Electric Current - AP Physics 2

In a wire, negative charges flow steadily to the east. Which statement best describes the direction of the conventional current?
Answer: It is to the west because conventional current is opposite electron motion. 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?
Answer: It is to the left, opposite the electrons' motion. 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.
A steady current flows in an electrolyte because positive ions drift to the right. Which statement best describes the conventional current direction?
Answer: It is to the right because conventional current is in the direction of positive charge motion. This question tests understanding of electric current. Electric current is the rate at which charge flows past a point in a circuit, and conventional current is defined as flowing in the direction that positive charges move. In an electrolyte solution, positive ions drifting to the right create a conventional current also flowing to the right, since conventional current follows positive charge motion. Unlike in metals where electrons carry current, electrolytes can have both positive and negative ions as charge carriers. Choice A incorrectly states that conventional current is always opposite to charge motion, which is only true for negative charge carriers like electrons. Always identify the sign of the charge carriers to determine conventional current direction.
A steady current is carried by positive ions drifting upward through a salt solution. The direction of the conventional current is most nearly
Answer: upward, because conventional current is in the direction of positive charge motion. This question tests understanding of electric current. Electric current represents the flow of charge through a conductor, with conventional current defined as flowing in the direction positive charges move. In this salt solution, positive ions drift upward, so the conventional current also points upward. Unlike in metals where electrons carry current, ionic solutions can have current carried by positive ions, negative ions, or both moving in opposite directions. Choice C reflects the misconception that slow-moving charges cannot create current, but current depends on the total charge passing a point per unit time, not individual particle speeds. When analyzing current direction, always identify the sign of the moving charges first.
In a wire, $Q=3.0\ \text{C}$ of positive charge passes a point in $6.0\ \text{s}$ steadily to the right. Which statement best describes the current direction?
Answer: It is to the right, in the same direction as the positive charge motion. This question tests understanding of electric current. Electric current is the rate of charge flow, calculated as I = Q/t = 3.0 C / 6.0 s = 0.50 A. Conventional current is defined to point in the direction positive charges move, so when positive charges move right, current also points right, making B correct. This contrasts with metals where electrons (negative charges) carry current opposite to their motion. Choice D incorrectly suggests current decreases as charge is "used up," misunderstanding that charge is conserved and current remains constant in a steady flow. Always remember that conventional current follows positive charge motion.
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?
Answer: $1.5\ \text{A}$ 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.
In a wire, a steady current exists while negative charges drift to the right. Which statement best describes the current direction?
Answer: To the left, opposite the motion of the negative charges. This question tests understanding of electric current. Electric current measures the flow of charge through a conductor per unit time, with conventional current defined as the direction positive charges would move. In this wire, negative charges (electrons) drift right, creating a conventional current flowing left—opposite to the electron motion. This convention, established before electrons were discovered, remains standard in electrical engineering and physics. Choice A reflects the misconception that current direction equals electron flow direction, confusing the actual charge carriers with the conventional definition. To avoid errors, always remember that in metals, conventional current opposes electron drift direction.
A steady current results when negative electrons drift downward through a vertical wire. Which statement best describes the conventional current direction?
Answer: It is upward, opposite the electron drift. This question tests understanding of electric current. Electric current is defined as the rate of flow of positive charge, which is opposite to electron flow in conductors. When negative electrons drift downward through a vertical wire, they carry negative charge downward, which is equivalent to positive charge flowing upward. Therefore, conventional current points upward, opposite to the electron drift. Choice A incorrectly follows electron motion, reflecting the misconception that current direction matches electron flow. Always distinguish between electron motion and conventional current direction.
In a metal wire, negative electrons drift steadily from left to right at a constant rate of $2.0\times10^{18}$ electrons/s. Conventional current is defined as the direction positive charge would move. Which statement best describes the direction of the conventional current in the wire?
Answer: It is from right to left because electrons move left to right. This question tests understanding of electric current. Electric current is defined as the rate of charge flow through a cross-section, measured in amperes (coulombs per second). Conventional current is defined as the direction positive charges would move, which is opposite to the direction of electron flow in metals. Since electrons (negative charges) move from left to right, conventional current flows from right to left. Choice A incorrectly assumes current follows electron motion, representing the common misconception that current direction always matches the direction of moving charges. Always distinguish between electron motion and conventional current direction by remembering that conventional current 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?
Answer: $-6.0\ \text{C}$ passes the point. 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.
In a metal conductor, negative charges drift steadily upward. Compared to the charge motion, the conventional current is directed
Answer: downward, because conventional current is opposite the motion of negative charges. This question tests understanding of electric current. Electric current represents the rate of charge flow through a conductor, with conventional current defined as the direction positive charges would flow. When negative charges (electrons) drift upward in a metal, they create a conventional current flowing downward—opposite to their motion. This opposite relationship is fundamental to understanding circuits, as conventional current was defined before the electron's discovery. Choice A commits the common error of assuming conventional current follows electron motion, rather than opposing it in metals. To correctly analyze circuits, always remember that conventional current points opposite to electron drift in metallic conductors.
Negative charges move steadily downward through a conducting solution. Which statement best describes the direction of the conventional current in the solution?
Answer: It is upward, opposite the motion of the negative charges. This question tests understanding of electric current. Electric current is defined as the rate of charge flow, with conventional current pointing in the direction positive charges would move. When negative charges move downward through a conducting solution, conventional current points upward, opposite to their motion, making B correct. This applies whether the conductor is a metal with electrons or a solution with ions. Choice A incorrectly assumes current follows any moving charge regardless of sign, missing the convention that defines current direction. Always apply the rule that conventional current is opposite to negative charge motion.
In a copper wire, negative charges move steadily left. Compared to this, the conventional current direction is
Answer: right, because conventional current is opposite the motion of negative charges. This question tests understanding of electric current. Electric current is the rate at which charge flows past a point in a circuit, and conventional current is defined as the direction positive charges would flow. In copper wire, current is carried by electrons (negative charges) moving left, which means conventional current flows right—opposite to the electron motion. This convention was established before the discovery of electrons and remains the standard in circuit analysis. Choice A incorrectly assumes current follows electron motion, a common misconception that confuses electron flow with conventional current. To avoid confusion, always remember that in metals, conventional current points opposite to electron drift direction.
In a wire, negative electrons drift left steadily. Which statement best describes the direction of conventional current and electron motion?
Answer: Current is right while electrons are left, because they have negative charge. This question tests understanding of electric current. Electric current is defined as the rate of positive charge flow, so conventional current points opposite to electron motion in conductors. When electrons drift left in a wire, they carry negative charge left, which is equivalent to positive charge moving right, so conventional current points right while electrons move left, making B correct. This opposite relationship is fundamental to understanding circuits. Choice A incorrectly assumes current follows electron motion, missing the conventional definition. Always remember that in metals, electron drift and conventional current point in opposite directions.
In a wire, negative electrons pass a cross-section at a steady rate of $5.0\times10^{17}$ electrons/s. Conventional current is defined opposite electron motion. Which statement best describes the magnitude of the current?
Answer: $0.0080\ \text{A}$ This question tests understanding of electric current. Electric current is the rate of charge flow, calculated as I = Q/t, where Q is the total charge and t is time. With 5.0×10¹⁷ electrons per second, and each electron carrying 1.6×10⁻¹⁹ C, the total charge per second is (5.0×10¹⁷)(1.6×10⁻¹⁹) = 8.0×10⁻³ C/s = 0.0080 A. The magnitude of current is the same whether we consider electron flow or conventional current; only the direction differs. Choice C incorrectly uses the charge of a single electron as the current, showing confusion between charge and charge flow rate. Always calculate current by multiplying the number of charges per second by the charge per particle.
A beam of positive ions moves steadily upward through a vacuum tube. Which statement best describes the conventional current direction in the tube?
Answer: It is upward, in the same direction as the positive ions move. This question tests understanding of electric current. Electric current is the rate at which charge flows past a point, and conventional current is defined as the direction positive charges move. In this vacuum tube, positive ions are the charge carriers moving upward, so conventional current is also upward, making A correct. Unlike metals where electrons carry current opposite to the conventional direction, here positive charges carry the current in the same direction as their motion. Choice C incorrectly assumes current requires metals, missing that any moving charge constitutes current. Always identify the charge carriers and their sign to determine current direction.
A steady current exists in a wire when negative charges drift left. Which statement best describes the conventional current direction?
Answer: Right, opposite the drift direction of the negative charges. This question tests understanding of electric current. Electric current measures charge flow rate through a conductor, defined as charge per unit time passing a cross-section. Conventional current is defined as flowing in the direction positive charges would move, which means opposite to electron (negative charge) motion in metals. With negative charges drifting left, conventional current points right. Choice A represents the misconception that current direction equals electron drift direction, confusing the physical charge carriers with the conventional definition used in circuit analysis. Always apply the fundamental rule: in metals, conventional current flows opposite to electron motion.
A steady current is observed in a wire even though electrons drift very slowly. Which statement best describes this situation?
Answer: It is possible, because current depends on charge per time, not drift speed alone. This question tests understanding of electric current. Electric current is the rate of charge flow (I = Q/t), not the speed of individual charges. Even though electrons drift slowly (typically mm/s), many electrons pass any point per second, creating substantial current, making B correct. The slow drift speed is compensated by the enormous number of free electrons in conductors. Choice A incorrectly assumes current requires fast-moving charges, confusing drift velocity with charge flow rate. Always distinguish between the drift speed of charges and the rate at which charge passes a point.
In a wire segment, negative electrons drift steadily to the right. Conventional current is defined as the direction of positive charge motion. Which statement best describes the conventional current direction?
Answer: To the left, because conventional current is opposite electron drift. This question tests understanding of electric current. Electric current is defined as the rate of charge flow, with conventional current defined as the direction positive charges would move. In metals, current is carried by negative electrons, and conventional current flows opposite to electron drift. Since electrons drift to the right, conventional current flows to the left. Choice A incorrectly states that current follows electron drift, representing the common misconception that current direction always matches particle motion. Always remember that conventional current is opposite to electron flow but follows positive charge flow.
A beam of positive ions carries a steady charge flow of $1.2\ \text{C}$ past a point in $0.30\ \text{s}$ . Which statement best describes the current magnitude?
Answer: It is $4.0\ \text{A}$ . This question tests understanding of electric current. Electric current is the rate at which charge flows past a point, calculated using I = Q/t where Q is the charge and t is the time interval. With 1.2 C passing in 0.30 s, the current magnitude is I = 1.2 C / 0.30 s = 4.0 A. The sign of the charge carriers (positive ions in this case) affects the direction of conventional current but not its magnitude. Choice A incorrectly divides in the wrong order (0.30/1.2), demonstrating confusion about which quantity goes in the numerator versus denominator. Always remember that current equals charge divided by time, not time divided by charge.

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