iTute 2009 question

[Milkshake]

Can someone explain to me how to do question 11 in the Electric Power section? Just not sure how to differentiate between options B and D.

The exam can be found here: http://www.itute.com/2009/07/05/free-download-2009-physics-trial-examination-2/

[Lasercookie]

So we have a transformer. Primary has 200 coils, secondary has 50 coils. The changing EMF in the primary will induce a EMF in the secondary. What does this mean? It means this equation is relevant:



So the larger the gradient/slope, the larger the rate of change of flux (the changing voltage causes a change in flux - if the voltage is D.C. there is no transformer action as you would know). We know the induced EMF is proportional to the rate of change in flux (i.e. the gradient), hence why the sizes of the lines are different.

edit: forgot a key word

[Milkshake]

Yeh, but after that, how do you find out whether the voltage starts as a positive or a negative voltage?

It should be D then, according to that equation shouldn't it?

[Lasercookie]

Yeh, but after that, how do you find out whether the voltage starts as a positive or a negative voltage?

It should be D then, according to that equation shouldn't it?

No, it wouldn't be D.

According to the equation, the graph would be the derivative of the original graph.

Look at the original graph.

You get a small +ve gradient at the start. Then you get a large -ve gradient. Then you get large +ve gradient (same size, different direction). Then a large -ve gradient again. Then you get a small +ve gradient.

D implies the size of the gradients are constant throughout. This clearly isn't the case with the original graph. You can eliminate D simply on this basis.

The different sizes of the gradients have to be taken into account.

Technically, the positive and negative sides of the graph aren't explicitly stated. This means you could consider any side of the graph to be positive or negative. This is one of the tricks that iTute has employed with option B.

If the top half of the graph was defined to be positive, and the bottom half negative - then all the graphs would be incorrect. I would still say that B is the most correct in the situation that it was defined. D changes the direction of the voltage more often than it actually does - while B replicates the changes in direction well.

[Milkshake]

Ok I get it now .

And oh my god lol, major facepalm. I actually didn't realise that the horizontal lines for D were longer than the ones on B

Thanks for the help!

[Lasercookie]

Ok I get it now .

And oh my god lol, major facepalm. I actually didn't realise that the horizontal lines for D were longer than the ones on B

Thanks for the help!

Haha yeah, I think that's what I like about iTute exams. They put in all this random stuff to throw you off. If you fall for their distractions,  the real answer is so obvious to the point that you never forget making that mistake and keep an eye out for it in future exams.