问题 计算题

水平放置的两个平行金属轨道相距0.2m,上面有一质量为0.04kg的不计内阻的均匀金属棒ab,电源电动势为6V,内阻为0.5Ω,滑动变阻器调到2.5Ω时,要在金属棒所在位置施加一个垂直于ab的匀强磁场,才能使金属棒ab对轨道的压力恰好为零。(g=10m/s2)

(1)求匀强磁场的大小和方向.

(2)若保持磁感应强度B的大小不变,方向顺时针转37°,此时ab仍然静止,则轨道对ab的支持力和摩擦力为多大?

答案

(1)根据闭合回路欧姆定律,电路中电流为

       (2分)

根据导体棒平衡,安培力与重力二力平衡,安培力方向竖直向上,大小

F=mg=0.4N

根据安培力性质

             (2分)

根据左手定则,磁感应强度方向水平向左。  (1分)

(2)当磁感应强度顺时针转θ=37º时,安培力也顺时针旋转θ,且大小不变,此时导体棒受力如图所示,根据导体棒水平方向受力平衡

Fsinθ="f       "

               

根据导体棒竖直方向受力平衡

Fcosθ+N="mg                  " (3分)

带入数据解得

N=0.08N,f="0.24N             " (2分)

单项选择题

阅读下列短文:从每题所给的A、B、C、D四个选项中,选出最佳选项,将正确的选项涂在答题卡上。B

Surviving Hurricane Sandy(飓风桑迪)

Natalie Doan,14, has always felt lucky to live in Rockaway, New York. Living just a few blocks from the beach, Natalie can see the ocean and hear the wave from her house. “It’s the ocean that makes Rockaway so special,” she says.

On October 29, 2012, that ocean turned fierce. That night, Hurricane Sandy attacked the East Coast, and Rockaway was hit especially hard. Fortunately, Natalie’s family escaped to Brooklyn shortly before the city’s bridge closed.

When they returned to Rockaway the next day, they found their neighborhood in ruins. Many of Natalie’s friends had lost their homes and were living far away. All around her, people were suffering, especially the elderly. Natalie’s school was so damaged that she had to temporarily attend a school in Brooklyn.

In the following few days, the men and women helping Rockaway recover inspired Natalie. Volunteers came with carloads of donated clothing and toys. Neighbors devoted their spare time to helping others rebuild. Teenagers climbed dozens of flights of stairs to deliver water and food to elderly people trapped in powerless high-rise buildings.

“My mom tells me that I can’t control what happens to me,” Natalie says. “but I can always choose how I deal with it.”

Natalie’s choice was to help.

She created a website page matching survivors in need with donors who wanted to halp. Natalie posted introduction about a boy named Patrick, who lost his baseball card collecting when his house burned down. Within days, Patrick’s collection was replaced.

In the coming months, her website page helped lots of kids: Christopher, who received a new basketball; Charlie, who got a new keyboard. Natalie also worked with other organizations to bring much-need supplies to Rockaway. Her efforts made her a famous person. Last April, she was invited to the White House and honored as a Hurricane Sandy Champion of Change.

Today, the scars(创痕)of destruction are still seen in Rockaway, but hope is in the air. The streets are clear, and many homes have been rebuilt. “I can’t imagine living anywhere but Rockaway,” Natalie declares. “My neighborhood will be back, even per than before.”

When Natalie returned to Rockaway after the hurricane ,she found().

A.some friends had lost their lives

B.her neighborhood was destroyed

C.her school had moved to Brooklyn

D.the elderly were free from suffering

单项选择题