What is the difference between tomato juice and passata? As far as I am aware passata is obtained by crushing tomatoes and then sieving the result without heating. That sounds an awful lot like juice to me. What is the difference (if any) between passata and tomato juice? Under what circumstances could they be substituted for each other? 

Pubs often sell small bottles of tomato juice so on camping trips etc I suspect it may be easier to get tomato juice than passata, although it hasn't been necessary yet.
Passata still contains the pulp of the tomato, whereas juice is literally just the juice.

So juice is thin like water whereas passata is thick like crushed tomatoes minus the seeds and skin. You probably wouldn't use tomato juice for making a marinara sauce.