Paul Hart's early season strugglers stretched their winning streak to four games with a comfortable victory over Barry Fry's under-achievers.
Chopra, on loan until the end of the season, took his tally to eight goals and after making a shaky start to life at Oakwell has finally won over the fans by taking his side to the brink of the play offs.
Peterborough made an impressive start, pinning back the home side in the first 15 minutes but then Chopra struck against the run of play.
Tony Vaughan's through ball was headed on by Mick Boulding and Chopra lifted a clever lob over the keeper.
Barnsley failed to cash in on their superiority before half time. Boulding was among the guilty as he scooped a shot over when Chris Shuker cut in a low cross.
But early in the second half, Barnsley went further in front when Shuker pounced in the 51st minute lashing in a 25-yard shot that curled inside the post.
Chopra grabbed the limelight again six minutes later after a slick move by the home side.
Shuker crossed into the area and when the ball fell to Chopra his first effort struck the post but he made no mistake with the follow up.
Substitute Daniel Nardiello almost made an immediate impact moments after coming on but his 75th minute shot was well saved.
In the 77th minute, Chopra completed the rout with his third goal of the afternoon.
Nardiello slipped the ball through the defence and Chopra kept his head, tucking a close-range effort past the keeper.