香港巴士論壇

A free electron travelling horizontally with
speed v enters a uniform vertical magnetic field
B. Which of the following statements is
correct?
(1) The path of the electron is a vertical circle.
(2) The speed of the electron remains
constant.
(3) The radius of curvature of the path is
inversely proportional to B.
A. (1), (2) and (3)
B. (1) and (2) only
C. (2) and (3) only
D. (1) only
E. (3) only

有無師兄打救

1)Incorrect

H>the path of moving election (Black in colour)
V>the direction of magnetic flux linkage (Red in colour)
So,D>the force acting on the election
(the force is into the paper or out of the paper)
As you can see the path of the election shown above,it is not vertically formed.

2)Correct
Electron undergos circular motion
(Change the direction but not the value of speed)

3)Correct

qvB=mv^2/r
qB=mv/r
r=mv/qB

m:Mass of the election (constant in all conditions)
v:velocity of the election (by statement 2)
q:charge of the election (also constant in all conditions)

So,r=k/B


The answer should be C

唔該哂師兄您畫左個圖
如果咁講vertical B-field 既意思即係同張紙成直角,而唔係同張紙上面成直角?