Physics marathon (hsc) (7 Viewers)

[barbernator]

I won't write a full answer but these are the main ideas.

- Photoelectric effect causes emission of electrons
- Each metal has a unique work function which is basically the energy required to liberate electrons from its surface -> the energy of incident photons (E=hf) must be above this in order to create a current
- Platinum has a higher work function than zinc so when the light is shone on the zinc, the energy of the photon is greater than the work function and photoemission occurs

Part B: The light would have to be of a higher frequency (and hence energy). For example, violet light might work or UV may be required, depending on the work function of platinum.

Question: Explain how maglev trains operate

good answer.

Maglev trains are trains that are not in contact with the ground due to the use of electromagnets for levitation and propogation

There are 2 types of maglev trains currently in operation, Electrodynamic suspension and electromagnetic suspension systems.

(ill just discuss EDS cos its the only one with superconductors)

For levitation, on the track, electromagnets are used that are not superconducting, just regular magnets. On the train, superconducting wire is used in the electromagnet to allow large currents to flow and hence create a powerful magnetic field. These are cooled by liquid nitrogen. By making the electromagnets repel each other (either by N facing N or S facing S) the train is able to levitate. (diagram)

For propulsion, equal strength electromagnetic interaction by magnets on the train and to the left and right of train are required. The electromagnets on the train are of fixed poles, while the electromagnets on the track are constantly alternating poles to create attraction and repulsion in phase with the motion of the train, necessary to either accelerate or decelerate the train. (diagram is a must)

Through these two sets of electromagnetic interaction, maglev trains are able to both levitate and be moved along a track.

 

[RishBonjour]

good answer.

Maglev trains are trains that are not in contact with the ground due to the use of electromagnets for levitation and propogation

There are 2 types of maglev trains currently in operation, Electrodynamic suspension and electromagnetic suspension systems.

(ill just discuss EDS cos its the only one with superconductors)

For levitation, on the track, electromagnets are used that are not superconducting, just regular magnets. On the train, superconducting wire is used in the electromagnet to allow large currents to flow and hence create a powerful magnetic field. These are cooled by liquid nitrogen. By making the electromagnets repel each other (either by N facing N or S facing S) the train is able to levitate. (diagram)

For propulsion, equal strength electromagnetic interaction by magnets on the train and to the left and right of train are required. The electromagnets on the train are of fixed poles, while the electromagnets on the track are constantly alternating poles to create attraction and repulsion in phase with the motion of the train, necessary to either accelerate or decelerate the train. (diagram is a must)

Through these two sets of electromagnetic interaction, maglev trains are able to both levitate and be moved along a track.

Do we have to know the other? I only know the one you discussed.


Explain why superconductors are advantageous to be used in computers.

 

[dawooddaman]

Explain why superconductors are advantageous to be used in computers.

Superconductive films may result in the miniaturization and increase speed of computer chips
This makes the integrated circuits made from superconductors far more powerful. Closer packing means less delay in signals and faster circuits.

 

[adzlark]

Question: An astronaut embarks on a mission to the Andromeda galaxy travelling at 99.99% the speed of light whilst his twin brother remains on Earth. When he returns to Earth he finds he is now much older than his twin. By considering inertial and non-inertial frames of reference explain why this outcome is not possible and explain the expected result.

 

[freeeeee]

Question: An astronaut embarks on a mission to the Andromeda galaxy travelling at 99.99% the speed of light whilst his twin brother remains on Earth. When he returns to Earth he finds he is now much older than his twin. By considering inertial and non-inertial frames of reference explain why this outcome is not possible and explain the expected result.

The principle of relativity states that no inertial frame of reference is given preference over another inertial frame of reference. Considering the situation from the Q, let the twin who has undergone space travel be T1 and the one who remained on Earth be T2. The misconception in the q arises from T1's frame of reference, T1 sees T2 disappearing away with the Earth at the relativistic velocity (0.9999c) and observe T2 to be younger on his/her return journey. Therefore T1 (the astronaut) finds that he is now older than his twin. However this is not true as T1's frame of reference is not inertail, the spacecraft has accelerated and decelerated and hence the inertial frame of reference of T2 (the twin brother on Earth) takes precedence and only that twin on Earth can apply time dilation in this situation so the expected outcome is that T1 (The astronaut) is actually younger than T2 (the Twin Brother on Earth).

 

[adzlark]

The principle of relativity states that no inertial frame of reference is given preference over another inertial frame of reference. Considering the situation from the Q, let the twin who has undergone space travel be T1 and the one who remained on Earth be T2. The misconception in the q arises from T1's frame of reference, T1 sees T2 disappearing away with the Earth at the relativistic velocity (0.9999c) and observe T2 to be younger on his/her return journey. Therefore T1 (the astronaut) finds that he is now older than his twin. However this is not true as T1's frame of reference is not inertail, the spacecraft has accelerated and decelerated and hence the inertial frame of reference of T2 (the twin brother on Earth) takes precedence and only that twin on Earth can apply time dilation in this situation so the expected outcome is that T1 (The astronaut) is actually younger than T2 (the Twin Brother on Earth).

Spot on mate *Thumbs up*

 

[dawooddaman]

Discus how do maglev train operate?

 

[Kimyia]

Can someone please tell me if this is right?
If you just have a magnet sitting on top of a superconductor (before its cooled before its critical temp) and you cool that superconductor below its critical temp, the magnetic will levitate because of the Meissner effect and the superconductor expelling all external magnetic fields such that it overcomes the weight force of the magnet and it levitates - nothing to do with induced currents?
But, if you have a superconductor cooled below its critical temp and you dropped a magnet from above the superconductor, then you'd induce 'perfect' eddy currents that repel the magnet's magnetic field (and is strong enough to overcome the weight of the magnet) such that it levitates???

 

[adzlark]

Discus how do maglev train operate?

The "magnetic levitation" or maglev train in Germany utilises the Electro-Magnetic Suspension system to operate the high-speed, frictionless vehicle. This operates by employing the property of electromagnetic repulsion, where electromagnets are positioned on the track and the vehicle itself. Due to the varying distances between the vehicle and the track, the EMS system requires constant computer interference to maintain balance of the vehicle.

The Electrodynamic Suspension system of the Japanese maglev train involves the Meissner effect to obtain its frictionless travel property. Superconducting plates are positioned upon the train itself with conduction pads along the track to provide the magnetic field for the Meissner effect to occur and result in the required magnetic levitation for high-speed, frictionless travel.

 

[adzlark]

Can someone please tell me if this is right?
If you just have a magnet sitting on top of a superconductor and you cool that superconductor below its critical temp, the magnetic will legitate because of the Meissner effect and the superconductor expelling all external magnetic fields such that it overcomes the weight force of the magnet and it levitates - nothing to do with induced currents.
But, if you have a superconductor cooled below its critical temp and you dropped a magnet from above the superconductor, then you'd induce 'perfect' eddy currents that repel the magnet's magnetic field (and is strong enough to overcome the weight of the magnet) such that it levitates???

In order for magnetic levitation to occur, there must exist a relative motion between the magnet and the super-conductor so that current (and ultimately magnetic field) can be generated within the super conductor for the Meissner effect to occur. If the magnet is just sitting there then nothing will happen.

 

[freeeeee]

Discus how do maglev train operate?

A Maglev train relies on superconductors for operation, superconductors are utilised both to levitate the maglev train and to propel the train. On the rails of a maglev train situate superconducting electromagnets which are fed with AC current, this means that the electromagnets have constantly changing polarity so they will alternately be pulled by magnets infront and pushed by magents behind. While on the train, superconducting coils are fed with large DC current setting up a strong constant magnetic field. The relative forward motion of the train causes changing flux in the coils on the train inducing large eddy currents, repelling the train, causing it to levitate.

 

[freeeeee]

In order for magnetic levitation to occur, there must exist a relative motion between the magnet and the super-conductor so that current (and ultimately magnetic field) can be generated within the super conductor for the Meissner effect to occur. If the magnet is just sitting there then nothing will happen.

I thought the whole point behind meisner effect was the expulsion of a magnetic field from within a superconducting matrerial

Anyways Question: Discuss two possible applications of superconductors in relation to computers, generators, motors, transmission of electricity (6marks)

 

[Kimyia]

In order for magnetic levitation to occur, there must exist a relative motion between the magnet and the super-conductor so that current (and ultimately magnetic field) can be generated within the super conductor for the Meissner effect to occur. If the magnet is just sitting there then nothing will happen.

But can't you see a magnet rising up off a superconductor once you start lowering its temperature below critical (provided the magnet is sitting on top the superconductor) - hence Meissner effect not induction because there is no relative motion? Someone help, I'm confused.

 

[dawooddaman]

Question: Discuss two possible applications of superconductors in relation to computers, generators, motors, transmission of electricity (6marks)

Superconductors are mainly used due to the following properties;
1. Efficient conduction – Energy is lost as heat when a current flows through a conductor. (P=I2R). However, since superconductors have zero resistance, then no heat loss will occur
2. Powerful magnets – A very strong magnetic field requires a very large current, which inevitably results in a significant amount of energy losses as heat. However, if an electromagnet with zero resistance is used, no heat loss will occur as the current flows. Hence this makes the whole process more energy efficient and allows the magnetic field to be stronger.
Also, once a current is established, due to the lack of resistance in the conductor, the flow of current will not diminish even if the power source is removed.

Supercomputers
The fact that superconductors have effectively zero resistance and therefore effectively zero heat production when electricity flows through them means that the devices made from superconductors can be integrated closer than those made from ordinary semiconductors. This makes the integrated circuits made from superconductors far more powerful. Closer packing means less delay in signals and faster circuits.

Motors and generators
When superconductors are used in motors, the low resistance to the flow of electricity means that with given amount of voltage, the net current flow in the motors will be bigger, which means the motors are more powerful. Furthermore no heat loss means the devices are more energy efficient

Transmission of electricity through power grids
Once transmission wires are made from superconductors, the resistance of the transmission wires is effectively reduced to zero. This minimises the heat lost, which means all of the energy produced at the generator can be transferred to households, making the process almost 100% efficient. Minimal energy losses enable the whole process to become more environmentally friendly. Also it enables power stations to be built further away from large cities, which reduces pollution near metropolitan areas. A major disadvantage is that it is very difficult to cool the wires, as they are open to the environment. Also, super conducting wires only transmit DC, which makes it hard to operate on our current AC system.

 

[freeeeee]

Superconductors are mainly used due to the following properties;
1. Efficient conduction – Energy is lost as heat when a current flows through a conductor. (P=I2R). However, since superconductors have zero resistance, then no heat loss will occur
2. Powerful magnets – A very strong magnetic field requires a very large current, which inevitably results in a significant amount of energy losses as heat. However, if an electromagnet with zero resistance is used, no heat loss will occur as the current flows. Hence this makes the whole process more energy efficient and allows the magnetic field to be stronger.
Also, once a current is established, due to the lack of resistance in the conductor, the flow of current will not diminish even if the power source is removed.

Supercomputers
The fact that superconductors have effectively zero resistance and therefore effectively zero heat production when electricity flows through them means that the devices made from superconductors can be integrated closer than those made from ordinary semiconductors. This makes the integrated circuits made from superconductors far more powerful. Closer packing means less delay in signals and faster circuits.

Motors and generators
When superconductors are used in motors, the low resistance to the flow of electricity means that with given amount of voltage, the net current flow in the motors will be bigger, which means the motors are more powerful. Furthermore no heat loss means the devices are more energy efficient

Transmission of electricity through power grids
Once transmission wires are made from superconductors, the resistance of the transmission wires is effectively reduced to zero. This minimises the heat lost, which means all of the energy produced at the generator can be transferred to households, making the process almost 100% efficient. Minimal energy losses enable the whole process to become more environmentally friendly. Also it enables power stations to be built further away from large cities, which reduces pollution near metropolitan areas. A major disadvantage is that it is very difficult to cool the wires, as they are open to the environment. Also, super conducting wires only transmit DC, which makes it hard to operate on our current AC system.

nice 6!

Question: Explain how the motor effect is utilised in loud speakers (4marks)

 

[adzlark]

But can't you see a magnet rising up off a superconductor once you start lowering its temperature below critical (provided the magnet is sitting on top the superconductor) - hence Meissner effect not induction because there is no relative motion? Someone help, I'm confused.

The relative motion I refer to is when you have the super conductor and then drop the magnet on top of it. Rather than falling and hitting the super conductor it will levitate i.e. stay suspended in the air not move upwards. Sorry if I'm confusing you haha

 

[dawooddaman]

Question: Explain how the motor effect is utilised in loud speakers (4marks)

A loudspeaker also works on the basis of the motor effect: it converts electrical energy to sound energy

A simple loudspeaker consists of a coil of wire between the pole pieces of the magnets which form the core. A paper diaphragm is then attached to this coil-magnet unit.

When signals are fed into the coil inside the loudspeaker, the coil experiences a force as a result of the motor effect. When the current flow from A to B we can apply the right hand palm rule and see the force acting on the wire pushes the wire out. Similarly when current flows from B to A the force will pull the coil in. Since AC signals vary in direction very rapidly, the coil should move in and out very rapidly as well. Furthermore, since the coil is very tightly wound on the magnetic pole piece, they will vibrate.

Add diagram if needed:

 

[barbernator]

Guys, maglev trains are NOT levitated due to the meissner effect.

 

[dawooddaman]

Guys, maglev trains are NOT levitated due to the meissner effect.

Can you please explain how? the only Dotpoint that confuses me =(

 

[barbernator]

Can you please explain how? the only Dotpoint that confuses me =(

refer to my above answer on this page