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Homework Statement
A Hertzian dipole is located at the origin of spherical coordinates and is aligned with the θ=0 direction. The dipole has strength I(subscript 0)[itex]\delta[/itex]l and oscillates with angular frequency [itex]\omega[/itex]. The magnetic field that it produces is given by the real part of the expression:
B(r)=[([itex]\mu[/itex](subscript 0) I(subscript 0) [itex]\delta[/itex] l)/(4pi)] sin[itex]\theta[/itex] [((-i [itex]\omega[/itex])/(rc))+(1/(r^2))]exp [i(kr-[itex]\omega[/itex]t)] phi-hat
When grouping terms by r-dependence, there are essentially three contribution to the fields produced by a Hertzian dipole: the r^-3 terms are effectively the electrostatic field; the r^-2 terms give rise to what is called the induction field; and, the radiation field is the single term r^-1.
i) Show that, in the limit of small distances r and zero angular frequency, the amplitude of the field given by this expression is consistent with the Biot-Savart law:
B(r)=(([itex]\mu[/itex](subscript 0) I)/(4pi) (([itex]\delta[/itex] l cross r-hat)/(r^2))
ii)Show also that in the limit of large r the form of the expression is consistent with that required for a radiation field.
The Attempt at a Solution
B(r)=[([itex]\mu[/itex](subscript 0) I(subscript 0) [itex]\delta[/itex] l)/(4pi)] sin[itex]\theta[/itex] [((1/(r^2))]exp [i(kr-[itex]\omega[/itex]t)] phi-hat
B(r)=[([itex]\mu[/itex](subscript 0) I(subscript 0) [itex]\delta[/itex] l)/(4pi)] sin[itex]\theta[/itex] [((1/(r^2))] cos(kr-[itex]\omega[/itex]t) phi-hat
B(r)=[([itex]\mu[/itex](subscript 0) I(subscript 0) [itex]\delta[/itex] l)/(4pi)] sin[itex]\theta[/itex] [((1/(r^2))] cos(kr) phi-hat
B(r)=[([itex]\mu[/itex](subscript 0) I(subscript 0) [itex]\delta[/itex] l)/(4pi)] sin[itex]\theta[/itex] [((1/(r^2))] cos(0) phi-hat
because r is small
B(r)=[([itex]\mu[/itex](subscript 0) I(subscript 0) [itex]\delta[/itex] l)/(4pi)] sin[itex]\theta[/itex] [((1/(r^2))] phi-hat
I suppose I need to find out what theta would be, but I am having trouble visualising the thing in my head. I suppose theta would have to be 90 degrees, but why?
How does one go from delta l to delta l cross r-hat? And how is the phi-hat gotten rid of?
Please help.