19.【答案】BC【解析】根据安培定则可知,直导线a中电流在直导线b处产生的磁场方向竖直向下,A项错误;设直导线a中电流在直导线b处产生的磁场磁感应强度大小为B,根据力的平衡可得直导线b中电流受到的安培力方向水平向右,有m30°=B,解得B=5mg,B项正确;根据左手定则可知,直导线b中电流方向垂直于纸面向外,C项正确;根据牛顿第三定律可知,直导线b对直导线a的作用力方向水平向左,D项错误。

书面表达One possible versionDear PeterI'm writing to invite you to see the Chinese Painting Exhibition to be held in our city.The exhibition will start at 8: 30 am on Saturday and last till 5: 00 pm on Sunday in the city museum It'ssaid that a large number of Chinese paintings, some of which are original works by famous painters like ZhangDaqian and Qi Baishi, will be on display. Besides, there will be various souvenirs to be sold. I know you re veryinterested in traditional Chinese culture, so I can t wait to tell you the news and hope we can go together.Looking forward to your reply.You

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