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I get quite confused with the matter that, when fields do negative work, an object gains energy.

So, for example. When we raise a book to a shelf, we are doing positive work equal to mgh and gravity is doing negative work equal to -mgh. Yet, the book gains potential energy. How?

Similarly, when we move a positive charge towards a negative plate through a distance s, we perform work equal to qEs, the field does -qEs and the charge gains potential energy. How?

Sorry if I am failing to spot anything really simple! I might have had a bit too much of studying today

Thanks in advance