Phys 133 -- Chapter 32 1 Chapter 33 Magnetic Field
Dec 15, 2015
Phys 133 -- Chapter 32 2
What are magnetic phenomena?
What causes magnetic fields?
Magnetic field of moving charges/currents
Magnetic force on moving charges/currents
Magnetic torque
Magnetic materials
The questions
Phys 133 -- Chapter 32 7
Magnetic field: wireCurrent produces a magnetic field too - same as magnet
Right thumb rule
Phys 133 -- Chapter 32 8
Thumb along, fingers around (think circles!)
Magnetic field: symbol conventionsIn general
Phys 133 -- Chapter 32 9
Magnetic field: vectors and field linesArrow indicates direction compass north would point.
Longer arrow, larger field Closer spacing, larger field
Phys 133 -- Chapter 32 11
Moving charges produce magnetic fields
Magnet field: cause
(cross product, 3D)
remember
Phys 133 -- Chapter 32 13
What are the magnetic field strength and direction at the dot in the figure?
Problem 32.7
Phys 133 -- Chapter 32 15
--pick a generic current segment
--field due to segment at location of interest
--convert current segment to coordinate
--sum them up, becomes integral
Magnetic field: current segmentCurrent (many moving charges) produce magnetic fields
To find magnetic field produced by a wire configuration
Phys 133 -- Chapter 32 16
Find an expression for the magnetic field at the center (P) of the circular arc.
Problem 32.50
Phys 133 -- Chapter 32 22
--always true
--sometimes helpful (high symmetry)
Ampere’s law
(Currents cause magnetic fields)
Phys 133 -- Chapter 32 23
Take a segment of path, dot product with field, write it down, find another segment, repeat, add up
Path integrals
Phys 133 -- Chapter 32 25
--always true
--sometimes helpful (high symmetry)
Ampere’s law
(Currents cause magnetic fields)
Phys 133 -- Chapter 32 26
A long wire is surrounded by a thin cylindrical shell of radius R1. The inner wire carries a current I0 in one direction and the outer shell carries a current I0 in the other direction. Use Ampere’s law to find the magnetic field strength everywhere.
Problem Ampere
Phys 133 -- Chapter 32 27
Problem Ampere (ans)
• x
•
•
•
I0 (in)
I0 (out)
R1
Side view
Choose concentric circle as path
B is circular (RTR)
Then
Ampere’s law becomes
r < R1
R1 < r
Phys 133 -- Chapter 32 33
An electron moves in the magnetic field B=0.50 i T with a speed of 1.0x107m/s in the direction shown. What is the magnetic force (in component form) on the electron?
Problem 32.31
Phys 133 -- Chapter 32 36
Find the strength and direction of electric and magnetic fields confined to the dashed region, such that a charged particle (q>0, m) coming from the left at speed v0 will pass through undeflected and without speeding up.
(easy one: E=0, B=0)
Problem Velocity Selector
Phys 133 -- Chapter 32 37
Problem Velocity Selector (ans) +++++++++++++
- - - - - - - - - - - - -
E
+
Put electric field down
Then
Need force up if proton is to be undeflected
Put magnetic field into page
Then
We want ay = 0