. . . numerically?¹

${\cal L}$ and ${\cal C}$ will, of course, vary from one kind of transmission line to another, but their product is a universal constant - check, for example, the cable in Exercise 7.13 on p. 319 - provided the space between the conductors is a vacuum. In the theory of transmission lines, this product is related to the speed at which a pulse propagates down the line ( $v = 1/\sqrt{\cal LC}$).

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. . . speed?²

Hint: see Exercise 4.6 on p. 183; by what factor does ${\cal L}$ change when an inductor is immersed in linear material of permeability $\mu$?

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. . . ). ³

Note: this is not an additional force, but rather an alternative way of calculating the same force - in (b) we got it from the force law, and in (d) we got it from conservation of momentum.

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