Physics Half Yearly Questions (2 Viewers)
- Thread starter coeyz
- Start date
Re: I screwed up my half yearlies
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.
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.
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.
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.
With that statement you would never obtain 3 in the HSC because you have used a wrong physics principle.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 exambut other things i wrote is fine) The galvanometer uses this effect to measure current.
This would have been ignored and is partially correct and partially misleading. It was being looked for in the second part of the question.
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.
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.
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.
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.
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.
Last edited:
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.
Re: I screwed up my half yearlies
Normally they will adapt the guidelines but if it is considered to be a fundamental error then they won't accept a change.
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.
Last edited:
Sure do
Do you still stick to your initial argument that coeyz answers were correct?
Helper and I sure do disagree.
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.Sure do
Do you still stick to your initial argument that coeyz answers were correct?
Oh and, Helper does seem like somebody who does know their stuff, you on the other hand, demonstrate the integral characteristics of a curr.
Last edited:
Re: I screwed up my half yearlies
), and yet proceeded to spout that:
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
"ROFL"ing at this guy who had the audacity to claim he knew something about Physics (sorryHey, here's the response from somebody who definitely knows what they're talking about - sorry
), and yet proceeded to spout that: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.
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
Last edited:
Re: I screwed up my half yearlies
See attachment.
I have to disagree. The torque is maximum when the coil is perpendicular to the magnetic field.
See attachment.
Re: I screwed up my half yearlies
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
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
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.
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.
Re: I screwed up my half yearlies
You have the same problem as the first. What part of the coil is parallel to the coil?
Re: I screwed up my half yearlies
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)
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.
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)
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.
Re: I screwed up my half yearlies
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.
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.
Re: I screwed up my half yearlies
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.
ok.... what do you want a medal?
Just because you do 4U maths doesn't mean you're good at it.
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.
Yeah I know! The way you just blindly used T=Fd was pretty frustrating, eh? LOL
Re: I screwed up my half yearlies
Already got one at prize giving last year.
You were patronising me. I know what the max/min values for trig functions are.
Which is what I said earlier. Look back a few pages.
No prize for seconds mate.
Re: I screwed up my half yearlies
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.
Yeah but no-one cares
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
Bored of
- Joined
- Feb 8, 2008
- Messages
- 1,133
- Gender
- Male
- HSC
- 2009
Re: I screwed up my half yearlies
But jm01 was talking about MAX FORCE, not MAX TORQUE..
Re: I screwed up my half yearlies
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.
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
Bored of
- Joined
- Feb 8, 2008
- Messages
- 1,133
- Gender
- Male
- HSC
- 2009
Re: I screwed up my half yearlies
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.
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.