Dynamics of contact electrification

Although the electrical charging of objects brought into contact has been observed for at least 2000 years, the details of the underlying mechanism are still not yet fully understood. The present paper deals with the very basic process of contact electrification between two metals. We have developed an experimental method to follow the charge of a small sphere bouncing on a grounded planar electrode on a time scale down to 1 μs. It reveals that the sphere is discharged in the moment of contact, which lasts about 6 to 8 μs. However, at the very moment of disruption of the electrical contact, it regains charge far beyond the expectation according to the contact potential difference. The excess charge rises with increasing contact area.

EXPRESS: Short-term upper limb immobilization impairs grasp representation

The present experiment aimed to gain more information on the effect of limb nonuse on the cognitive level of actions and, more specifically, on the content of the motor program used for grasping an object. For that purpose, we used a hand-grasping laterality task that is known to contain concrete information on manipulation activity. Two groups participated in the experiment: an immobilized group, including participants whose right hand and arm were fixed with a rigid splint and an immobilization vest for 24 hours, and a control group, including participants who did not undergo the immobilization procedure. The main results confirmed a slowdown of sensorimotor processes, which is highlighted in the literature, with slower response times when the participants identified the laterality of hand images that corresponded to the immobilized hand. Importantly, the grip-precision effect, highlighted by slower response times for hands grasping a small sphere versus a large sphere, is impaired by 24 hours of limb nonuse. Overall, this study provided additional evidence of the disengagement of sensorimotor processes due to a short period of limb immobilization.

The Reactive and Radiation Electromagnetic Energies of Antennas: A New Formulation

<p>It is required to calculate the stored reactive energy of an antenna in order to evaluate its Q factor. Although it has been investigated for a long time, some issues still need further clarification. The main difficulty involved is that the reactive energy of an antenna tends to become infinitely large when integrating the conventionally defined energy density in the whole space outside a small sphere containing the antenna. The reactive energy is usually made to be bounded by subtracting an additional term associated with the radiation fields. However, there exists no well-accepted accurate definition for this additional term that is valid for all cases. By re-checking the definition of reactive energies, a new formulation is proposed in this paper which can separate the reactive energy and the radiation energy explicitly based on source-potentials. The clearly defined reactive energy is bounded without necessity to subtract that additional term, and the resultant formulae are easy to implement.</p>

Plastic Flow Verification in a Tool Cavity for Production of Test Sample for Wettability Solders Measurement

The paper describes the structural design, simulation and proper manufacture of a tool for manufacturing of test samples in spherical form. This shape is essential for an innovative assessment of solder wettability, since the small sphere by its ideal shape represents the total non-wettability of substrate surface. In order to verify the design of the shaping tool, a simulation software, type Deform was used, which allowed the FEM analysis of plastic flow in the tool cavity. Just after successful computer simulation, a shaping tool was designed and then manufactured in laboratory conditions, thus the fabrication of test samples in spherical form was successfully tested.