AP Physics 2 Quiz: Electric Current

What this quiz covers

This quiz focuses on Electric Current, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Physics 2.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. It is to the right, because current is opposite the motion of any charge.
2. It is to the left, because conventional current follows positive charge motion. (correct answer)
3. It is zero, because current is used up as charges move through the conductor.
4. It alternates direction, because charges slow down as they move.

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.

Question 2

1. $-6.0\ \text{C}$ passes the point. (correct answer)
2. $+6.0\ \text{C}$ passes the point.
3. $-0.060\ \text{C}$ passes the point.
4. $0\ \text{C}$ 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.

Question 3

1. To the right, because conventional current is opposite any charge motion.
2. To the left, because conventional current follows positive charge motion. (correct answer)
3. To the left, but it decreases as the charges are used up.
4. 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.

Question 4

1. To the left, because conventional current follows electron motion.
2. To the right, because conventional current is opposite electron motion. (correct answer)
3. To the left, but it becomes smaller as electrons are used up.
4. 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.

Question 5

1. It is from Y to X because conventional current follows electron flow.
2. It is from X to Y because positive charge flows from X to Y. (correct answer)
3. It is zero because only negative charges can produce current in metals.
4. It is from X to Y but decreases as charge is used up along the wire.

Explanation: This question tests understanding of electric current. Electric current is the rate at which charge flows past a point, with conventional current defined as the direction positive charges move. When positive charges flow from X to Y, the conventional current is also from X to Y, making this a straightforward case where charge motion and current direction align. Unlike with electron flow, positive charge motion directly indicates conventional current direction. Choice A incorrectly states that conventional current follows electron flow, revealing confusion between positive charge motion and electron motion. Always identify the type of charge carrier first, then apply the rule that conventional current follows positive charge motion or opposes negative charge motion.

Question 6

1. It is $0.80\ \text{A}$ to the left.
2. It is $0\ \text{A}$ because electrons are moving slowly.
3. It is $5.0\times10^{18}\ \text{A}$ to the right.
4. It is $0.80\ \text{A}$ to the right. (correct answer)

Explanation: This question tests understanding of electric current. Electric current is the rate of charge flow, calculated as I = Q/t, where Q is the charge and t is time. With 5.0×10¹⁸ electrons per second moving left, the charge flow rate is (5.0×10¹⁸)(1.6×10⁻¹⁹ C) = 0.80 C/s = 0.80 A. Since electrons move left, conventional current (defined as positive charge flow) points right with magnitude 0.80 A. Choice C incorrectly uses the electron count as current, showing the misconception of confusing particle count with charge flow rate. Always distinguish between electron motion and conventional current direction.

Question 7

1. It is to the right, because current is always the direction electrons drift.
2. It is to the left, because conventional current is opposite electron motion. (correct answer)
3. It is zero, because drift motion does not count as current.
4. It is to the right near the battery but is used up along the wire.

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.

Question 8

1. It is to the right, because current points in the direction electrons move.
2. It is to the left, opposite the motion of the electrons. (correct answer)
3. It is zero, because electrons move too slowly to make a current.
4. 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.

Question 9

1. It is to the left, in the same direction as the positive ions. (correct answer)
2. It is zero, because ions do not count as moving charge.
3. It is to the right, because current follows electron flow.
4. 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.

Question 10

1. Zero, because steady drift means charges are not accelerating.
2. To the left, opposite the motion of the negative charges. (correct answer)
3. To the right, because current is the direction electrons move.
4. To the left near the source, but it is used up along the wire.

Explanation: 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.

Question 11

1. $2.0\ \text{A}$ (correct answer)
2. $0.20\ \text{A}$
3. $20\ \text{A}$
4. $0\ \text{A}$ because electrons drift slowly in metals

Explanation: This question tests understanding of electric current. Electric current is the rate of charge flow, calculated as I = nq/t, where n is the number of charges, q is the charge per particle, and t is time. With 1.25×10¹⁹ electrons per second and each electron carrying 1.6×10⁻¹⁹ C, the current is I = (1.25×10¹⁹)(1.6×10⁻¹⁹) = 2.0 A. The magnitude is independent of whether we consider electron flow or conventional current direction. Choice B incorrectly divides by 10, showing a calculation error or unit confusion. Always multiply the particle flow rate by the fundamental charge to find current magnitude.

Question 12

1. $0.080\ \text{A}$
2. $0.0080\ \text{A}$ (correct answer)
3. $8.0\times10^{-19}\ \text{A}$
4. $0\ \text{A}$ because electrons drift slowly in a conductor

Explanation: 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.

Question 13

1. It is from left to right because electrons move left to right.
2. It is from right to left because electrons move left to right. (correct answer)
3. It is zero because current exists only if charges move quickly.
4. It is from left to right but decreases as electrons are used up.

Explanation: 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.

Question 14

1. To the right, because current is the same direction as electron drift.
2. To the left, because conventional current is opposite electron drift. (correct answer)
3. To the right, but it decreases along the segment as electrons are used up.
4. There is no current unless electrons move at near light speed.

Explanation: 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.

Question 15

1. Downward, because current is opposite the motion of positive charges.
2. Upward, because current is in the direction of positive charge motion. (correct answer)
3. Upward, but it decreases with height as charge is used up.
4. Zero, because current requires electrons rather than positive charges.

Explanation: This question tests understanding of electric current. Electric current is defined as the rate of charge flow through a conductor, with conventional current defined in the direction of positive charge motion. When positive charges move upward, the conventional current is also upward—this is the straightforward case where particle motion and current direction align. Unlike electron flow, positive charge motion directly indicates conventional current direction. Choice A incorrectly states that current opposes positive charge motion, confusing the rule for electrons with that for positive charges. Always remember that conventional current follows positive charge motion but opposes negative charge (electron) motion.

Question 16

1. It is to the left, because conventional current is opposite any charge motion.
2. It is to the right, in the same direction as the positive charge motion. (correct answer)
3. It is zero, because current exists only when charges move extremely fast.
4. It is to the right at first but then becomes smaller as charge is used up.

Explanation: 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.

Question 17

1. $1.5\times10^{-3}\ \text{A}$
2. $6.0\times10^{-9}\ \text{A}$
3. $1.5\ \text{A}$ (correct answer)
4. $0\ \text{A}$ 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.

Question 18

1. It is zero because negative charges cannot produce conventional current.
2. It is to the west because conventional current is opposite electron motion. (correct answer)
3. It is to the east because current follows the motion of any charges.
4. It is to the west only if the electrons speed up as they move.

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.

Question 19

1. It is zero because electrons are negative charges.
2. It is to the left, opposite the electrons' motion. (correct answer)
3. It is to the right, the same as the electrons' motion.
4. It is to the left only if the electrons move very quickly.

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.

Question 20

1. $10\ \text{C}$ passes the point. (correct answer)
2. $0.63\ \text{C}$ passes the point because current is used up.
3. $4.0\ \text{C}$ passes the point because seconds measure charge.
4. $0\ \text{C}$ passes unless the charges move quickly.

Explanation: This question tests understanding of electric current. Electric current is the rate of charge flow, and we can rearrange I = Q/t to find charge: Q = I × t. With a current of 2.5 A flowing for 4.0 s, the charge is Q = 2.5 A × 4.0 s = 10 C. Current represents a rate (coulombs per second), so multiplying by time gives the total charge that flows. Choice B incorrectly divides instead of multiplying, showing confusion about rearranging the current equation. Always remember that charge equals current times time (Q = It), just as distance equals speed times time.