浙江大学大学物理甲下 chapter 32

1、Chapter 32 The Magnetic Field1Chapter 32 The Magnetic FieldChapter 32 The Magnetic Field232-1 Magnetic Interactions and Magnetic Poles Keep in our mind many important similarities between electric and magnetic interactions and many important differences between them. Magnetic field of the EarthChapt

2、er 32 The Magnetic Field3Magnetic field of a bar magnet1. Magnetism and moving chargesDoes there exist in nature a “magnetic charge”? No .Magnetic field of a current-carrying wireChapter 32 The Magnetic Field4 Can we use an electric test charge to probe a magnetic field? Yes, but only if the charge

3、is moving relative to the source of the magnetic field. Can a electric charge in motion produce magnetic field? Yes. Is there a “magnetic potential energy” associated with a moving electric test charge in a magnetic field? No, because the magnetic force is not conservative force.Moving electric char

4、geMoving electric chargeMagnetic fieldChapter 32 The Magnetic Field5 2. Magnetic poles north pole and south pole like poles and unlike poles the magnetic pole of the EarthChapter 32 The Magnetic Field632-2 The Magnetic Force on a Moving ChargeBqFB vB is called the magnetic induction or magnetic flux

5、 density. B is the more fundamental quantity and is regarded the magnetic field.)(EqFEUnite of B is tesla (T) in SI.Chapter 32 The Magnetic Field7qvBF Magnetic field cannot be determined quite so simply with a single measurement.Note: Magnetic force is always a sideway deflecting force.BqFB v Magnet

6、ic force cannot change the magnitude of v, only its direction. Magnetic force do no work on charged particle q.Chapter 32 The Magnetic Field8Combined electric and magnetic forceforce Lorentz BqEqFvExample: A modern version of J.J.Thomsons apparatus for measuring the charge-to-mass ratio of the elect

7、ron. (1897)BeeEv22222 ,2 LByEmemeELyvVelocity selector Chapter 32 The Magnetic Field932-3 Circulating ChargesqBmrrmBqvvv 2. and on t independen is )or ( vrfTrmqBfqBmrT2 22vmrBqmK2212222vChapter 32 The Magnetic Field10qBmr11 v 1. The cyclotronqBmr22 v final, 2mqBRqBmTmvChapter 32 The Magnetic Field11

8、2228BUme In our laboratory, we use the magnetic focuser for measuring the charge-to-mass ratio of the electron. 2. The magnetic focusTh/sin)0 if( ,cosvvvvvvvBRh/vv v Chapter 32 The Magnetic Field12 3. The magnetic mirrorChapter 32 The Magnetic Field13Van Allen radiation beltInner radiation belt103km

9、Outer radiation belt2 104kmauroraChapter 32 The Magnetic Field14Quick quiz-1 A charged particle is moving perpendicular to a magnetic field in a circle with a radius r. An identical particle enters the field, with v perpendicular to B, but with a higher speed v than the first particle, the radius of

10、 the circle for the second particle is (1) smaller, (2) larger, (3) equal in size. Chapter 32 The Magnetic Field15Quick Quiz-2 A charged particle is moving perpendicular to a magnetic field in a circle with a radius r. The magnitude of the magnetic field is increased. Compared to the initial radius

11、of the circular path, the radius of the new path is (1) smaller, (2) larger, (3) equal in size.Chapter 32 The Magnetic Field1632-4 The Hall Effect (1879)In figure 32-18 :If the charge carriers is electronFBFEHEIf the charge carriers is positiveFBFEHEChapter 32 The Magnetic Field17BwwEVBEBqqEdHHdHdHv

12、vv , Measure B from VHcarriers. charge theofdensity number themeasure toHVqtiBnBwtnqiBnqjBEwVdHHv nt.cooefficie shall called is 1 ;1nqtiBnqVHChapter 32 The Magnetic Field18The quantized hall effect (1980)resistance Hall nqtBiVRHH), 3 , 2 , 1( 2kkehRHAt high magnetic field: 1-10T and low temperature:

13、 1K 42eh1 KlitzingChapter 32 The Magnetic Field1932-5 The Magnetic Force on a Current-carrying Wire Amperes force BLiBnALeBNeFddBvv If the wire is not straight or the field is not uniform, BLiFBLiFBBddd) ,d(dEqFEqFEEcurrent. ofdirection in thepoint ofdirection Thesegment. oflength is of magnitude Th

14、eLLChapter 32 The Magnetic Field20Sample problem 32-5:Solution: 31iLBFF. owardradially tdirection its ),d(dd2ORiBsiBF 2dsinsind)(sind00022iBRiBRiBRFF)(2321RLiBFFFFChapter 32 The Magnetic Field21Quick Quiz-3 The four wires shown in Figure. All carry the same current from point A to point B through th

15、e same magnetic .field. In all four parts of the figure, the point A and point B are 10 cm apart. Which wires on them exerted the magnitude of the magnetic force is greatest?(1)(2)(3)(4)Chapter 32 The Magnetic Field22Quick Quiz-4 A wire carries current in the plane of this paper toward the top of th

16、e page. The wire experiences a magnetic force toward the right edge of the page. The direction of the magnetic field causing this force is (1) in the plane of the page and toward the left edge. (2) in the plane of the page and toward the bottom edge. (3) upward out of the page.(4) downward into the

17、page. iFChapter 32 The Magnetic Field2332-6 The Torque on a Current Loop The net force on the loop is zero, so its The net torque on the loop is not zero.rule. hand-right by the determined isdirection its r,unit vecto is nBniAsin2)(2biaBFrabbsinsiniABiabBChapter 32 The Magnetic Field24Quick Quiz-5 Which torque, acting on the rectangular loop shown edge-on in Figure, is greatest? All loops are identical and carry the same current.pmpmpmMagnetic moment pm=iAn Bpm(1)(2)(3)Chapter 32 The Magnetic Field25Quick Quiz-6 Which force, acting on the rectangular loop