This might be a naive question but if my job is 9 to 5, can I log out at 5 even though a supervisor is asking me to work more?

I am in an entry level position. I was offered a 9 to 5 job. However, it seems like my boss expects me to to work until whenever he deems as acceptable. Sometimes it’s 5:30, which is fine, but sometimes it’s 8:00. Even though he asks me to continue working, can I log off at 5:30? I don’t understand the working culture. Why would I work past my hours if I am not getting paid for them.

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