Compel vs. impel

What’s the difference between compel and impel? In practical usage it’s a matter of degree, with compel being stronger than impel.

Impel

A person who is impelled has been persuaded and acts at least partially on his or her own volition—for example:

Surely we won’t need a third cataclysm to impel us, at long last, to take serious action? [Weekly Volcano]

The bridge-burning effect is the underlying fear that seems often to impel journalists not to print unflattering information about their sources . . . [The Daily Athenaeum]

Compel

Compel implies that the person being compelled has no choice in the matter and is being coerced—for example:

Schwarzenegger then sought a court order to compel Chiang to honor the order. [Reuters]

He stressed investigators cannot legally compel anyone to talk. [The Chronicle-Telegram]

Compel is sometimes used where impel would make more sense—for example:

Ryan insists the right on-air talent will help transform WCFS into more of a “companion” to listeners and compel them to stay tuned in longer. [Chicago Sun-Times]

And in this weird case, the writer, apparently in an effort to avoid using propulsive and propel so close to each other, uses compel where he clearly means propel:

. . . and composer Hans Zimmer, whose propulsive score helps compel the action forward. [LA Times]