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Date November 2018 Marks available 2 Reference code 18N.3.SL.TZ0.2
Level Standard level Paper Paper 3 Time zone 0 - no time zone
Command term Suggest Question number 2 Adapted from N/A

Question

In an experiment to measure the specific latent heat of vaporization of water Lv, a student uses an electric heater to boil water. A mass m of water vaporizes during time t. Lv may be calculated using the relation

L v = V I t m

where V is the voltage applied to the heater and I the current through it.

Outline why, during the experiment, V and I should be kept constant.

[1]
a.

Outline whether the value of Lv calculated in this experiment is expected to be larger or smaller than the actual value.

[2]
b.

A student suggests that to get a more accurate value of Lv the experiment should be performed twice using different heating rates. With voltage and current V1, I1 the mass of water that vaporized in time t is m1. With voltage and current V2I2 the mass of water that vaporized in time t is m2. The student now uses the expression

 

L v = ( V 1 I 1 V 2 I 2 ) t m 1 m 2

 

to calculate Lv. Suggest, by reference to heat losses, why this is an improvement.

[2]
c.

Markscheme

to provide a constant heating rate / power

OR

to have m proportional to t

a.

due to heat losses «VIt is larger than heat into liquid» ✔

Lv calculated will be larger ✔

b.

heat losses will be similar / the same for both experiments

OR

heat loss presents systematic error ✔

 

taking the difference cancels/eliminates the effect of these losses

OR

use a graph to eliminate the effect ✔

c.

Examiners report

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

Syllabus sections

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