# Tag Info

56

The Earth's climate isn't quite as stable as you think. The Earth's climate has toggled back and forth between a greenhouse Earth and an icehouse Earth for the last 600 million years or so. During the icehouse Earth phases, the climate can enter an ice age, an extended period of time during which the climate in oscillates between glaciations and ...

23

A slightly simpler version of David Hammen's (as usual excellent) answer: Earth is "big enough" to have sufficient pull on the atmosphere: gravity stops it from escaping Earth is "close enough" to the sun to keep liquid water (and liquid core) Core is sufficiently magnetic that it acts to protect against solar wind (which would otherwise strip the ...

9

There are two parts to this question (even when you cut out the bonus bits). How much energy is stored in the earth's magnetic field (ramp up the magnet) How much power to keep that field going (drive current through big loop) The former is given by the $\frac12 L I^2$ - so we need to estimate the inductance of the coil needed and its current. A single ...

7

The main reason Earth still has its water and Mars doesn't is gravity. Earth is big enough so that typical thermal speeds of hydrogen in the upper atmosphere don't get to escape velocity. On Mars, this is not the case, Mars having significantly less mass and therefore lower escape velocity. As a result, over a long time, much of the hydrogen that was ...

3

I don't consider myself an expert in MRI, but let me try (since nobody else has stepped up in the last hour...) You are right with your first assertion: the spin precesses about the B vector (this is why you get resonance in the first place). However, on average there is a net component of the magnetic moment aligned with the B field. This is what gives ...

3

A charge radiates every time is accelerated. The power radiated is given by the Larmor formula. Putting this into the introductions to the motion of a charge in electromagnetic fields would be a meaningless complication, as much as considering air friction. But yes, a charge in a magnetic field would not spin indefinitely, even in vacuum.

2

It's not the case. In the second, the electron will radiate. This is how light species lose energy, and cool in Penning traps, and one of the factors that limit the energies of particles in circular particle accelerators. For a reference see: http://en.wikipedia.org/wiki/Cyclotron_radiation

2

The motivation for pursuing fusion is clear, but there are currently several main physics and engineering challenges: Confinement time: An operational reactor requires a long energy confinement time, $\tau_E$. An empirical scaling law for confinement time has been found to depend on the size of the tokamak as $\tau_E \propto R^{2.04} a^{1.04}$, where $R$ ...

2

If them magnetic field is creating electric field and they combine to form EM waves, why does a compass show a magnetic field around the wire? To produce EM waves, we do need the current to be time varying such that the magnetic field is time varying which induces a time varying electric field etc. However, it isn't that case that the entire ...

2

What's probably happening here is the following: The fundamental or microscopic fields $\mathbf{E}$ and $\mathbf{B}$ are technically called the electric field strength and the magnetic induction, while $\mathbf{D}$ and $\mathbf{H}$, their macroscopic counterparts, are called the electric displacement and the magnetic field, a quite weird nomenclature, since ...

2

Floris's answer gives you an excellent description of the forces that would be present in both a magnetic and gravitational field, whilst MaxGraves's Answer gives you a clear and careful discussion of how you should use the word weight. In more the spirit of MaxGraves's Answer, something that seems a little pedantic but may be interesting to you is the ...

2

The trapping technology used by the ALPHA group at CERN is described in this article, and specifically the trapping of the neutral anti-hydrogen is described here. The trapping is done by an Ioffe trap - I couldn't find a simple article describing how this works, but the search I've linked finds lots of related articles that are worth reading. In brief, the ...

1

The earth's field is too weak to cause any damage. But sometimes it must be screened to avoid errors in delicate physical measurements. But a very useful application is its property to align compass needles in the north-south direction.

1

$\Phi$ is the flux of $\vec B$ through one surface $$\Phi = BA$$ where A is the area bounded by a loop (turn). But there are $n$ turns and thus $n$ surfaces that are pierced by $\vec B$ so the the flux linkage $\lambda$ is $$\lambda = n \Phi$$

1

Yes, a high magnetic field can massively reduce the entropy of the dipoles for a given temperature. When the dipoles enter a field they become hotter (analogous to a gas when its volume is compressed) so they dissipate heat into their environment (into lattice vibrations and other degrees of freedom). Once the dipoles have returned to thermal equilibrium ...

1

This is perhaps a comment to the @Floris answer. (I can move it there.) First about T1, It not only describes the decay of the Z-magnetization. But if you suddenly turn on a B field it also describes how long it takes the spins to become polarized in that direction. (Spins are not immediately polarized.) Concerning the decay of x-y magnetization. (I ...

1

If there is a magnetic field present, it will dictate the direction of the magnetization, as you anticipated and as @user3683367 said. This is then not referred to as spontaneous symmetry breaking but the external magnetic field breaks the symmetry explicitly. In the absence of an external magnetic field, the alignment is indeed random. You intuition that ...

1

Besides Mars having a weaker gravity field than Earth, it also has a much weaker magnetic field. Therefore, Mars' atmosphere was not protected from the solar wind. NASA scientists put forth the theory that the small sparse magnetic field that Mars does have actually helped the solar wind to drive off the atmosphere. ...

1

Every change in a magnetic field automatically creates an electric field and vice versa. For technical purposes, however, magnetic antennas create a stronger magnetic near field (i.e the field that can be measured less than a wavelength away from the antenna), while "electrical" antennas create a stronger electric near field. So depending on application, ...

1

This particular orientation sensing protocol is not wonted to me, but the following, given the data you cite, will indeed give you your orientation in space: Magnetometer gives $\vec{N}_0$ north direction (in general, not parallel to the "ground" because it has magnetic dip included); Gravity accelerometer gives $\vec{D}$ "down" direction; Then ...

1

In every negative acceleration electron loos energy, and this of course in the form of photons. This is not surprising because negative acceleration could be only after positive acceleration, the electron has to move befor he could be stopped or declined. And how the electron can be accelerated? By electric fields where the electron get the kinetic energy ...

1

Is there a center-of-charge kind of concept where that point remains stationary when a neutral particle splits into two or more charged particles ? Not when there are external forces acting on the system. As soon as the particle breaks down, the two charged particles experience magnetic force in the same direction, so their sum momentum changes and the ...

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