HecticLad said:When the current is turned on in X, at the start a magnetic field is induced to oppose the increase in voltage, and therefore generate a back EMF, however it will eventually reach a constant voltage as it is a DC circuit and there will eventually be no magnetic field. A similar field will be created to oppose the voltage change when the switch is turned off but it will be in the opposite direction. Because there is only a changing magnetic field when the switch is turned off and on and not while it is operating at full voltage, a current will only be induced in coil Y when this field is changing, so therefore only when the switch at X is turned on and off. The induced voltage in Y when the switch is turned on will be in the opposite direction as that induced when the switch is turned off, so it would be C.
The diagram you showed for the LHS v/t graph is pretty unrealistic for a Year 12 student to recognise because the ending portion of it will not be something that they are familiar with. The content given in the question contain assumptions that are taught to students (as assumptions which are clearly made known at times) within the syllabus itself and does not go out of the scope of the syllabus. Both graphs in the question are pretty stock-standard HSC level Module 6 questions because a lot of the mathematical theory behind Mod 6 formulae are taken for granted and students are just expected to memorise them for what they are. Surprisingly, I'd say the new syllabus is much more rigorous than the old one, especially with the removal of electives, and the module 6 content is definitely the most conceptually challenging in the syllabus. Yes, it's unfortunate that students need to take some things for how it is taught (and force to memory) but that's the reality of introducing a calculation heavy module in a course that isn't allowed to have a minimum restriction on the level of mathematics taken by candidates (i.e. integration, etc is not assumed knowledge so it cannot be used in any question or concept within HSC Physics).Geeze, I hate these questions that are written by supposedly experienced HSC teachers, who have no specialist knowledge of electricity.
Yes the best answer is (C) but the graphs for the primary emf and secondary emf are wrong.
The actual graphs for emf would look like this:
View attachment 37582
When the switch is closed, the data logger will be connected to the applied DC voltage, through conductors that have negligible series resistance, so the applied voltage will rise pretty much instantaneously to the maximum DC value. When the switch suddenly opens, the collapsing magnetic field will induce a large back e.m.f. in the primary winding. Anyone who has ever done this in a Physics lab will have seen the spark that is induced at the switch when the voltage spike appears in the primary winding. This transient is actually utilised in internal combustion engines. The Kettering ignition system uses a coil and switching DC voltage to create a very high voltage spark to ignite the fuel/air mixture.
I don't like unrealistic problems being given to students in Stage 6, if they don't have the necessary Physics and Mathematics to understand fully what is going on.
To properly understand the behaviour of an inductor that is subject to a step-change in applied emf, we need 2nd Year university AC Circuit Theory for current and e.m.f. in an inductor. You would also need more data, such as the series resistance R of the primary winding.
(end of rant)
I take your points about taking much of the mathematical theory of electromagnetism for granted in the HSC Physics syllabus. However you can teach everything required by the syllabus in transformers using well-behaved continuous functions like sine or cosine. There is no need to introduce transient step-functions and other discontinuous functions that complicate the mathematics, in my opinion. The syllabus doesn't require it, and it should not be examinable at high school level. The other problem with erroneous marking schemes like the one above is that less experienced Science teachers will be misled into believing that's how self-induction actually works, which is very dangerous. That's my 2-cents worth.The diagram you showed for the LHS v/t graph is pretty unrealistic for a Year 12 student to recognise because the ending portion of it will not be something that they are familiar with. The content given in the question contain assumptions that are taught to students (as assumptions which are clearly made known at times) within the syllabus itself and does not go out of the scope of the syllabus. Both graphs in the question are pretty stock-standard HSC level Module 6 questions because a lot of the mathematical theory behind Mod 6 formulae are taken for granted and students are just expected to memorise them for what they are. Surprisingly, I'd say the new syllabus is much more rigorous than the old one, especially with the removal of electives, and the module 6 content is definitely the most conceptually challenging in the syllabus. Yes, it's unfortunate that students need to take some things for how it is taught (and force to memory) but that's the reality of introducing a calculation heavy module in a course that isn't allowed to have a minimum restriction on the level of mathematics taken by candidates (i.e. integration, etc is not assumed knowledge so it cannot be used in any question or concept within HSC Physics).
At the end of all of this, don't get me the wrong way, I completely agree with where you're coming from actually; but it's the reality of the HSC syllabus without higher level math unfortunately.
yep i agree with the end half, however, overall its a recipe for disaster since Standard Mathematics students are not aware of the cosine and sine function graphs as it is, so honestly, how simple can we really make Mod 6 to cater for this. but definitely i agree with everything you said, it's overall very problematic hahaI take your points about taking much of the mathematical theory of electromagnetism for granted in the HSC Physics syllabus. However you can teach everything required by the syllabus in transformers using well-behaved continuous functions like sine or cosine. There is no need to introduce transient step-functions and other discontinuous functions that complicate the mathematics, in my opinion. The syllabus doesn't require it, and it should not be examinable at high school level. The other problem with erroneous marking schemes like the one above is that less experienced Science teachers will be misled into believing that's how self-induction actually works, which is very dangerous. That's my 2-cents worth.