If we reverse the direction of the current, the magnetic field around the wire reverses (surprise!). This reverses the force, so the wire deflects down:
A rather neat result of this effect is that if we pass alternating current (ac) through the wire, the direction of the force will continuously change. In the UK, ac is at a frequency of 50 Hz, so the wire can be seen vibrating up and down. This effect is used in speakers!
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