# how to calculate the current in this circuit ( opamp)

how to calculate the current in this circuit (show with the green arrow) Can someone explain with a bit of details. In the books i got from school, they dont come close to what this looks like.

• What is this cross symbol supposed to be ? I can guess it's a lamp, so I'll assume its pretty much a 1k resistor. The voltage across the 2k resistor would be 0v (due to ideal opamp assumptions), hence the current though R would be 10/2k = 5mA
– Mike
Jan 11 '15 at 17:28
• @Mike, the voltage across the 2k resistor would be 10 V due to ideal opamp assumptions. Jan 11 '15 at 17:57
• It would be 0v referenced to ground. And 10v referenced to the op amp negative rail.
– Mike
Jan 11 '15 at 23:42

In this circuit the op amp will drive its output up or down until the - input is at the same voltage as the + input (or until the output hits one of the supply rails). The + input is connected to ground, so the - input will become 0V and R will have 10V across it. 10V / 2k = 5mA.

• ...and that is correct because on top of the 1k resistor/lamp you would have 5V, that is below 10V. If the 2k resistor was a 200$\Omega$ resistor, the solution 10V/200$\Omega$=50mA would have been wrong. Jan 11 '15 at 22:22