HSC Physics Marathon 2013-2015 Archive (2 Viewers)

[Drsoccerball]

re: HSC Physics Marathon Archive

Cross-sectional area?

ye

 

[Crisium]

re: HSC Physics Marathon Archive

(a) There has been a significant debate regarding whether cathode rays were either waves or composed of particles. Various experiments were performed to provide evidence for either side. The Maltese cross experiment showed that cathode rays cast a shadow over glass, suggesting they travel in straight lines, a property of waves. However, the Paddle wheel experiment showed that cathode rays are able to cause a paddle wheel to move, suggesting they have momentum, and consequently mass. As a result, they were classified as waves. The debate was soon over following Thomson's experiment which showed that cathode rays were negatively charged particles.

Is this enough for 4 marks? Should I include an outline of another experiment?

You should acknowledge how the debate arose in regards to specific scientists as well

The conflicting results of the experiments regarding the nature of cathode rays performed by the Germans (i.e. Hertz and co.) and the British (i.e. Crookes and co.) arose from inadequacies in experimental design (i.e. Lack of technology, namely the technology to evacuate a glass tube - This is why Hertz was unable to deflect the cathode rays with electric fields) and the current state of knowledge about the nature of atoms.

Bolded parts are optional ~

Also be more specific with what experiment Thomson is using here because he did carry out many others, such as the one which resulted in the discovery of the radiometric effect in 1903, etc.

This debate was settled in 1897 when J.J Thomson's experiment to determine the charge-mass ratio of an electron was successfully carried out. This experiment showed that cathode rays have a measurable mass, thus providing definitive evidence for the particle nature of cathode rays, as waves do not have mass.

 

[Fizzy_Cyst]

re: HSC Physics Marathon Archive

Analyse the impact of using granular ferrites in the core of a transformer (4 marks)

 

[astroman]

re: HSC Physics Marathon Archive

Analyse the impact of using granular ferrites in the core of a transformer (4 marks)

allow magnetic flux to change easily
high resistance to eddy currents

 

[Fizzy_Cyst]

re: HSC Physics Marathon Archive

allow magnetic flux to change easily
high resistance to eddy currents

Totally give you 1/4 for that!

Answer properly!

 

[astroman]

re: HSC Physics Marathon Archive

Totally give you 1/4 for that!

Answer properly!

thats harsh

 

[Fizzy_Cyst]

re: HSC Physics Marathon Archive

thats harsh

Ikr?!?! But all you have done is identify reasons why granular ferrites are used. Not analysing the impact of using them.

You need to think about structure, young padawan.

 

[InteGrand]

re: HSC Physics Marathon Archive

Totally give you 1/4 for that!

Answer properly!

thats harsh

In my opinion, that's actually quite generous! lol

 

[iforgotmyname]

re: HSC Physics Marathon Archive

when soft iron core of transformer is subjected to change in flux, eddie currents will be formed. The eddie currents will heat up the soft iron core due to collision between moving charges and lattice caused by lattice vibrations. This changes the kenetic energy of electrons into thermal energy therefore heating up the soft iron core. which translate to powerloss as ke of electron is supplied by electrical energy.

The granular ferrites is layered into the core of transformer to minimise eddie currents by restricting large eddie currents and only allowing smaller eddie currents to form. This significantly reduces the powerloss in transformers due to production of eddie currents

 

[PhysicsMaths]

re: HSC Physics Marathon Archive

when soft iron core of transformer is subjected to change in flux, eddie currents will be formed. The eddie currents will heat up the soft iron core due to collision between moving charges and lattice caused by lattice vibrations. This changes the kenetic energy of electrons into thermal energy therefore heating up the soft iron core. which translate to powerloss as ke of electron is supplied by electrical energy.

The granular ferrites is layered into the core of transformer to minimise eddie currents by restricting large eddie currents and only allowing smaller eddie currents to form. This significantly reduces the powerloss in transformers due to production of eddie currents

Eddy

 

[PhysicsMaths]

re: HSC Physics Marathon Archive

when soft iron core of transformer is subjected to change in flux, eddie currents will be formed. The eddie currents will heat up the soft iron core due to collision between moving charges and lattice caused by lattice vibrations. This changes the kenetic energy of electrons into thermal energy therefore heating up the soft iron core. which translate to powerloss as ke of electron is supplied by electrical energy.

The granular ferrites is layered into the core of transformer to minimise eddie currents by restricting large eddie currents and only allowing smaller eddie currents to form. This significantly reduces the powerloss in transformers due to production of eddie currents

It seems you've analysed the generation of heat in transformers due to eddy currents, but not the impact of granular ferrites.

 

[PhysicsMaths]

re: HSC Physics Marathon Archive

Assess the impact of the invention of transistors on society with particular reference to their use in microchips and microprocessors

 

[porcupinetree]

re: HSC Physics Marathon Archive

Lets get this thread up and running again!

Assess the impact of the invention of transistors on society with particular reference to their use in microchips and microprocessors

You didn't specify how many marks, so I'll assume...say, 4 marks sounds about right.

The development of transistors in the mid 1900s allowed for the replacement of thermionic devices (valves), which were the current technology for storing and manipulating electrical information. Solid state devices (made from transistors) replaced valves because they were smaller, more reliable, more durable, required no warm up time, were cheaper and more power efficient.
Transistors are used in, and are essential to, microchips, which form the basis of modern computers: bipolar transistors are used in analog devices while Metal Oxide Semiconductor Field Effect Transistors (MOSFET) are used in digital circuits. Microprocessors, i.e. microchips with enough transistors to perform arithmetic, logic and control operations, were/are integral in the development of modern computers. Hence transistors have allowed modern computers and other technologies (phones, tables ets), which have hugely impacted worldwide communication, have increased the quality of life for billions, and have ushered in an 'information age'.

Next question:
Describe Einstein's contributions to Special Relativity and to Quantum Theory and how these contributions changed the direction of scientific thinking in the 20th century. 6 marks

 

[Drsoccerball]

re: HSC Physics Marathon Archive

Lets get this thread up and running again!



You didn't specify how many marks, so I'll assume...say, 4 marks sounds about right.

The development of transistors in the mid 1900s allowed for the replacement of thermionic devices (valves), which were the current technology for storing and manipulating electrical information. Solid state devices (made from transistors) replaced valves because they were smaller, more reliable, more durable, required no warm up time, were cheaper and more power efficient.
Transistors are used in, and are essential to, microchips, which form the basis of modern computers: bipolar transistors are used in analog devices while Metal Oxide Semiconductor Field Effect Transistors (MOSFET) are used in digital circuits. Microprocessors, i.e. microchips with enough transistors to perform arithmetic, logic and control operations, were/are integral in the development of modern computers. Hence transistors have allowed modern computers and other technologies (phones, tables ets), which have hugely impacted worldwide communication, have increased the quality of life for billions, and have ushered in an 'information age'.

Next question:
Describe Einstein's contributions to Special Relativity and to Quantum Theory and how these contributions changed the direction of scientific thinking in the 20th century. 6 marks

I wish i had time to type up a solution D: english is overpowering me...

 

[mrpotatoed]

re: HSC Physics Marathon Archive

Lets get this thread up and running again!
Next question:
Describe Einstein's contributions to Special Relativity and to Quantum Theory and how these contributions changed the direction of scientific thinking in the 20th century. 6 marks

Poor Einstein has been waiting almost 2 months

Einstein's contribution to special relativity was to that he interpreted the null result of the Michelson and Morley experiment as meaning that the speed of light was constant from all frames of reference. This meant that time, mass, and distance had to vary to maintain the constant speed of light. He directed scientific thought away from the search for a frame of a reference at absolute rest (ie: aether), by showing how all frames of reference were relative to one another. The energy mass equivalence formula he developed (E=MC^2) also provided a further breakthrough in scientific knowledge, by showing that mass was simply a reservoir of energy, and helped scientists to understand how the big bang could have lead to the formation of the universe.

His work in quantum theory was to apply Planck's proposal, that blackbody radiation was quantised, and apply it to light to explain the photoelectric effect. He determined that metals have a work function and that the energy of the photon must exceed the work function to cause photoemission. He also showed that there with a one to one ratio between photons and photoelectrons, meaning that one photon can only release one photoelectron, regardless of how much energy the photon has. The energy of the photon is proportional to its frequency. This explained why increasing the rate of photons per second would not cause photoemission, if the photons were below the work function. By applying quantum theory to solve a problem that classical physics could not, Einstein showed the significance of quantum physics. His contribution therefore moved the direction of scientific thought away from classical physics and towards quantum physics.

------

In an experiment conducted by two physics students, a magnet was dropped in a hollow plastic tube, and then the same magnet dropped in a copper tube of equal length and diameter to the plastic tube. It was observed to fall with a constant acceleration in both the plastic and cooper tube, but the time recorded for the magnet to reach the bottom of the cooper tube was considerably longer. Assess the accuracy of the student's observations. (6 marks)

makeshift marking criteria in white font below

1 mark for each:
constant acceleration due to gravity in plastic tube
Faraday's law induces current in cooper tube
Lenz's law: current flows to oppose the relative motion
current in tube produces magnetic field that provides a lifting force on magnet
lifting force partly negates the weight force and results in a slower fall that takes longer.
provides an assessment that strongly communicates the accuracy of the student's observation as being consistent with accepted laws.

Note: if there is no assessment, max mark is 4 (ie; can't reach top band of 5-6 marks), and 1 mark taken off for other subsequent missing points.

 

[porcupinetree]

re: HSC Physics Marathon Archive

In an experiment conducted by two physics students, a magnet was dropped in a hollow plastic tube, and then the same magnet dropped in a copper tube of equal length and diameter to the plastic tube. It was observed to fall with a constant acceleration in both the plastic and cooper tube, but the time recorded for the magnet to reach the bottom of the cooper tube was considerably longer. Assess the accuracy of the student's observations. (6 marks)

When dropping the magnet in the copper tube, the magnet accelerates at -9.8ms^-2 (due to gravity), creating a changing magnetic flux which induces an emf in the copper tube. Due to Lenz's law, the eddy currents generated in the copper tube by this emf will create a magnetic field to oppose the cause on induction. In this case, the eddy currents will create a magnetic field to oppose the falling of the magnet. Thus the magnet undergoes an upwards force, and is slowed at a rate which is proportional to the rate that its magnetic field changes relative to the copper tube. This relationship results in the magnet accelerating at a constant rate downwards, but at a lower rate than 9.8ms^-2 due to the electromagnetic braking effect.
When the magnet is dropped in the plastic tube of equal dimensions, there is no emf induced in the non-conductive plastic. Thus there is no electromagnetic braking effect, and thus the magnet accelerates downwards at a constant rate of 9.8ms^-2.
Hence, the students' observations are accurate and reflect the accepted physics principle of Lenz's Law / electromagnetic braking, along with gravitational acceleration.

 

[InteGrand]

re: HSC Physics Marathon Archive

When dropping the magnet in the copper tube, the magnet accelerates at -9.8ms^-2 (due to gravity), creating a changing magnetic flux which induces an emf in the copper tube. Due to Lenz's law, the eddy currents generated in the copper tube by this emf will create a magnetic field to oppose the cause on induction. In this case, the eddy currents will create a magnetic field to oppose the falling of the magnet. Thus the magnet undergoes an upwards force, and is slowed at a rate which is proportional to the rate that its magnetic field changes relative to the copper tube. This relationship results in the magnet accelerating at a constant rate downwards, but at a lower rate than 9.8ms^-2 due to the electromagnetic braking effect.
When the magnet is dropped in the plastic tube of equal dimensions, there is no emf induced in the non-conductive plastic. Thus there is no electromagnetic braking effect, and thus the magnet accelerates downwards at a constant rate of 9.8ms^-2.
Hence, the students' observations are accurate and reflect the accepted physics principle of Lenz's Law / electromagnetic braking, along with gravitational acceleration.

The acceleration isn't actually constant in the copper tube, it actually decays exponentially (becoming virtually constant fairly quickly though in practice). See:

How would you draw a graph for the acceleration of a magnet as it falls through a metal tube?
From my understanding:

1. Before entering the tube, it's acceleration would be 9.8
2. As it enters the tube, it's acceleration would become lower
3. As it is traveling through the tube, it's acceleration would go back to 9.8
4. As it exits the tube, it's acceleration would decrease
5. When it is completely out of the tube, it's acceleration would go back to 9.8

However, my teacher said the acceleration as it goes through the tube is not constant, and that it should be a curve. So, what does it actually look like?

It is indeed a curve.

From Lenz's Law, the differential equation governing the motion of the magnet as it falls through the tube is going to be

, where is the velocity of the magnet (we are taking downwards as the positive direction), is the acceleration, m is the mass of the magnet, g is the acceleration due to gravity, and k is a damping constant which depends on the magnet, pipe material, and pipe geometry. Empirically, that differential equation is what we get.

This can be solved to give velocity as a function of time: . (Assuming v is 0 at t = 0.)

Differentiating with respect to t gives us our acceleration as a function of time: .

So this means acceleration starts off at gravity's acceleration, and attenuates exponentially, becoming 0 in the limit as time goes to infinity.

Eventually of course, your magnet will exit the tube. After this time, the acceleration will quickly get back to g. To calculate exact times, you'll need to calculate k for a given magnet and tube by constructing a simple geometric model of the situation and looking up the conductivity of the metal of the tube.

For the purposes of your a-t graph, it'll look something like this for the period of time that the magnet is inside the tube: http://www.graphsketch.com/?eqn1_co..._lines=1&line_width=4&image_w=850&image_h=525

Further reading: http://arxiv.org/pdf/physics/0702062.pdf, http://en.wikipedia.org/wiki/Magnetic_damping

 

[InteGrand]

re: HSC Physics Marathon Archive

(So, from that thread, the a-t graph looks something like this for the magnet dropped through the copper tube:

Ah okay, so overall, the graph would be more like this?:

)

 

[Drsoccerball]

re: HSC Physics Marathon Archive

The acceleration isn't actually constant in the copper tube, it actually decays exponentially (becoming virtually constant fairly quickly though in practice). See:

I love extension 2 mechanics and how applicable it is

 

[Mr_Kap]

re: HSC Physics Marathon Archive

Analyse the impact of using granular ferrites in the core of a transformer (4 marks)

please give us a 4 mark answer Fizzy_Cyst.