Suppose a magnetic monopole qm passes through a resistanceless loop of wire with self-inductance L . What current is induced in the loop?

Short Answer

Expert verified

The induced current in the loop isl=μ0qmL.

Step by step solution

01

Induction law

This law is used to determine the electromotive force (emf) generated due to the interaction between a magnetic field and an electric conductor.

Based on this law, the amount of emf induced in a conductor directly relies upon the changes in the magnetic flux

02

Given information

The magnetic monopole is,qm.

The self-inductance of the resistanceless loop of wire is, L .

03

Current induced in the loop

The generalised equation of Faraday’s law for the resistanceless loop of wire is given by,

×E=-μ0Jm-Bt

Here,represents the electric field,μ0is the permeability of free space,Jmis the current of magnetic charge andBtis the change in magnetic field.

Integrating both sides of equation over the da surface,

(×E).da=-μ0Jm-Bt.daE.dl=-μ0Jm.da-Bt.daε=-μ0Jm.da-ddtB.daε=-μ0lmenc-dΦdt

Here,the induced emf,lmencis induced electro-magnetic current anddΦdtis the change in the magnetic flux.

Also, the induced emf in the wire loop is given by,

ε=-Ldldt

Equating both values,

-Ldldt=-μ0lmenc-dΦdtdldt=-μ0Llmenc+1LdΦdtl=μ0LQm+1LΦ

Here,Qmis the total magnetic charge passing through the surface,Φis the change in flux through the surface.

For theresistanceless loop of wire, and .

l=μ0L×qm+1L×0l=μ0qmL

Hence, the induced current in the loop isl=μ0qmL.

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Most popular questions from this chapter

(a) Show that Maxwell's equations with magnetic charge (Eq. 7.44) are invariant under the duality transformation

E'=Ecosα+cBsinα,cB'=cBcosα-Esinα,cq'e=cqecosα+qmsinα,q'm=qmcosα-cqesinα,

Where c1/ε0μ0and αis an arbitrary rotation angle in “E/B-space.” Charge and current densities transform in the same way as qeand qm. [This means, in particular, that if you know the fields produced by a configuration of electric charge, you can immediately (using α=90°) write down the fields produced by the corresponding arrangement of magnetic charge.]

(b) Show that the force law (Prob. 7.38)

F=qe(E+V×B)+qm(B-1c2V×E)

is also invariant under the duality transformation.

A transformer (Prob. 7.57) takes an input AC voltage of amplitude V1, and delivers an output voltage of amplitude V2, which is determined by the turns ratio (V2V1=N2N1). If N2>N1, 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 M2=L1L2, where Mis the mutual inductance of the coils, and L1,L2, are their individual self-inductances.

(b) Suppose the primary is driven with AC voltage Vin=V1cos(ωt), and the secondary is connected to a resistor, R. Show that the two currents satisfy the relations

L1=dl1dt+Mdl2dt=V1cos(ωt);L1=dl2dt+Mdl1dt=-I2R.

(c) Using the result in (a), solve these equations for localid="1658292112247" l1(t)and l2(t). (Assume l2has no DC component.)

(d) Show that the output voltage (Vout=l2R)divided by the input voltage (Vin)is equal to the turns ratio: VoutVin=N2N1.

(e) Calculate the input power localid="1658292395855" (Pin=Vinl1)and the output power (Pout=Voutl2), 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 ±V02atϕ=±π, and a steady current flows over the surface, as indicated in Fig. 7.51. According to Ohm's law, then,

V(a,ϕ)=V0ϕ2π,(-π<ϕ<+π)

Figure 7.51

(a) Use separation of variables in cylindrical coordinates to determine V(s,ϕ) 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 σ=and is J finite, then

Bt=×(v×B)

(b) Let S be the surface bounded by the loop (P)at time t , and S'a surface bounded by the loop in its new position (P')at time t+dt (see Fig. 7.58). The change in flux is

=S'B(t+dt)da-SB(t)da

Use ·B=0to show that

S'B(t+dt)da+RB(t+dt)da=SB(t+dt)da

(Where R is the "ribbon" joining P and P' ), and hence that

=dtSBt·da-RB(t+dt)da

(For infinitesimal dt ). Use the method of Sect. 7.1.3 to rewrite the second integral as

dtP(B×v)·dI

And invoke Stokes' theorem to conclude that

dt=S(Bt-×v×B)·da

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:

I(t)={(1-t)I0for0t1/afort>/a

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