An urgent mc

用戶85377

2010-12-09 3:06 am

Explain the following options clearly.

1. A beam of charged particles passes through a vacuum chamber and strikes the centre of the fluorescent screen at the end of the chamber. When a magnetic field B is applied in the direction shown, two spots appear on the screen, one 1 cm above the centre, and one 8 cm above the centre. Which of the following can be possible constituents of the beam? i) protons and alpha particles, traveling at the same speedii) protons traveling with speed v and alpha particles traveling with speed 4viii) electrons and protons, traveling at the same speediv) electrons and alpha particles, traveling at the same speed

更新1:

the direction of B is into the paper

回答 (1)

天同

2010-12-09 5:24 am

✔ 最佳答案

Where is the diagram showing the direction of the magnetic field?

Assuming the field is pointing into the paper, so that an upward force is given to the particles when travelling from let to right.

Then we have,,,,,,
MV^2/R = QVB
where M is the mass of the particel
V is the speed of the particle
R is the radius of the arc traversed by the particle
Q is the charge on the particle
B is the magnetic flux density

hence, R = MV/QB = (V/B).(M/Q)

For a proton,
Rp = (m/e).(v/B)
where m is the mass of proton
e is the electronic charge
v is the speed of proton

For an alpha particle, assume travelling at the same speed v,
Ra = (4m/2e).(v/B) = 2(m/e).(v/B) = 2Rp

If the speed of the alpha particel is 4v, then
Ra = (4m/2e).(4v/B) = 8.(m/e),(v/B) = 8Rp

Therefore, it is possible that the particle constituents described in (ii) is correct.Options (iii) and (iv) are definitively wrong. Because the two particles are of opposite polarity, they would strike the screen at opposite side of the centre.

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