how can we determine that the current is negative or positive.

HI dear,

You can electrify  a glass rod by rubbing it with silk. Now suspend this rod with another glass rod electrified in the same manner. The two rods repel each other. Similarly, repeat the experiment with two ebonite rods( when rubbed with fur) . They repel each other.

Hence, early scientists decided to call the charge , a negative charge. As the charge on glass rod when rubbed with silk as positive charge. Since, like charges repel, two -ve or two +ve repel each other. On the other hand, a -ve and a +ve charge attract each other.

Hope this helps.

Regards

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