Post on 22-Jan-2021
PHYS.1440Lecture16 A.DanylovDepartmentofPhysicsandAppliedPhysics
Lecture 16
Chapter 29
Magnetism
Physics II
Bye, bye, Electricity.You won’t be missed.
Course website:https://sites.uml.edu/andriy-danylov/teaching/physics-ii/
A)CapacitorA.B)CapacitorB.C)Theydischargeatthe
samerate.D)Can’tsaywithout
knowingtheinitialamountofcharge.
Whichcapacitordischargesmorequicklyaftertheswitchisclosed?
ConcepTest RC circuit 1
time constant = RC
= 12 µs = 15 µs
So the capacitor A discharges faster than B
A)5sB)4sC)2sD)1s
Whatisthetimeconstantforthedischargeofthecapacitorshowninthefigure?
ConcepTest RC circuit 3
time constant by definition = ReqC
+ =
= ReqC =4Ωx1F=4 seconds
How about this?
= ReqCeq
PHYS.1440Lecture16 A.DanylovDepartmentofPhysicsandAppliedPhysics
Today we are going to discuss:
Chapter 29:
Section 29.1-3 Section 29.5 (skip) Section 29.7
PHYS.1440Lecture16 A.DanylovDepartmentofPhysicsandAppliedPhysics
Sources of Magnetic FieldWehavelearnedthatelectricchargeqproducesanelectricfieldE
qE
Itturnsoutthatthereisanotherfield,magneticfield B,whichisproducedbyamovingelectricchargeq.
q v
(I)MovingqB B
Current
B (II)Current(manyq) B (III)Magnet B(orbitingelectrons)
B
So,therearethree systemsproducingBfield.Let’slookatthemseparately.
PHYS.1440Lecture16 A.DanylovDepartmentofPhysicsandAppliedPhysics
(III) Permanent Magnet
PHYS.1440Lecture16 A.DanylovDepartmentofPhysicsandAppliedPhysics
Magnets interact (repel/attract, observation)
The north pole of one magnet exerts anattractive force on the south pole ofanother magnet.
Like poles repel Unlike poles attract.
If the north pole of one magnet is broughtnear the north pole of another magnet,they repel each other.
PHYS.1440Lecture16 A.DanylovDepartmentofPhysicsandAppliedPhysics
There are no magnetic monopolesIf you cut a magnet in half, you don’t get a north pole and a south pole – you get two
smaller magnets.
Sheldon: “if I’m able to detect slow‐moving magneticmonopoles there, I will be the scientist to confirm string theory.People will write books about me. Third‐graders will createmacaroni‐art dioramas depicting scenes frommy life.”
Hisfriendstamperedwithhisexperimentonmagneticmonopolesbyusinganelectriccanopener.
There is no magnetic monopole
Oneofthe“BigBangTheory”episodeswasarrangedaroundthisfact.
Thereisnoexperimentalevidencethatmagneticmonopolesexist.
Anumberofattemptshavebeenmadetodetectmagneticmonopoles.SheldonCooperhasalsomadeone:
PHYS.1440Lecture16 A.DanylovDepartmentofPhysicsandAppliedPhysics
Probe object. Magnet Pole Convention
If we position a compass needle in the Earth field, one side will point to the geographical North pole.
(I) The end of a needle that points to the North geograph. pole is called the north pole (by CONVENTION).The end of a magnet that points south is called the south pole.As the result, if we follow the convention, the Earth’s “Geogr. North Pole” is a south magnetic pole, as the north end of the compass needle is attracted to it.The magnetic poles are slightly offset from the geographical poles.
*The Earth is a huge magnet due to currents in the molten iron core.
North Magnetic Pole
South Magnetic Pole
Again?
Now, we know that the magnetic field (B) exists. How can we detect B? Which object can be used as a probe system?
(0) A compass needle will be used as a probe system.
Which side of the needle should be called the North pole?Lets use the Earth’s Magnetic field*
(II) The North pole of a compass shows the direction of the magnetic field at the point.
PHYS.1440Lecture16 A.DanylovDepartmentofPhysicsandAppliedPhysics
Magnetic Fields Lines
Magneticfieldscanbevisualizedusingmagneticfieldlines,whicharealwaysclosedloops(consequenceofthefactthattherearenomagneticmonopoles).
Howtofindthesemagneticlines? With a compass needlelike a “probe charge”
B
Density of magn. lines is smaller so the magn. field is weaker
The North Pole of a compass needle shows the direction of the magnetic field
https://www.youtube.com/watch?v=8llkHQtaOlg
(II) Density of magn. lines is higher so the magnetic field is stronger
Demo:Iron filings are suspended in liquid in a clear rectangle box.
Now,sincewehaveaprobesystem,acompassneedle,wecanuseittomapamagneticfieldof,forexample,apermanentmagnet.
PHYS.1440Lecture16 A.DanylovDepartmentofPhysicsandAppliedPhysics
Vector notation and the cross (vector) product
Before we proceed with magnetic field we need to look at
PHYS.1440Lecture16 A.DanylovDepartmentofPhysicsandAppliedPhysics
Notation for Vectors and Currents Perpendicular to the Page
Magnetismrequiresathree‐dimensionalperspective,buttwo‐dimensionalfiguresareeasiertodraw.
Wewillusethefollowingnotation:
Bad karma
PHYS.1440Lecture16 A.DanylovDepartmentofPhysicsandAppliedPhysics
The Vector (Cross) Product
=(ab sinθ,directiongivenbytheright‐handrule)
Fromnowon,almostallequationswillhaveacrossproduct
Thecrossproductofvectorsa andb isavectorperpendiculartobotha andb.
[I]Pointfingersinthedirectionofthe1st (a)vector,thenbendtheminthedirectionofthe2nd one(b).Theoutstretchedthumbwillgiveadirectionofthecrossproduct
[II]Usethreefingersasshowninthefigure
Orderisimportantinthecrossproduct:
ConcepTest Cross Product
Find a direction of a cross product
PHYS.1440Lecture16 A.DanylovDepartmentofPhysicsandAppliedPhysics
Thank you