Cathode Rays

Describe cathode rays and cathode ray tubes

This is an additional dotpoint included to provide a quick general overview of what cathode rays are.

Cathode rays were first observed by Faraday in 1838, who noticed light emission from within the vacuum tube he was experimenting with. This led to the ongoing research into cathode rays that forms the majority of this HSC topic. A cathode ray tube is simply a vacuum tube with electrodes at either end. The electrodes are simply pieces of conductive metal, and have contacts outside the tube. When a high potential difference is applied to the tube, by passing high voltage electricity through the tube (by connecting the electrodes to a circuit), electrons jump from one electrode to the other, crossing the tube. This cannot occur in normal air because the high density of air molecules prevents the electrons from travelling large distances. However, this is not an issue in a vacuum tube. From this description, it is clear that cathode rays are in fact negatively charged electrons. The emission of light occurs when the electrons collide with particles inside the tube, causing the particles to emit light as they absorb and then release the energy carried by the electron (which is transferred to the particle in the collision). The appearance of the light, such as its shape and colour, is dependent on both the chemical composition of the gas inside the tube and on the gas pressure.

figure 3

Remember- Cathode rays are the stream of electrons produced between electrodes in a vacuum tube.

Explain that cathode ray tubes allowed the manipulation of a stream of charged particles

Cathode ray tubes allowed the manipulation of a stream of charged particles in several ways. Firstly, and most importantly, cathode ray tubes are a source of a steady stream of charged particles, a prerequisite to their manipulation. The manipulation of charged particles can either be done remotely via electric and magnetic fields, or directly by obstructing the charged particles (examples include with thin metals, thick metals like the Maltese cross, and small paddlewheels). Cathode ray tubes enabled the manipulation of charged particles in both these ways. Obstructions could be placed inside the tube to block the cathode rays, and fields could operate within the tube by placing charged plates or field coils next to the tube. In this way, cathode ray tubes allowed the manipulation of a stream of charged particles.

Remember- Cathode ray tubes allowed the manipulation of charged particles because objects could be placed inside the tubes, and because fields could permeate the tubes.

Explain why the apparent inconsistent behaviour of cathode rays cause debate as to whether they were charged particles or electromagnetic waves

In a test you can write your answer in dot points.

Cathode rays had properties that could classify it as a wave or as a particle. As a wave, they

  • Travelled in straight lines
  • Produced a shadow when obstructed by objects
  • Could pass through thin metal foils without damaging them As a particle, they
  • Left the surface of the cathode at 90 degrees, not radiating like a wave
  • Were deflected by magnetic fields
  • Could turn a wheel in the path of the ray (i.e. they have momentum)
  • Travelled far slower than light

The reason the debate ensued is because scientists wanted to determine the nature of cathode rays to the extent where they could classify it as a wave or particle, and the fact that cathode rays had conflicting properties made this very difficult. Crookes insisted it was a particle while Hertz maintained it was a wave. The debate was resolved when an electric field was used to deflect the rays by Thompson, which had been impossible up to that point because older vacuum pumps were not strong enough to remove enough air to make the effect visible, and because the electric fields that were used before were not strong enough. This evidence was strong because scientists knew it was impossible to deflect electromagnetic waves with an electric field, and since cathode rays were deflected this was taken as proof they were not electromagnetic waves, and were therefore particle streams.

Remember- Cathode rays had both wave and particle properties, and it wasn’t until Thompson showed that they could be deflected with electric fields that the debate was resolved.

Perform an investigation to demonstrate and identify properties of cathode rays using discharge tubes containing a Maltese cross, electric plates, a fluorescent screen, a glass wheel, and analyse the information gathered to determine the sign of the charge of cathode rays

To perform this experiment we had several discharge tubes each with element from the list above.

The Maltese cross tube had an anode mounted on the base of the tube, underneath the Maltese cross which was situated between the end of the tube and the cathode. When cathode rays travelled from the cathode, they did so in a straight line, and were obstructed by the Maltese cross. This caused a shadow to be formed, showing that the cathode rays could be blocked relatively easily. Also, the shadow had a very sharp edge, indicating that diffraction was not occurring and that therefore cathode rays could be particles, not waves. The shadow also indicated that the cathode rays travelled in straight lines.

When electric plates were set up, the cathode ray beam was deflected. To perform this experiment, the tube had a curved screen set up inside it so that the horizontal path of the beam was visible. When we applied an electric field, we were able to bend the beam, showing the beam was electrically charged. As the beam deflected towards the positive plate, we determined the cathode rays to be negatively charged. We also deflected the beam with a magnetic field from a bar magnet.

Setting up a fluorescent screen in the path of the cathode ray beam caused it to light up as it was struck. This suggested that the cathode rays carried enough energy to produce the reaction in the screen necessary to produce light, a property exploited in many TVs and computer monitors. Lastly, when a glass paddlewheel was mounted inside the tube on runners so it was able to move, the cathode rays striking the wheel caused it to rotate and roll along the tube. The movement was away from the cathode, showing that the rays were emitted from the cathode. Through conservation of momentum, the fact that cathode rays could move a wheel by colliding with it strongly suggested that they had mass, and were therefore particles.

figure 4

Remember- The negatively charged cathode rays were blocked by the Maltese cross, could spin a paddlewheel, caused a fluorescent screen to emit light, and were deflected by electric and magnetic fields.

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

Most resources simply say ‘less air’ and ‘still less air’ when referring to the middle two tubes. There- fore, 2% and 0.5% are arbitrary figures here. Of course, the best option is to check when you’re performing the experiment the pressure on the tubes (the pressures will depend on the exact tubes used, so there will probably be variation between schools etc.), but if you didn’t, just remember the figures here.

Striation patterns refer to light and dark areas inside a discharge tube. Electrons colliding with air particles release light dependant on the energy of the electrons, but also on the amount of gas inside the tube. As the pressure of the gas changes, so too do the striation patterns. In this experiment, we had 4 discharge tubes each with different air pressures- 5%, 2%, 0.5%, and 0.01% (measured as a percentage of standard atmospheric pressure). With 5% air, glowing purple/pink streamers formed, extending all the way from the cathode to the anode. At 2%, the pattern changed to a series of alternating light and dark bands running perpendicular to the length of the tube. At 0.5%, the dark gaps between the lines widened (i.e. There were fewer lines), with the pink-purple glow concentrated around the anode, and a blue glow forming at the cathode. At 0.01%, there were no striations. Instead, the glass around the anode glowed yellow-green. The exact nature of the striation patterns varies depending on what gas is used eg. Normal air, hydrogen etc.

figure 5

Remember- The striation patterns formed in a vacuum tube depend both on the gas inside the tube and on the pressure

Identify that moving charged particles in a magnetic field experience force

When a moving charged particle travels through a magnetic field, it experiences a force related to its velocity and its direction of travel relative to a field. If the particle is travelling along with or parallel to field lines, there is no applied force. Maximum force is applied if the particle is travelling at 90 degrees to the field lines.

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

For a point charge, the electric field strength depends entirely on the magnitude of charge the object has. The field extends outward in all directions and so obeys inverse square law, rapidly diminishing as distance from the charge increases. For a positive charge, the field lines radiate outwards, indicative of the direction in which a positive test charge would experience force. For a negative charge, the field is identical except the field lines run in the opposite direction, pointing inwards to the point charge, indicative of the fact that a positive charge would be attracted to the negative charge. Oppositely charged parallel plates have a uniform field (in both direction and strength) running between them from the positive plate to the negative plate. Unlike a point charge where the direction of the electric field changes depending on where the field is being examined, the electric field lines between parallel plates always run in the same direction. Also, unlike a point charge where the field exists all around the point charge, the electric field from parallel charged plates only exists in between the plates. The spacing of field lines between the plates indicates field strength.

figure 6

Remember- The field lines point away from a positive charge, towards a negative charge, and run from positive to negative between charged plates.

Identify that charged plates produce an electric field

See 3.1.9 for a mathematical description of the field between the plates.

Charged plates- that is, plates with a potential difference between them, produce an electric field running between them. The field lines run from the positive plate to the negative plate, are parallel, and the field strength is equal at all points between the plates. The field does not exist outside the space between the plates.

Describe quantitatively the force acting on a charge moving through a magnetic field, using F = qvBsinθ (including ”Describe quantitatively the electric field due to oppositely charged parallel plates”)

When a charged particle moves through a magnetic field, it experiences a force that is equal to qvBsinθ. This shows that the force experienced by a charged particle depends on 4 things- its velocity, its charge, magnetic field strength, and the angle that it makes with the field. The right- hand palm rule is used to calculate the direction in which this force is applied. To make the force larger, all of these attributes can be increased, including the angle, making force directly proportional to all of them.

The field between the parallel plates depends on only two things- the potential difference between the plates and the distance between them. It is calculated according to E = \frac {V}{d} , where E is the field strength, V is the potential difference and d is the distance separating the plates in metres. From this, E is proportional to V and inversely proportional to d. The field is at right angles to the plates in all directions and is uniform in strength.

Outline Thompson’s experiment to measure the charge/mass ratio of an electron

An examination of the mathematics behind Thompson’s experiment is not vital to addressing this dotpoint, and hence has not been included here. 

Thompson carried out vitally important work to determine the charge-to-mass ratio of an electron. He accomplished this using a modified cathode ray tube. The first part contained a thermionic cathode (a thermionic cathode is one which is heated by a separate heating circuit, in order to release more electrons) and an anode with a small hole through the centre to produce a thin stream of electrons travelling into space rather than between a potential difference. The second part was a velocity filter, consisting of charged plates above and below the beam set to deflect the electrons upward, and a Helmholtz coil mounted on either side of the tube producing a magnetic field to deflect the electrons downwards. Finally, at the end of the tube was a fluorescent screen which indicated how the electrons were being deflected, if at all. Thompson used both the fields simultaneously and balanced them so that the electrons travelled on the original path they took when the fields were off, as gauged by the fluorescence on the screen. This also had the effect of filtering electron velocities such that only electrons with a single particular velocity travelled through the system uninterrupted. By equating the two field strengths, a formula for the electron velocity was produced related to the strength of both fields. By equating the potential energy and kinetic energy of the electrons at the cathode and anode respectively, and substituting the potential difference across the tube and the velocity of the electrons, he was able to calculate the charge-to-mass ratio of the electron.

Outline the role of electrodes in the electron gun, the deflection plates or coils and the fluorescent screen in the cathode ray tube of conventional TV displays and oscilloscopes

The cathode ray tube in a display uses 3 parts- the electron gun, deflection plates and fluorescent screen, to form an image. The electron gun is used to produce a fast-moving stream of electrons. The electrodes have two roles- firstly to emit electrons to form the stream (as performed by the heated thermionic cathode) and secondly to accelerate the electrons to very high speeds (accomplished by a very large potential difference between the anode and the cathode). The anode has a hole in it to allow the stream of electrons to leave the electron gun. The deflection plates or coils are used to change the direction of the electron beam. This is because to form an image on the fluorescent screen (as opposed to a dot), the electron beam must sweep over the screen rapidly. This is accomplished by the deflection plates that guide the electron beam to particular parts of the screen. Deflection coils are used in TVs because magnetic fields can deflect the beam through larger angles which is necessary when using a large screen. The fluorescent screen is coated with phosphors that emit light when struck by electrons, which makes the electron beam visible. This is vital to the formation of an image that is viewed on an oscilloscope or TV.

Remember- The cathode ray tube in TVs and oscilloscopes uses electrodes in an electron gun to produce a beam of electrons, deflection plates/coils to steer the beam, and a fluorescent screen to turn the beam into light.