How do I do this question?
Thank you!
(Image removed from quote.)
Right, so this one is relying on the mathematical version of Faraday's Law:
Now to my knowledge,
this isn't directly examinable either (lots of weird questions tonight, or maybe I've just forgotten the whole syllabus
) - But not too difficult if you have learned that formula!
As a side note, this formula doesn't apply exactly this way exactly each time - But since it is a basic coil situation I'm fairly sure we just do this, and I see no other way anyhow! 
So the change in time is 0.2 seconds - What's the change in magnetic flux? Well we relate flux to flux density (field strength) and area with the formula \(\phi=BA\).
Initially, the area of the circular loop is is \(A_i=\pi r^2=0.04\pi\) (using SI units). The final area will be half of this value, \(0.02\pi\), so the change in area is \(0.02\pi\). So \(\Delta\phi=BA=2\times0.02\pi=0.04\pi\).
We use the formula to obtain:
I'm
loosely confident this is the approach - Does it match any solutions you might have to this question?
Irrespective of this, the graph we can do. Remember that \(A=\pi r^2\), so decreasing radius at a constant rate will actually result in an accelerating change to area! Think of it this way, we know the area at 20cm is \(0.04\pi\). At 15cm, it will be \(0.0225\pi\). At 10cm, it will be \(0.01\pi\). The decrease in radius from 20cm to 15cm has a slightly larger effect on area than the decrease from 15cm to 10cm. Hence, the rate of decrease of area is
highest initially, slowly decreasing.
Thus, the graph we sketch should have a peak right at the start, then a slow decrease, as the rate of change of area slowly decreases (and thus, so will the induced emf)
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