The paper presents the results of the study in physics learning during the pandemic in Croatian schools. The aim of the study was to identify students' views on physics lessons and problems with the online study process, as well as their knowledge. Data were collected by an online questionnaire in the school year 2020/2021 and knowledge tests for each level of pre-university education for two testing cycles, school year 2020/2021 and 2021/2022. The results of the questionnaire show that, from the students' point of view, teaching during the pandemic was as detailed as in regular classes, and experiments were conducted very rarely. The knowledge test results show the difference in the level of physics knowledge in the pre-pandemic period, during online and hybrid classes.

Formal, non-formal, and in-formal learning is very intertwined for younger learners in primary school. Therefore, it is worth exploring how to benefit from these three formats of learning in an organized and efficient way. The paper discusses what is understood by formal, non-formal, and informal learning and what their characteristics are. As the article focuses on primary school, we also discuss the meaning of this name, as “primary school” often has different meanings in different countries. Finally, we briefly discuss how pre-service and in-service teachers should be prepared so that they can benefit from what these three different types of learning offer and thus help to better support students.

Physics teachers’ training and professional development play a crucial role in preparing schools for the countless challenges they face. One of the central issues in the preparation of teachers is to provide them with tools, strategies, and methods to be able to build bridges between school and society, particularly to find fruitful links between formal, non-formal, and informal teaching, starting from primary school. In this paper, we report on some discussions developed during the Malta 2020 webinar by the Work Group 4 (WG4) participants, together with some questions and answers that emerged.

In Croatia, as in Slovenia, all schools were closed on 16 March 2020 due to the SARS-CoV-2 pandemic (COVID 19). This has changed the way of teaching and thus has an impact on physics teaching, which encourages experimentation. The paper presents the results of the study on physics teaching during the pandemic in Croatia and Slovenia at the pre-university level. The data were collected using an e-questionnaire. The results show how physics was taught by 332 teachers during the pandemic and highlight the problems they faced during online teaching, focusing on experimental work. It is evident that teachers from both countries tried their best to deliver physics lessons through direct teaching as well. Moreover, they tried to include the experiments and they were very flexible about the students’ obligations and much of what they learned during this time will be implemented in regular classes.

The paper presents the analysis of static and dynamic properties of achiral polar smectic doped with chiral dopant. In this system the two phases with equal symmetries exist, the SmAP*(A) and the SmCAP*(A) phase, which continuously transform one to the other upon temperature changes. The analysis of dynamical properties allowed for the development of the criterion which determines the temperature ranges of the two phases. Besides, the analysis of collective modes revealed that they are either amplitude or phase. However, in many of them the character changes from polar to tilt or vice versa. The collective modes occur in pairs of opposite chiralities. The chiral character of modes explains also the gaps which occur between the amplitude modes. Copyright (C) 2021 EPLA

The phenomenological continuum model is used to study the director structure and dielectric response of the ferroelectric smectic-A phase in thin planar cells. The frequency of the phase mode and the dielectric permittivity are calculated numerically as a function of the bias external DC electric field, cell thickness, the strength of polar surface anchoring, and the ratio between the bend and splay elastic constants. The theoretically obtained dependencies are in agreement with the recently reported experimental measurements, which show that in thin planar cells both the phase mode frequency and the dielectric permittivity decrease with increasing external bias electric field.

Water transport in tall trees is an everyday phenomenon, seldom noticed and not completely understood even by scientists. As a topic of current research in plant physiology it has several advantages for presentation within school physics lectures: it is interdisciplinary and clearly shows the connection between physics and biology; the construction of an artificial tree is an ideal laboratory project, which enables detailed studies of several phenomena related to water transport in an artificial tree model; it also clearly shows the failures of widespread ideas about the origins of the upward water flow. We present the construction of the laboratory tree, suggest measurements that illustrate water transport and present a few additional experiments which clearly show why water transport in trees higher than 10 m is still an ongoing debate amongst plant physiologists.

Free energy of thin smectic films in an antiferroelectric system is analyzed within the discrete phenomenological model. The effect of chirality within the region of uniplanar structures and on the structure is studied. As a result the regions with less tilted surface layers widen and uniplanar structures transform into wound structures having less tilted inner layers.

A mechanical model is a simple device made out of helical springs. Using the mechanical model one can visualize the difference between the first and the second order phase transitions. The analogy between the mechanical model and phase transitions in liquid crystals will be discussed. The parameters of the simple device (length, frequency, amplitude, tilt, etc.) can serve as an analogy for the parameters used in theories of phase transitions. The simple mechanical model is also convenient for the presentation and visualization of the dynamics near SmA-SmC phase transition in liquid crystals.

A discrete model is used to study the dielectric response of chiral smectic liquid crystals under the influence of external harmonic electric field. In this model, the order parameter is the tilt parameter ξj which describes the magnitude and direction of the director in layer-j, where inclines at an angle to the smectic layer normal. The effect of quadrupolar interactions on dielectric loss of chiral smectic liquid crystal phases is calculated numerically. Analytical expressions for the dielectric loss in terms of eigen-frequencies of the phase and amplitude modes can be derived from the free energy expansion. Numerical results of frequency dependence of dielectric loss are also obtained.

We consider various bulk liquid crystal systems in the SmC alpha phase, each assumed to consist of N layers and having different helical pitch P. Using the phenomenological Landau free-energy model in discrete form, the total free energy of each system is calculated. In the free energy expansion we include only the usual Van der Waals intra-layer interaction, the nearest-neighbour and next-nearest-neighbour inter-layer interactions. In order to investigate the structural stability of the system under the application of an electric field, we simulate numerically evolution of the structure of each system from an initial state to an unwound state. The total energy of each system is calculated and compared. We investigate five LC structures with pitches over 3, 4, 5, 6, and 7-layers using the values of the zero-field equilibrium five-layers structure parameters. Plots of the free energy of these structures in dependence of electric field are illustrated. Based on these plots, stability of structures throughout the range of the applied electric fields is discussed.

The effect of a chiral dopant or chiral molecular properties on the polarly ordered smectic system made of achiral bent-core molecules is studied theoretically. Two phenomena are predicted: a) A non-tilted antiferroelectrically ordered system becomes tilted upon chiral doping. b) Magnitudes of the tilt and polarization differ in layers with opposite chiralities, if the synclinic antiferroelectric structure is stable. Both phenomena are induced by the chiral field of dopants or the chiral field of chiral bent-core molecules themselves. Therefore the name chiroclinic effect is suggested for the phenomena.

We use the discrete phenomenological model to study theoretically the phase diagrams in antiferroelectric liquid crystals (AFLCs) as a function of optical purity and temperature. High sensitivity of the phase sequence in the AFLCs to optical purity is attributed to the piezoelectric coupling which is reduced if optical purity is reduced. We limit our study to three topologically equal smetic (Sm)phases: Sm-C(*), Sm-C(*)(alpha), and Sm-C(*)(A) and show that the reduction of optical purity forces the system from the antiferroelectric to the ferroelectric phase with a possible Sm- C(*)(alpha) between them. The effect of the flexoelectric and the quadratic coupling is considered as well. If the phase diagram includes only two phases, Sm-C* and Sm- C*(A), the flexoelectric coupling is very small. The materials which exhibit the Sm-C(*)(alpha) in a certain range of optical purity and temperature, can be expected to have a significant flexoelectric coupling that is comparable with the piezoelectric coupling. Upon lowering the temperature the phase sequence Sm-A-->Sm-C(*)(alpha)-->Sm-C*-->Sm-C(*)(A) is possible in systems where quadratic coupling is very strong.

Two different types of isostructural transitions between two homogeneously tilted ferroelectric liquid crystalline structures are discussed. A strong biquadratic coupling between the tilt and the polarization leads to the discontinuous jump of the tilt magnitude when the electric field is increased. A strong anisotropic quadratic coupling with the electric field leads to the continuous structural transition associated with a change of the direction of the tilt.

We consider a discrete phenomenological model description of tilted smectic phases, in which the simple Landau free energy expression includes only the usual van der Waals interaction, nearest-neighbour interaction (J(1)), next nearest-neighbour interaction (J(2)) and the chiral term (Y). We study the effect of the chirality on the angles between the adjacent layers by minimizing the free energy and assuming the constant amplitude approximation. The effects of the chirality are investigated for three different situations: J(1) > J(2), J(1) < J(2) and J(1) = J(2). The results obtained are illustrated and discussed.

A theoretical possibility of the re-entrancy of the antiferroelectric properties in antiferroelectric liquid crystals under the influence of the high external electric field is discussed.

The behaviour of polar, broken-layer-type columnar phases made of bent molecules (B(1Rev) and B(1RevTilted)) was studied under an applied electric field. There are two competing mechanisms of ferroelectric switching in the polar B(1RevTilted) columnar phase: collective rotation around the long molecular axis and collective rotation around the tilt cone. The proposed model shows that the main factor discriminating the type of switching is the width of the column cross-section.

The position of the bent-shaped molecule when it is found in a chiral structure is discussed. Although the bent-shaped molecule is achiral by itself, its position and orientation can be recognized as structurally chiral. It is shown that due to the special shape and structural chirality, chiral structures of initial system become more enhanced.

Critical freezing of molecular rotation in an achiral smectic phase, which leads to polar ordering through the second order paraelectric-antiferroelectric (Sm-A-->Sm-APA) phase transition is studied theoretically and experimentally. Strong softening of the polar mode in the Sm-A phase and highly intensive dielectric mode in the Sm-APA phase are observed due to weak antiferroelectric interactions in the system. In the Sm-APA phase the dielectric response behaves critically upon biasing by a dc electric field. Such a behavior is found general for the antiferroelectric smectic phase with significant quadrupolar interlayer coupling.

ABSTRACT When an external electric field is applied to short pitch polar smectics, their helical structures distort and unwind. The final transition to completely unwound structure can be continuous or discontinuous. Within the discrete phenomenological model the nature of the transition depends on the existence of the next-nearest neighboring layers interactions. If they are present, the transition is discontinuous. We try to elucidate this phenomenon.