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18 Οκτ 2013 (πριν από 3 χρόνια και 7 μήνες)

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Chapter
1
6



Charge and Field



17
.1

Linear accelerators


Learning outcomes



Electric field strength E =
F

q



in a UNIFORM FIELD E = V/d



Electric potential V =
electrical potential energy

charge



energy transfer to/from charge when it moves through a p.d. is
W = q



the velocity at non
-
relativistic velocities (v << c) of a charge q when it has moved through a
potential difference V is





electric field strength =
-

potential gradient

and is given by




an electric field can be represented by field lines and equipote
ntial surfaces, which are
perpendicular to each other



the evidence for the discreteness of the charge on an electron (that it comes in multiples of 1.6 x
10
-
19

C)


Lesson 1
:


Accelerators, electric field and potential


Objectives
:

-



you know that there

is a field there because there is a force on a charge

-

in a uniform field E = V/d

-

field lines are parallel in uniform field, equipotentials are at right angles to them

-

at points where equipotentials are close together the field is strong

-

because field is
potential gradient


Starter: animation of CERN


how does it work

Display Material 40O 'The linear accelerator'


Demo 10D 'A flame between charged plates'

Demo 20D 'Using a foil strip to look at uniform electric fields'

Demo 30D 'Exploring potential differ
ences in a uniform field'

-

optional

Define field, uniform field, field = potential gradient


Dis 10O 'The electric field between parallel plates'

Dis 20O 'Acceleration: gravitational and electrical'

Dis 30O 'Two ways of describing electrical forces'

Warm

10W 'The uniform electric field'


R
eal life situations…
Exp 60E 'Measuring potentials in a uniform field conducting paper'

Dis 50O 'Field lines and equipotential surfaces'

Dis 60O Field strength and potential gradient'



Lesson
2
:
.


Using the uniform fiel
d
-

Millikan



Objectives:

-


charged objects (oil drops) can be held still/fall with constant velocity in uniform
field

-

So you can measure the charge


as Millikan did



A
pplet for millikan
http://physics.wku.edu/~womble/phys260/millikan.html


Dis 70O 'Millikan's experiment'


Use applet or
Data 70D 'Millikan's oil drop experiment'

to work out value of e.


Lesson 3


More

practice


SAQ 60S 'Using uniform electric fields'

MC
20M 'The uniform electric field and its effect on charges'



Demo 180D 'Electrical breakdown.'

Strong fields make sparks


Homework

SoftAct 70S 'Relating field and potential'

Comp 50C 'Electrical breakdown in a vacuum'

Reading 20T Text to Read 'Atmospheri
c electricity'

Comp 40C 'Thunder clouds and lightning conductors'

Extension:
a) Reading 30T Text to Read 'The ultimate speed' + Dis 80O 'The ultimate speed
-

Bertozzi's demonstration'



1
6.
2
Deflecting Charged Beams


Learning outcomes



a charge follows a pa
rabolic path in uniform
electric

field



the force on a moving charge in a uniform magnetic field, F = qvB



a charge follows a circular trajectory in a uniform
magnetic

field, radius r =
mv


qB



F =

q
1
q
2


and E
e

=

q
1
q
2


4

0
r
2




4

0
r



E =

q


and V =

q

4

0
r
2



4

0
r

in a central field: use of analogy and symmetry arguments



graphs of all these equations and show how they relate to one another



electric and magnetic fields ar
e used in particle accelerators



Lesson 4
Parabolic paths

in electric fields


Objectives

-

electron beams follow parabolic paths between parallel plates


…in cathode ray tubes
, CROs

Dis Material 110O 'Deflection plates in an oscilloscope'


Demo 110D
'Deflecting electron beams in an electric field'

Dis 100O 'How an electric field deflects an electron beam'

SoftAct 130S 'Charged particles between plates'


Work out

using

the maths


constant acceleration so s = ½ a t
2
. Work out a value for s using the
se
t up on the deflection tube and see if you get it right.



Lesson 5:

Circular paths

in magnetic fields


Objectives

-

force on a charged particle in a magnetic field F = BQv

-

charges go in circles

-

and this is useful in accelerators, particle detectors, m
edical equipment and
mass spectrometers


Demo 120D Demonstration 'Deflecting electron beams in a magnetic field'

Warm 80W 'Getting F = q v B'


link to Chapter 15


Dis 120O 'How a magnetic field deflects an electron beam'

Dis 130O 'Force on current: force
on moving charge'

SoftAct Activity 140S 'Circular motion in a magnetic field'

Dis 140O 'Measuring the momentum of moving charged particles'

Dis 150O 'Principle of the synchrotron accelerator'

Dis 160O 'Electromagnetic waves generated by accelerating charge
s'



The

use of magnetic fields in accelerators, detectors, scanners and spectrometers.


SAQ Question 90S 'Deflection with electric and magnetic fields'



Lesson 6:
Questions to try it out

and applications of BQv


Objectives

-

know how to apply equatio
ns to situations involving deflection in magnetic and
electric field


Starter:
applet of use of magnets in mass spectrometer, scanner, and bubble chamber pictures.
Discussion of use of magnetic fields.

http://www.physics.upenn.edu/courses/gladney/mathphys/java/sect5/section5_1.html

http://teachers.web.cern
.ch/teachers/archiv/HST2000/teaching/resource/bubble/bubble.htm



SAQ Question 100S 'The cyclotron'

MC 110M 'Charged particles in electric and magnetic fields

SAQ 150S 'Charged particles moving in magnetic fields'

Tracks in bubble chambers, and another
look at the cyclotron


S
ummary poster activity


deflection using F = VQ/d on the left, F = BQv on the right.


Lesson 7:
Detecting charges: non
-
uniform electric fields


Objectives

-

fields and potential with spherical geometry.


R
evise equations from Ch
apter 11


gravitational fields.
C
harge equations by analogy. F equation
is Coulomb’s Law, like Newton’s Law.

SoftAct 270S 'Radial force, field and potential'

Dis 210O 'Force, field, energy and potential'


Pres 210P 'The 1/r hill: Slope and force'

T
he hill

can be used to demonstrate the equations


Demo 200D 'Exploring potential differences round a charged sphere'

-

optional


Exp 220E 'Plotting potentials in non
-
uniform fields''

Exp 230E 'Measuring potential differences between concentric conductors'


E fiel
ds are concentrated around points.

Dis 170O 'Shapes of electrical fields'

Dis 180O 'Electrical fields with cylindrical symmetry'



Lesson 8:
Coulomb’s Law
, e.m. waves

and equation practice



Objectives

-

know how you test Coulomb’s Law



-

be able to u
se the equations


Question 210D: Data Handling Testing Coulomb’s law


SAQ 180S 'Non
-
uniform electric fields'

Covers all the ideas in this episode: use of symmetry to complete a field pattern, relationship
between equipotentials and field strength, Coulomb
's law, potential in a radial field.


Moving charges in dipoles make electromagnetic waves.

Show applets

http://www.phys.hawaii.edu/~teb/java/ntnujava/emWave/emWave.html


Dat
a 170D 'The electric dipole 'using Excel

if time or for homework



Lesson 9:
Further practice



Objectives

-

be able to use the equations


SAQ 200S 'Using the 1/r 2 and 1/r laws for point charges'

MC 220M 'Relationships for force and field, potential a
nd potential energy'

SAQ 190S Short Answer 'Charged spheres: Force and potential'


Homework


Data 130D 'The proton synchrotron' not Q13 + 14 optional


Reading 40T Text to Read 'Some information about LEP at CERN'

SoftAct 240S 'Mapping inverse square vecto
r fields'

SoftAct 250S 'Summing vector fields'

M
CQ 240M 'Fields and charged particles'

Question 260E Estimates 'Estimating with fields'

Extension
: a) Data 130D 'The proton synchrotron' Q13 + 14

b) ExExQ 230X 'The inverse square law applied to nuclear phenomena'

c) ExExQ 120X 'Deflection of charged particles in a magnetic field'

d) SAQ 250S 'Controlling charged particles'

e) The Hall Effect (link
s to magnetism, chemistry etc.)
-

examples include finding e/m and the mass
spectrometer.

SoftAct 150S ''Velocity filters on a spreadsheet'

Demo 160D 'Measuring the charge to mass ratio for an electron'

SoftAct 170S 'Making a velocity filter'

SAQ 140S 'The

Hall effect'

SAQ 160S 'Fields in nature and in particle accelerators'