Charge and Field
Electric field strength E =
in a UNIFORM FIELD E = V/d
Electric potential V =
electrical potential energy
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 =
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
Accelerators, electric field and potential
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
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'
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'
10W 'The uniform electric field'
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'
Using the uniform fiel
charged objects (oil drops) can be held still/fall with constant velocity in uniform
So you can measure the charge
as Millikan did
pplet for millikan
Dis 70O 'Millikan's experiment'
Use applet or
Data 70D 'Millikan's oil drop experiment'
to work out value of e.
SAQ 60S 'Using uniform electric fields'
20M 'The uniform electric field and its effect on charges'
Demo 180D 'Electrical breakdown.'
Strong fields make sparks
SoftAct 70S 'Relating field and potential'
Comp 50C 'Electrical breakdown in a vacuum'
Reading 20T Text to Read 'Atmospheri
Comp 40C 'Thunder clouds and lightning conductors'
a) Reading 30T Text to Read 'The ultimate speed' + Dis 80O 'The ultimate speed
Deflecting Charged Beams
a charge follows a pa
rabolic path in uniform
the force on a moving charge in a uniform magnetic field, F = qvB
a charge follows a circular trajectory in a uniform
field, radius r =
and V =
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
in electric fields
electron beams follow parabolic paths between parallel plates
…in cathode ray tubes
Dis Material 110O 'Deflection plates in an oscilloscope'
'Deflecting electron beams in an electric field'
Dis 100O 'How an electric field deflects an electron beam'
SoftAct 130S 'Charged particles between plates'
constant acceleration so s = ½ a t
. Work out a value for s using the
t up on the deflection tube and see if you get it right.
in magnetic fields
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
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
use of magnetic fields in accelerators, detectors, scanners and spectrometers.
SAQ Question 90S 'Deflection with electric and magnetic fields'
Questions to try it out
and applications of BQv
know how to apply equatio
ns to situations involving deflection in magnetic and
applet of use of magnets in mass spectrometer, scanner, and bubble chamber pictures.
Discussion of use of magnetic fields.
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
ummary poster activity
deflection using F = VQ/d on the left, F = BQv on the right.
Detecting charges: non
uniform electric fields
fields and potential with spherical geometry.
evise equations from Ch
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'
can be used to demonstrate the equations
Demo 200D 'Exploring potential differences round a charged sphere'
Exp 220E 'Plotting potentials in non
Exp 230E 'Measuring potential differences between concentric conductors'
ds are concentrated around points.
Dis 170O 'Shapes of electrical fields'
Dis 180O 'Electrical fields with cylindrical symmetry'
, e.m. waves
and equation practice
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.
a 170D 'The electric dipole 'using Excel
if time or for homework
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'
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
SoftAct 250S 'Summing vector fields'
CQ 240M 'Fields and charged particles'
Question 260E Estimates 'Estimating with fields'
: 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
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
SAQ 160S 'Fields in nature and in particle accelerators'