HSC Physics Question Thread

[jamonwindeyer]

Hey guys!

So I started learning about Cathode Ray Tubes and I was wondering how the control electrode actually controls the number of electrons coming off the cathode? And also, how does the accelerating anode work? Like I get how the electrons are attracted to the anode but how does the electric field produced by the accelerating anode actually speed up the electrons? Many thanks!

Neutron

Hey Neutron! First off, super impressed into the detail you are going into learning this stuff, this is definitely extra knowledge you are asking! Love it, I'll do my best to answer 

Your second question first, how does the accelerating anode speed up the electrons? Basically, the accelerating anode is usually cylindrical in shape, around the path of the electrons. The electric field produced by that anode will pull an electron towards it and accelerate it. But why? The answer is simple. Electrons are negatively charged, and the anode has an extremely large positive charge. Keep in mind, a positive voltage means an accrual of positive charge (i.e. - a complete absence of electrons), and a voltage generates an electric field. In any case, positive charge and negative charge attracts, so the electron is accelerated.

There is a formula governing the force experienced by the electron, linked to the net field strength of the accelerating anode. It is:



where F is the force, E is the field strength (in volts per metre), and q is the charge on, in this case, the electron.

It sounds like you understand most of this already, does this help at all? Any particular part of this you are confused about?

In terms of controlling the number of electrons coming off the cathode, it is completely dependent on the electric fields/voltages involved. Apply a larger potential difference between the cathode and anode, and you get more electrons. We also use a technique called thermionic emission, where heat energy excites electrons and emits them from the cathode. This can be used to increase electron emission at a set potential difference 

Hope this helps!! 

[Happy Physics Land]

Hey guys!

So I started learning about Cathode Ray Tubes and I was wondering how the control electrode actually controls the number of electrons coming off the cathode? And also, how does the accelerating anode work? Like I get how the electrons are attracted to the anode but how does the electric field produced by the accelerating anode actually speed up the electrons? Many thanks!

Neutron

Hey Neutron!

I've just started doing Ideas into Implementation as well. I did a bit of research on control electrode and how it works. It is situated at the cathode which contains a heating filament/small heater (depending on whether you are talking about CRO/TV). This heating filament would heat up and "boil" off electrons from the cathode, producing electron clouds. What the control electrode does is that it would vary the output of electrons according to the varying signals that are applied. So to put it into simplest terms, if you require low brightness on the screen you can just emit less electrons through the control electrode and if you want a high brightness you just emit more electrons using control electrode. The actual mechanism of control electrode is not examinable in the scope of the syllabus so I will just briefly explain it. Essentially this control electrode is connected to the external voltage circuit of the cathode and depending on the amount of input voltage (i.e. the amount of input signal), the amount of electrons released can be controlled.

In regards to the accelerating anodes, we will have to recall a concept we learnt in year 11: currents flow from higher potential to lower potential. The current here refers to positive charges (i.e. conventional current). Now, we have two anodes, one is the focusing anode and another is the accelerating anode. The accelerating anode has HIGHER VOLTAGE than the focus anode, meaning that for a positive charge it will travel from accelerating anode to focusing anode. However, because cathode rays are NEGATIVELY CHARGED ELECTRONS, therefore they will travel from the focusing anode to the accelerating anode.

Now, recall that higher voltage leads to higher electric field (E = V/d, hence E is directly proportional to V) and this will lead to higher force exerted upon the electrons (F=Eq, increase in E --> increase in F). Since the mass of electrons is constant, and force applied is constant (due to constant supply of voltage), then it will accelerate according to F=ma. Because accelerating anode has higher voltage than focusing anode, there is more force being applied to the electrons and hence it will accelerating as it travels through the accelerating anode.

[Happy Physics Land]

Hey Neutron! First off, super impressed into the detail you are going into learning this stuff, this is definitely extra knowledge you are asking! Love it, I'll do my best to answer 

Your second question first, how does the accelerating anode speed up the electrons? Basically, the accelerating anode is usually cylindrical in shape, around the path of the electrons. The electric field produced by that anode will pull an electron towards it and accelerate it. But why? The answer is simple. Electrons are negatively charged, and the anode has an extremely large positive charge. Keep in mind, a positive voltage means an accrual of positive charge (i.e. - a complete absence of electrons), and a voltage generates an electric field. In any case, positive charge and negative charge attracts, so the electron is accelerated.

There is a formula governing the force experienced by the electron, linked to the net field strength of the accelerating anode. It is:



where F is the force, E is the field strength (in volts per metre), and q is the charge on, in this case, the electron.

It sounds like you understand most of this already, does this help at all? Any particular part of this you are confused about?

In terms of controlling the number of electrons coming off the cathode, it is completely dependent on the electric fields/voltages involved. Apply a larger potential difference between the cathode and anode, and you get more electrons. We also use a technique called thermionic emission, where heat energy excites electrons and emits them from the cathode. This can be used to increase electron emission at a set potential difference 

Hope this helps!! 

Combine my answer with Jamon's you will basically understand to a professional level how an electron gun works...

[jamonwindeyer]

Combine my answer with Jamon's you will basically understand to a professional level how an electron gun works...

The teamwork is strong  again I'll stress, the HSC will never test to this level of detail. You guys are definitely pushing for a Band 6 with this kind of knowledge 

[Neutron]

Yesss thank you so much you two! I too, did some research online and I discovered that the control electrode can either be positive or negative (this helps in controlling the amount of electrons and thus brightness etc), meaning if the control electrode is:
Negative- It repels the electrons back into the cathode (so not as many electrons can move onto the anode and thus be focussed and accelerated)
Positive- Encourages electrons to come out of the cathode and thus, increases the amount of electrons

Yeah, so I guess the control electrode uses more than just thermionic emission! Just thought I might throw that out there heh

Although if you guys don't mind answering another question, I was wondering how Hertz's antenna actually generated radiowaves? I was reading up on how spark-gap transmitters work but they just say that it sparks across the gap and produces electromagnetic radiation but I'm having trouble understanding how D: And also, how are electromagnetic waves produced naturally? So what actually provides the initial changing magnetic or electric fields? Thanks you guys, you're legends <3

Neutron

[RuiAce]

Yesss thank you so much you two! I too, did some research online and I discovered that the control electrode can either be positive or negative (this helps in controlling the amount of electrons and thus brightness etc), meaning if the control electrode is:
Negative- It repels the electrons back into the cathode (so not as many electrons can move onto the anode and thus be focussed and accelerated)
Positive- Encourages electrons to come out of the cathode and thus, increases the amount of electrons

Yeah, so I guess the control electrode uses more than just thermionic emission! Just thought I might throw that out there heh

Although if you guys don't mind answering another question, I was wondering how Hertz's antenna actually generated radiowaves? I was reading up on how spark-gap transmitters work but they just say that it sparks across the gap and produces electromagnetic radiation but I'm having trouble understanding how D: And also, how are electromagnetic waves produced naturally? So what actually provides the initial changing magnetic or electric fields? Thanks you guys, you're legends <3

Neutron

Quoted off Physics in Focus:

"An accelerating or oscillating charge produces EMR."
"EMR can cause charges to accelerate or oscillate."

As for how this phenomena is true, that you may want to research. (For the most part I am no longer a physicist.)

The extremely compact version of the long story is that Hertz's spark gap basically had charges that oscillated. Basically, the current that was fed INTO the induction coil was oscillating.

[Happy Physics Land]

Yesss thank you so much you two! I too, did some research online and I discovered that the control electrode can either be positive or negative (this helps in controlling the amount of electrons and thus brightness etc), meaning if the control electrode is:
Negative- It repels the electrons back into the cathode (so not as many electrons can move onto the anode and thus be focussed and accelerated)
Positive- Encourages electrons to come out of the cathode and thus, increases the amount of electrons

Yeah, so I guess the control electrode uses more than just thermionic emission! Just thought I might throw that out there heh

Although if you guys don't mind answering another question, I was wondering how Hertz's antenna actually generated radiowaves? I was reading up on how spark-gap transmitters work but they just say that it sparks across the gap and produces electromagnetic radiation but I'm having trouble understanding how D: And also, how are electromagnetic waves produced naturally? So what actually provides the initial changing magnetic or electric fields? Thanks you guys, you're legends <3

Neutron

Hey Neutron!

Thats some really interesting information on control electrode! Turns out that it functions somewhat like a control grid in thermionic triodes!!! Good stuff there!

Ah yeah lesson 3 is confusing aye, apparently the most difficult lesson in the book. I will begin with answering the electromagnetic radiation part. In nature, radio waves can be produced through lightning. Why? Because when lightning strikes the air, the air molecules are ionised and charged particles are released. Because lightning hits with an extremely high voltage, it would put charged particles into motions. When the charged particles are travelling in a certain direction, a current is formed. Using our Right Hand Coil Rule, we ascertain the existence of a magnetic field around the current. What does this tell us? This tells us that when a charged particle is put into motion (i.e. a current is formed), there is a changing electric field and consequently a magnetic field is produced. James Maxwell further proposes that a changing magnetic field will then produce a electric field which will produce a magnetic field and so on. So this explains the fact that electromagnetic waves are self-sustaining mutual generation of electric field and magnetic fields.

However keep in mind, that only CHANGING magnetic field or CHANGING electric field may induce a changing electric field or changing magnetic field. This means that the charged particle will HAVE TO BE IN MOTION (i.e. oscillate) in order for a changing electric field to be produced. This is why electromagnetic waves propagate! Because the electric field is constantly changing due to motion!

Think about this: if an electron is travelling at constant speed in a consistent direction, will a magnetic field be produced? No. Because its motion is constant which DOES NOT lead to a CHANGING electric field. So in order for it to produce a changing electric field, it must be accelerated or decelerated. Therefore the fundamental mechanism responsible for the production of electromagnetic waves is the acceleration of charged particles which produce changing electric field. This theory can be proven through Heinrich Hertz's experiments. Going back to lightning producing EM waves, since lightning provide the charged particles with a load of force, the particles will accelerate (F=ma) and hence producing changing electric fields for producing changing magnetic fields and so on.

The spark gap transmitter itself isnt the thing that creates the radiowave, but sure it does contribute to the production of EM wave through providing a sufficient voltage in the inductor coil, hence maintaining an arc across the spark gap. The actual formation of EM wave is this: one electrode is connected to the positive terminal of the power source, another electrode is connected to the negative terminal of the power source. Now, this spark gap transmitter acts like a step-up transformer, providing a high voltage to both electrodes. So the positive electrode would have a very high positive potential whilst the negative electrode would have a very high negative potential. The significant difference between these two potentials allows air to actually become a conductor as current passes from negative to positive electrode, ionising all the air molecules along the way, causing discharge arcs. These ionised air particles (charged particles) keep on fluctuating inside the gap because there is a constant forward and backward exchange of energy into the capacitor. This exchange of energy resembles alternating current in nature and hence a radio wave is created (Alternating current provides constant acceleration of discharge ions, hence constantly changing electric field --> constantly changing magnetic field --> constantly changing electric field and so on).

[Happy Physics Land]

Quoted off Physics in Focus:

"An accelerating or oscillating charge produces EMR."
"EMR can cause charges to accelerate or oscillate."

As for how this phenomena is true, that you may want to research. (For the most part I am no longer a physicist.)

The extremely compact version of the long story is that Hertz's spark gap basically had charges that oscillated. Basically, the current that was fed INTO the induction coil was oscillating.

Downfall of all actuaries

[jamonwindeyer]

Hey Neutron!

Thats some really interesting information on control electrode! Turns out that it functions somewhat like a control grid in thermionic triodes!!! Good stuff there!

Ah yeah lesson 3 is confusing aye, apparently the most difficult lesson in the book. I will begin with answering the electromagnetic radiation part. In nature, radio waves can be produced through lightning. Why? Because when lightning strikes the air, the air molecules are ionised and charged particles are released. Because lightning hits with an extremely high voltage, it would put charged particles into motions. When the charged particles are travelling in a certain direction, a current is formed. Using our Right Hand Coil Rule, we ascertain the existence of a magnetic field around the current. What does this tell us? This tells us that when a charged particle is put into motion (i.e. a current is formed), there is a changing electric field and consequently a magnetic field is produced. James Maxwell further proposes that a changing magnetic field will then produce a electric field which will produce a magnetic field and so on. So this explains the fact that electromagnetic waves are self-sustaining mutual generation of electric field and magnetic fields.

However keep in mind, that only CHANGING magnetic field or CHANGING electric field may induce a changing electric field or changing magnetic field. This means that the charged particle will HAVE TO BE IN MOTION (i.e. oscillate) in order for a changing electric field to be produced. This is why electromagnetic waves propagate! Because the electric field is constantly changing due to motion!

Think about this: if an electron is travelling at constant speed in a consistent direction, will a magnetic field be produced? No. Because its motion is constant which DOES NOT lead to a CHANGING electric field. So in order for it to produce a changing electric field, it must be accelerated or decelerated. Therefore the fundamental mechanism responsible for the production of electromagnetic waves is the acceleration of charged particles which produce changing electric field. This theory can be proven through Heinrich Hertz's experiments. Going back to lightning producing EM waves, since lightning provide the charged particles with a load of force, the particles will accelerate (F=ma) and hence producing changing electric fields for producing changing magnetic fields and so on.

The spark gap transmitter itself isnt the thing that creates the radiowave, but sure it does contribute to the production of EM wave through providing a sufficient voltage in the inductor coil, hence maintaining an arc across the spark gap. The actual formation of EM wave is this: one electrode is connected to the positive terminal of the power source, another electrode is connected to the negative terminal of the power source. Now, this spark gap transmitter acts like a step-up transformer, providing a high voltage to both electrodes. So the positive electrode would have a very high positive potential whilst the negative electrode would have a very high negative potential. The significant difference between these two potentials allows air to actually become a conductor as current passes from negative to positive electrode, ionising all the air molecules along the way, causing discharge arcs. These ionised air particles (charged particles) keep on fluctuating inside the gap because there is a constant forward and backward exchange of energy into the capacitor. This exchange of energy resembles alternating current in nature and hence a radio wave is created (Alternating current provides constant acceleration of discharge ions, hence constantly changing electric field --> constantly changing magnetic field --> constantly changing electric field and so on).

Nice HPL! Only thing I'd add is just a stressed fact: The EM Waves are not exclusively emitted from the spark itself. Rather, the whole antennae emits the waves. As HPL correctly describes, the spark itself is caused by the voltage exceeding some value and causing part of the air itself to become conductive. This is called electrical breakdown. When this happens, the circuit goes from having zero current, to having a huge current, virtually instantaneously.

What this does is what I'm studying right now. Basically, such a huge change happening so quickly causes oscillations. Picture holding a pendulum in your hand, and quickly moving your hand forward, like throwing a punch. This virtually instantaneous change does not just let the pendulum hang, it swings. This is a very poor analogy for what happens in an electrical circuit which has a HUGE current applied instantaneously (for those interested, research the Fourier Transform and the Dirac Delta function). These oscillations occur in the whole circuit at a variety of frequencies, and as HPL correctly states, it is these oscillations which generate EM waves (with the different frequencies of oscillation being what causes the different wavelengths of EM radiation).

Remember, charges produces electric fields. Moving charges produce magnetic fields. Accelerating (oscillating) charges produce EM waves.

Hopefully between Rui, HPL and myself, we have given you a solid understanding of what is a very complex phenomenon. Again, you have gone way beyond what you need to know for HSC Physics! 

[Happy Physics Land]

Nice HPL! Only thing I'd add is just a stressed fact: The EM Waves are not exclusively emitted from the spark itself. Rather, the whole antennae emits the waves. As HPL correctly describes, the spark itself is caused by the voltage exceeding some value and causing part of the air itself to become conductive. This is called electrical breakdown. When this happens, the circuit goes from having zero current, to having a huge current, virtually instantaneously.

What this does is what I'm studying right now. Basically, such a huge change happening so quickly causes oscillations. Picture holding a pendulum in your hand, and quickly moving your hand forward, like throwing a punch. This virtually instantaneous change does not just let the pendulum hang, it swings. This is a very poor analogy for what happens in an electrical circuit which has a HUGE current applied instantaneously (for those interested, research the Fourier Transform and the Dirac Delta function). These oscillations occur in the whole circuit at a variety of frequencies, and as HPL correctly states, it is these oscillations which generate EM waves (with the different frequencies of oscillation being what causes the different wavelengths of EM radiation).

Remember, charges produces electric fields. Moving charges produce magnetic fields. Accelerating (oscillating) charges produce EM waves.

Hopefully between Rui, HPL and myself, we have given you a solid understanding of what is a very complex phenomenon. Again, you have gone way beyond what you need to know for HSC Physics! 

Fourier transform

We learnt about this thing called effective inductance and capacitance today, not too sure what they mean. Apparently they are specific to electrical engineering?

[jamonwindeyer]

Fourier transform

We learnt about this thing called effective inductance and capacitance today, not too sure what they mean. Apparently they are specific to electrical engineering?

Yep! My life for the next few years 

As a quick run down, inductance is an electrical property whereby the circuit element in question resists changes in current through it. Note that this is different to resistance. Capacitance is the ability to store charge.

Both inductors and capacitors store energy. Inductors are normally large coils of wire, where energy is stored in the form of magnetic fields. Capacitors are normally two plates, separated by some distance, where energy is stored in the form of an electric field.

Both are only really useful for AC circuits. At DC, an inductor essentially disappears (since the magnetic fields required to store energy require a changing current), and a capacitor essentially breaks the circuit (we reach a point where the capacitor is fully charged and current cannot flow anymore). They can be used in AC circuits to transfer what we call reactive power between circuit elements, they are in essence, an energy storage mechanism 

[Happy Physics Land]

Yep! My life for the next few years 

As a quick run down, inductance is an electrical property whereby the circuit element in question resists changes in current through it. Note that this is different to resistance. Capacitance is the ability to store charge.

Both inductors and capacitors store energy. Inductors are normally large coils of wire, where energy is stored in the form of magnetic fields. Capacitors are normally two plates, separated by some distance, where energy is stored in the form of an electric field.

Both are only really useful for AC circuits. At DC, an inductor essentially disappears (since the magnetic fields required to store energy require a changing current), and a capacitor essentially breaks the circuit (we reach a point where the capacitor is fully charged and current cannot flow anymore). They can be used in AC circuits to transfer what we call reactive power between circuit elements, they are in essence, an energy storage mechanism 

Well l didnt really get that last sentence but I guess Im not gonna go anywhere near electrical engineering. I would imagine now why so many people drop out after the first year --- its really damn hard.

[jamonwindeyer]

Well l didnt really get that last sentence but I guess Im not gonna go anywhere near electrical engineering. I would imagine now why so many people drop out after the first year --- its really damn hard.

Happy to explain it a little better, but it is totally irrelevant to anything HSC related  yes, Elec is quite intense, extremely mathematical! One of my subjects this year has something like a 20% failure rate, it's very difficult stuff, definitely not for anyone who isn't into their math 

[Planck]

Hey guys!

I was wondering how the equation w=qv was derived for work done in an electric field? Thanks

[Happy Physics Land]

Hey guys!

I was wondering how the equation w=qv was derived for work done in an electric field? Thanks

Hey Professor Max Planck!

I thought you are the professor in this area, not sure why you would be asking us for help but I guess you are too great to remember minute details such as this. After all I'm sure you are busier with your investigation into the blackbody radiation and E=hf. But it is certainly my honour to help a person like you who have receive nobel prizes for your significant contribution to our physics world. (Dont worry, I know you are not Max Planck hehehehe)

Ok so the simpler version of this derivation is simply substituting into the formula for work, where the force is the electric force experienced by a charged particle in an electric field and displacement is distance the charged particle has moved as a result of this  electric force.

Work done = Force x Displacement
Work = qE x d
Work = qV

The more complicated version is actually more intuitive for me, so if you think you can follow my logic here, you can have a read. I feel like for me this is a better way of truly understanding how W=qV really came about.

Recall that 1 Voltage = 1 Joule of energy available to move 1 Coulomb of charge. When voltage is applied to a pair of parallel plates, an electric field is produced ( E = V/d ). Now, in this electric field, a charged particle will experience a force ( F = qE ). This force will cause the charged particle to accelerate ( F = ma ) and hence the velocity of this charged particle will increase. Because the velocity increases, the charged particle's kinetic energy also increase ( KE = 1/2 mv² ). Since kinetic energy is a form of work done on an object, it has the unit Joules. If we now compare the unit for voltage, which is Joules/Coulomb with the unit for kinetic energy (work), which is Joules, then we can see that all we need to do to convert for voltage to work is to multiply voltage by q (i.e. J/C x C = J). Hence formula for work is W = qV.

Best Regards
Happy Physics Land