Daniel Powell

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Jun 05 2014

Diffusion in Solids, Liquids, Gases and Jelly

By Daniel Powell in Miscellaneous
June 5, 2014

Diffusion in Solids, Liquids, Gases and Jelly

Background information
Diffusion in liquids: When substances dissolve in liquids (like salt dissolving in water) the substances spread out. We call this spreading out of dissolved particles diffusion. The end result of this is that the solute particles that have dissolved in water will spread out evenly. This movement or spreading out is due to the fact that in liquids the particles are moving randomly.

Diffusion in gases: When two or more different gases, like oxygen and nitrogen are mixed they will mix themselves evenly. We call this mixing of particles diffusion. This is due to the random movement of particles.

Diffusion in solids: Diffusion does not happen in solids because the particles are not free to move around and so they cannot inter-mix.

Diffusion in jelly: Jelly is a liquid before it has set and looks like a solid when it has set. However the truth is a little more interesting. After it has set jelly is not really a solid or a liquid, it is in fact a mixture of both of them. As shown in the diagram below there are long fibres of protein or carbohydrate which form the solid part of the jelly and between these fibres there are spaces where water molecules are free to move around. This is why substances can diffuse through jelly.

Using Jelly in science experiments… Because it allows diffusion through it, jelly is very useful as it allows us to track the movement substances through the jelly for example in Bioassay experiments testing the effectiveness of antibiotics as shown in the photograph below. In this photo it is easy to see which antibiotic is the best at killing bacteria (the biggest clear area). The antibiotic has diffused through the jelly.

The rate of Diffusion is affected by a number of factors:

Temperature
Concentration
The surface area of the exchange surface
The size of the particles

Experiment to determine how temperature affects the rate of diffusion through jelly

Materials

Cork-borer
Petri dishes
Agar jelly with Universal indicator mixed into it
Hydrochloric acid of the following concentrations: 1.0M, 0.8M, 0.6M, 0.4M 0.2M.
Stop clock
Ruler with mm graduation

Procedure

1) Pour equal volumes of the hot Agar/Indicator mix into a different petri dish, leave to cool and set overnight.

2) Use the cork borer to cut 5 wells into the jelly making sure all the jelly is removed from the well.

3) Use a pipette to carefully fill the first well with 1.0 M Hydrochloric acid and start stop-clock.

4) As the acid diffuses through the agar the indicator will turn red. After 5 minutes use the ruler to measure how far the red colour has moved.

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Permanent link to this article: https://animatedscience.co.uk/2014/diffusion-in-solids-liquids-gases-and-jelly

May 29 2014

Cheese Rolling – Gravitational Potential Energy to Kinetic

By Daniel Powell in GCSE Physics, Miscellaneous
May 29, 2014

Cheese Rolling – Gravitational Potential Energy to Kinetic

Funny really as I always think of this as a simple topic. However, my students always find it hard, especially the formulae.

First place to start is the hill near to the village (Brockworth) where I grew up where they still do Cheese Rolling every year… http://www.cheese-rolling.co.uk/index1.htm. Even my primary school teacher wrote a book on the topic. (However, it is not focused on the Physics!)

So now you have the idea think about a man who lifts a cheese and himself up to the top of a hill. His muscles have to do work as he is moving himself and a cheese to a point further away from the surface of the Earth. This is because the man and cheese are in the influence of a “gravitational field” which causes anything with mass to feel weight or acceleration towards the centre of the object.

So the formulae we employ to work out the work done in climbing the hill is the change in height x distance moved against the field x mass. #

We often write this as…

E_p = mgh or sometimes as mgΔh to show “a change in height”.

So where has the energy come from…. well simple the muscles in the body of the man have contracted and converted chemical energy to movement energy to push the man away from the field.

So what happens to the energy as you release the cheese? Well we think of another idea of “kinetic energy”. As you roll down the hill and gain in velocity you exchange your gained E_p to E_k so then most of the energy is coverted according to the rule ½ mv² .

Often we write that mgΔh = ½ mv²so if it was a 100% transfer we could work out the maximum velocity of a cheese falling down the hill!

Try out these animations in flash to help you out…

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Permanent link to this article: https://animatedscience.co.uk/2014/cheese-rolling-gravitational-potential-energy-to-kinetic

May 29 2014

New Scientist Videos?

By Daniel Powell in Miscellaneous
May 29, 2014

How We Became Human?

Here is a video from MacGregor Campbell who is a Portland-based science/tech journalist, animator, and maker of weird sounds. He has made several video’s on Science and asked me to share with everyone on Animated Science so here one is…