The voltage difference between the input terminals of an ideal op-amp that is used in a circuit that employs negative feedback is zero.
When the ideal op amp is used in a circuit without a feedback loop, does the assumption that the voltage difference between the input terminals is zero still hold?
Without negative feedback to close the op-amp loop, the input voltage difference can be anything i.e. it is subject to the vagaries of the circuit.
From what I understand, this is only true if there is a feedback loop connecting the output and input terminals.
Usually true but, without the feedback, the circuit could force the inputs to be the same value such as by connecting them together.