Q: A 2.0-mm-diameter copper wire carries a current of 1.0 A. If the conduction-electron density for copper is $$8.5 \times 10$$^{28}$$ \ \mathrm{m}^{-3}$$, what is the electron drift speed in the wire?

$$7.5 \times 10$$^{-5}$$ \ \mathrm{m/s}$$

Question 1

A 2.0-mm-diameter copper wire carries a current of 1.0 A. If the conduction-electron density for copper is $$8.5 	imes 10$$^{28}$$ \ \mathrm{m}^{-3}$$, what is the electron drift speed in the wire?

Accepted Answer

$$7.5 	imes 10$$^{-5}$$ \ \mathrm{m/s}$$

Question 2

A wire carries a current $$I. If $$both the wire's diameter and the electron drift speed are doubled, by what factor does the electron current increase?

Accepted Answer

It increases by a factor of 4.

Question 3

Which of the following best explains why a wire carrying current gets warm?

Accepted Answer

Collisions of electrons with atoms transfer energy to the atomic lattice, increasing thermal energy.

Question 4

A 120 C charge passes through a wire in 1 minute. What is the current in the wire?

Accepted Answer

$$2 \ \mathrm{A}$$

Question 5

Which of the following statements about current in a circuit is correct?

Accepted Answer

Current is the same at all points in a series circuit due to conservation of charge.

Question 6

A wire is made of a material with resistivity $$\rho$$, length $$L$$, and cross-sectional area $$A. $$What is the resistance $$R of $$the wire?

Accepted Answer

$$R = \dfrac{\rho L}{A}$$

Question 7

A 9.0 V battery is connected to a resistor, and a current of 15 mA flows. What is the resistance of the resistor?

Accepted Answer

$$600 \ \Omega$$

Question 8

Which of the following devices is nonohmic?

Accepted Answer

A diode

Question 9

If the resistivity of a wire is doubled and its length is halved, while the cross-sectional area remains the same, how does its resistance change?

Accepted Answer

It remains the same.

Question 10

A device measures body fat by sending a small current through the body. Why does this work?

Accepted Answer

Fat and muscle have different resistivities, so the current varies depending on the fat-to-muscle ratio.

Question 11

Which equation correctly relates current density $$J$$, conductivity $$\sigma$$, and electric field $$E in a $$conductor?

Accepted Answer

$$J = \sigma E$$

Question 12

A wire is replaced with another wire of the same material but twice the length and twice the diameter. What is the ratio of the new resistance $$R_2 to $$the original resistance $$R_1$$?

Accepted Answer

$$\dfrac{1}{2}$$

Question 13

Which of the following best describes the direction of conventional current in a metal wire?

Accepted Answer

It is opposite to the direction of electron flow.

Question 14

A junction in a circuit has three wires entering with currents of 2 A, 3 A, and 5 A, and one wire leaving. What is the current in the wire leaving the junction?

Accepted Answer

10 A

Question 15

Which of the following statements about superconductors is correct?

Accepted Answer

They have zero resistance below a certain critical temperature.

Question 16

If a wire has a current density $$J$$ and cross-sectional area $$A$$, what is the total current $$I$$?

Accepted Answer

$$I = JA$$

Question 17

A copper wire and an aluminum wire of equal length and cross-sectional area are connected to the same potential difference. Which wire carries more current?

Accepted Answer

The copper wire, because it has higher conductivity.

Question 18

Which of the following is a correct statement of Ohm’s law?

Accepted Answer

$$I = \dfrac{\Delta V}{R}$$

Question 19

A segment of wire has a smaller cross-sectional area than another segment, but both are made of the same metal and carry the same current. How does the current density $$J in $$the thinner segment compare to the thicker segment?

Accepted Answer

$$J is $$greater in the thinner segment.

Question 20

A battery is connected to a resistor with ideal wires. Which of the following best describes the potential difference across the resistor?

Accepted Answer

It is equal to the battery's potential difference.

Current and Resistance: Study Notes (Physics for Scientists and Engineers, Ch. 27)

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