如图所示,圆环a和圆环b的半径之比为2∶1,两环用同样粗细、同种材料制成的导线连成闭合回路,连接两环的导线电阻不计,匀强磁场的磁感应强度变化率恒定.则在a、b环分别单独置于磁场中的两种情况下,M、N两点的电势差之比为( )
A:4∶1
B:1∶4
C:2∶1
D:1∶2
Do you kow that animals have love for each other and their children just like us humans? Let me tell you some moving stories of great animal parents. Several years ago a heavy rain hit a town and made the river go up. When the rain stopped, people found that a dog swam to an island in the river twice a day for two weeks. Why did she do so? Her four children were there. The mother swam there every day to feed her babies. This true story moved many people. Another story is about chimpanzees(猩猩). A scientist named Jane Goodall spent 4 years living with chimpanzees in Africa. She found that chimpanzees also cared for those that were hurt or got lost. “It is not only humans who have duties, animals also do,” she said. Two birds in Chengdu also showed their parental(父母的) love. Their baby was hurt and fell on the street in the center of the city. Cars were driving past but the brave parents rushed down to the road and took the little bird away with their claws(爪子).
如图的秦代砖文为“海内皆臣,岁登成熟,道毋饥人,践此万岁”。 其中“海内皆臣”所反 映的历史信息主要是( )
1911年,一位科学家用一束平行的高速运动的α粒子(带2个单位正电荷的氦原子核)轰击金箔时,出现了三种实验现象:
(1)有一小部分α粒子改变了原来的运动路径,原因是α粒子途经金原子核附近时,受到了斥力而稍微改变了运动方向;
(2)大多数α粒子不改变原来的运动方向,顺利地穿过了金箔,原因____ ;
(3)极少数α粒子轰击金箔时被弹了回来,原因是____ .