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Question: Let pab(t)be the probability of finding a particle in the range (a<x<b),at time t.

(a)Show that

dpabdt=j(a.t)-j(b,t),

Where

j(x,t)ih2m(*x-*x)

What are the units of j(x,t)?

Comment: j is called the probability current, because it tells you the rate at which probability is "flowing" past the point x. Ifpab(t) is increasing, then more probability is flowing into the region at one end than flows out at the other.

(b) Find the probability current for the wave function in Problem 1.9. (This is not a very pithy example, I'm afraid; we'll encounter more substantial ones in due course.)

Short Answer

Expert verified

(a)The unit of j(x,t)is j(a,t)-j(b,t).

(b)The probability current of wave function isj(x,t)=0

Step by step solution

01

Define the Schrödinger equation

An equation that accounts for the electron's nature as a matter-wave inside of an atom describes the electron's energy and position in space and time.

*t-ih2m2*x2+ih2*x2V* ....(1)

*tih2m2*x2-ihV* ....(2)

02

Determine the units of  j ( x , t )

(a)

From Schrodinger equation

t-h22m2x2+V2

Now,

Pab=-2dx=ab*dxdPabdt=ddtab*dxdPabdt=abt*dx=ab*t+*tdx

Substitute from Schrodinger equation and its conjugate,

dPabdt=ab-ih2m2*x2+ih痴蠄*+*ih2m2x2+ih痴蠄dx=-ih2mab2*x2-*2x2dx=-ih2mabx*x-*xdx=-ih2m*x-*xab=-ih2m(b,t)*(b,t)x-*(b,t)(b,t)x-(a,t)*(a,t)x-*(a,t)(a,t)x=j(a,t)-j(b,t)

Therefore, the unit of j ( x , t ) is j ( a , t ) -j ( b , t ) .

03

Determine the probability current of the wave function

(b)

From problem 1.9 we have the wave-function

(x,t)=Ae-amx2/h+it

therefore,

J(x,t)=ih2mA2e-amx2/h+it-2amxhe-amx2/h+it-e-amx2/h+it-2amxhe-amx2/h+it

Hence, the probability current of wave function is J ( x , t ) = 0

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At time t = 0 a particle is represented by the wave function

(x,0)={A(x,0),0xa,A(bx)/(ba),axb,0,otherwise,where A, a, and b are (positive) constants.

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