2,922 research outputs found
Effects of dark energy on the efficiency of charged AdS black holes as heat engine
In this paper, we study the heat engine where charged AdS black holes
surrounded by dark energy is the working substance and the mechanical work is
done via term in the first law of black hole thermodynamics in the
extended phase space. We first investigate the effects of a kind of dark energy
(quintessence field in this paper) on the efficiency of the RN-AdS black holes
as heat engine defined as a rectangle closed path in the plane. We get
the exact efficiency formula and find that quintessence field can improve the
heat engine efficiency which will increase as the field density grows.
At some fixed parameters, we find that bigger volume difference between the
smaller black holes() and the bigger black holes( ) will lead to a
lower efficiency, while the bigger pressure difference will make the
efficiency higher but it is always smaller than 1 and will never be beyond
Carnot efficiency which is the maximum value of the efficiency constrained by
thermodynamics laws, this is consistent to the heat engine in traditional
thermodynamics. After making some special choices for thermodynamical
quantities, we find that the increase of electric charge and normalization
factor can also promote heat engine efficiency which would infinitely
approach the Carnot limit when or goes to infinity.Comment: 28 pages, 16 figures, refernces added, discussion and computation
improve
Note on the Radion Effective Potential in the Presence of Branes
In String Theory compactification, branes are often invoked to get the
desired form of the radion effective potential. Current popular way of doing
this assumes that the introduction of branes will not modify the background
geometry in an important way. In this paper, we show by an explicit example
that at least in the codimension 2 case, the gravitational backreaction of the
brane cannot be neglected in deriving the radion effective potential. Actually,
in this case, the presence of branes will have no effect on the dynamics of
radion.Comment: 6 pages, no figures. Some discussion clarified, conclusion unchanged.
To appear in Phys. Rev.
Gravitational potential in Palatini formulation of modified gravity
General Relativity has so far passed almost all the ground-based and
solar-system experiments. Any reasonable extended gravity models should
consistently reduce to it at least in the weak field approximation. In this
work we derive the gravitational potential for the Palatini formulation of the
modified gravity of the L(R) type which admits a de Sitter vacuum solution. We
conclude that the Newtonian limit is always obtained in those class of models
and the deviations from General Relativity is very small for a slowly moving
source.Comment: 5 pages, no figure
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