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4) Suppose I have a motor, and I apply 115V(AC RMS) to its terminals.Then
the average emf across its terminals is 115V. That tells me the emf,not
the back emf. If I measure the current under these conditions, thatgives
me one point on the I-V curve. Call this point (V1,I1). One point isnot
enough; I need another point in order to pin down the first-ordercertain
properties of this device.
To get a second data point, I briefly open-circuit the terminals. The
still-spinning motor will act like a generator. It will produce a
voltage (i.e. emf) on its terminals. This is the open-circuitemf".
voltage. Call this point (V0,I0) where I0=0 and V0 is called the "back
Given these two points, (V1,I1) and (V0,I0), we can draw a straight line
and have a nice first-order model of the behavior of the device.