How can I relate the idea of 'knowledge is power' to the play Othello with textual evidence of Iago's power and the powerlessness of others?
Providing texual evidence and anaalysis of Iago and power/powerlessness of other characters
Othello is a play of contrasts. Good and evil, love and hate, appearance and reality and even black and white, to a degree. The drama of the play centers on these themes and no less on reason and passion.
Iago knows how to manipulate situations and people. He is astute in his recognition of the failures and flaws of all the other characters, especially Othello. Even Othello's strengths are used against him. His high sense of honor and therefore his misguided trust of "honest" Iago, decorated in war, contributes to his downfall.
Ironically, Iago tells his audience from the start - "I am not what I am" (I.i.65) and unfortunately he intends to usurp this characteristic. Shakespeare allows Iago to speak most freely in his soliloquies and even the audience is forewarned that disaster will strike.
I'll pour this pestilence into his ear(II.iii.351)
This is a sharp indicator that Iago will persevere until Othello is destroyed. Iago knows he cannot simply tell Othello about Desdemona's (supposed) infidelity and he sets up a web of events that would convince anyone - and certainly a person predisposed to jealousy. Iago knows this all to well as he has the same character flaw.
At the start of Act IV, Iago paints visual images for Othello to envision, such as finding Desdemona and Cassio in an embrace or in bed together. He then allows Othello's own imagination to take over
fanning the flames of murderous jealousy.
Iago uses Roderigo's feelings towards Desdemona to manipulate him to do his bidding. He gets Cassio to seek restitution with Othello through Desdemona and he uses Desdemona's kind and willing nature against her as she will gladly help her friend Cassio.
There may have been other manipulative characters in Othello and indeed such people are in our midst. It is Iago's consistency and drive to be vindicated for something he cannot even explain that makes him so dangerous. The punishment certainly does not fit the crime.
It is this consistency that fools the other characters into thinking that he has their best interests at heart when in fact he is "an eternal villain."