Michael J. Sandel, the instructor for “Justice,” is recognized for his ability to capture the attention of his students. But when he announced in his first lecture that the course would introduce a Friday class this year, even his captivating cases on trolleys and torture could not stop roughly half of the 800 students from going back to their shopping lists to look at other course offerings.

While Ethical Reasoning 22: “Justice” saw a sharp decrease from its usual student enrollment numbers, Computer Science 50: “Introduction to Computer Science I” grew to become the second largest class in undergraduate enrollment. Economics 10: “Principles of Economics” remained the class with the highest undergraduate enrollment, with 715 College students.

Sandel, a government professor, said that once he announced the addition of a Friday lecture, the number of students in the class “immediately dropped.”

The new lectures are part of what Sandel called an “educational experiment” where class lectures are broadcasted to several universities abroad, creating a “global classroom” that he hopes will encourage discussions of justice “across cultures and national boundaries.”

“I don’t think this reflects any lack of enthusiasm for the ‘global classroom’ experiment, but rather an aversion to Friday classes,” he wrote in an email, as he could not be reached by phone.

This semester, 396 students enrolled for Justice, down from 797 in the fall of 2010.

CS50 increased by 20 percent from last fall, growth that CS50 instructor David J. Malan ’99 attributed to increasing interest from students specializing in the humanities, the life sciences, and the physical sciences.

“These are exciting times for computer science at Harvard, and we’re thrilled to have so many students with us this term,” Malan, who could not be reached by phone, wrote in an email.

“Trends in industry have certainly gotten students here and elsewhere all the more excited about computer science.”

This surge in students, however, is part of CS50’s steady climb in enrollees over the past few years. CS50 displayed a 32 percent rise in enrollees last fall, going from 478 undergraduates in 2010 to 579 in 2011.

Ec10 still remained the class with the most enrolled undergraduates, experiencing little variation in number of enrollees from last fall.

“Learning the basics of economics is essential for being an informed citizen, and it is a good foundation for many career paths,” wrote Mankiw in an email, as he could not be reached by phone.

“As long as students appreciate those facts, Ec10 will continue to be among the largest classes at Harvard.”

Ethical Reasoning 18: “Classical Chinese Ethical and Political Theory” collected 501 undergraduate enrollees, placing the class as the third most enrolled class.

The course’s teacher, Chinese history professor Michael Puett, said he attributes the course’s steady climb in enrollment to its rich texts and, he hoped, word of mouth.

Science of the Physical Universe 27: “Science and Cooking” saw the most dramatic increase in student enrollment, experiencing a 65 percent increase from last fall,

First offered in the fall of 2010, the course then had nearly 700 applicants and was forced to hold a lottery. Course instructor and physics professor David A. Weitz, said that he attributes the drop in 2011 enrollment to 171 undergraduates to the former perception that the class was difficult to get into, an impression that receded after the lottery was abolished.

“I’m happy,” Weitz said. “It’s going to be fun.”

—Staff writer Gina K. Hackett can be reached at ghackett@college.harvard.edu.