问题 问答题

如图所示,一质量为m=0.016kg、长L=0.5m、宽d=0.1m、电阻R=0.1Ω的矩形线圈,从h1=5m的高处由静止开始下落,然后进入高度为h2(h2>L)的匀强磁场.下边刚进入磁场时,线圈正好作匀速运动.线圈的下边通过磁场所经历的时间t=0.15s.取g=10m/s2

(1)求匀强磁场的磁感应强度B.

(2)求线圈的下边刚离开磁场的瞬间,线圈的加速度的大小和方向.

(3)在线圈的下边通过磁场的过程中,线圈中产生的焦耳热Q是多少?通过线圈的电荷量q是多少?

答案

(1)设线圈刚进入磁场时的速度为v0,则据机械能守恒定律可得:

mgh1=

1
2
m
v20

所以有:v0=

2gh1
=10m/s

线框产生的感应电动势为:E=Bdv0

感应电流为:I=

Bdv0
R

线框所受的安培力大小为:F=BdI

则得:F=

B2d2v0
R

根据平衡条件可得mg=F,即为:mg=

B2d2v0
R

代人数据解得:B=0.4T

(2)因为线圈的下边进入磁场后先做匀速运动,用时为:t1=

L
v0
=0.05s,

所以线圈做加速运动的时间为:t2=t-t1=0.1s

线圈的下边刚离开磁场的瞬间的速度为:v=v0+gt2=11m/s;

线圈的加速度的大小为:a=

B2d2v-mgR
mR
=1m/s2,方向向上.

(3)在线圈的下边通过磁场的过程中,线圈中产生的焦耳热为:Q=mgL=0.08J

电量为:q=It=

E
R
t=
△φ
R
=0.2C

答:(1)匀强磁场的磁感应强度B为0.4T.

(2)线圈的下边刚离开磁场的瞬间,线圈的加速度的大小为1m/s2,方向向上.

(3)在线圈的下边通过磁场的过程中,线圈中产生的焦耳热Q是0.08J,通过线圈的电荷量q是0.2C.

单项选择题
单项选择题

The question of whether war is inevitable is one, which has concerned many of the world’s great writers. Before considering this question, it will be useful to introduce some related concepts. Conflict, defined as opposition among social entities directed against one another, is distinguished from competition, defined as opposition among social entities independently striving for something, which is in inadequate aupply. Competitors may not be aware of one another, while the parties to a conflict are. Conflict and competition are both categories of opposition, which has been defined as a process by which social entities function is the disservice of one another.

Opposition is thus contrasted with cooperation, the process by which social entities function in the service of one another. These definitions are necessary because it is important to emphasize that competition between individuals or groups in inevitable in a world of limited resources, but conflict is not. Conflict, nevertheless, is very likely to occur, and is probably an essential and desirable element of human societies.

Many authors have argued for the inevitability of war from the premise that in the struggle for existence among animal species, 0nly the fittest survive. In general, however, this struggle in nature is competition, not conflict. Social animals, such as monkeys and cattle, fight to win or maintain leadership of the group. The struggle for existence occurs not in such fights, but in the competition for limited feeding areas and for occupancy of areas free from meat-eating animals. Those who fail in this competition starve to death or become victims to other species. This struggle for existence does not resemble human war, but rather the competition of individuals for jobs, markets, and materials. The essence of the struggle is the competition for the necessities of life that are insufficient to satisfy all.

Among nations there is competition in developing resources, trades, skills, and a satisfactory way of life. The successful nations grow and prosper; the unsuccessful decline. While it is true that this competition may induce efforts to expand territory at the expense of others, and thus lead to conflict, it cannot be said that war-like conflict among nations is inevitable, although competition is.

In the first paragraph, the author gives the definitions of some terms in order to()

A. argue for the similarities between animal societies and human societies

B. smooth out the conflicts in human societies

C. distinguish between two kinds of opposition

D. summarize the characteristic features of opposition and cooperation