Backups in SQL Server don't work that way, they aren't just copies of the database files in another location but instead they are backups of the database content stored in a backup file with a different structure than the database files (you could make a backup of just a database file but even in that case the backup format would be different).
To perform a backup the most basic way is doing something like this:
BACKUP DATABASE DBName TO DISK='C:\Backup directory\DB_backup.bak' WITH FORMAT
The backup files can't be used directly by SQL Server, you need to restore it on a database first (either overwriting an existing database or creating a new one):
RESTORE DATABASE DBName FROM DISK='C:\Backup directory\DB_backup.bak' WITH REPLACE
You could need additional backup/restore parameters depending of the DB properties and the exact operation you are doing (also you can perform and restore backups from SQL Server Management Studio)