Short demonstrations are designed for users who have little time available. They present interactive extracts from the full demonstration modules.
Contents and Explanation Take this everyday phenomenon: to slide
a box across the floor, energy has to be expended to overcome the opposing
frictional force between their surfaces. Intuitively we assume that this
frictional force results from the interaction between rough surfaces.
So we would expect friction to be absent from smooth surfaces on the atomic
scale. However, friction does exist even for such smooth surfaces; this
is explained by the Tomlinson mechanism.
In 1929, the British scientist G. Tomlinson put forward an explanation for atomic-scale friction: as the box in our example is pushed across the floor, there is a mutual “plucking” action between the atoms on both surfaces. But the atoms spring back to the resting state if the maximum tension is exceeded, and then oscillate around their initial position. At the same time, their kinetic energy is dissipated as heat.
The demonstration translates Tomlinson’s explanation into a visual model with clear, easy-to-follow sequences: a ball attached to a spring is dragged across a hill and valley structure. The ball and spring together represent the atoms at the surface of the box, the spring represents chemical bonding, and the hill and valley structure is the floor.
The model demonstrates that the movement of the ball cannot be continuous: at a certain point the ball jumps to another position and vibrates. In the same way, the actual atom jumps back to its initial position and vibrates; this vibration manifests itself at the atomic level as heat.
Handling the Application As shown in the position chart (Fig. 1, left) the position of the ball support may be varied by moving the bar to which the spring is attached.
The energy level chart (Fig. 2, middle) shows the total energy of the system for different spring constants; this is not an interactive model.
In the field chart (Fig. 3, right) the spring constant can be changed by rotating the blue line.