Electric Current
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AP Physics 2 › Electric Current
A steady stream of positive charges moves to the left through a conductor. Which statement best describes the conventional current direction?
It is to the left, because conventional current follows positive charge motion.
It alternates direction, because charges slow down as they move.
It is to the right, because current is opposite the motion of any charge.
It is zero, because current is used up as charges move through the conductor.
Explanation
This question tests understanding of electric current. Electric current is the rate of charge flow, with conventional current defined to point in the direction positive charges move. When positive charges move left through a conductor, conventional current also points left, making B correct. This is the straightforward case where actual charge motion matches conventional current, unlike in metals where electrons move opposite to current. Choice A incorrectly states current opposes any charge motion, overgeneralizing the rule for negative charges. Always identify whether charge carriers are positive or negative to correctly determine current direction.
A steady stream of positive ions carries $3.0\times10^{-6}\ \text{C}$ of charge past a point every $2.0\ \text{ms}$. Which statement best describes the current magnitude?
$1.5\times10^{-3}\ \text{A}$
$6.0\times10^{-9}\ \text{A}$
$1.5\ \text{A}$
$0\ \text{A}$ because ions move too slowly to make current
Explanation
This question tests understanding of electric current. Electric current is the rate of charge flow, calculated as I = Q/t. With 3.0×10⁻⁶ C passing every 2.0 ms (2.0×10⁻³ s), the current is I = (3.0×10⁻⁶)/(2.0×10⁻³) = 1.5 A. The calculation is the same whether charges are positive ions or negative electrons; current magnitude depends only on the amount of charge per unit time. Choice B incorrectly multiplies instead of dividing, showing confusion about the relationship between charge, time, and current. Always divide the total charge by the time interval to find current, regardless of the charge carrier type.
Electrons drift steadily to the right in a metal wire. Which statement best describes the direction of the electric current in the wire?
It is zero, because drift motion does not count as current.
It is to the right near the battery but is used up along the wire.
It is to the left, because conventional current is opposite electron motion.
It is to the right, because current is always the direction electrons drift.
Explanation
This question tests understanding of electric current. Electric current is the rate of charge flow through a conductor, with conventional current defined as the direction positive charges would move. In metal wires, electrons (negative charges) are the charge carriers, so when they drift right, conventional current points left, opposite to electron motion, making B correct. This convention was established before the discovery of electrons and remains standard. Choice A incorrectly equates current direction with electron drift, a fundamental misconception about conventional current. Always remember that in metals, current direction is opposite to electron flow.
A steady current of $0.60\ \text{A}$ flows through a conductor for $10\ \text{s}$. The moving charges are negative electrons. Which statement best describes the net charge that passes a point?
$-6.0\ \text{C}$ passes the point.
$+6.0\ \text{C}$ passes the point.
$-0.060\ \text{C}$ passes the point.
$0\ \text{C}$ passes because current is used up in the wire.
Explanation
This question tests understanding of electric current. Electric current is the rate of charge flow, and the total charge passing a point is Q = It, where I is current and t is time. With I = 0.60 A and t = 10 s, the total charge is Q = (0.60)(10) = 6.0 C. Since the moving charges are negative electrons, -6.0 C of negative charge passes the point. The sign indicates the type of charge, not the direction of conventional current. Choice B incorrectly assigns positive charge, showing the misconception that current magnitude determines charge sign rather than the type of charge carrier. Always identify whether the charge carriers are positive or negative to determine the sign of the net charge that passes.
A steady current of $0.40\ \text{A}$ flows because positive charges move to the left in a conductor. Conventional current is defined in the direction of positive charge motion. Which statement best describes the conventional current direction?
To the left, because conventional current follows positive charge motion.
To the left, but it decreases as the charges are used up.
To the right, because conventional current is opposite any charge motion.
No current exists unless the charges move very fast.
Explanation
This question tests understanding of electric current. Electric current is the rate of charge flow, with conventional current defined as the direction positive charges move. When positive charges move to the left, conventional current also flows to the left—this direct correspondence makes positive charge problems more intuitive than electron flow problems. The current magnitude of 0.40 A tells us the rate of charge flow but not the direction, which depends on the charge carrier type. Choice A incorrectly states that conventional current opposes any charge motion, confusing the electron rule with a general principle. Always determine the charge carrier type first: conventional current follows positive charges but opposes negative charges.
In a conductor, a steady current of $2.0\ \text{A}$ is due to negative electrons moving left. Conventional current is defined as the direction positive charges would move. Which statement best describes the conventional current direction?
To the right, because conventional current is opposite electron motion.
To the left, because conventional current follows electron motion.
To the left, but it becomes smaller as electrons are used up.
No direction can be assigned because current exists only for positive charges.
Explanation
This question tests understanding of electric current. Electric current is the rate of charge flow, with conventional current defined as the direction positive charges would move. When negative electrons move left, they carry negative charge leftward, which is equivalent to positive charge moving rightward. Therefore, conventional current flows to the right, opposite to electron motion. Choice A incorrectly states that conventional current follows electron motion, a common misconception among students learning about current. Always visualize conventional current as the direction positive charges would move, which is opposite to electron flow in conductors.
In a metal wire, negative electrons drift steadily to the right at constant speed. Which statement best describes the conventional current direction?
It is to the left, opposite the motion of the electrons.
It is zero, because electrons move too slowly to make a current.
It is to the right, because current points in the direction electrons move.
It is to the right at first but is used up as electrons travel.
Explanation
This question tests understanding of electric current. Electric current is defined as the rate of flow of electric charge, measured in amperes (coulombs per second). Conventional current is defined as the direction positive charges would flow, which is opposite to the direction electrons actually move in metals. Since electrons (negative charges) drift to the right, conventional current points to the left, making B correct. Choice A incorrectly assumes current follows electron motion, a common misconception that confuses the historical convention with actual charge carrier movement. Always remember that conventional current is opposite to electron flow in conductors.
Positive ions drift steadily to the left in a saltwater tube. Which statement best describes the conventional current direction in the tube?
It is to the right, because current follows electron flow.
It is zero, because ions do not count as moving charge.
It is to the left, in the same direction as the positive ions.
It decreases to the left as positive charge is used up.
Explanation
This question tests understanding of electric current. Electric current is defined as the rate of flow of positive charge in a specific direction. When positive ions drift to the left in a saltwater tube, they carry positive charge to the left, so the conventional current is also to the left, matching the positive ion motion. This differs from metallic conductors where current opposes electron motion. Choice C incorrectly claims ions don't count as moving charge, showing the misconception that only electrons create current. Always distinguish between electron motion and conventional current direction.
In a wire, negative charges flow steadily to the east. Which statement best describes the direction of the conventional current?
It is to the west only if the electrons speed up as they move.
It is zero because negative charges cannot produce conventional current.
It is to the east because current follows the motion of any charges.
It is to the west because conventional current is opposite electron motion.
Explanation
This question tests understanding of electric current. Electric current is defined as the rate of charge flow, and conventional current is defined to flow in the direction that positive charges would move. When negative charges flow to the east, they are equivalent to positive charges flowing to the west, so conventional current flows to the west (opposite to electron motion). This convention was established before the discovery of electrons and remains the standard in circuit analysis. Choice A incorrectly assumes current follows the motion of any charges regardless of sign, missing the key distinction between electron flow and conventional current. Always determine conventional current by asking which way positive charges would need to move to create the same effect.
In a copper wire, a steady stream of $2.0\times10^{18}$ electrons passes a point each second from left to right. Which statement best describes the conventional current direction?
It is to the left, opposite the electrons’ motion.
It is to the left only if the electrons move very quickly.
It is to the right, the same as the electrons’ motion.
It is zero because electrons are negative charges.
Explanation
This question tests understanding of electric current. Electric current is defined as the rate of charge flow past a point, measured in amperes (coulombs per second). Conventional current is defined as the direction positive charges would flow, which is opposite to the direction electrons actually move in a conductor. Since electrons are moving from left to right, conventional current flows from right to left (opposite direction). Choice C incorrectly assumes that negative charges cannot produce current, when in fact current magnitude depends only on the amount of charge flowing per unit time, regardless of sign. Always remember that conventional current direction is opposite to electron flow direction in conductors.