or http://puu.sh/kXo0a/308f226426.png Question 9 -- please show full work. This

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or http://puu.sh/kXo0a/308f226426.png

Question 9 -- please show full work.

This question and the next two pertain to the following situation: An electron of mass m and charge q is accelerated to the right (in the plane of the page) from rest through a potential difference V. The electron then enters a region, defined by x > 0, containing a uniform magnetic field. V= 1500 V B = 0.3 T out of page q =-1.6 Times 10^-19 C m =9.1 Times 10^-31 kg When the electron enters the region with the magnetic field the force on it is directed When the electron is in the magnetic field region, its speed v. How much time T does the electron spend in the magnetic field region?

Explanation / Answer

magnetic force Fm = q ( v xB)

v is towwards x axis (along i+ vector)

B = is out of page ( along +k vector)

Fm = - ( i x k) = - (-j) = j

Ans. towards top of the page .

8)
as magnetic force wwill always be perpendicular to both velocity and magentic field.

so electron will move in circle with constant speed.

so speed remain constant. (Ans (c) )

9)
as electron goes through potential diff 1500V

KE = qV

mv^2 / 2 = qV

9.1 x 10^-31 x v^2 /2   = 1.6 x 10^-19 x 1500

v = 2.30 x 10^7 m/s

for ciccular motion,

qvB = mv^2 / R

R = (9.1 x 10^-31 x 2.30 x10^7) / (1.6 x 10^-19 x 0.3)

R = 4.35 x 10^-4 m

and electron moves half circle only in magnetic field after that it moves out of field.

t = pi R / v = (pi x 4.35 x 10^-4) / (2.30 x 10^7)

t = 5.96 x 10^-11 s

Ans (c)

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