An electrolyte contains equal numbers of positive and negative ions in solution.

Question

An electrolyte contains equal numbers of positive and negative ions in solution. When an electric field is applied, the positive ions move in the direction of the field and the negative ions move opposite the field.

If the negative ions move faster than the positive ions, is the net current in the direction of the field, is it opposite the field, or do the two ion currents cancel to give no net current? Select the correct answer and explanation.

A) If negative charges move opposite the field, we say it is a positive current in the direction of the field. When positive charges move in the direction of the field, we say it is a positive current in the direction of the field. So both the positive ions and the negative ions contribute to a current in the direction of the field. The net current is in the direction of the field.

B) If negative charges move opposite the field, we say it is a negative current in the direction of the field. When positive charges move in the direction of the field, we say it is a positive current in the direction of the field. So the positive ions contribute to a current in the direction of the field while negative ions cancel this contribution. The two ion currents cancel to give no net current.

C) If negative charges move opposite the field, we say it is a negative current in the direction opposite to the direction of the field. When positive charges move in the direction of the field, we say it is a positive current in the direction opposite to the direction of the field. So neither the positive ions nor the negative ions contribute to a current in the direction of the field. The net current is in the direction opposite to the direction of the field.

D) If negative charges move opposite the field, we say it is a positive current in the direction opposite to the direction of the field. When positive charges move in the direction of the field, we say it is a positive current in the direction of the field. So the positive ions contribute to a current in the direction of the field, while the negative ions contribute to a current in the opposite direction. Since the negative ions move faster, the net current is in the direction opposite to the direction of the field.

B) If negative charges move opposite the field, we say it is a negative current in the direction of the field. When positive charges move in the direction of the field, we say it is a positive current in the direction of the field. So the positive ions contribute to a current in the direction of the field while negative ions cancel this contribution. The two ion currents cancel to give no net current.

C) If negative charges move opposite the field, we say it is a negative current in the direction opposite to the direction of the field. When positive charges move in the direction of the field, we say it is a positive current in the direction opposite to the direction of the field. So neither the positive ions nor the negative ions contribute to a current in the direction of the field. The net current is in the direction opposite to the direction of the field.

D) If negative charges move opposite the field, we say it is a positive current in the direction opposite to the direction of the field. When positive charges move in the direction of the field, we say it is a positive current in the direction of the field. So the positive ions contribute to a current in the direction of the field, while the negative ions contribute to a current in the opposite direction. Since the negative ions move faster, the net current is in the direction opposite to the direction of the field.

Explanation / Answer

Option A is correct.

Because, current is defined as the rate of charge flow. It by default means, rate of positive charge flow. A negative sign is added to get the direction of current because of the movement of NEGATIVE charges. Beacuse, say, if negative charges are going towards left, it is equivalent to positive charges of same magnitude going towards right. i.e in short - Current is in the direction of motion of positive charges and opposite to the direction of motion of negative charges. Take it as a thumb rule.

In the given situation, the positive charges move in the direction of the field giving rise to a current in the direction of the field and the negative charges move in the direction opposite to the direction of field giving rise to a current in the direction opposite to the direction of their motion which again is the direction along the field. So, both these currents (due to positive and negative charges) add up to give a net current in the direction of the electric field.

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