I have a newbie question and would appreciate anyone helping.

I have read up on the basics of bitcoin, and I have learnt that if someone is given a valid 51-character private key:

e.g. 5SomeMadeUpSamplePrivateKey.....................123456789

and it is the only thing he knows i.e. no public address, no wallet amount, no computer file etc

Is it the only thing he needs to access and use whatever how many bitcoins are at that address?

Thanks v much!