According to the scene of this topic, the metal bar in the magnetic field will be forced to move after being electrified, so it can explain the role of electrified wires in the magnetic field, that is, the magnetic field has a powerful effect on the current; The motor is made according to this principle; At the same time, in this experiment, the direction of stress on the conductor is related to two factors, namely, the direction of current and the direction of magnetic field, so only changing the direction of current will change the deflection direction of the straight conductor.

So the answer is: magnetic field; Motor; Change.