May be there is a bug.
See the video. 清华大学edx公开课-电路原理：
and more on edx
The Original Link: http://baham.github.io/10_05_qing-hua-da-xue-zai-xian-ke-cheng-dian-lu-yuan-li-di-%5B%3F%5D-jiang-edx.html
If you want to reprint it, please do under the CC BY-NC-SA 4.0