If two you have two unconnected batteries (one 3 v and the other 1.5 v), would there be a 1.5 v difference between their positive terminals? I thought this at first, but I saw an example of it in a textbook and it says there is no voltage difference. Is this because there is no common ground between them?

 

there is no voltage difference. Is this because there is no common ground between them?

Yes, they would have to have there negative terminals connected together, so see the voltage difference at their positive terminals.
All electricity requires a complete path for use or measurement.

 

A voltage measurement is always with respect to a reference point.
As an analogy, it makes no sense to ask what is the mileage at New York City. You can ask what is the mileage from Boston to NYC.
With voltage measurements you have to establish your reference point.

 

Remember also, that by design a voltmeter has a high impedance between the probes, so as not to disturb the circuit under test. Current still needs a complete path to flow.

 

If two you have two unconnected batteries (one 3 v and the other 1.5 v), would there be a 1.5 v difference between their positive terminals? I thought this at first, but I saw an example of it in a textbook and it says there is no voltage difference. Is this because there is no common ground between them?

There is a voltage difference between them, but to determine it you need to determine what the electric field is along some path between them and then integrate it along that path.

However, the electric field is so delicate that anything you do to determine it will change it. Therefore, for all practical purposes, the voltage difference is indeterminate. This changes when there is something, along some path, that will establish a firmer electric field along it.