Presentation on theme: "MIT visualizations: Biot Savart Law,"— Presentation transcript:
1MIT visualizations:Biot Savart Law,Integrating a circular current loop on axis
2Change of policy:Quizzes: you are allowed one 3x5 index card to write anything you wish on it, front and back.CoursIf you did not receive the class s, please give me an address after class!Class s are posted on the website – see the “Solutions/Class s tab” link
3Notes on how to study:Give a chapter a cursory read before lecturePrint out lecture notes ahead of class ( if available). Take notes on slides/back-side.Notes on how to do homework:Read some section(s) and take notes from bookWork those homework problems pertaining to section(s) away from computer!Login to computer terminal and check answersSpend a moment reflecting on the concepts the homework problem examinedReiterate through out week and finish homework on timeQuizzes and Exams will presume know how to work all problems.If you don't do the homework, you will be killed by exams and quizzesNotes on how to study for quizzes:Review homework and compare with posted solutionsReview your book notes + lecture notesWrite up your 3x5 cardMinimize memorization in many cases by remembering principle(s) + derivationNotes on how to study for exams:Review book notes + Lecture notes + the homework problems you have written
4Last time:-Moving charges (and currents) create magnetic fields-Moving charges (and currents) feel a force in magnetic fieldsDefines the B-field; Examples - cyclotron frequency, velocity selector,mass spectrometer- Force on a wire segment:- Force on a straight wire segment in uniform B-field:- Torque on current loop in uniform magnetic field:- Potential energy of magnetic dipole (current loop) in magnetic field
8Applying the Biot-Savart Law Arbitrary shaped currents difficult to calculateSimple cases, one can solve relatively easily with pen and paper:current loops, straight wire segmentsLast time, we found B-field for straight wire segment with current in x- direction:And limit xf=-xi=L-> infinity for an infinitely long straight wire:
9Applying the Biot-Savart Law: Circular arc Circle
11Applying the Biot-Savart Law: On-axis of circular loop
12Applying the Biot-Savart Law: On-axis of circular loop
13Applying the Biot-Savart Law: On-axis of circular loop
14Ampere’s Law: An easier way to find B-fields (in very special circumstances)
15Ampere’s Law: An easier way to find B-fields (in very special circumstances)
16Ampere’s Law: An easier way to find B-fields (in very special circumstances)
17Ampere’s Law: An easier way to find B-fields (in very special circumstances)For any arbitrary loop (not just 2-D loops!):Ampere’s LawUse Ampere’s Law to find B-field from current (in very special circumstances)Current that does not go through “Amperian Loop” does not contribute to the integralCurrent through is the “net” current through loop3. Try to choose loops where B-field is either parallel or perpendicular to ds, the Amperian loop. To do this, remember that symmetry is your friend!Review pages !!
18Ampere’s Law: Example, Finite size infinite wire Calculate the B-field everywhere from a finitesize, straight, infinite wire with uniform current.From symmetry,Br=0 (reverse current and flip cylinder)Bz=0 (B=0 at infinity, Amperian rectangular loop from infinity parallel to axial direction implies zero Bz everywhere)Only azimuthal component exists. Therefore, amperian loops are circles such that B parallel to ds.
19Ampere’s Law: Example, Finite size infinite wire Calculate the B-field everywhere from a finitesize, straight, infinite wire with uniform current.
20Ampere’s Law: Example, Finite size infinite wire Calculate the B-field everywhere from a finitesize, straight, infinite wire with uniform current.
21Ampere’s Law: Example, Infinitely long solenoid As coils become more closely spaced, and the wires become thinner, and the length becomes much longer than the radius,The B-field outside becomes very, very small (not at the ends, but away from sides)The B-field inside points along the axial direction of the cylinderSymmetry arguments for a sheet of current around a long cylinder:Br=0 – time reversal + flipping cylinder, but time reversal would flip Br!Azimuthal component? No, since if we choose Amperian loop perpendicular to axis, no current pierces it.B=0 everywhere outside: any Amperian loop outside has zero current through it
22Ampere’s Law: Example, Infinitely long solenoid Cross sectional view:
23Ampere’s Law: Example, Toroid Solenoid bent in shape of donut.B is circumferential.
24Force Between two parallel, straight current carrying wires: Parallel currents attract, Opposite currents repel.
27Permanent magnets related to (tiny) currents: Current loops look like magnets, and vice versa:Permanent magnets can be thought of as a many tiny current loops created by the ‘spin’ of the electron. These tiny current loops (magnetic moments) tend to line up creating a macroscopic, large magnetic field. Note that, at least from a classical point of view, a charged sphere spinning creates a circulating current magnetic field.