VCAA 2009 physics querie

[rk]

VCAA 2009
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Section 1: Magnets How do they work?

Question 3 - I got the right working out until the last step, then I divided their final answer by 2. I did that because they asked for the magnitude of the AVERAGE voltage and all the data provided and used is for the max magnetic flux not the average. Why was this step wrong?

Is question 4 a lenz law question or is it that just in general with the drawing of field lines they go 'in' the direction of the actual current then the field lines polarity that is 'out' goes around and does like a backflip...Ihave no idea if what I'm saying makes any sense.

[CharlieW]

yeah don't divide ur final answer by 2

i haven't seen the question, coz im saving vcaa 09 for tommorrow.

but i presume its one of those induced voltage questions. in that case, u take the change in flux from quarter rotation. and thats ur average voltage even over full rotation, something with symmetry of voltage graph. im no expert though, ask appianway

[SixWinged]

The voltage you obtain from (delta flux)/(delta time) is average in the same sense that average velocity is determined from (delta distance)/(delta time).

[coletrain]

with the question 4 did u use RHG rule for solenoids?

[schnappy]

Question 4 is simply an application of the RHG rule for solenoids. Grab your your hand, make a fist with your thumb pointing up. Your thumb is pointing at the north pole of the solenoid, and your fingers are the direction of conventional current around the permanent magnet. Note that current flows from +ve to -ve of the cell/battery, in this question it's going 'down' on the left side of the diagram, then 'up' the coil.

You now know that they top of the magnet is north, now draw in mag field lines like a perm bar magnet, indication the direction (from north to south).

[Blakhitman]

how do you do this exactly:

"your fingers are the direction of conventional current around the permanent magnet"?

That seems strange I thought that fingers were magnetic field and these are usually not the direction of the conventional current?

Follow the wire from the positive and around the solenoid and you wrap your fingers around the solenoid in the direction which the current is travelling.