The penalty for filing between one day and one month late is 5% of the amount you owed. Then, there is interest on the tax plus the penalty at about a 4% APR. The IRS will send you a bill. If you have never owed a penalty before, you can request a one-time penalty abatement. You would apply for the abatement after you get the penalty bill.
it's really going to depend on how much was unpaid on 7/15. if you owed a very small amount the IRS might not bill you because it would cost more to bill you and process the subsequent payment then the amount of the bill. But it's really up to the IRS.