex: plug in tools/ motion appliance DC motors: current doesn t reverse so do it by reversing the battery connections every ½ cycle Ex: battery powered tools

10 Right-Hand Rules Current-Carrying Carrying Wire: Thumb: points in direction of I, Fingers: curl in direction of B in concentric circles around wire. Loop: Fingers: curl in direction of current Thumb: points in direction of B. Force on Moving Charge: Thumb: direction of v Fingers: direction of B Palm: pushes in direction of F (or use middle finger!) Note all 3 Quantities are at 90 degrees!

11 Right-Hand Rules Hints * I is in direction of positive charges as move from + battery to battery ends (for electrons flip or use left hand) * Force is zero if the magnetic field is exactly parallel to the flow of charge * B magnetic field flows from N to S pole

12 Example Problem current loop in a magnetic field Helga s Super Vacuum F =? (magnitude & direction!) B = 200 Teslas I = 20 Amps l= 5.8 m of copper wire in motorbrush = into page (tail feathers of arrow) = out of page (tip of arrow)

13 Magnetic Induction The Missing Link: -Oersted discovered Electric Currents produce Magnetism. Will Magnetism produce Currents? Lenz s Law: -A changing B field through a circuit will induce an opposing B field. Why opposing? Mechanical energy to change B will result in electrical energy! Energy is conserved! Otherwise, infinite free energy!

14 Generators The generator effect: the magnetic force pushes the charges along the wire inducing voltage since the entire wire is pushed down (by steam) see p. 583 in book! N move I S

15 Magnetic Force on Moving Charges v v v F = qv B magnetic magnetic x x v v v F = Il B Units: F = Force in Newtons (N) q = charge in Coulombs (C) B = magnetic field in Teslas (T) I = current in Amperes (A) l= length in meters (m) v B v F v Il magnetic = = Definition of Magnetic Field (B): 1 Tesla = 1 N / Am = 1 N / (Cm/s) 1 Tesla = 10,000 Gauss v F magnetic v qv

16 Faraday s Law Emf (voltage)= - (NBA) / t Faraday s Law of Magnetic Induction: emf= Electro-Motive Force (Volts) N = # of loops in circuit A = Area of loop (m 2 ) B = magnetic field (Teslas) t = time (seconds)

17 Summary 1. Moving charge (current) produces magnetic field -magnetic field wraps around current wire - used in electromagnets (junk car lifter) - direct current produces a constant B -alternating creates an alternating B field, which then creates a counter (eddy) current (ac electromagnet- floating rings demo, remote charging of pacemakers)

18 Transformers Emf depents on # of coils Must use ac currents! Power in = power out I*V = I*V V1/turns = V2 /turns