Phys 133 -- Chapter 321 Chapter 33 Magnetic Field.

Phys 133 -- Chapter 32 1

Chapter 33

Magnetic Field

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 3

Demo magnets

Phys 133 -- Chapter 32 4

Orients a compass

Magnetism: experiments 1

Phys 133 -- Chapter 32 5

Magnetism: experiments 2

Phys 133 -- Chapter 32 6

Do Workbook 32.1, 3 & 4

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 10

Do Workbook 32.7 & 8

Phys 133 -- Chapter 32 11

Moving charges produce magnetic fields

Magnet field: cause

(cross product, 3D)

remember

Phys 133 -- Chapter 32 12

Do Workbook 32.11 & 12

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 14

Problem 32.7 (ans)

Direction-RHR = out of page

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 17

Problem 32.50 (ans)

1

2

3

For entire loop

Direction into page (RHR)

Phys 133 -- Chapter 32 18

d

Magnetic fields: typical situations

Phys 133 -- Chapter 32 19

Demo loop

Phys 133 -- Chapter 32 20

Magnetic dipoleA current loop

Phys 133 -- Chapter 32 21

Do Workbook 32.18,19, 20

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 24

Do Workbook 32.23 & 24

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 28

Solenoid

Phys 133 -- Chapter 32 29

Uniform inside, “zero” out

Solenoid

Phys 133 -- Chapter 32 30

Force on moving charge

Phys 133 -- Chapter 32 31

Do Workbook 32.27 & 28

Phys 133 -- Chapter 32 32

Demo Tesla Coil

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 34

Problem 32.31 (ans)

Direction RHRDirection RHR

Phys 133 -- Chapter 32 35

Do Workbook 32.29 & 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

Phys 133 -- Chapter 32 38

Force on current-carrying wire

Phys 133 -- Chapter 32 39

Demo wire in magnetic field

Phys 133 -- Chapter 32 40

Do Workbook 32.35 & 37

Phys 133 -- Chapter 32 41

Force/torque on current loop

Phys 133 -- Chapter 32 42

Do Workbook 32.40 & 41

Phys 133 -- Chapter 32 43

Magnetic material and induced dipole