The laws list
H


h to Huygen's construction.


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H.

 h

See Planck constant.
 Hall effect

When charged particles flow through a tube which has both an
electric field and a magnetic field (perpendicular to the electric
field) present in it, only certain velocities of the charged
particles are preferred, and will make it undeviated through the
tube; the rest will be deflected into the sides. This effect is
exploited in such devices as the mass spectrometer and in the
Thompson experiment. This is called the Hall effect.
 Hawking radiation (S.W. Hawking; 1973)

The theory that black holes emit radiation like any other hot
body. Virtual particleantiparticle pairs are constantly being
created in supposedly empty space. Occasionally, a pair will
be created just outside the event horizon of a black hole.
There are three possibilities:
 both particles are captured by the hole;
 both particles escape the hole;
 one particle escapes while the other is captured.
The first two cases are straightforward; the virtual
particleantiparticle pair recombine and return their energy
back to the void via the uncertainty principle.
It is the third case that interests us. In this case, one
of the particles has escaped (and is speeding away to infinity),
while the other has been captured by the hole. The escapee
becomes real and can now be detected by distant observers. But
the captured particle is still virtual; because of this, it has
to restore conservation of energy by assigning itself a
negative massenergy. Since the hole has absorbed it, the hole
loses mass and thus appears to shrink. From a distance, it
appears as if the hole has emitted a particle and reduced in
mass.
The rate of power emission is proportional to the inverse
square of the hole's mass; thus, the smaller a hole gets, the
faster and faster it emits Hawking radiation. This leads to
a runaway process; what happens when the hole gets very small
is unclear; quantum theory seems to indicate that some kind
of "remnant" might be left behind after the hole has emitted
away all its massenergy.
 Hawking temperature

The temperature of a black hole caused by the emission of Hawking
radiation. For a black hole with mass m, it is
T = (hbar c^{3})/(8 pi G k m).
Since blackbody power emission is proportional to the area of the
hole and the fourth power of its thermodynamic temperature, the
emitted power scales as m^{2}  that is, as the inverse square of
the mass.
 hbar

See Dirac constant.
 Heisenberg uncertainty principle

See uncertainty principle.
 henry; H (after W. Henry, 17751836)

The derived SI unit of inductance, defined as the inductance of a
closed circuit in which an electromotive force of 1 V is produced
when the electric current varies uniformly at a rate of 1 A/s; it
thus has units of V s/A.
 hertz; Hz (after H. Hertz, 18571894)

The derived SI unit of frequency, defined as a frequency of 1
cycle per s; it thus has units of s^{1}.
 Hooke's law (R. Hooke)

The stress applied to any solid is proportional to the strain it
produces within the elastic limit for that solid. The constant of
that proportionality is the Young modulus of elasticity for that
substance.
 hoop conjecture (K.S. Thorne, 1972)

The conjecture (as yet unproven, though there is substantial
evidence to support it) that a nonspherical object, nonspherically
compressed, will only form a black hole when all parts of the
object lie within its event horizon; that is, when a "hoop"
of the event horizon circumference can be rotated in all
directions and will completely enclose the object in question.
 Hubble constant; H_{0} (E.P. Hubble; 1925)

The constant which determines the relationship between the
distance to a galaxy and its velocity of recession due to the
expansion of the Universe. Since the Universe is selfgravitating,
it is not truly constant. In cosmology, it is defined as
H = (da/dt)/a,
where a is the 4radius of the Universe. When evaluated for the
present, it is written
H_{0} == H(t = now).
The Hubble constant is not known to great accuracy (only
within about a factor of 2), but is believed to lie somewhere
between 50 and 100 km/s/Mpc.
 Hubble's law (E.P. Hubble; 1925)

A relationship discovered between distance and radial velocity.
The further away a galaxy is away from is, the faster it is
receding away from us. The constant of proportionality is the
Hubble constant, H_{0}. The cause is interpreted as the expansion
of spacetime itself.
 Huygens' construction; Huygens' principle (C. Huygens)

The mechanical propagation of a wave (specifically, of light) is
equivalent to assuming that every point on the wavefront acts
as point source of wave emission.

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