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Difficulty: HardElectrical Current and Electron Flow

An ammeter connected in series with a direct-current (DC) circuit registers a steady current of 250 mA250\text{ mA}. Which of the following statements correctly describes both the rate of electric charge transfer through the conductor and the physical direction of electron flow?

  1. A
    A charge of 0.25 coulombs0.25\text{ coulombs} passes through the cross-section each second, with electrons drifting from the positive terminal toward the negative terminal.
  2. B
    The electromotive force drives 4 coulombs4\text{ coulombs} of energy each second, with electron flow maintaining constant potential throughout the conductor.
  3. A charge of 0.25 coulombs0.25\text{ coulombs} passes through the cross-section each second, with electrons drifting from the negative terminal toward the positive terminal.Answer
  4. D
    A charge of 250 coulombs250\text{ coulombs} alternates direction each second, causing electrons to oscillate back and forth without net directional movement.

Answer

A charge of 0.25 coulombs0.25\text{ coulombs} passes through the cross-section each second, with electrons drifting from the negative terminal toward the positive terminal.
One ampere equals one coulomb of electrical charge passing a point per second (1 A=1 C/s1\text{ A} = 1\text{ C/s}). Therefore, a direct current of 250 mA250\text{ mA} (0.25 A0.25\text{ A}) represents an electric charge transfer rate of 0.25 coulombs per second0.25\text{ coulombs per second}. Furthermore, because electrons are negatively charged particles, electrostatic repulsion and attraction cause them to drift from the negative terminal of the DC source, through the external circuit, and into the positive terminal.

Step-by-Step Solution

1
Convert the current measurement from milliamperes (mA\text{mA}) to standard SI units of amperes (A\text{A}).
250 mA=250×10−3 A=0.25 A250\text{ mA} = 250 \times 10^{-3}\text{ A} = 0.25\text{ A}
The ampere is the standard unit of current where 1 A=1000 mA1\text{ A} = 1000\text{ mA}.
2
Relate the current in amperes to the rate of electric charge flow using the fundamental formula I=QtI = \frac{Q}{t}.
0.25 A=0.25 coulombs per second0.25\text{ A} = 0.25\text{ coulombs per second}
One ampere is defined strictly as the flow of one coulomb of electrical charge per second.
3
Determine the physical direction of electron flow in the external circuit.
Electrons flow from the negative terminal to the positive terminal.
Electrons carry a negative charge and are repelled by the negative terminal and attracted toward the positive terminal of the DC power source.

Key Concept

Electric current is the time rate of charge flow (1 A=1 C/s1\text{ A} = 1\text{ C/s}), and physical electron flow travels from negative to positive potential in a DC circuit.

Alternative Method

Calculate the total time required for a standard unit charge (1 C1\text{ C}) to pass: t=QI=1 C0.25 A=4 secondst = \frac{Q}{I} = \frac{1\text{ C}}{0.25\text{ A}} = 4\text{ seconds}. This verifies that each second carries 14 C=0.25 C\frac{1}{4}\text{ C} = 0.25\text{ C}.
Estimated Time:45s
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Topics

Electrical Current and Electron Flow

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