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Description
The programme looks at (l) the states of matter and the forces which bind atoms together (2) Distribution of molecular velocities in a gas and a method for measuring this.
| 13 | 00:00:51,000 | 00:00:56,000 | ..set up an analogy using our old friend, the air table. |
| 10 | 00:00:52,000 | 00:01:00,000 | Now I’ll speed up the tray. I’m increasing the kinetic energy of the simulated atoms. |
| 9 | 00:00:54,000 | 00:00:57,000 | But how should I add more bearings? |
| 10 | 00:00:54,000 | 00:00:58,000 | To escape from its cell it must either wait until there is a gap in the walls, |
| 11 | 00:00:59,000 | 00:01:04,000 | or until it has enough energy to push its neighbours apart and escape to an adjacent cell. |
| 14 | 00:01:01,000 | 00:01:05,000 | (Gerald Elliott) You see the motion of this white puck. |
| 11 | 00:01:01,000 | 00:01:05,000 | Apart from a small amount of evaporation at the edges, the material remains solid. |
| 10 | 00:01:02,000 | 00:01:06,000 | Should I perhaps take this one and flick it in gently? |
| 12 | 00:01:05,000 | 00:01:10,000 | Escape is comparatively rare, so diffusion in liquids is a slow process. |
| 15 | 00:01:06,000 | 00:01:10,000 | It’s moving in a random way around the field of view… |
| 12 | 00:01:06,000 | 00:01:09,000 | Watch what happens as I continue to give the bearings more kinetic energy. |
| 11 | 00:01:07,000 | 00:01:10,000 | Or should I really go to town on it? |
| 13 | 00:01:10,000 | 00:01:13,000 | In other words, as I raise the temperature. |
| 12 | 00:01:11,000 | 00:01:14,000 | Well that’s just it, it’s the speed with which this one comes in… |
| 13 | 00:01:15,000 | 00:01:19,000 | ..that determines whether the material is a solid, a liquid or a gas. |
| 17 | 00:01:17,000 | 00:01:22,000 | But if we change the illumination conditions we will see that the… |
| 14 | 00:01:19,000 | 00:01:24,000 | The regular crystal structure breaks down. We’ve evidently gone past the melting point. |
| 18 | 00:01:23,000 | 00:01:27,000 | ..motion of the white puck is in fact caused because round it there are… |
| 15 | 00:01:25,000 | 00:01:30,000 | I'll repeat the melting experiment but this time I’ve added a single white bearing… |
| 19 | 00:01:28,000 | 00:01:32,000 | ..lots of other black pucks and these are impacting with it in a random way… |
| 16 | 00:01:31,000 | 00:01:33,000 | ..to make it easier for you to see what’s happening. |
| 20 | 00:01:33,000 | 00:01:36,000 | ..and it’s the motion of the black pucks which is forcing… |
| 21 | 00:01:37,000 | 00:01:41,000 | ..the random motion of the white puck in the system. |
| 22 | 00:01:42,000 | 00:01:46,000 | This is, of course, a straight analogy with Brownian motion… |
| 17 | 00:01:44,000 | 00:01:48,000 | There’s clearly more space available for atomic movement. |
| 23 | 00:01:46,000 | 00:01:50,000 | ..as Brown saw it in his microscope. |
| 18 | 00:01:49,000 | 00:01:51,000 | Just how much more free space can best be seen by… |
| 24 | 00:01:51,000 | 00:01:54,000 | You’ll see that the particles are moving about at random… |
| 19 | 00:01:52,000 | 00:01:55,000 | ..comparing photographs of our simulated liquid and solid. |
| 25 | 00:01:55,000 | 00:01:58,000 | ..as they moved about at random in our analogue. |
| 26 | 00:01:59,000 | 00:02:03,000 | You’ll see that they are going in and out of focus in our microscope… |
| 27 | 00:02:03,000 | 00:02:06,000 | ..as they go up and down in the solution. |
| 28 | 00:02:08,000 | 00:02:12,000 | So we see that the effect of Brownian motion is caused by… |
| 29 | 00:02:13,000 | 00:02:18,000 | ..the impact of literally billions of billions of atoms and what we’re seeing here is… |
| 30 | 00:02:20,000 | 00:02:28,000 | ..the influence of the atomic motion of the molecules in our fluid, the gas or the liquid. |
| 16 | 01:01:11,000 | 00:01:16,000 | ..and there are apparently no forces to cause this motion. |
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