Many high school students struggle to understand the relationship of unemployment and inflation through the Phillips curve — even those who are taking the college-level Advanced Placement course in macroeconomics.

Others stumble on electromagnetic fields in AP physics. Or they get lost in area and volume analysis in AP calculus.

Now educators are exploring a way to help students across the country, and around the world, master such difficult concepts through online lessons posted on a Web site that specializes in delivering college-level courses for free.

On Wednesday, Davidson College in North Carolina announced a partnership with edX — a Web site overseen by Harvard University and the Massachusetts Institute of Technology — and the College Board that will create lessons for some the toughest subjects in those three AP courses. The nonprofit College Board oversees the AP program.

The plan is to develop and road-test the instructional “modules” in the Charlotte-Mecklenburg public schools in North Carolina and then make them available to the public as early as fall 2015.

“This project serves as a scalable model within the evolving education landscape as schools, teachers, organizations and families strive to make educational opportunity real for all of our kids,” said Davidson President Carol Quillen in a news release. Davidson, with about 1,800 students, is a selective, private liberal arts college near Charlotte.

In the past two years, edX and other Web sites, such as Coursera, have drawn widespread notice for offering free online courses from elite colleges and universities such as Harvard, MIT, Stanford and many others. The new edX venture with Davidson presents a push into another fast-growing area — online high school instruction.

Anant Agarwal, president of edX, said numerous high school students have taken what are called massive open online courses, or MOOCs, through the Web site since it launched in 2012. Some, he said, are “actually getting credit from their schools” for passing the MOOCs. Developing AP lessons, he said, is “a natural next step.”

The AP program offers high school courses in more than 30 subjects, from the arts to world languages, that aim to give students a taste of work at the college level. Students who pass tests in those subjects can earn college credit.