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Consider the motion of a particle with mass m and electric charge qein the field of a (hypothetical) stationary magnetic monopole qmat the origin:

B=04qmr2r^

(a) Find the acceleration of qe, expressing your answer in terms of localid="1657533955352" q, qm, m, r (the position of the particle), and v(its velocity).

(b) Show that the speed v=|v|is a constant of the motion.

(c) Show that the vector quantity

Q=m(rv)-0qeqm4r^

is a constant of the motion. [Hint: differentiate it with respect to time, and prove-using the equation of motion from (a)-that the derivative is zero.]

(d) Choosing spherical coordinates localid="1657534066650" (r,,), with the polar (z) axis along Q,

(i) calculate , localid="1657533121591" Q^and show that is a constant of the motion (so qemoves on the surface of a cone-something Poincare first discovered in 1896)24;

(ii) calculate Qr^, and show that the magnitude of Qis

Q=04|qeqm肠辞蝉胃|;

(iii) calculate Q^, show that

诲蠒dt=kr2,

and determine the constant k .

(e) By expressing v2in spherical coordinates, obtain the equation for the trajectory, in the form

drd=f(r)

(that is: determine the function )f(r)).

(t) Solve this equation for .r()

Short Answer

Expert verified

(a) The acceleration of qeis 0qm4mr2qvr^.

(b) The speed v=vis a constant of the motion.

(c) The vector quantity is a constant of the motion.

(d)

(i) Q^is and is a constant of the motion.

(ii) The value of Qr^is Qcosand the magnitude of Qis proved.

(iii) The value of Q^is -Qsinand ddt=kr2is proved.

The constant k is Qm.

(e) The function f (r) is rrvk2-sin2.

(f) r()is AcosarcosAr0+sin(-0.

Step by step solution

01

Identification of the given data

The given data is listed below as:

  • The mass of the particle is, m
  • The electric charge of the particle is, qe
  • The stationary magnetic monopole is qm,
02

Significance of the magnetostatics

Magnetostatics is described as the study of the magnetic fields in the systems where the currents remain steady. However, magnetostatics is described as the electrostatics鈥 magnetic analogue where there are stationary charges.

03

(a) Determination of the acceleration

The equation of the force of the particle is expressed as:

F= ma

鈥(颈)

Here, m is the mass of the particle and a is the acceleration of the particle.

The equation of the force of the particle can also be written as:

F=0qm4r2qvr^

鈥(颈颈)

Here, qmis the stationary magnetic monopole, v is the velocity of the particle, r is the position of the particle, q is the particle鈥檚 charge, role="math" localid="1657535676962" 0is the permeability constant r^and is the position vector.

Equalling the equation (i) and (ii).

role="math" localid="1657536121138" ma=0qm4r2qvr^a=0qm4mr2qvr^

Thus, the acceleration of qeis 0qm4mr2qvr^.

04

(b) Determination of the speed

The magnitude of the velocity of the particle is conserved. Hence, the equation of the speed of the particle is expressed as:

av=0

Here, is the acceleration of the particle andv is the velocity of the particle.

The above equation can also be written as:

a.v=dvdtv=12ddt(v2)=0

Hence, the equation is proved.

Thus, the speed v=vis a constant of the motion.

05

(c) Determination of the prove that the vector quantity is a constant of the motion

The equation of the vector quantity is expressed as:

Q=m(rv)-0qeqm4r^

鈥(颈颈颈)

Here, qmis the stationary magnetic monopole, v is the velocity of the particle, r is the position of the particle, qeis the particle鈥檚 electric charge, 0is the permeability constant, m is the mass of the particle and is r^the position vector.

Differentiating the above equation with respect to time.

dQdt=m(vv+ra)-0qeqm4vr-rrr2=mr0qeqm4mr2vr^-0qeqm4r2(vr-rr)=0qeqm4r2(r(vr^))-0qeqm4r2(vr-rr) 鈥(颈惫)

For the triple product, the BAC-CAB rule is used.

r(vr)=v(rr^)-r^(rv)=vr-r^rr=vr-rr^

Substitute the above value in the equation (iv).

dQdt=0qeqm4r2(vr-rr)-0qeqm4r2(vr-rr)

= 0

As the derivate is zero, hence it can be stated that the vector quantity is a constant of the motion.

Thus, the vector quantity is a constant of the motion.

06

(d) (i) Determination of Q.ϕ¯

The equation of the cross product of the particle鈥檚 position and the velocity is expressed as:

rv=r(rr^+r^+rsin^)=r2^-r2sin^

Substitute the values from the above equation in the equation (iii).

Q=mr2^-mr2sin^-0qeqm4r^

鈥(惫)

The dot product of the above equation with is expressed as:

Q^=mr2^-0qeqm4r^^=mr2=0

As both the variables are mutually orthogonal. Hence, localid="1657620419227" =0and it is constant of motion.

Thus, localid="1657620427016" Q.^is 0 and is a constant of the motion.

07

(d) (ii) Determination of Q.r^

The dot product of the equation (v) with r^is expressed as:

Qr^=mr2^r^-mr2sin^r^-0qeqm4r^r^=Qcos

The above equation can also be expressed as:

Qcos=0qeqm4Q=0qeqm4cos

Thus, the value of Qr^is Qcosand the magnitude of Qis proved.

08

(d) (iii) Determination of Q.θ^

The dot product of the equation (v) with is expressed as:

Q.^=mr2^^-mr2sin^-0qeqm4r^^=-Qsin

The above equation can also be expressed as:

-Qsin=-mr2sin=Qmr2

鈥(惫颈)

As the dot product Q^is , -Qsinthen the constant is kexpressed as:

k=Qm

Substitute the value of the above equation in equation (vi).

kr2ddt=kr2

Thus, Q^the value of-Qsin is and ddt=kr2is proved.

The constant kis Qm.

09

(e) Determination of the function f (r)

The equation of expressing the square of the velocity in the spherical coordinates is expressed as:

v2=r+r2sin22+r22

Substitute 0 for and Qmr2for in the above equation.

role="math" localid="1657618825571" v2=r2+r2sin2Q2m2r4=r2+C2r2C2=sin2Q2m2

The chain rule has been applied in the differential equation of the curve.

r=drdt=drdddt=drdr=drd

鈥(惫颈颈)

The equation of the position of the particle is expressed as:

r=v2-C2r2

Substitute v2-C2r2for randQmr2for in the equation (vii).

drd=rmQr2v2-C2=rrvk2-sin2=f(r) 鈥(惫颈颈颈)

Thus, f(r)the function is rrvk2-sin2.

10

(f) Solving the equation

The equation (viii) can be recalled as:

drd=rrvk2-sin2d=drrrvk2-sin2

Integrating the above equation with the limits.

od=rordrrrvk2-sin2

-0kvrordrrr2-A2

Substitute sin2k2v2for in the above equation.

-o=kvAarccosArrro=1sinarccosAr-arccosAr0arccosAr=arccosAro+sin(-o)cosarccosAr=cosarccosAro+sin(-o

Hence, further as:

Ar=cosarccosAr0+sin(-0r=AcosarccosAr0+sin(-0)

The term Ar0is the integration constant and it rotates around the orbit.

Thus, r()is AcosarccosAro+sin(-o).

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Most popular questions from this chapter

A uniformly charged solid sphere of radius R carries a total charge Q, and is set spinning with angular velocity w about the z axis.

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