Transcript 9.4 From Ideas to Implementation Contextual outline

9.4 From Ideas to Implementation
Contextual outline
By the beginning of the twentieth century, many of the pieces of the physics puzzle
seemed to be falling into place. The wave model of light had successfully explained
interference and diffraction, and the wavelengths at the extremes of the visible
spectrum had been estimated.
The invention of a pump that would evacuate tubes to 10-4 atmospheres allowed the investigation
of cathode rays. X- rays would soon be confirmed as electromagnetic radiation and patterns in the
Periodic Table appeared to be nearly complete. The nature of cathode rays was resolved with the
measurement of the charge on the electron soon to follow. There was a small number of
experimental observations still unexplained but this, apparently complete, understanding of the
world of the atom was about to be challenged.
The exploration of the atom was well and truly inward bound by this time and, as
access to greater amount s of energy became available,the journey of physics
moved further and further in to the study of subatomic particles . Careful
observation , analysis, imagination and creativity throughout the early part of the
twentieth century developed a more complete picture of the nature of
electromagnetic radiation and matter.
The journey taken into the world of the atom has not remained isolated in laboratories. The
phenomena discovered by physicists have, with increasing speed, been channelled into technologies
such as computers, to which society has ever-increasing access. These technologies have, in turn,
often assisted physicists in their search for further knowledge and understanding of natural
phenomena at the sub-atomic level.
This module increases students’ understanding of the history, nature and practice of
physics and the applications and uses of physics, the implications of physics for society
and the environment, and the current issues, research and developments in physics.
1. Increased
understandings of
cathode rays led to
the development of
television
Perform an investigation and
gather first-hand information to
observe the occurrence of different
striation patterns for different
pressures in discharge tubes
Using discharge tubes
A common piece of apparatus used for this investigation is a set of glass discharge tubes at
different pressures, arranged side-by-side on a board. The tubes have been sealed after having
had varying amounts of air pumped out of them (the more air pumped out, the lower the air
pressure).
Each tube contains an electrode at each end to allow the application of a large voltage, which is
provided by an induction coil. The high voltage causes an electrical discharge through the air in
the tube, causing the air to glow. Different discharge patterns are formed at different pressures.
Sample observations:
At 5% of atmospheric pressure, long, thin red-purple streamers appear between the two
electrodes.
At lower pressure, these streamers give way to a soft red glow.
Upon further pressure reduction, the glow is broken into striations, bands of light and
dark. The amount of dark space between the glowing bands increases with further
reductions.
At 0.01% of atmospheric pressure, the dark space extends throughout the tube. At this
very low pressure, the glass near the anode glows a yellow-green colour.
Explain that cathode ray tubes
allowed the manipulation of a
stream of charged particles
A cathode ray tube is a highly evacuated glass tube
containing two electrodes. A high voltage applied
across the electrodes causes cathode rays, streams
of negatively charged particles (electrons), to flow
from the cathode towards the anode, with little
obstruction from the few remaining gas particles.
Structures built into or around the cathode ray
tube allow the cathode rays to be manipulated.
Further electrodes can be built into the cathode
ray tube to create an electric field to change the
path of the cathode rays. Magnetic fields can be
applied to the cathode rays through the glass
from outside the tube. Solid objects can also be
placed inside the tube to block the path of the
rays. (NSW HSC On-line)
Identify that moving charged
particles in a magnetic field
experience a force
A charged particle moving perpendicular to (across) a
magnetic field experiences a force which is perpendicular to
the motion and the field. Just like you have hundreds of times
in Lab Newton, use your right hand palm rule for positive
particles and your left hand for negative particles.
The force, F, on the charge is:
– proportional to the size of the charge, q;
– proportional to the velocity of the charge, v;
– proportional to the magnetic field strength, B; and
– proportional to the sine of the angle, q , between the velocity and the magnetic field
It is a maximum when q is 90° (velocity at right angles to the field), and zero when q is
zero (velocity parallel to the field).
Describe quantitatively the
force
acting on a charge moving
through a magnetic field
F =Bqv sinq
A proton travelling at 5.0 x 104 m s-1 enters a magnetic field of strength 1.0 Tesla at 90°.
Determine the magnitude of the force experienced by the proton.
Solution:
F =Bqv sinq = 1.0 * (1.6 x 10-19 )* (5.0 x 104) * 1
= 8 x 10-15 N
The path of a helium nucleus, travelling at 3.0 x 103 m s-1, makes an angle of 90° to a
magnetic field. The electron experiences a force of 1.2 x 10-15 N while in the field.
Calculate the strength of the field.
Can you arrive at the solution B = 1.25 T ?
Explain why the apparent inconsistent behaviour of cathode
rays caused debate as to whether they were charged
particles or electromagnetic waves
Early experiments with cathode rays provided apparently inconsistent evidence
about the nature of cathode rays, which seemed to behave both as waves and as
streams of particles.
Heinrich Hertz performed an experiment in 1883 that appeared to
show that cathode rays were not deflected by electric fields. His
experimental results were incorrect, however his result was used as
evidence that cathode rays were electromagnetic waves, just like
light which is not deflected by electric fields.
J. J. Thomson performed an experiment that showed
that a cathode ray beam was visibly deflected by an
electric field. This was interpreted as indicating that
cathode rays were charged particles.
In 1892, Hertz also showed that cathode rays penetrated thin metal foils. This was
interpreted to mean that cathode rays were electromagnetic waves.
These apparently conflicting results arose from inadequacies in
experimental design and the then current state of knowledge
about the nature of atoms. The properties of cathode rays were
clarified by later experiments. (NSW HSC On-line)
Perform an investigation to demonstrate and identify properties of cathode rays
using discharge tubes:
– containing a maltese cross
– containing electric plates
– with a fluorescent display screen
– containing a glass wheel
and analyse the information gathered to determine the sign of the charge on cathode rays
You performed an investigation that was planned by me. Make sure you can identify the
safe work practices with induction coils and discharge tubes used during this investigation.
You gathered first-hand information by observing and recording the way cathode rays
can be manipulated in various cathode ray tubes. A table would be a suitable format for
recording your observations. List each feature of the various cathode ray tubes you use
and describe how each demonstrates a property of cathode rays.
Analyse the information gathered by using your observations to justify inferences and
conclusions about the properties of cathode rays.
Sample observations
The Maltese cross is placed in the path of the cathode rays, causing a clearly defined shadow at the
end of the tube. This effect is used to infer that cathode rays travel in straight lines and are blocked
by solid objects.
Pairs of electric plates cause the cathode rays to bend towards the positive plate. This shows that
cathode rays are associated with negative charges.
A fluorescent screen shows that cathode rays can cause fluorescence. This demonstrates that cathode
rays have energy. A fluorescent screen can also be used to trace the path of cathode rays being
manipulated by other means.
A lightweight glass paddle wheel, able to rotate freely, is placed in the path of the cathode rays so
that the rays strike one edge of the wheel at a tangent. The cathode rays cause the wheel to spin and
move away from the cathode. This demonstrates that the cathode rays must have momentum, and
therefore mass, and that they are emitted from the cathode. (NSW HSC On-line)
Identify that charged plates
produce an electric field
An electric field exists in any region in which an electrically
charged object experiences a force. The observation that
charged plates exert a force on other charged objects
brought close to them indicates that an electric field is
associated with charged plates (NSW HSC On-line)
+
M. Edwards 2003
Discuss qualitatively the
electric field strength due to a
point charge, positive and
negative charges and
oppositely charged parallel
plates
+
-
The strength of the electric field due to a
point charge decreases with distance
from the object. The direction of the
field is defined as pointing radially away
from a positive point charge and towards
a negative point charge.
M. Edwards 2003
The closeness of the lines drawn to represent a field at any point indicates the electric
field strength at that point. The stronger the field, the closer the lines.
The electric field between two oppositely
charged parallel plates is uniform in strength
and direction. The field direction is defined as
at right angles to the plates and away from
the positive plate.
+
M. Edwards 2003
Describe quantitatively the
electric field due to
oppositely
charged parallel plates
Electric field strength, E, between two oppositely charged parallel plates is:
– proportional to the potential difference, V, between the plates;
– inversely proportional to the separation, d, between the plates;
– the same at all points in the region between the plates; and
– at right angles to the plates everywhere in the region between the plates.
+
d
V
M. Edwards 2003
E = V/d , where
E is electric field strength between two oppositely charged parallel plates,
V is the potential difference between the plates in volts, and
d is the distance between the plates in metres.
Solve problem and analyse
information using:
F = B qv sin q, F=qE and
E=V/d
The following diagram shows a charged particle of moving with a velocity of 6.4 x 10 5
ms-1 through a magnetic field of magnitude 0.10 T and two parallel plates with a potential
difference of 200 V, 2 cm apart. The mass of the particle is 4.0 x 10-25 kg
(a) 3 marks
FB = qvB
FB = 6.4 x 105 x 0.1
FB = 6.4 x 104q N upwards
FE = qV/d
FE = 200 x q/0.02
FE = 1.0 x 104q N downwards
The particle will be deflected upwards.
(b) 1 mark
Given the extremely small size of the particle, the
(a) Ignoring gravity, describe which way the particle be deflected.
order of magnitude of the particles weight force would
(b) Is it a reasonable assumption to ignore gravity? Explain your answer.
be approximately – 24. This is much smaller than the
force due to the magnetic and the electric fields.
Therefore it is a reasonable assumption to ignore
gravity.
Outline Thomson’s
experiment to measure the
charge/mass ratio of an
electron
The following diagram represents Thompson’s experiment to determine the charge to
mass ratio of cathode rays.
(a) Outline Thompson’s experiment.
(b) Explain the significance of the charge to mass ratio, given that the same ratio was
determined using different cathode materials.
1 (a) 4 marks
Electrons were emitted from the cathode and accelerated to the anode.
Electrons that passed through the anode were deflected by both a magnetic field from the electromagnet and by the
electrode plates. These were arranged in such a way that the deflection due to the plates was in the opposite direction
to the deflection due to the magnetic field. The potential difference across the plates was altered until the deflection
due to the plate was equal but opposite to the deflection due to the magnetic field.
Mathematically,
F = qE = qvB
v = E/B
since E = V/d
v = V/Bd
Also F = qvB = mv2/r
q/m = v/Br ------>
q/m = V/B2dr
The potential difference, magnetic field strength and distance between the plates could be measured. When the
electromagnet could be turned off the radius could be measured by the deflection, therefore the charge to mass ratio
could be determined.
(b) 1 mark
Given that the same ratio was measured for different cathodes, cathode rays can be thought of as a fundamental
particle of matter.
Outline the role of:
– electrodes in the electron gun
– the deflection plates or coils
– the fluorescent screen
in the cathode ray tube of conventional TV displays and
oscilloscopes
A television picture tube is essentially a cathode ray tube in which the cathode rays (beams
of electrons) are focused onto a fluorescent screen to form a moving picture. The picture is
formed by combining and synchronising the actions of horizontal and vertical magnetic
fields, with modulation of the electron beam by the electron gun, in response to signals
from the television station or video player.
The horizontal field causes the beam to sweep uniformly across the screen from one side to
the other, then rapidly back to the start. The vertical field moves the beam down slightly
for each successive sweep, filling the screen with a series of horizontal lines, until it
reaches the bottom, then returning rapidly to the top. This process is repeated 50 or 60
times a second.
The intensity of the electron beam is modulated by the input signal to cause lighter and
darker spots along each line on the fluorescent screen, forming the detail of the picture.
The picture is constantly refreshed, giving the appearance of smooth motion. (NSW HSC on-line)
A cathode ray oscilloscope (CRO) uses cathode rays to form an image on a fluorescent screen of the waveform of a
periodically varying voltage. This allows the waveform to be analysed for frequency, amplitude,irregularities, etc.
An internal variable time-base uses an electric field to make the beam sweep uniformly from left to right, then
rapidly back to the start. The periodically varying voltage of the input signal makes the beam move up and down
with the same frequency as the input signal. The glowing trace on the fluorescent screen forms a visual
representation of the waveform of the input signal. (NSW HSC on-line)
The diagram below shows an electron gun.
(a) Explain how the beam of electrons is produced.
(b) Describe how the cathode ray is utilised in an electron microscope.
(a) 1 mark
The electrons are emitted from the cathode when it is connected to a potential difference. They can flow easily in
the evacuated tube. The electrons pass through slits in the anodes which accelerate the electrons.
(b) 2 marks
The cathode ray passes through a magnetic lens which aligns the rays so that they are parallel. The rays pass
an object and then pass through another magnetic lens which focuses the rays. A final magnetic lens projects the
rays onto a fluorescent screen to produce the image.
The following diagram illustrates a paddle wheel in a discharge tube.
Outline the importance of this discharge tube in changing the understanding of
electromagnetic radiation.
Cathode rays were thought to be light waves. The momentum gained by the paddle
wheel came from the cathode rays indicating that the cathode rays had mass.