Potential difference question – Immediate Help Please!

The emf and internal resistance are shows in the attached image. When the terminal voltage, Vba, is equal to -40V, what is the current through the battery, including its direction?

Solution:

VT = emf – iR (general)

In this case, VT = 40V, when considering VT = Vab = Vb – Va

VT = Vext – 28V – i(1.5Ω)

Now, I’m not entirely certain of where I went wrong, but don’t seem to know much about the current besides that it is going from a to b in the cell.

The solution shows the answer is 8.0A but I can only get that if I do 0 = 40V – 28V – i(1.5Ω) but I don’t quite see how you can apply the 40V inside (using Kirchoff’s loop rule) if that’s the terminal voltage instead of the applied voltage from an external source. Any clarification on what I am doing wrong or what I should do, and clarification on VT would be helpful.

This is what I quickly used as a reference: http://ift.tt/1maA6nM

Attached Images
File Type: png Screen Shot 2014-04-15 at 9.50.43 PM.png (32.5 KB)

http://ift.tt/QbJj45

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