Physics Half Yearly Questions (3 Viewers)

[cutemouse]

So you do still stick by your initial argument that was supported by a plasma engineer?

 

[helper]

Re: I screwed up my half yearlies

**************
With reference to the physical principles involved, explain the operation of the galvanometer (3marks)

Firstly without the marking criteria it is impossible to say what the mark should be. It may be marked as a band 6 answer, which can explain a zero or if less I would have expected part marks.

Faraday's Law states that when there is a change in flux, a voltage will be induced. (This bit is wrong i should say motor effect. My brain went wrong during exam but other things i wrote is fine) The galvanometer uses this effect to measure current.

With that statement you would never obtain 3 in the HSC because you have used a wrong physics principle.

A coil is placed in a radial magnetic field produced by a pair of curved magnets. So the magnetic field is always perpendicular to the coi, ie. the torque will always be maximum.

This would have been ignored and is partially correct and partially misleading. It was being looked for in the second part of the question.

As the current is flow through the coil, it will experience a corce and the pointer attached will therefore turn and against the calibrated scale.

I would normally have expected a 1 there but you haven't explained why the force causes rotation or where on the coil this force is being applied or torque.

The spring will produce an opposing torque to stop the pointer. The soft iron core is used to increase sensitivity.

Opposing to what?
Why does the opposing torque stop it? Just because it is a net torque of zero, doesn't mean it will stop.
Is the spring torque constant or is it increasing or decreasing as the spring coils up?
Why does it stop at different points for different currents?

I wouldn't have expected all that but at least some.

It is an explain question, so you would have to go from the physic principle to the effect and directly linked. In that answer you haven't fully done that with any.

So I would have said probably 1 at a max.

The next question :
The magnet is shaped so as to produce a uniform radial magnetic field. What is the advantage of having such a field? (1mark)

The field will always perpendicular to the coil, and therefore the torque will always be at maximum.
I got 0 mark here again.

Firstly you haven't explained which part of the coil is perpendicular. Some is perpendicular (the two sides) even if it wasn't a radial field. The ends are always parallel because it radial.

More importantly the use of the word maximum. I know what you mean and it normally wouldn't be picked up in the HSC. You mean the maximum for a fixed current as it rotates. Make sure you mention rotate. A better way of saying it would be for a fixed current the torque will be a constant.

Finally you haven't said why this is a benefit, that a constant torque enables a uniform scale on the galvanometer.

So it would be a zero because you haven't stated the benefit at all in the application. The earlier bits may or may not count against you in the marking.

Ye i have showed the success book answer to her.
wtf then she said : Im not responsible for that book. The answer there is wrong. I havnt explained the "area of the coil" thing to her yet coz she was away 1week before holiday :mad1:

Typical and exactly what the board of studies does. No text book is endorsed by the board of studies and as such, if there is an error in the text book, it isn't a reason to change the marking criteria.

As I haven't got Success One here, I am not going to comment on its answer but once again, the level of answer depends on the band level the teacher is marking to.

Then i talked to my own physics teacher,
I did emphasise the word "AREA of the coil" and showed to formulas to him, then i said if i write "the coil is parallel to the field, so the torque is maximum" in the hsc i will get wrong.
Then he said, "well then make sure u say "area of the coil" to make it clear"
The teachers are just not willing to help you to gain any extra marks :mad1:

No they are trying to get you to be precise in your answers, so that in the HSC you are as well, so you don't drop marks if a question is being marked to a high band level.

 

[cutemouse]

Re: I screwed up my half yearlies

if there is an error in the text book, it isn't a reason to change the marking criteria.

Actually it is... If a HSC textbook shows incorrect information (eg. Dotpoint Physics, especially about the MM experiement), they have to... I don't know why but my teacher said that this is the case.

 

[helper]

Re: I screwed up my half yearlies

Actually it is... If a HSC textbook shows incorrect information (eg. Dotpoint Physics, especially about the MM experiement), they have to... I don't know why but my teacher said that this is the case.

Strange that I'm wrong when I have been in discussions about this and it hasn't occurred and what I said is a quote from the head of the exam committee.

Normally they will adapt the guidelines but if it is considered to be a fundamental error then they won't accept a change.

 

[gutzeit]

So you do still stick by your initial argument that was supported by a plasma engineer?

Sure do
Do you still stick to your initial argument that coeyz answers were correct?

Helper and I sure do disagree.

 

[cutemouse]

Sure do
Do you still stick to your initial argument that coeyz answers were correct?

This is coming from somebody who didn't even know about the application of the motor effect to the moving coil galvanometer (and of course someone who wasn't smart enough to pick up on some really obvious sarcasm), something that the syllabus requires.

Oh and, Helper does seem like somebody who does know their stuff, you on the other hand, demonstrate the integral characteristics of a curr.

 

[rheyn]

Re: I screwed up my half yearlies

Hey, here's the response from somebody who definitely knows what they're talking about - sorry

"ROFL"ing at this guy who had the audacity to claim he knew something about Physics (sorry ), and yet proceeded to spout that:

The galvanometer works by magnetic attraction – the coil forms an electromagnet, and the curved magnet is the other magnet. The north pole of the curved magnet attracts the south pole of the electromagnet and vice versa.

Mate you have no clue about how galvanometers work! If your "plasma physicist" gave you this explanation, please tell him to stop inhaling the plasma and do something useful with it instead.

I strongly suggest you reread what I posted - perhaps look here for further explanation - Galvanometer - MSN Encarta

Hey I checked out that link and nowhere does it say anything about a galvanometer working by magnetic attraction, in fact it says that it works by the "interaction of the coil’s magnetic field with the magnetic field of the permanent magnet" (i.e. the motor effect)

Hi Coeyz, for the first question I would imagine the marking criteria goes:

1. Explanation of the movement of coil with reference to the motor
effect - 1 mark

2. Explanation of the advantage of a radial magnetic field [e.g. the magnetic field is always parrallel (not perpendicular) to the plane of the coil, ensuring that torque is constant and at a maximum, in turn ensuring that the scale of the galvanometer is linear] must make reference to the formula for torque: torque = nBIAcosx (=> cosx is at a maximum as x=0)
N.B. deduct mark/crucify them if they use the formula F=nBILsinx as sin0=zero and is not the correct formula for torque in any case - 1 mark

3. The needle is rotated until the magnetic force that is acting on the spring is equalled by a counter balancing spring with reference to a physics principle such as: because net force here is balanced therefore according to Newton's First Law "an object will persists its state of rest or of uniform motion unless acted upon by an external unbalanced force." the needle will stay in its state of rest. - 1 mark

So ...(sigh) you would lose one mark because you confused Faraday's Law with the Motor Effect

you would lose another mark because you claimed that the torque is maximum when the magnetic field is perpendicular to the coil, in fact it is maximum when the magnetic field is parrallel to the coil.

Probably another mark lost because you did not reference a physics principle for the counter balancing spring (I'm a little ambivalent about this though)

Now for question 2: 0/1
As you may already know your answer is incorrect because torque is maximum when B is parallel to the coil.

QED

 

[cutemouse]

Re: I screwed up my half yearlies

you would lose another mark because you claimed that the torque is maximum when the magnetic field is perpendicular to the coil, in fact it is maximum when the magnetic field is parrallel to the coil.

I have to disagree. The torque is maximum when the coil is perpendicular to the magnetic field.

See attachment.

 

[rheyn]

Re: I screwed up my half yearlies

I have to disagree. The torque is maximum when the coil is perpendicular to the magnetic field.

See attachment.

Are you serious boy!? Go check out your textbook; in Jacaranda Physics it clearly says that "the plane of the coil will always be parallel to the magnetic field and the torque will be constant no matter how far the coil is deflected"

Ok lets look at the formula for torque: torque = nBIAcosx right?

So if the coil was perpendicular to the field then the angle x would be 90 degrees, following me?

So when you plug it into the formula
=> torque= nBIAcos90=nBIA multiplied by 0 = 0 (because as you know cos90=0)

Now as you can clearly see: torque is not maximum when torque equals zero.

However if the field was parallel to the plane of the coil, then angle x would = 0 degrees.

Therefore by substituting x=0 degrees
=> torque = nBIAcos0 = nBIA (because cos0 = 1)

As the maximum value of cosx is 1 and n, B, I and A are constants therefore torque will be at a maximum when angle x = 0; i.e. when plane of the coil is parallel to the magnetic field, torque will be at a maximum.

QED

 

[helper]

Re: I screwed up my half yearlies

Jm was correct in his diagram, where the magnetic field is perpindicular to the side of the coil, resulting in a maximum force and torque, if the plane of the coil in parallel to the magnetic field as shown in his diagram as well.

Rheyn, you are correct when you say plane of the coil is parallel to the magnetic field, torque will be at a maximum. That isn't what you said originally.

What you are debating is the point that its important to say what feature of the coil is parallel or perpindicular to the magnetic field.

 

[rheyn]

Re: I screwed up my half yearlies

That isn't what you said originally.

Lol what did you think I said originally?

 

[helper]

Re: I screwed up my half yearlies

Lol what did you think I said originally?

in fact it is maximum when the magnetic field is parrallel to the coil.

You have the same problem as the first. What part of the coil is parallel to the coil?

 

[cutemouse]

Re: I screwed up my half yearlies

Are you serious boy!? Go check out your textbook; in Jacaranda Physics it clearly says that "the plane of the coil will always be parallel to the magnetic field and the torque will be constant no matter how far the coil is deflected"

And?

If a maths textbook stated that 4 over 2 is 2, then would I be wrong by saying that 4 divided by 2 is 2?

My point is that I've said the exact same thing, the textbook obviously explains it in a difficult way of understanding it. Note the key word "the PLANE of the coil" (IOW the AREA of the coil)

Ok lets look at the formula for torque: torque = nBIAcosx right?
....

I hate it when people just blindly apply formulas without knowing what it means... I do 4U mathematics, you honestly think I don't know how to do simple substitutions and the max/min values of trig functions?

Look at the formula... T=nBIAcosθ where A is the AREA of the coil.

The definition of Torque is the turning effect/moment of a force, and is given by T=Fd, in the case of a current carrying coil, F=BILsinθ... Note that sin90deg=1 and sin0=0. Also note that when Force is zero, Torque is also zero by this defintion. So torque is a maximum when the coil is perpendicular to the magnetic field. Obviously if you're describing this in terms of the AREA of the coil, then that's different.

 

[helper]

Re: I screwed up my half yearlies

And?
The definition of Torque is the turning effect/moment of a force, and is given by T=Fd, in the case of a current carrying coil, F=BILsinθ... Note that sin90deg=1 and sin0=0. Also note that when Force is zero, Torque is also zero by this defintion. So torque is a maximum when the coil is perpendicular to the magnetic field. Obviously if you're describing this in terms of the AREA of the coil, then that's different.

Yes but the angle between the magnetic field and the current carrying sides causing the force is not changing as the coil rotates. It doesn't matter if its a radial field or not.

The perpendicular distance from the axis of rotation to the line of action of the force is changing, which is what causes a change in torque if the magnetic field isn't radial.
Thus why its important for people to be specific about talking about the plane of the coil remaining parallel and a the torque remaining constant if a radial magnetic field is being used, enabling a constant scale on the galvanometer.

 

[rheyn]

Re: I screwed up my half yearlies

I do 4U mathematics

ok.... what do you want a medal?

you honestly think I don't know how to do simple substitutions and the max/min values of trig functions?

Just because you do 4U maths doesn't mean you're good at it.

The definition of Torque is the turning effect/moment of a force, and is given by T=Fd, in the case of a current carrying coil, F=BILsinθ... Note that sin90deg=1 and sin0=0. Also note that when Force is zero, Torque is also zero by this defintion. So torque is a maximum when the coil is perpendicular to the magnetic field.

Actually if you really did know what torque is then you would know that the F in T=Fd stands for the force that is perpendicular to the coil plane (ie F is perpendicular to d).

Therefore torque is a maximum when the force is perpendicular to the magnetic field NOT "whenthe coil is perpendicular to the magnetic field".

You only showed that torque is maximum when force is perpendicular to the magnetic field; it is wrong to say torque is maximum when the coil (I'm assuming you mean coil plane, because your explanation using T= Fd refers to the coil plane) is perpendicular to the magnetic field.

Because force is perpendicular to the coil plane and because Torque is maximum when force is perpendicular to the magnetic field, therefore Torque is maximum when the coil plane is parallel to the magnetic field.

I hate it when people just blindly apply formulas without knowing what it means...

Yeah I know! The way you just blindly used T=Fd was pretty frustrating, eh? LOL

 

[cutemouse]

Re: I screwed up my half yearlies

ok.... what do you want a medal?

Already got one at prize giving last year.

Just because you do 4U maths doesn't mean you're good at it.

You were patronising me. I know what the max/min values for trig functions are.

Actually if you really did know what torque is then you would know that the F in T=Fd stands for the force that is perpendicular to the coil plane (ie F is perpendicular to d).

Which is what I said earlier. Look back a few pages.

Yeah I know! The way you just blindly used T=Fd was pretty frustrating, eh? LOL

No prize for seconds mate.

 

[rheyn]

Re: I screwed up my half yearlies

Already got one at prize giving last year.

Yeah but no-one cares

Which is what I said earlier. Look back a few pages

if you have a radial magnetic field, then the coil will always experience maximum force, which only happens when the coil is perpendicular to the magnetic field.

LOL and you were WRONG back there as well, good work!

Because force is perpendicular to the coil plane and because Torque is maximum when force is perpendicular to the magnetic field, therefore Torque is maximum when the coil plane is parallel to the magnetic field.

 

[study-freak]

Re: I screwed up my half yearlies

LOL and you were WRONG back there as well, good work!

Because force is perpendicular to the coil plane and because Torque is maximum when force is perpendicular to the magnetic field, therefore Torque is maximum when the coil plane is parallel to the magnetic field.

But jm01 was talking about MAX FORCE, not MAX TORQUE..

 

[rheyn]

Re: I screwed up my half yearlies

But jm01 was talking about MAX FORCE, not MAX TORQUE..

Firstly, when there there is maximum force there is maximum torque (assuming d stays constant)

Secondly, he said that "maximum force, which only happens when the coil is perpendicular to the magnetic field."


Which is still wrong because there is maximum force when the the force is perpendicular to the magnetic field.

When the force is maximum (i.e. when the the force is perpendicular to the magnetic field), the coil plane must be parallel to the magnetic field.

 

[study-freak]

Re: I screwed up my half yearlies

Firstly, when there there is maximum force there is maximum torque (assuming d stays constant)

Secondly, he said that "maximum force, which only happens when the coil is perpendicular to the magnetic field."


Which is still wrong because there is maximum force when the the force is perpendicular to the magnetic field.

When the force is maximum (i.e. when the the force is perpendicular to the magnetic field), the coil plane must be parallel to the magnetic field.

I don't know the context (I haven't been reading this thread and cbb to read all), but I presume that you and jm01 are talking about force/torque in a motor? (Tell me if my presumption is wrong)

"Firstly, when there there is maximum force there is maximum torque (assuming d stays constant)" True, if and only if d stays constant, which is not the case when a motor operates.

"Secondly, he said that "maximum force, which only happens when the coil is perpendicular to the magnetic field."


Which is still wrong because there is maximum force when the the force is perpendicular to the magnetic field."
Yes, you're right. That statement by jm01 is wrong.

BUT
"there is maximum force when the the force is perpendicular to the magnetic field"
sounds very odd.
We say that maximum force occurs when the direction of current or movement of charge is perpendicular to the magnetic field.

"When the force is maximum (i.e. when the the force is perpendicular to the magnetic field), the coil plane must be parallel to the magnetic field."
Not really in a motor.