When to use "Job" and "work"

[Unregistered]

Can anyone tell me a straight-forward rule of when to use "job" and "work"? I am a native English speaker, but I have trouble communicating to my students WHEN and WHY to use which one.

For example, why would you use work in this sentence:
"I am going to visit my friend in the hospital after work".
You can't say, "After job", but why? Is there a rule?

And what about
"More jobs are needed because a great number of people are out of work".
Why do you use "More jobs are needed"...

If anyone could help, I'd appreciate it.

[Tdol]

A job is a position, so in the second we use it. 'Work' is an activity, so when I finish work, I have finished what I need to do that day, and not left employment.