Physics-1997-Set-III

Q1) Horizontal component and vertical component of Earth’s magnetic field at a place are equal. what is the value of angle of dip at this place? (Marks 1)
Ans1) BV = BH
tan = BV/BH = 1
or = 45o

Q5) Is the ratio of number of holes and the number of conduction electrons in a p-type extrinsic conductor more than, less than or equal to 1? (Marks 1)
Ans5) The ratio of number of holes to the number of conduction electrons in a p-type extrinsic conductor more than one as holes are the majority charge carriers.

Q9) What percentage of a given mass of a radioactive substance will be left undecayed after four half-life periods? Justify your answer. (Marks 2)
Ans9) N/No = (1/2)n, where n is no. of half-lives.
n = 4
… N = No(1/2)4
or N/No = (1/2)4 = 1/16
or N/No = 1/16 x 100% = 6.25%

Q12) Use mirror formula to show that for an object lying between F and 2F of a concave mirror, a real image is formed beyond 2F.
(Marks 2)

Ans12) According to mirror formula
1/f = 1/v + 1/u
For a concave mirror f is -ve , u is -ve
… -1/|f| = 1/v – 1/|u|
or 1/v = 1/|u| – 1/|f| – (i)
object lies between F and 2F
So |f| < |u| < |2f|
or 1/|f| > 1/|u| > 1/|2f| – (ii)
or 0 > 1/|u| – 1/|f| > -1/|2f|
using Equation (i)
0 > 1/v > -1/|2f|
1/v < 0 or v < 0 i.e. v is -ve, hence a real image is formed.
Also 1/v > -1/|2f|
or v < -|2f| or |v| > |2f| i.e. image is formed beyond 2F.

Q13) An astronomical telescope of magnifying power 7 consists of two thin lenses 40 cm apart. Calculate the focal length of the lenses. (Marks 2)
Ans13) 7 = fo/fe or fo = 7fe
Also L = fo + fe
40 = 7fe + fe = 8fe
or fe = 5 cm
& fo = 7fe = 35 cm.

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