问题 问答题

如图所示,带正电的粒子以水平速度v0从平行金属板MN间中线OO′连续射入,MN板间接有如图乙所示的随时间变化的电压uMN,令电场只存在两板间,紧邻金属板右侧有垂直纸面向里的匀强磁场B,CD为分界线、EF为屏幕,已知金属板间距、磁场宽度、极板长均为0.2m,每个带正电粒子速度v0=105m/s,比荷为q/m=108C/kg,B=5×10-3T,粒子重力不计,在每个粒子通过电场区域的极短时间内,可认为电场是恒定不变的,求:

(1)带电粒子进入磁场做圆周运动的最小半径;

(2)带电粒子射出电场的最大速度;

(3)带电粒子打在屏幕EF上的范围.

答案

(1)t=0时刻射入电场的带电粒子不被加速,进入磁场做圆周运动的半径最小,

粒子在磁场中运动时有qv0B=m

v20
r

rmin=

mv0
qB
=
105
5×10-3×108
m=0.2m

(2)因带电粒子通过电场时间t=

l
v0
=2×10-6s<T,所以带电粒子通过电场过程中可认为电场恒定不变.

设两板间电压为U1时,带电粒子能从N板右边缘飞出,

则 

d
2
=
1
2
U1q
dm
(
l
v0
)2

得  U1=

md2
v20
ql2
=100V

在电压低于或等于100V时,带电粒子才能从两板间射出电场,故U1=100V时,

带电粒子射出电场速度最大,q

U1
2
1
2
m
v2m
-
1
2
mv02

解得:vm=

v20
+
qU1
m
=1.41×105m/s

(3)t=0时刻进入电场中粒子,进入磁场中圆轨迹半径最小,打在荧光屏上最高点E,

O′E=rmin=0.2m

从N板右边缘射出粒子,进入磁场中圆轨迹半径最大,

qvmB=m

v2m
rm

解得:rm=

mvm
qB
=
2
5
m

因vm=

2
v0,故tanθ=
vy
v0
=1,θ=45°,

O2P=2×

2
d
2
=0.2
2
m=rmax

所以从P点射出粒子轨迹圆心O2正好在荧光屏上且O2与M板在同一水平线上,0′O2=

d
2
=0.1m,

O′F=rm-O2O′=

2
5
-0.1=0.18(m)

带电粒子打在荧光屏AB上范围为:EF=O′E+O′F=0.38m

答:(1)带电粒子进入磁场做圆周运动的最小半径为0.2m;

(2)带电粒子射出电场时的最大速度为1.41×105m/s;

(3)带电粒子打在屏幕EF上的范围为离屏幕中心上0.2m,下0.18m,长度为0.38m.

阅读理解

This is the VOA Special English News Report.

Single--handed(独自的)

Zhou Yang,18,China

Against three strong Republic of Korea competitors,Zhou single—handed took the l,500一meter short speed skating gold to become China’s youngest winter Olympics gold medalisi.According to Coach Li Yan,Zhou,who is usually a gentle girl,took some risks and skated fiercely to take the gold.This is the third gold the Chinese have won at the Vancouver Olympics.

Unobtrusive(不显眼的)

Liu Xiang,26,China   

The Olympic gold medalist said on Friday that he’s not the same old him,after setting his worst record in years——8.08 seconds for 60一meter hurdles.Liu said he still had some way to go before he’s back to his best.Many fans have been expecting his taking--off in the 2010 Doha,but Liu said unobtrusively“Robles is still the best in the 60一meter and 1l0—meter hurdles.

I’m no match now”.He added the performances of Terrence Trammell(US)and Robles would be the highlight of the Doha meet.

Coveted(令人垂涎的)

Michael Jackson,1958—2009,US

Thanks to a whopping(庞大的) $350,000 bid,Michael Jackson’s rhinestone—encrusted(镶满人造钻石的) glove has gone to a Macao company.Jackson wore the sparking glove when he did his first“moonwalk” dance in 1983,so it was a really coveted piece of equipment.The pre—auction estimate put its value at $40,000~60,000,but that wasn’t even close . The glove and nine other items from the auction will be on display in a Macao hotel.

1.From the first news we can learn that Zhou Yang __________

A. likes taking chances in the competitions

B.won three  gold medaIs at the olympics.

C.is not a gentle but a fierce girl

D.is a hard--working and strong--minded girl

2.From the above news,we can infer Liu Xiang__________

A.is not popular with his fans any longer

B.is still out of condition at present

C.is feeling discouraged and depressed

D. is sure of the highlight of the Doha meet

3.The underlined part“but that wash’t even close”probably means __________

A.the pre--auction estimate may be totally wrong

B.the actual price may be far higher than the estimate

C.the glove is not worth the pre--auction estimate

D.the pre—auction estimate is probably correct

4.Where can we probably learn the passage?

A.Broadcast    B.  Announcement    C.Advertisement   D.Newspaper

单项选择题