How to find the direction of an induced current when the direction of movement is unknown? (1 Viewer)

[floppyspud]

In this question of the 2017 HSC:



I'm unsure how to find both points' polarity with the direction of movement not given, nor do I know how to derive the direction of movement from the rest of the question. Googling it isn't any help, as it only comes up with examples of a given movement. Any help is appreciated, and I'm sorry if I missed something easy.

Spoiler: Answer

P is positive, Q is negative

The answer doesn't give any reasoning other than "consistent with Lenz's law."

 

[liamkk112]

In this question of the 2017 HSC:

View attachment 43050

I'm unsure how to find both points' polarity with the direction of movement not given, nor do I know how to derive the direction of movement from the rest of the question. Googling it isn't any help, as it only comes up with examples of a given movement. Any help is appreciated, and I'm sorry if I missed something easy.

Spoiler: Answer

P is positive, Q is negative

The answer doesn't give any reasoning other than "consistent with Lenz's law."

during the period of max flux change (which by faraday's law produces maximum voltage / induced emf) the flux through the loop decreases right? so this means that the magnetic field into the page is decreasing in strength. so to counteract this, you want the current in the loop to generate a magnetic field into the page, so that the flux change is opposed. doing right hand rule here tells us that the current should thus flow from Q to P (clockwise from this perspective). taking conventional current (as is expected in HSC questions) this means that a bunch of the positive charges will end up flowing from Q and ending up at P, making P positive and Q negative

i think you're confused that a loop always has to be moving for an emf to be generated, this is not the only time emf has to be generated. a change in magnetic field strength will also cause an emf to be generated. this is because magnetic field strength is really "magnetic flux density" aka how many fluix lines are passing through a unit area. so if magnetic field strength changes, then the number of flux lines through the loop will change, meaning the magnetic flux changes which by faraday/lenz law means that emf has to be generated

 

[wizzkids]

Where does it say that the loop is moving? The loop isn't moving. You need to practice reading exam questions carefully and answering exactly the question you are given, not the question that you have memorised.