我想問下呢題磁場數點計??中六phy題~

用戶20139

2008-04-14 5:49 am

hi~我想問下你呢題點計?
一帶電量為+q,質量為m的小球從一傾角為X的光滑斜面上由靜止開始滑下,斜面處於磁場感應強度B的勻強磁場中,磁場方向垂直於紙面向外,求小球在斜面上滑行的速度范圍和最大距離。

回答 (1)

天同

2008-04-26 5:01 am

✔ 最佳答案

As there is no diagram given, I presume the magnetic field is pointing OUT of the paper perpendicualr to the direction of motion of the ball down the inclined plan.

The magnetic force (Lorentz force) is opposite to the downward weight component normal to the inclined plane. When the two forces are equal, the ball begins to lift off from the plan. This happens when

q.v.B = mg.cos(x)
where v is the velocity of the ball when lifting occurs and g is the acceleration due to gravity.

thus, v = mg.cos(x)/q.B
Thus, the velocity of the ball increases from zero up to mg.cos(x)/q.B

Since acceleration a = g.sin(x)
using equation of motion, v^2 = u^2 + 2.a.s
[mg.cos(x)/q.B]^2 = 2.(g.sin(x)).s

thus s = [mg.cos(x)/q.B]^2/2.g.sin(x)

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