Chapter 7: Q39P (page 339)
Suppose a magnetic monopole passes through a resistanceless loop of wire with self-inductance L . What current is induced in the loop?
Short Answer
The induced current in the loop is
Chapter 7: Q39P (page 339)
Suppose a magnetic monopole passes through a resistanceless loop of wire with self-inductance L . What current is induced in the loop?
The induced current in the loop is
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Get started for free(a) Show that Maxwell's equations with magnetic charge (Eq. 7.44) are invariant under the duality transformation
Where and is an arbitrary rotation angle in “E/B-space.” Charge and current densities transform in the same way as and . [This means, in particular, that if you know the fields produced by a configuration of electric charge, you can immediately (using ) write down the fields produced by the corresponding arrangement of magnetic charge.]
(b) Show that the force law (Prob. 7.38)
is also invariant under the duality transformation.
A transformer (Prob. 7.57) takes an input AC voltage of amplitude , and delivers an output voltage of amplitude , which is determined by the turns ratio . If , the output voltage is greater than the input voltage. Why doesn't this violate conservation of energy? Answer: Power is the product of voltage and current; if the voltage goes up, the current must come down. The purpose of this problem is to see exactly how this works out, in a simplified model.
(a) In an ideal transformer, the same flux passes through all turns of the primary and of the secondary. Show that in this case , where is the mutual inductance of the coils, and , are their individual self-inductances.
(b) Suppose the primary is driven with AC voltage , and the secondary is connected to a resistor, . Show that the two currents satisfy the relations
(c) Using the result in (a), solve these equations for localid="1658292112247" and . (Assume has no DC component.)
(d) Show that the output voltage divided by the input voltage is equal to the turns ratio: .
(e) Calculate the input power localid="1658292395855" and the output power , and show that their averages over a full cycle are equal.
Question: A rare case in which the electrostatic field E for a circuit can actually be calculated is the following: Imagine an infinitely long cylindrical sheet, of uniform resistivity and radius a . A slot (corresponding to the battery) is maintained at , and a steady current flows over the surface, as indicated in Fig. 7.51. According to Ohm's law, then,
Figure 7.51
(a) Use separation of variables in cylindrical coordinates to determine inside and outside the cylinder.
(b) Find the surface charge density on the cylinder.
Prove Alfven's theorem: In a perfectly conducting fluid (say, a gas of free electrons), the magnetic flux through any closed loop moving with the fluid is constant in time. (The magnetic field lines are, as it were, "frozen" into the fluid.)
(a) Use Ohm's law, in the form of Eq. 7.2, together with Faraday's law, to prove that if and is J finite, then
(b) Let S be the surface bounded by the loop at time t , and a surface bounded by the loop in its new position at time t+dt (see Fig. 7.58). The change in flux is
Use to show that
(Where R is the "ribbon" joining P and P' ), and hence that
(For infinitesimal dt ). Use the method of Sect. 7.1.3 to rewrite the second integral as
And invoke Stokes' theorem to conclude that
Together with the result in (a), this proves the theorem.
A square loop, side a , resistance R , lies a distance from an infinite straight wire that carries current l (Fig. 7.29). Now someone cuts the wire, so l drops to zero. In what direction does the induced current in the square loop flow, and what total charge passes a given point in the loop during the time this current flows? If you don't like the scissors model, turn the current down gradually:
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