I would like to apologize for my lack of preparation during tutorial just now, and I hope I did not confuse you when trying to explain question 6. Here is an attempt to be clearer:
There are two methods of doing question 6, and in fact, these same two methods apply for all questions of energy transfer. The first method involves substitution, and the second method involves taking ratios.
Substitution Method:
Let Ir = k(8x)2
Because of the inverse square law, ¼ Ir = I2r [Equation 2]
Let I2r = k(A)2
Substitute [Equation 1] and [Equation 3] into [Equation 2]: ¼ k(8x)2
Solve for A and you should get 4x.
[Addendum 17/7: There was previously a typo. Please check if you got the correct answer.]
Ratio Method:
Ir/Ir2 = (Ar/Ar2)2
But also, Ir/Ir2 = (r2/r1)2 [By inverse square law]
So therefore, (Ar/A2r)2 = (r2/r1)2
Taking square root, Ar/A2r = r2/r1
Since r2/r1 = 2
Ar/A2r = 2
Ar = 8x, therefore A2r = 4x
Which method is better?
Both methods are acceptable, and you should familiarize yourself with both. In an exam situation, stick with the method you're more comfortable with.
The substitution method is a more "direct" method and therefore some of you might find it easier. The downside of the substitution method is that you keep on introducing new variables (in this case, k and A) into your working, which might potentially confuse you. The ratio method does not introduce as much new variables in your working, but one potential danger is forgetting to invert your ratios when things are inversely proportional.
Announcement
For next tutorial (21st July Monday) I am likely to be absent because I need to report for my service to the country. I will give you all more instructions when I have more information. In the meanwhile, prepare notes for Section 4 (Polarization) and Section 3.1 (CRO). Prepare questions from Bond & Hughes ("challenging drill questions"):
Ch 10 Pg 11 Q 1,2,3
Ch 10 Pg 13 Q 1,2
[Addendum: I have removed Ch 10 Pg 12 Q1 because this question is no longer in the syllabus.]
Announcement
For next tutorial (21st July Monday) I am likely to be absent because I need to report for my service to the country. I will give you all more instructions when I have more information. In the meanwhile, prepare notes for Section 4 (Polarization) and Section 3.1 (CRO). Prepare questions from Bond & Hughes ("challenging drill questions"):
Ch 10 Pg 11 Q 1,2,3
Ch 10 Pg 13 Q 1,2
[Addendum: I have removed Ch 10 Pg 12 Q1 because this question is no longer in the syllabus.]
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1 comment:
it's okay yanndao.. we understand that u were trying to help us understand the qns better. :)
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