Hello once again everyone! Time again for another HSC Physics Guide. We are in to Ideas and Implementation now, an interesting topic, but a little difficult, lots of content to cover! Like the others, these guides will have a focus on summarising the content, and addressing the common exam questions likely to pop up in your exam! Squeezing everything in to these guides is tricky, so read slowly, and let me know if there are any areas you need a little extra help with. This guide will look at Cathode Rays.
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.Ok, so cathode ray tubes look like this:
Basically, they are an evacuated glass tube with an electrode at either end (the negatively charged plate is called the
cathode , the positively charged plate is called the
anode ). Due to the low air pressure, air does not act as an insulator inside these tubes. So, if a large enough potential difference is applied, electrons begin to flow from the cathode to the anode. These are
cathode rays . These rays make different patterns, called
striations , as they collide with the air particles still in the tube. These striations change based on the air pressure in the tube (you aren’t required to know details here, just identify that changes occurred with air pressure).
Now, you would have done several experiments with cathode rays. Some key ones to recall are listed below. These experiments were done in the 19th century to determine the nature of cathode rays.
There was argument as to whether they were a particle, or a wave. Indeed, this is a
very common HSC question. You may be asked to compare or present arguments concerning the nature of cathode rays. Whatever the wording, this is what thy mean and here are some arguments you can list.
Some arguments for the ‘wave’ theory:
- If you place a barrier in the path of the cathode rays, that barrier creates a ‘shadow,’ much like you would expect for visible light. It shows that the rays, like waves, travel in straight lines.
- The rays are observed to induce fluorescence in phosphors, such as those on certain screens. This is a property of electromagnetic waves.
Arguments for the ‘particle’ theory:
- The rays can be observed to cause the movement of a paddle wheel (or similar) where friction is minimised. Thus, it can be deduced that the rays have momentum.
- The rays are deflected by both magnetic and electric fields, although, the latter was not proved immediately. Both of these suggested that the rays were negatively charged particles
Before we move on, we need to do a bit of electric and magnetic field theory.
Magnetic fields surround all, well, magnetic substances (shocker!). Now, any ferromagnetic material, or other magnet, in this field will experience a force in response to the field. However, there is a relationship between electricity and magnetism (something you don’t need to know about). What this means, is that a
moving electric charge will experience a force in a magnetic field. Note that, it MUST be moving. This is something which links to the Motor Effect studied earlier.
The formula we use is as follows, where
is the charge on the object in coulombs,
is speed,
is magnetic field strength, and
is the angle made with the field lines.
A common way this formula is asked is the following.
We have to equate centripetal and magnetic force to get our radius:
Not too bad, and again, very common. Be on the lookout.
Electric fields are a little simpler. The force experienced by a charged particle is equal to the electric field strength, multiplied by the charge on the particle.
You also have to be able to analyse the electric field between two charged plates, simiilar to the situation in a cathode ray tube. The important thing here:
the electric field is constant in strength at all points between the plates . This is not like the field surrounding point charges, which diminish in strength over distance.
The electric field strength between two large plates is
, where V is the potential difference across the plates, and d is the distance between them. Obviously, the plates must have opposite charges for this to be valid, otherwise a potential difference won’t exist. As the plates get closer, the field is stronger, but no matter how strong, it is constant at all points!
Let’s combine these ideas to look at this multiple choice question:
Now, this is a multiple choice question, no need for complicated math, let’s make the very accurate approximation that neutrons and protons have the same mass. Now, the electric field strength between the plates is
, and the force experienced is
. So,
. Read that over if you need to.
Now, let’s consider the proton.
, but
, so
.
Therefore, by rearranging,
.
Now, the same formula applies to the alpha particle. Except, the charge is twice as much as the proton, and the mass is four times as much. So, replace
with
, and
with
, and we get:
So, the answer is B!
Right, now electric field theory is out of the way, we can more easily understand Thompson’s experiment. JJ Thompson conducted an experiment to determine the charge/mass ratio of cathode rays. He had observed that the rays were deflected by electric fields, and thus must have been negatively charged particles. He wanted their charge/mass ratio.
First, Thompson set up a cathode ray tube, with the rays subjected to both electric and magnetic fields. These fields would create a force, but Thompson set them up so the force acted in opposite directions. So, if he modified the fields so that the ray maintained it’s original path, then the two forces must have been equal. So, he can equate magnetic and electric forces:
Next, Thompson removes the electric field. The magnetic field creates a force perpendicular to the motion of the particle. Sound familiar? Uniform circular motion occurs. So Thompson can equate magnetic and centripetal force.
Thompson knows the velocity from the previous set, he knows the magnetic field strength, and he can measure the radius. So, he has the charge/mass ratio! This was, in essence, the discovery of the electron.
This is a complicated experiment, but it is asked frequently. Read this over, ask me any questions, make sure you remember the process and can do these mathematical manipulations yourself.
The final part of understanding cathode rays is to understand how they are actually used. There are three key parts to understand.
The first is the electron gun. This is the part that actually generates the cathode rays. It consists of a cathode and anode as normal. Normally, in addition, the cathode is heated to increase the number of electrons ejected. This is called
thermionic emission . The electrons accelerate towards the anode, and pass through a hole, out the end of the electron gun.
Next comes the deflection plates. These are sets of plates which create a set of either magnetic or electric fields (not both). These manipulate the moving electrons. One set of plates deflects the beam vertically. The other deflects it horizontally. Thus, the beam can be moved up and down, as well as left to right, to hit any point on…
The fluorescent screen. Screens in appliances with cathode ray tubes utilise phosphors, which fluoresce for a short time after being struck by an electron. By manipulating the electron gun to sweep across the screen very quickly, a picture can be shown!
For example, if you were to slow down a cathode ray television and look at it 1000 times slower, you would see the electron gun hit the pixel in the bottom left corner. Then the next one. Then the next one. Then the next one. All the way to the top, where it loops back and repeats. This happens so fast, that you only see a moving picture.
Putting all of this in one question is a big ask. The electron gun or deflection plates are normally the ones specified, so be prepared to draw a diagram of a cathode ray tube, explain how the plates are used to deflect the beam (and why), and just describe how they work in general. It can be easy to lose marks here, be careful that you explain things clearly and logically!
So, that’s Cathode Rays! Stay tuned for more on Ideas and Implementation, next up is the Photoelectric Effect. Be sure to register and ask questions, as many as you like, I am happy to help! Happy study! 
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