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Magnetic effects and star formation

We would expect the magnetic effects to be important when the energy associated with the presence of the magnetic field is comparable to the gravitational energy in magnitude. In cgs units, the energy density (erg/cm3) associated with a magnetic field B is uB = B2/ 8w

Example 15.4 Magnetic energy For what magnetic field strength B does the magnetic energy of a cloud equal the absolute value of the gravitational potential energy? Assume a spherical cloud with a radius R = 10 pc, and a density of molecular hydrogen n(H2) = 300 cm~3.

solution

The magnetic energy UB is the energy density, multiplied by the volume of the cloud:

B2R3 6

The magnitude of the (negative) gravitational potential energy is

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