This research investigates the influence of computer simulations (virtual experiments) on one hand and real experiments on the other hand on the conceptual understanding of electrical charging. The investigated sample consists of students in the second year (10 th grade) of three gymnasiums in Macedonia. There were two experimental groups and one control group. In one of the experimental groups, called Sim group, the instruction was realized by means of computer simulations. In the other experimental group, called Real group, the instruction was realized by means of real experiments. The difference between pre-test results and post-test results revel that the approaches used in experimental groups give more quality knowledge and skills than the one in the control group. The results in the Real group and Sim group are very similar. There are slight differences caused by the different features of the two approaches. Some differences occur based on the feature difference between real experiments and computer simulations.

Equal Channel Angular Pressing (ECAP) is a very interesting method for modifying the microstructure in producing materials with ultrafine grain in sub-micrometer or nanometre range. Experiments demonstrate that these ultrafine structures may exhibit, by comparison with large -grained polycrystals, major differences in some fundamental properties. A nanostructured AA 3004 alloy was used to investigate the evolution of microhardness and microstructure on the cross-sectional plane X, after processing by ECAP at room temperature for up to six passes. The measurements show the average microhardness increases significantly after two passes with additional increases in subsequent passes. Microhardness values increase by more than two times after 6 passes. There is a reduction in grain size from of ~ 50µm in the initial condition to ~ 12 µm after 6 passes. The results also show that the microstructure and the microhardness evolve with increasing strain so that, after a total of 6 passes, the structure is almost homogenous throughout the cross-sectional plane of the billet.Keywords: Al alloy; SPD; ECAP; micro-hardness; homogeneity IntroductionExtensive studies of equal-channel angular pressing (ECAP) and other severe severe plastic deformation (SPD) techniques, over the past two decades [1][2][3] have established the processing techniques and have shown the possibility of producing ultrafine grain (UFG) structure in many metals and alloys. Grain size reduction is one of the most attractive ways of improving the mechanical properties of metallic materials. It is known that the strength of all polycrystalline materials is related to the grain size, through the Hall-Petch equation which predict an increase in yield strength (σ y ) with a decrease in grain size (d): σ y = σ 0 + kd -1/2 [4,5]. Several different SPD procedures are now available but the most popular appears to be Equal Channel Angular Pressing (ECAP), that was first proposed in the 80 s, by Segal. In processing by ECAP the sample is pressed through a die containing two channels equal in cross-section, intersecting at an angle φ that is generally close to 90 o (Fig. 1). Since the sample emerges from