I saw the description on the internet.
Both of them have the mean beat back or drive away, So I am confused about their using situation.
Which situation is suitable for which one?
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According to Etymonline, both words trace back to the Latin verb repellere (drive back, reject), with "repulse" arising from the past participle, repulsus.
In day-to-day speech in Canadian and American English, the difference lies in their connotations.
Repulse has two primary uses outside of academia:
Indicating that something drives someone away by evoking a sense of disgust, drawing more from the "reject" meaning of the Latin root.
(In my experience, this intuitively gets justified by perceiving "repulsive" as the primary concept with "repulse" being thought of as a derivative of it. From there, "repulsive" is seen as being a synonym for "disgusting" unless further qualified as in "repulsive force" to indicate that an academic interpretation was intended.)
(eg. "He was repulsed by what he saw")
In related words such as "repulsion" or "repulsor", which are commonly used in science-fiction when discussing devices which hover, emit blasts of force, or otherwise repel things.
(Primarily because words like "repulsion" or "repulsor" have a construction that feels more like historical academic uses of Latin and, thus, sound more appropriate as jargon than "repelling" or "repeller".)
Repel then serves all uses outside those two roles as the default choice for a native speaker.
Roughly speaking, "repel" is more intentional, and suggests that you have successfully compelled someone (usually - though not always - a threat) to leave:
"The woman repelled the burglar's attack using a can of mace" "At the Battle of Warsaw, the Poles repelled the Russian advance."
Repulsed is more that you are unintentionally compelling people to leave you, because you are so disgusting (perhaps through no fault of your own) that people don't want to associate with you:
"I wanted to ask Katie on a date, but she is so repulsed by me that she won't even speak to me." "I was repulsed by Thomas' communist views." "Her body odour is repulsive."