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Date May 2018 Marks available 2 Reference code 18M.3.SL.TZ2.2
Level Standard level Paper Paper 3 Time zone 2
Command term Explain Question number 2 Adapted from N/A

Question

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.

This relationship can also be written as follows.

1 I = K x + K C

Show that  K = 2 π P .

[1]
a.

Estimate C.

[2]
b.i.

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

[4]
b.ii.

Explain the disadvantage that a graph of I versus 1 x 2  has for the analysis in (b)(i) and (b)(ii).

[2]
c.

Markscheme

combines the two equations to obtain result

 

«for example  1 I K2(Cx)2 4 π P (C + x)2»

OR

reverse engineered solution – substitute K 2 π P  into 1 I = K2(Cx)2 to get I P 4 π ( C + x ) 2

 

There are many ways to answer the question, look for a combination of two equations to obtain the third one

[1 mark]

a.

extrapolating line to cross x-axis / use of x-intercept

OR

Use C y  - intercept gradient

OR

use of gradient and one point, correctly substituted in one of the formulae

 

accept answers between 3.0 and 4.5 «cm»

 

Award [1 max] for negative answers

[2 marks]

b.i.

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]

b.ii.

this graph will be a curve / not be a straight line

 

more difficult to determine value of K

OR

more difficult to determine value of C

OR

suitable mathematical argument

 

OWTTE

[2 marks]

c.

Examiners report

[N/A]
a.
[N/A]
b.i.
[N/A]
b.ii.
[N/A]
c.

Syllabus sections

Core » Topic 1: Measurements and uncertainties » 1.1 – Measurements in physics
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