HSC Physics Marathon 2013-2015 Archive (2 Viewers)

[anomalousdecay]

re: HSC Physics Marathon Archive

Eddy currents are formed as an application of Lenz's law, wherein circular currents are induced in sheets of metal due to a change in flux. According to Lenz's law, the eddy currents would provide a magnetic field that opposes the initial motion of the conductor. When attached to a moving object, like a train or roller-coaster, the eddy currents induced will therefore provide a breaking force, known as electromagnetic breaking. As the object slows, the rate at which the metal cuts the flux decreases, resulting in smaller eddy currents and smaller resistive forces, thus providing smooth braking.

Nice succinct answer. Clear cause, effect and reason.

Expand upon this point a bit: the eddy currents induced will therefore provide a breaking force, known as electromagnetic braking." You can use an equation here and state that this is also as a result of conservation of energy (so expanding upon Lenz's Law).

Change this part thought to make it clearer: "the rate at which the metal cuts the flux decreases".

And remember, it's "braking" not "breaking" !

 

[anomalousdecay]

re: HSC Physics Marathon Archive

Is this related to this question: http://puu.sh/kVO51/bf1e392ee2.png
If so please explain

Yeah.

When it enters the field, there is a change in flux through the split ring. This will induce a voltage in a certain direction, which is actually unknown as the voltmeter orientation applied is not known. However, we know that it will induce a current so that it avoids entering the field.

When it leaves the field, there is a change in flux of the same magnitude but opposite direction. So it will induce a voltage in the other direction. Hence the answer is A.

Ok so just to clarify, when a conductor is moving at a constant velocity through a magnetic field with an uniform flux density, then there is no change in magnetic flux?

yup

 

[anomalousdecay]

re: HSC Physics Marathon Archive

any way, I better do my assignment

 

[godofindolence]

re: HSC Physics Marathon Archive

Yeah.

When it enters the field, there is a change in flux through the split ring. This will induce a voltage in a certain direction, which is actually unknown as the voltmeter orientation applied is not known. However, we know that it will induce a current so that it avoids entering the field.

When it leaves the field, there is a change in flux of the same magnitude but opposite direction. So it will induce a voltage in the other direction. Hence the answer is A.



yup

The thing is, the pendulum is swinging, so it is moving in simple harmonic motion. Wouldn't that mean that the velocity is constantly changing and thus constantly creating change in flux, resulting in constant induced EMF?

 

[godofindolence]

re: HSC Physics Marathon Archive

any way, I better do my assignment

Thank you so much! have fun!

 

[anomalousdecay]

re: HSC Physics Marathon Archive

The thing is, the pendulum is swinging, so it is moving in simple harmonic motion. Wouldn't that mean that the velocity is constantly changing and thus constantly creating change in flux, resulting in constant induced EMF?

Good point but in the question the effect would be minimal compared to the initial expected larger change in flux due to the magnetic field (the multiple choice options are suggesting this). The velocity change relative to the magnetic field would be quite small in my opinion, so that in this question it was deemed negligible. Basically you can expect a graph like this if you look at it closer.

 

[Crisium]

re: HSC Physics Marathon Archive

lol that sounds like some theory of relativity stuff, think you should explicitly say "when a conductor experiences a change in magnetic flux" instead of that relative movement thing since you referred to "magnetic field opposing the initial change in flux" but sounded a bit vague in the first part of your sentence

When there is relative movement between a conductor and a magnetic field, the change in magnetic flux will induce a EMF in the conductor (Faraday) which gives rise to a current that will flow in such a way to produce a magnetic field (Oersted) that will oppose the motion that that created it (Lenz's Law).

 

[Kaido]

re: HSC Physics Marathon Archive

When there is relative movement between a conductor and a magnetic field, the change in magnetic flux will induce a current in the conductor (Faraday) and this current will flow in such a way to produce a magnetic field (Oersted) that will oppose the motion that that created it (Lenz's Law).

Technically, it induces an emf, which gives rise to a current

 

[Kaido]

re: HSC Physics Marathon Archive

How do intrinsic semiconductors carry current? (3)

 

[Kaido]

re: HSC Physics Marathon Archive

Describe an investigation to help distinguish between non-inertial and inertial frames of reference (3)

 

[TQuadded]

re: HSC Physics Marathon Archive

Describe an investigation to help distinguish between non-inertial and inertial frames of reference (3)

I'll give it a go.

A inertial frame refers to a frame of reference in which no experiment can be done to determine whether you're moving at a constant speed or at rest. A non-inertial frame of reference refers to one that can be observed to be moving at a velocity relative to the inertial frame. Therefore, a simple procedure to distinguish between non-inertial and inertial frames of reference can be followed:
1.) Hang a small mass from a string.
2.) Hold the opposite end of the string so that the mass hangs down. Do not move it with your hand.
3.) If the mass stays hanging vertically, then you are in an inertial frame of reference. If the mass hangs at an angle other than being vertically downwards, then you are in a non-inertial frame of reference.

How'd I go?

 

[Mr_Kap]

re: HSC Physics Marathon Archive

Should mention specifically that a non-inertial is one that is "ACCELERATING". I know you practically said this.... but mention that word. Other than that it seems legit --> As Kaido said, hang it on the roof

 

[Kaido]

re: HSC Physics Marathon Archive

I'll give it a go.

A inertial frame refers to a frame of reference in which no experiment can be done to determine whether you're moving at a constant speed or at rest. A non-inertial frame of reference refers to one that can be observed to be moving at a velocity relative to the inertial frame. Therefore, a simple procedure to distinguish between non-inertial and inertial frames of reference can be followed:
1.) Hang a small mass from a string.
2.) Hold the opposite end of the string so that the mass hangs down. Do not move it with your hand.
3.) If the mass stays hanging vertically, then you are in an inertial frame of reference. If the mass hangs at an angle other than being vertically downwards, then you are in a non-inertial frame of reference.

How'd I go?

A little bit of refining, as Kap mentioned.
Also, holding the string is not valid, you should hang it on the roof (if you're in a car etc.)

 

[Mr_Kap]

re: HSC Physics Marathon Archive

How do you convert eV/Hz into Js????

 

[atargainz]

re: HSC Physics Marathon Archive

How do you convert eV/Hz into Js????

1 eV (electron volt) = 1.6x10^-19 J, so just times by charge of electron, not sure about Hz

 

[Kaido]

re: HSC Physics Marathon Archive

How do you convert eV/Hz into Js????

Hertz is the emasure of frequency
Since E=hf, you can calc it that way

 

[Crisium]

re: HSC Physics Marathon Archive

Technically, it induces an emf, which gives rise to a current

Yeah that was meant to be EMF haha my bad

 

[InteGrand]

re: HSC Physics Marathon Archive

How do you convert eV/Hz into Js????

1/Hz is equal to s (seconds), so all you need to do is convert eV to J, and I think the conversion for that is given in the data sheet.

 

[TQuadded]

re: HSC Physics Marathon Archive

A little bit of refining, as Kap mentioned.
Also, holding the string is not valid, you should hang it on the roof (if you're in a car etc.)

Woohoo! Band 6 here I come! (I thought to myself that wasn't valid, and I'm an idiot for forgetting that it's not impossible to hang it on the roof of a moving vehicle.)

 

[Mr_Kap]

re: HSC Physics Marathon Archive

Assess the effects of the development of AC generators on today's society and the environment. 6 MARKS