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Measurements II - Physics Form 2 Coursework e-Content CDs

Background

In Measurements I, you may have learnt about SI units of measurement and performed experiments on length, area, volume, mass, time and density. In this topic our focus will be on estimating the thickness of an oil molecule, using the concept of volume that was learnt earlier.



Introduction

You will recall that matter is made up of tiny particles which are in constant motion. These tiny particles are called atoms. Groups of atoms joined together are called molecules. Like other substances, oil is made up of molecules. In this sub-topic, we shall learn how to estimate the size or thickness of an oil molecule.

Specific Objectives

By the end of this sub-topic, the learner should be able to:

    • Experimentally estimate the size of an oil molecule using oil film experiment
    • Determine the size of an oil molecule from a given set of data

     

Estimating the size of an oil molecule

1. Obtaining a small drop of oil and estimating its diameter
The following animation shows how to obtain a small oil drop using a thin wire.

The diameter of the oil drop can then be estimated by holding the drop against a millimetre scale. Play the video and observe how this is done.


2. Estimating the diameter of oil drop
Click on NEXT to measure the diameter of the oil drop against a millimetre scale. What is the diameter of the oil drop as read from the millimetre scale?

VIDEO showing how to estimate diameter of drop against a mm scale

Fig. 2.2: Estimating the diameter of oil drop

 

2. Obtaining oil film and estimating its diameter

A thin layer of lycopodium powder is sprinkled uniformly on the surface of water held in wide tray. The drop of oil is then introduced gently onto the surface and allowed to spread, pushing the lycopodium powder radially outwards to form a circular patch. Play the video clip and note the procedure.

What have you observed? Compare the result with the following animation.

 

A drop placed at the centre of the water surface spreads out into a large circular patch or film.


4. Estimating diameter of oil film

Click on NEXT to observe the process of estimating the diameter of the oil film. The waxed brooms are moved slowly till they are at the right and left edge of the patch.


VIDEO SHOWING THE PROCESS OF ESTIMATING THE DIAMETER OF OIL FILM

3. Calculating the thickness of oil film (This equals the diameter of a molecule)

Using the results obtained, we can determine the thickness of oil film as follows.

The drop is spherical; so its

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