VCAA 2010 exam 2 Question

[liamh96]

Hi all,

http://www.vcaa.vic.edu.au/Documents/exams/physics/2010physics2-w.pdf
Regarding question 3 (Electric power) from the exam above. How would one know the direction of the magnetic field in the solenoid? VCAA merely stated "Using the right-hand rule, the force on side PQ was vertically down. " I am leaning towards Lenz's law, but still not sure.

Thanks in advance,
Liam

[speedy]

Hi all,

http://www.vcaa.vic.edu.au/Documents/exams/physics/2010physics2-w.pdf
Regarding question 3 (Electric power) from the exam above. How would one know the direction of the magnetic field in the solenoid? VCAA merely stated "Using the right-hand rule, the force on side PQ was vertically down. " I am leaning towards Lenz's law, but still not sure.

Thanks in advance,
Liam

Not lenz's law, there's no changing flux. It's a two-stepped application of both the right hand grip rule and right hand slap rule.

For a solenoid, fingers are current (and we curl them around) and thumb is field. So to find the direction of field within the solenoid, and thus passing through the loop, use right hand grip rule, curl fingers around the solenoid, thus thumb points to the left. You know you have to curl your fingers over, because of the dotted line on the diagram - look at the left hand side of the solenoid, the dotted portion clearly indicates that the current is flowing away, ie. flowing out that end. On the right hand side the line is solid, thus flowing in that end.

So know that we know the direction of the field, we can simply apply the slap rule to determine the force to be down. 

[liamh96]

Thanks for the explanation. I didn't realize that the full solenoid was given in figure 1, which states the direction of the current.  I was only looking at figure two which does not clearly show the direction of the current in the solenoid :/