题干

如图所示,水平放置的两根平行金属导轨相距L=0.4m,上面有一金属棒PQ垂直导轨放置,并处于竖直向上匀强磁场中,磁场的磁感应强度大小B=0.5T,与导轨相连的电源电动势E=4.5V,内阻r=1.0Ω,电阻R=8.0Ω,其他电阻不计.闭合开关S后,金属棒PQ仍然静止不动.求:

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    Do you often communicate with your parents? Some parents are always comparing their1with themselves when they were young or with other people when they were children.
    "When I was your age," a father said to his sixteen-year-old son one day, "I was at the top of my class every year. And I2what I wanted to do when I left school. I had goals (目标). You are always at the bottom (最后) of your class3you have no goals."
    The man's son said nothing. He had already heard this from his father many times. "And when your mother was your age, " the young man's father4, " she had a good job and she5lots of money. You don't even have a part-time job .You earn nothing. " The man's son still said nothing. He had heard this before, too. "
    And when your6brother was your age, "the young man's father said, "he studied hard. But you just waste your time."
And the man's son still said nothing.
     Finally, the young man's father said, "And when Abraham Lincoln was your age, he worked all day and studied at night."
This time the son7stay silent. "
    And when Abraham Lincoln was your age, father," he said, "He was the President of8United States.9are you?"
    His father had no answer10this.
    Are your parents always comparing you with others? And what’s your reply? Do you have a better way to communicate with each other?