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Question 18M.3.SL.TZ2.2b.ii

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Date May 2018 Marks available [Maximum mark: 4] Reference code 18M.3.SL.TZ2.2b.ii
Level SL Paper 3 Time zone TZ2
Command term Determine Question number b.ii Adapted from N/A
b.ii.
[Maximum mark: 4]
18M.3.SL.TZ2.2b.ii

A student carries out an experiment to determine the variation of intensity of the light with distance from a point light source. The light source is at the centre of a transparent spherical cover of radius C. The student measures the distance x from the surface of the cover to a sensor that measures the intensity I of the light.

M18/4/PHYSI/SP3/ENG/TZ2/02

The light source emits radiation with a constant power P and all of this radiation is transmitted through the cover. The relationship between I and x is given by

I = P 4 π ( C + x ) 2

The student obtains a set of data and uses this to plot a graph of the variation of  1 I  with x.

(b.ii)

Determine P, to the correct number of significant figures including its unit.

[4]

Markscheme

ALTERNATIVE 1

Evidence of finding gradient using two points on the line at least 10 cm apart

Gradient found in range: 115–135 or 1.15–1.35

Using P =  4 π K 2  to get value between 6.9 × 10–4 and 9.5 × 10–4 «W» and POT correct

Correct unit, W and answer to 1, 2 or 3 significant figures

 

ALTERNATIVE 2

Finds I ( 1 y 2 ) from use of one point (x and y) on the line with x > 6 cm and C from(b)(i)to use in I =  P 4 π ( C + x ) 2  or  1 I = Kx + KC

Correct re-arrangementto get P between 6.9 × 10–4 and 9.5 × 10–4 «W» and POT correct

Correct unit, W and answer to 1, 2 or 3 significant figures

 

Award [3 max] for an answer between 6.9 W and 9.5 W (POT penalized in 3rd marking point)

Alternative 2 is worth [3 max]

[4 marks]

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