Agenda for Today. Elements of Physics II. Forces on currents

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1 Forces on currents Physics 132: Lecture e 20 Elements of Physics II Agenda for Today Currents are moving charges Torque on current loop Torque on rotated loop Currents create B-fields Adding magnetic fields Force between wires Physics 201: Lecture 1, Pg 1

2 Force on charged particle by B-field A charged particle will feel a force when it s ina B-field if: The charge is moving Velocity of charge is not completely parallel to the B-field Charge B-field velocity No Force Physics 201: Lecture 1, Pg 2

3 Force on charged particle by B-field A charged particle will feel a force when it s ina B-field if: The charge is moving Velocity of charge is not completely parallel to the B-field Charge velocity B-field There is a Force Physics 201: Lecture 1, Pg 3

4 Direction of Force on charged particle by B-field Magnitude of force: F = qvbsin The direction of the force is perpendicular to both the velocity vector and the B-field vector Velocity and B-field make a plane, force goes through this plane. Charge velocity B-field Force is perpendicular to screen! Physics 201: Lecture 1, Pg 4

5 Right hand rule The direction of the force can be found with the right hand rule Thumb in direction of velocity, fingers in direction of fbfi B-field, palm points where force goes Charge velocity B-field Force goes into page! Physics 201: Lecture 1, Pg 5

6 To find the direction of the force felt by a charged particle moving in a magnetic-field Flat Right Hand Rule Thumb = velocity Fingers = B-field. Palm = force If charge is negative force is opposite! Physics 201: Lecture 1, Pg 6

7 Clicker Question 1: A proton is shot straight at the center of a long, straight wire carrying current into the screen. The proton will A. Go straight into the wire. B. Hit the wire in front of the screen. C. Hit the wire behind the screen. D. Be deflected d over the wire. E. Be deflected under the wire. Physics 201: Lecture 1, Pg 7

8 Clicker Question 2: Which magnetic field causes the observed force? Physics 201: Lecture 1, Pg 8

9 Clicker Question 3: A long straight wire is carrying current from left to right. Near the wire is a charge q with velocity v. Compare the strength of the magnetic force on q in (a) vs. (b) a) (a) has the larger force b) (b) has the larger force c) force is the same for (a) and (b) a) r v I v b) r I Physics 201: Lecture 1, Pg 9

10 Electric vs Magnetic Electric Magnetic Source: Charges Moving Charges Act on: Charges Moving Charges Magnitude: F=Eq Direction: Parallel E F = q v B sin( ) Perpendicular to v,b Physics 201: Lecture 1, Pg 10

11 Motion of q in uniform B field Force is perpendicular to B,v Uniform circular motion B does no work! (W=F d cos ) Speed is constant (W= KE K.E. ) F m Calculate R v 2 v qvbsinθ m R 2 R R mv qb v x x x x x x x R x x x x x x x x F x x x x x x x x x x x x x F x x x x x x x v x x x x x x x Uniform x B into x x page x x x x Physics 201: Lecture 1, Pg 11

12 Particle Moving in an External B-Field If the particle s velocity is not perpendicular to the field, the path followed by the particle is a spiral The spiral path is called a helix Physics 201: Lecture 1, Pg 12

13 Aurora Physics 201: Lecture 1, Pg 13

14 Clicker Question 4: Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity 75 m/s up, and follows the dashed trajectory. What is the direction of the magnetic field in region 1? a) up b) down c) left d) into page e) out of page 1 2 v = 75 m/s q = +25 mc Physics 201: Lecture 1, Pg 14

15 Clicker Question 5: Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity 75 m/s up, and follows the dashed trajectory. Compare the magnitude of the magnetic field in chambers 1 and 2 A) B 1 > B 2 B) B 1 = B 2. C) B 1 < B v = 75 m/s q = +25 mc Physics 201: Lecture 1, Pg 15

16 Clicker Question 6: Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity 75 m/s up, and follows the dashed trajectory. What is the speed of the particle in chamber 2. A) v 2 < 75 m/s B) v 2 = 75 m/s C) v 2 > 75 m/s 1 2 v = 75 m/s q = +25 mc Physics 201: Lecture 1, Pg 16

17 Force on a Current F = q v B sin( ) Out of the page (RHR) + v F =(q/t)(vt)b sin( ) = I L B sin( ) Flat right hand rule + B v I = q/t L = vt Physics 201: Lecture 1, Pg 17

18 Clicker Question 7: Suppose a current pointing up is applied. In which direction will the wire feel a force? (a) Right (b) Left (c) Out of the page (d) Into the page (e) There will be no force Physics 201: Lecture 1, Pg 18

19 Force between wires carrying current I towards us B F Another I towards us Conclusion: Currents in same direction attract! I towards us B F Another I away from us Conclusion: Currents in opposite directions repel! Note: this is different from the Coulomb force between like or unlike charges. Physics 201: Lecture 1, Pg 19

20 Clicker Question 8: What is the direction of the force on the top wire, due to the two below??assume the wires carry equal magnitude of current A. Left B. Right C. Up D. Down E. Zero Physics 201: Lecture 1, Pg 20

21 Example: The wires are located at (-2,0) meters and (2,0) meters. The former carries 3 A coming out from the sheet of the paper, and the latter 3 A going into it. What is the x-component of the force on a 5-meterlong segment of wire B due to the magnetic field from wire A? (a) F = N (b) F = N (c) F = 0 N (d) F = N (e) F = N Physics 201: Lecture 1, Pg 21

22 Example: The wires are located at (-2,0) meters and (2,0) meters. The former carries 3 A coming out from the sheet of the paper, and the latter 3 A going into it. What is the x-component of the force on a 5-meter-long segment of wire B due to the magnetic field from wire A? Physics 201: Lecture 1, Pg 22

23 Clicker Question 9: A rectangular loop of wire is carrying current as shown. There is a uniform magnetic field parallel to the sides ab and cd. The loop will: (a) move to the right (b) move to the left (c) move up (d) move down (e) rotate d c B a b Physics 201: Lecture 1, Pg 23

24 Torque on Rotated Loop More general case B Magnitude Direction: = I A B sin is angle between normal and b-field The torque tries to line up its B-field with the external B! Even if the loop is not rectangular, as long as it is flat = IAB sin N # of loops (area of loop) Physics 201: Lecture 1, Pg 24

25 Clicker Question 10: If released from rest, the current loop will A. Move upward. B. Move downward. C. Rotate clockwise. D. Rotate counterclockwise. E. Do something not listed here. Physics 201: Lecture 1, Pg 25

26 MRI Physics 201: Lecture 1, Pg 26

27 Magnetic Resonance Imaging Physics 201: Lecture 1, Pg 27

28 Types of Magnetic Materials Soft magnetic materials are easily magnetized (Iron) Lose their magnetism easily Hard magnetic materials are difficult to magnetize (nickel) Retain their magnetism Physics 201: Lecture 1, Pg 28

29 Atomic Magnets A plausible explanation for the magnetic properties of materials is the orbital motion of the atomic electrons. The figure shows a simple, classical model of an atom in which a negative electron orbits a positive nucleus. In this picture of the atom, the electron s motion is that of a current loop! An orbiting electron acts as a tiny magnetic dipole, with a north pole and a south pole. Physics 201: Lecture 1, Pg 29

30 Magnetic Effects of Electrons Spins Electrons also have spin The classical model is to consider the electrons to spin like tops It is actually a quantum effect Physics 201: Lecture 1, Pg 30

31 Magnetic Effects of Electrons Orbits An individual atom should act like a magnet because of the motion of the electrons about the nucleus Each electron circles the atom once in about every seconds This would produce a current of 1.6 ma and a magnetic field of about 20 T at the center of the circular path However, the magnetic field produced by one electron in an atom is often canceled by an oppositely revolving electron in the same atom Physics 201: Lecture 1, Pg 31

32 Magnetic Effects of Electrons Spins, cont The field due to the spinning is generally stronger than the field due to the orbital motion Electrons usually pair up with their spins opposite each other, so their fields cancel each other That is why most materials are not naturally magnetic Physics 201: Lecture 1, Pg 32

33 Magnetic Effects of Electrons Domains In some materials, the spins do not naturally cancel Such materials a are called ferromagnetic e Large groups of atoms in which the spins are aligned are called domains When an external field is applied, the domains that are aligned with the field tend to grow at the expense of the others This causes the material to become magnetized Physics 201: Lecture 1, Pg 33

34 Domains, cont Random alignment, a, shows an unmagnetized material When an external field is applied, the domains aligned with B grow, b Physics 201: Lecture 1, Pg 34

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