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9.4 From ideas to implementation: 1. Cathode rays

Syllabus reference (October 2002 version)
1. Increased understandings of cathode rays led to the development of television	Students learn to: explain why the apparent inconsistent behaviour of cathode rays caused debate as to whether they were charged particles or electromagnetic waves explain that cathode ray tubes allowed the manipulation of a stream of charged particles identify that moving charged particles in a magnetic field experience a force identify that charged plates produce an electric field describe quantitatively the force acting on a charge moving through a magnetic field discuss qualitatively the electric field strength due to a point charge, positive and negative charges and oppositely charged parallel plates describe quantitatively the electric field due to oppositely charged parallel plates outline Thomson's experiment to measure the charge/mass ratio of an electron 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	Students: perform an investigation and gather first-hand information to observe the occurrence of different striation patterns for different pressures in discharge tubes 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 charge on the cathode rays solve problem and analyse information using: F = qE and

Extract from Physics Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW
[Edit: 14 Aug 08]

Prior Learning: Preliminary modules 8.2 (subsections 1 and 3) 8.3 (subsections 2 and 5)

identify that moving charged particles in a magnetic field experience a force

Any moving charged particles that move in a magnetic field will experience a force.

perform an investigation and gather first-hand information to observe the occurrence of different striation patterns for different pressures in discharge tubes

You may be performing an investigation that has been planned by your teacher. One suitable investigation is described below. Take note of the safe work practices required when any discharge apparatus is operating.
Gather information by observing and recording observations in accurate summary form. Simple diagrams can assist your recollection of phenomena observed.

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.

The following web site has some simple diagrams showing various striation patterns and also provides information on the properties of cathode rays.

Investigating new rays Dr. E.P. Scarlett High School, Calgary Board of Education, Canada

The following web site has an example of alternative apparatus that can be used to show the above effects.

Gases that emit light Chemical of the week, Science is Fun in the Lab of Shakhashiri

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.

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 may be performing an investigation that has been planned by your teacher. Make sure you identify and use safe work practices with induction coils and discharge tubes during this investigation.

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 sign of the charge on cathode rays.

The cathode ray tubes used to demonstrate the properties of cathode rays are fairly standard across schools and below are some web sites that outline common procedures you may follow:

Investigating new rays Dr E.P. Scarlett High School, Calgary Board of Education, Canada

Cathode Ray Tube Catharine H Colwell, Physics LAB online

Sample information

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.

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.

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.

discuss qualitatively the electric field strength due to a point charge, positive and negative charges and oppositely charged parallel plates

An electric field has both strength and direction.

The strength of the electric field due to a positive point charge diminishes with distance from the object. The direction of the field is defined as pointing radially away from a positive point charge.

The strength of the electric field due to a negative point charge diminishes with distance from the object. The direction of the field is defined as pointing radially towards a negative point charge.

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.

The number of the lines drawn to represent a field at any point indicates the electric field strength at that point. The stronger the field, the more lines are drawn in a given space.

The strength of the electric field due to a positive point charge diminishes with distance from the object. The direction of the field is defined as pointing radially away from a positive point charge.
The strength of the electric field due to a negative point charge diminishes with distance from the object. The direction of the field is defined as pointing radially towards a negative point charge.
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.
The number of the lines drawn to represent a field at any point indicates the electric field strength at that point. The stronger the field, the more lines are drawn in a given space.

The following web sites have simple interactive examples of electric field strength. You may need to be patient if you are using an early version computer.

Electric Force Fields University of Colorado, Boulder, Colorado, USA.

Electric Field Shapes David Hoult, Open Door Web Site, Ecole Active Bilingue Jeannine Manuel in Paris, France.

describe quantitatively the electric field due to oppositely charged parallel plates

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.
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.

describe quantitatively the force acting on a charge moving through a magnetic field

, where
- F is the force on the charged object,
- q is the charge in coulombs,
- v is the velocity of the charged object in m s^-1,
- B is the magnetic field flux density (or magnetic induction) in teslas, and
- is the angle between the velocity and the magnetic induction.
Force, F, on a charge moving through a magnetic field is:
- proportional to the size of the charge, q;
- proportional to the velocity of the charge, v;
- proportional to the magnetic induction, B; and
- proportional to the sine of the angle, , between the velocity and the magnetic induction, being a maximum when is 90° (that is, when the velocity is at right angles to the field), and zero when is zero (that is, when the velocity is parallel to the field).

solve problems and analyse information using F=qE and

Solve problems by selecting the appropriate equation, rearranging the equation where necessary, substituting known data for the variables, and computing the answer.

A sample problem:

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:

Another sample problem, requiring rearrangement of the equation:

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.

Solution:

Here is a sample problem for you to try:

Two parallel plates, placed 0.1 m apart, are connected to a 6-volt battery. Determine the electric field strength between the plates.

Solution

Analyse information by examining relationships between the variables in the equations given.

Sample analysis questions:
1. If the potential difference between two charged parallel plates is kept the same, while the plates are moved closer together, how will this affect the electric field strength between the plates?
  
  Analysis: E = V / d. Since E is inversely proportional to d, if d is reduced E will be increased. For example, if the separation is halved, the field strength will be doubled.
2. A proton and an alpha particle are travelling parallel to each other with the same speed in the same magnetic field. Which will experience the greater force?
  
  Analysis: . Since F is directly proportional to q, and v and B are equal for both particles, the alpha particle, with two protons, will experience a greater force than the proton alone.

outline Thomson's experiment to measure the charge/mass ratio of an electron

Cathode rays were passed through two narrow slits to make a thin parallel beam aimed at the centre of a fluorescent screen. Electrodes were placed to create a uniform electric field that exerted a downward force on the beam. Electromagnets were placed to produce a uniform magnetic field that exerted an upward force on the beam.
Thomson manipulated the strengths of the two fields until the beam passed through both fields undeflected. This showed that the two forces on the particles (electrons) in the beam were equal and opposite. By equating the expressions for these two forces, Thomson calculated the velocity of the particles.
Thomson removed the electric field and calculated the radius of the circular path followed by the particles in the uniform magnetic field alone. By equating the force due to the magnetic field to the centripetal force, he was able to calculate that all cathode ray particles (electrons) had the same charge / mass ratio of 1.76 x 10¹¹ C kg^-1.

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

The electron gun produces a narrow beam of electrons. The electrodes in the gun accelerate the electrons.
The deflection plates or coils establish an electric field that controls the deflection of the electron beam from side to side and up and down.
The fluorescent screen is coated with a material that emits light when struck by electrons in the cathode ray. This allows the position of the beam to be seen where it strikes the screen.