问题 问答题

在“描绘电场等势线”的实验中,有同学想模拟负点电荷附近电场在一个平面上的等势线,他在木板上依次铺上白纸、复写纸、导电纸,并用图钉固定,然后在导电纸中央平放上一个小圆柱形电极A,如图所示.则通电时应将电极A接在电源的______(选填“正极”或“负极”),本实验还需要一个______电极(请写出形状)连接电源.

答案

静电场与恒定电流场相似,用恒定电流场模拟静电场,两电极模拟两个异种电荷.要想模拟负点电荷附近电场在一个平面上的等势线,圆柱形电极A就要接电源负极,圆环电极B接电源正极,可以在AB之间形成一个四周空间流向电极A的辐射状的电流,产生恒定的电流场,可以用此电流场模拟负点电荷的静电场.

故答案为:负极,圆环形

判断题
单项选择题

It’s a typical Snoopy card: cheerful message, bright colors, though a little yellow and faded now. Though I’ve received fancier, more expensive cards over the years, this is the only one I’ve saved. One summer, it spoke volumes to me.
I received it during the first June I faced as a widow to raise two teenage daughters alone. In all the emotional confusion of this sudden single parent- hood, I was overwhelmed with, of all things, the simplest housework: leaky taps, oil’ changes, even barbecues (烧烤). Those had always been my husband’s jobs. I was embarrassed every time I hit my thumb with a hammer or couldn’t get the lawnmower (割草机) started. My uncertain at- tempts only fueled the fear inside me: How could I be both a father and mother to my girls Clearly, I lacked the tools and skills.
On this particular morning, my girls pushed me into the living room to see something. (I prayed it wasn’t another repair job). The "something" turned out to be an envelope and several wrapped bundles on the carpet. My puzzlement must have been plain as I gazed from the colorful packages to my daughters’ bright faces.
"Go ahead! Open them!" they urged. As I unwrapped the packages, I discovered a small barbecue grill (烧烤架) and all the necessary objects in- eluding a green kitchen glove with a frog pattern on it.
"But why" I asked.
"Happy Father’s Day!" they shouted together.
"Moms don’t get presents on Father’s Day," I protested.
"You forgot to open the card," Jane reminded. I pulled it from the envelope. There sat Snoopy, on top of his dog house, merrily wishing me a Happy Father’s Day. "Because," the girls said, "you’ve been a father and mother to us. Why shouldn’t you be remembered on Father’s Day"
As I fought back tears, I realized they were right. I wanted to be a "professional" dad, who had the latest tools and knew all the tricks of the trade. The girls only wanted a parent they could count on to be there, day after day, performing repeatedly the maintenance tasks of basic care and love.
The girls are grown now, and they still send me Father’s Day cards, but none of those cards means as much to me as that first one. Its simple message told me being a great parent didn’t require any special tools at all--just a willing worker.

The girls gave their mother a barbecue set probably because

A. it was what their mother wanted
B. it was a proper Father’s Day gift
C. barbecue was their favorite food
D. they wanted their mother to barbecue