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Date November 2020 Marks available 1 Reference code 20N.3.SL.TZ0.1
Level Standard level Paper Paper 3 Time zone 0 - no time zone
Command term Identify Question number 1 Adapted from N/A

Question

A spherical soap bubble is made of a thin film of soapy water. The bubble has an internal air pressure Pi and is formed in air of constant pressure Po. The theoretical prediction for the variation of Pi-Po is given by the equation

(Pi-Po)=4gR

where γ is a constant for the thin film and R is the radius of the bubble.

Data for Pi-Po and R  were collected under controlled conditions and plotted as a graph showing the variation of Pi-Po with 1R.

Suggest whether the data are consistent with the theoretical prediction.

[2]
a.

Show that the value of γis about 0.03.

[2]
b(i).

Identify the fundamental units of γ.

[1]
b(ii).

In order to find the uncertainty for γ, a maximum gradient line would be drawn. On the graph, sketch the maximum gradient line for the data.

[1]
b(iii).

The percentage uncertainty for γ is 15%. State γ, with its absolute uncertainty.

[2]
b(iv).

The expected value of γ is 0.027. Comment on your result.

[1]
b(v).

Markscheme

«theory suggests» Pi-Po is proportional to 1R 


graph/line of best fit is straight/linear «so yes»
OR
graph/line of best fit passes through the origin «so yes»

 

MP1: Accept ‘linear’

MP2 do not award if there is any contradiction
eg: graph not proportional, does not pass through origin.

a.

gradient = «4γ» =0.10
OR
use of equation with coordinates of a point 


γ=0.025 

 

MP1 allow gradients in range 0.098 to 0.102

MP2 allow a range 0.024 to 0.026 for γ

 

b(i).

kgs-2

 

Accept kgs2

 

b(ii).

straight line, gradient greater than line of best fit, and within the error bars

b(iii).

«15% of 0.025» = 0.00375
OR
«15% of 0.030» = 0.0045

rounds uncertainty to 1sf
±0.004
OR
±0.005

 

Allow ECF from (b)(i)
Award
[2] marks for a bald correct answer

b(iv).

Experimental value matches this/correct, as expected value within the range
OR
experimental value does not match/incorrect, as it is not within range

b(v).

Examiners report

Many students obtained full marks here although a significant number did not acknowledge that the graph was through the origin and lost a mark.

a.

Very well answered either by obtaining the gradient or replacing with the coordinates of a point.

b(i).

Although the question was specifically about the fundamental units, several candidates lost the mark by answering Pa m.

b(ii).

Almost all candidates were able to draw the correct maximum gradient line.

b(iii).

Well answered. A significant number did not round the uncertainty to match the value of gamma.

b(iv).
[N/A]
b(v).

Syllabus sections

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