It's rare, in most departments, that business is slow for very long. Some days, though, the patients don't come. And then, it's important to know that you cannot be made to go home without pay.
If the option arises and you would like to go home, you can volunteer to do so; you can use time from your bank, or you can simply go unpaid. However, if you prefer to stay at work, your supervisor can work with you to find additional tasks, which could involve floating. If there are no tasks for you to do, then you may be sent home with pay. Here's what the SHARE contract says . . .
Time Off Without Pay (TOWOP)
No SHARE unit member shall be required to take TOWOP. When all other options have been explored, and there is no need to float, the SHARE unit member may take TOWOP or may use earned time or vacation/personal leave, whichever is applicable. If the SHARE member does not want to leave work (either unpaid, or using their own ET, or vacation/personal time whichever is applicable), the employee may choose to stay and be assigned reasonable other duties for which they are qualified (in this case, contract and practice restrictions on cross-campus and/or intra-campus floating will not apply.) If there are no such duties, and the employee is sent home, the employee will be paid.