The most important example of powerful vs. powerless in Disgrace comes when David Lurie forces himself on Melanie. Although she has flirted with him, his lusts and his inability to see beyond immediate gratification leads him to commit an act that is described as "not quite rape."

She does not resist. All she does is avert herself: avert her lips, avert her eyes. She lets him lay her out on the bed and undress her: she even helps him, raising her arms and then her hips.
[...]
Not rape, not quite that, but undesired nonetheless, undesired to the core.
(Coetzee, Disgrace, Google Books)

His position as her teacher leads her to an unwilling acceptance of his attentions, as she does not want either the humiliation of accusation or his possible retribution. Despite this, after the fact she comes to stay with him for a time, during which she passively submits to his desires. He is powerful; she is not powerless, but she feels powerless because of her position as student and as a knowing accomplice to their first sexual encounter. She cannot accuse him of rape, not in her own mind, because of their first encounter, and so she finds herself unable to reconcile her feelings. For his part, David feels power in his lusts, and only after he is fired  does he begin to realize that he lost his power through his own choices.

In Disgrace, how is the relationship between the powerful and powerless portrayed?

In this novel, apartheid, the separation of the black and white races in South Africa, is coming to an end. Under apartheid, the black population had no rights and were powerless against the wealthy white population. David Lurie represents the powerful in the novel who now must accept the changes that occur with the end of apartheid. This is best seen when David's daughter refuses to prosecute the men who rape her. She feels the men who raped her did so to assert the only kind of authority they had in their repressive society. It's ironic that Lurie demands justice for his daughter in a society that does not extend justice to the largest part of its population.