In the last 30 days

• Open/close A blackbody is not a blackbox

  Matteo Smerlak 2011 Eur. J. Phys. 32 1143 Tag this article

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  We discuss carefully the blackbody approximation, stressing what it is (a limit case of radiative transfer), and what it is not (the assumption that the body is perfectly absorbing, namely black). Furthermore, we derive the Planck spectrum without enclosing the field in a box, as is done in most textbooks. Although convenient, this trick conceals the nature of the idealization expressed in the concept of a blackbody: first, the most obvious examples of approximate blackbodies, stars, are definitely not enclosed in boxes; second, the Planck spectrum is continuous, while the stationary modes of radiation in a box are discrete. Our derivation, although technically less elementary, is conceptually more consistent, and brings the opportunity to introduce to advanced undergraduate and graduate students the important concept of the local density of states via the resolvent formalism.

• Open/close The Bravais pendulum: the distinct charm of an almost forgotten experiment

  V M Babović and S Mekić 2011 Eur. J. Phys. 32 1077 Tag this article

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  In the year 1851 in Paris, the apparent change of the plane of oscillation of a linear pendulum was observed by Léon Foucault. In the same year, at the same place, the unequal duration of the oscillations of a right- and left-handed conical pendulum was observed by Bravais. Today, the Foucault pendula are common at universities, the Bravais pendula not at all. We have revisited and experimentally tested Bravais's method and found it worthy of revitalization in student labs.

• Open/close Mechanical study of a modern yo-yo

  Charles de Izarra 2011 Eur. J. Phys. 32 1097 Tag this article

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  This paper presents the study of a modern yo-yo having a centrifugal clutch allowing the free rolling. First, the mechanical parts of the yo-yo are measured, allowing us to determine analytically its velocity according to its height of fall. Then, we are more particularly interested in the centrifugal device constituted by springs and small masses. The physics of this toy is suitable for undergraduate students, illustrating the concepts of dynamics of rigid bodies and of potential energy.

• Open/close The photoelectric effect: experimental confirmation concerning a widespread misconception in the theory

  Darren Wong et al 2011 Eur. J. Phys. 32 1059 Tag this article

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  The photoelectric effect is widely taught in schools and institutions. It is common knowledge that in order for photoelectrons to be emitted, the energy of the incoming photons must be greater than the work function of the irradiated metal (i.e. hν > _emitter). However, what may not be as commonly known is that the ' ' in Einstein's photoelectric equation ( eV _s = hν − ) is often implicitly and mistakenly taken as the work function of the emitter (or anode) rather than that of the collector (or cathode). This paper provides experimental evidence for the more correct but the non-intuitive form of the photoelectric equation eV _s = hν − _collector, which can also be theoretically derived by taking into account the contact potential difference between the electrodes. This equation is shown experimentally to be invalid for hν < _emitter and hν > _collector, even though in such a case a positive V _s (stopping potential) is obtained by the equation, no V _s is obtainable experimentally. However, the equation is valid for hν < _collector and hν > _emitter where a negative V _s is obtained from the equation, which corresponds to an experimentally observed 'starting potential'.

• Open/close The vertical oscillations of coupled magnets

  Li Kewei et al 2011 Eur. J. Phys. 32 S1 Tag this article

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  The International Young Physicists' Tournament (IYPT) is a worldwide, annual competition for high school students. This paper is adapted from the winning solution to Problem 14, Magnetic Spring, as presented in the final round of the 23rd IYPT in Vienna, Austria. Two magnets were arranged on top of each other on a common axis. One was fixed, while the other could move vertically. Various parameters of interest were investigated, including the effective gravitational acceleration, the strength, size, mass and geometry of the magnets, and damping of the oscillations. Despite its simplicity, this setup yielded a number of interesting and unexpected relations. The first stage of the investigation was concerned only with the undamped oscillations of small amplitudes, and the period of small amplitude oscillations was found to be dependent only on the eighth root of important magnet properties such as its strength and mass. The second stage sought to investigate more general oscillations. A numerical model which took into account magnet size, magnet geometry and damping effects was developed to model the general oscillations. Air resistance and friction were found to be significant sources of damping, while eddy currents were negligible.

• Open/close Twin paradox in de Sitter spacetime

  Sebastian Boblest et al 2011 Eur. J. Phys. 32 1117 Tag this article

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  The 'twin paradox' of special relativity offers the possibility of making interstellar flights within a lifetime. For very long journeys with velocities close to the speed of light, however, we have to take into account the expansion of the universe. Inspired by the work of Rindler on hyperbolic motion in curved spacetime, we study the worldline of a uniformly accelerated observer in de Sitter spacetime and the communication between the travelling observer and an observer at rest. This paper is intended to give graduate students who are familiar with special relativity and have some basic experience of general relativity a deeper insight into accelerated motion in general relativity, into the relationship between the proper times of different observers and the propagation of light signals between them, and into the use of compactification to describe the global structure of a relativistic model.

• Open/close Slipping and rolling on an inclined plane

  Cina Aghamohammadi and Amir Aghamohammadi 2011 Eur. J. Phys. 32 1049 Tag this article

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  In the first part of the paper, using a direct calculation two-dimensional motion of a particle sliding on an inclined plane is investigated for general values of friction coefficient (μ). A parametric equation for the trajectory of the particle is also obtained. In the second part of the paper, the motion of a sphere on the inclined plane is studied. It is shown that the evolution equation for the contact point of a sliding sphere is similar to that of a point particle sliding on an inclined plane whose friction coefficient is . If , for any arbitrary initial velocity and angular velocity, the sphere will roll on the inclined plane after some finite time. In other cases, it will slip on the inclined plane. In the case of rolling, the centre of the sphere moves on a parabola. Finally the velocity and angular velocity of the sphere are exactly computed.

• Open/close Elementary analysis of the special relativistic combination of velocities, Wigner rotation and Thomas precession

  Kane O'Donnell and Matt Visser 2011 Eur. J. Phys. 32 1033 Tag this article

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  The purpose of this paper is to provide an elementary introduction to the qualitative and quantitative results of velocity combination in special relativity, including the Wigner rotation and Thomas precession. We utilize only the most familiar tools of special relativity, in arguments presented at three differing levels: (1) utterly elementary, which will suit a first course in relativity; (2) intermediate, to suit a second course; and (3) advanced, to suit higher level students. We then give a summary of useful results and suggest further reading in this often obscure field.

• Open/close An experiment in heat conduction using hollow cylinders

  M Ortuño et al 2011 Eur. J. Phys. 32 1065 Tag this article

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  An experimental apparatus was designed and built to allow students to carry out heat conduction experiments in hollow cylinders made of different materials, as well as to determine the thermal conductivity of these materials. The evolution of the temperature difference between the inner and outer walls of the cylinder as a function of time is analysed, and when the process reaches the steady state regime the thermal conductivity can be easily calculated. Several materials such as wood, plastic and metals are considered and the values of their thermal conductivities, obtained experimentally, are compared with those given in the reference list.

• Open/close Rotations with Rodrigues' vector

  E Piña 2011 Eur. J. Phys. 32 1171 Tag this article

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  The rotational dynamics was studied from the point of view of Rodrigues' vector. This vector is defined here by its connection with other forms of parametrization of the rotation matrix. The rotation matrix was expressed in terms of this vector. The angular velocity was computed using the components of Rodrigues' vector as coordinates. It appears to be a fundamental matrix that is used to express the components of the angular velocity, the rotation matrix and the angular momentum vector. The Hamiltonian formalism of rotational dynamics in terms of this vector uses the same matrix. The quantization of the rotational dynamics is performed with simple rules if one uses Rodrigues' vector and similar formal expressions for the quantum operators that mimic the Hamiltonian classical dynamics.