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鈥淒erive鈥 the Lorentz force law, as follows: Let chargeqbe at rest inS, so F=qE, and let Smove with velocityv=vxwith respect to S. Use the transformation rules (Eqs. 12.67 and 12.109) to rewrite Fin terms of F, and Ein terms of E and B. From these, deduce the formula for F in terms of E and B.

Short Answer

Expert verified

The Lorentz force is deduced asF=qE+q(vB)

Step by step solution

01

Expression for Maxwell’s equation:

Using equation 12.67, write the equation for the transformation of forces from one frame to another frame.

F1=1yFF1=F1

Here, y is the constant pertains to the relative motion between the two frames.

02

Deduce the Lorentz force law:

Write the expression for the force acting on the charge in the frame S.

F=qE

Here, q is the charge and Eis the electric field.

Write the above expression in a vector form.

F=qExx^+qEyy^+qEzz^

Here, Ex,Eyand Ezare the components of an electric field in the frame .

Write the expression for the force acting on the charge in frame S.

F=Fxx^+Fyy^+Fzz^ 鈥︹ (1)

Here, Fx,Fyand Fzare the components of the forces in frame S.

Write the equations for the component of the forces of frame S in terms of the component of the forces of the frame .

Fx=qExFy=1qEyFz=1qEz

Using equation 12.109, the above component of the forces becomes,

Fx=qExFy=1q((Ey-vBz))=q(Ey-vBz)Fz=1q((Ez+vBz))=q(Ez-vBy)

Substitute qExfor Fx,q(Ey-vBz)for Fyand q(Ez+vBy)Fzfor Fzin equation (1).

F=qExx^+q(Ey-vBz)y^+q(Ez+vBy)z^F=q(Exx^+Eyy^+Ezz^)-q(vBz)y^+(vBy)z^.......(2)

Here, q(vBz)y^+(vBy)z^and q(Exx^+Eyy^+Ezz^)=qE.

Hence, the equation (2) becomes,

F=qE+q(vB)

Therefore, the Lorentz force law is deduced.

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Most popular questions from this chapter

12.48: An electromagnetic plane wave of (angular) frequency is travelling in the xdirection through the vacuum. It is polarized in the ydirection, and the amplitude of the electric field is Eo.

(a) Write down the electric and magnetic fields, role="math" localid="1658134257504" E(x,y,z,t)and B(x,y,z,t)[Be sure to define any auxiliary quantities you introduce, in terms of , Eo, and the constants of nature.]

(b) This same wave is observed from an inertial system Smoving in thexdirection with speed vrelative to the original system S. Find the electric and magnetic fields in S, and express them in terms of the role="math" localid="1658134499928" Scoordinates: E(x,y,z,t)and B(x,y,z,t). [Again, be sure to define any auxiliary quantities you introduce.]

(c) What is the frequency of the wave in S? Interpret this result. What is the wavelength of the wave in S? From and , determine the speed of the waves in S. Is it what you expected?

(d) What is the ratio of the intensity in to the intensity in? As a youth, Einstein wondered what an electromagnetic wave would like if you could run along beside it at the speed of light. What can you tell him about the amplitude, frequency, and intensity of the wave, as approaches ?

(a) What鈥檚 the percent error introduced when you use Galileo鈥檚 rule, instead of Einstein鈥檚, withvAB=5mi/handvBC=60mi/hand?

(b) Suppose you could run at half the speed of light down the corridor of a train going three-quarters the speed of light. What would your speed be relative to the ground?

(c) Prove, using Eq. 12.3, that ifvAB<candvBC<cthenvAC<cInterpret this result.


Prove that the symmetry (or antisymmetry) of a tensor is preserved by Lorentz transformation (that is: if tvis symmetric, show thattv is also symmetric, and likewise for antisymmetric).

Define proper acceleration in the obvious way:

=dd=d2xd2

(a) Find0and 伪 in terms of u and a (the ordinary acceleration).

(b) Expressin terms of u and a.

(c) Show that=0.

(d) Write the Minkowski version of Newton鈥檚 second law, in terms of. Evaluate the invariant productK.

In classical mechanics, Newton鈥檚 law can be written in the more familiar form F=ma. The relativistic equation, F=dpdt, cannot be so simply expressed. Show, rather, that

F=m1-u2/c2[a+uuac2u2]

where a=dudt is the ordinary acceleration.

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