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What does the negative sign tell?

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- Thread starter Zubair Ahmad
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What does the negative sign tell?

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Does it mean I has to be always kept positive?

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What does the negative sign tell?

This is perfectly VAGUE! You need to make some effort in describing the situation.

Current is defined as the rate of charge flow across a cross-sectional surface. If the charge passing through per unit time is decreasing, dq/dt is still positive, but dI/dt is negative! This means that current is decreasing over time.

dq/dt will have a negative value if (i) q is a negative charge OR (ii) the positive charge is moving in the OPPOSITE direction. This implies that current is in the opposite direction.

Zz.

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NoDoes it mean I has to be always kept positive?

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In the standard justification for ##I=dq/dt##, one imagines amount of charge ##dq## moving by a fixed point in time ##dt##. In that respect, current ##I## is like a one-dimensional speed ##v=ds/dt##. If a one-dimensional speed (for whatever reason) turns out to be negative, we say that the velocity is "opposite" to the assumed direction; we never say that the speed is in the opposite direction. Likewise, it seems to me that, for consistency and to avoid confusing the vector with its magnitude, when the current ##I## or ##dq/dt## turn out to be negative, it is appropriate to think of the current density (or direction of carrier flow) being opposite to the assumed direction.

I prefer to think of current direction in terms of ##\vec J## in which case the sign of the charge carriers does not matter. It helps me keep negative signs sorted out.

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