Q: A copper wire has a diameter of 2.0 mm and an electron drift speed of $$1.0 \times 10$$^{-4}$$ m/s. If the conduction electron density is 8.5$$ \times $$10^{28} m$^{-3}$$, what is the electron current $$i_e in $$the wire?

$$2.7 \times 10$$^{19}$$ electrons/s

Q: The mean time between collisions for electrons in a copper wire is $$2.8 \times 10$$^{-14}$$ s. What is the average number of collisions per second?

$$3.5 \times 10$$^{13}$$ s$$^{-1}$$

Question 1

Which of the following best describes the role of conduction electrons in a metal wire?

Accepted Answer

They form a fluid-like sea that can move throughout the solid, allowing electrical conduction.

Question 2

A copper wire has a diameter of 2.0 mm and an electron drift speed of $$1.0 	imes 10$$^{-4}$$ m/s. If the conduction electron density is 8.5$$ 	imes $$10^{28} m$^{-3}$$, what is the electron current $$i_e in $$the wire?

Accepted Answer

$$2.7 	imes 10$$^{19}$$ electrons/s

Question 3

If both the diameter of a wire and the electron drift speed are doubled, by what factor does the electron current increase?

Accepted Answer

4

Question 4

Why do real capacitors discharge almost instantaneously, even though the drift speed of electrons is very slow?

Accepted Answer

Because the wire is already full of electrons and only a slight rearrangement is needed.

Question 5

Which equation correctly relates the electron current $$i_e to $$the number density of electrons $$n_e$$, cross-sectional area $$A$$, and drift speed $$v_d$$?

Accepted Answer

$$i_e = n_e A v_d$$

Question 6

A wire carries a current of 8 A and has a square cross-section of $$2 	ext{ mm} 	imes 2 	ext{ mm}. $$What is the current density $$J in $$the wire?

Accepted Answer

$$2 	imes 10^6$ A/m^2$

Question 7

Which of the following statements about the direction of current in a metal wire is correct?

Accepted Answer

The conventional current is opposite to the direction of the electron current.

Question 8

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 9

Which of the following materials is considered ohmic?

Accepted Answer

A metal wire

Question 10

The resistance $$R of a $$wire of length $$L$$ and cross-sectional area $$A is $$given by which formula?

Accepted Answer

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

Question 11

Wire 2 is twice the length and twice the diameter of wire 1. What is the ratio $$R_2$$/$$R_1 of $$their resistances?

Accepted Answer

$$1/2$$

Question 12

A current of 120 C flows through a wire in 1 minute. What is the current in amperes?

Accepted Answer

2 A

Question 13

Which of the following best explains why a steady electric field is needed to maintain a constant current in a wire?

Accepted Answer

Because electrons experience collisions with atoms, requiring a continuous force to keep them moving.

Question 14

The mean time between collisions for electrons in a copper wire is $$2.8 	imes 10$$^{-14}$$ s. What is the average number of collisions per second?

Accepted Answer

$$3.5 	imes 10$$^{13}$$ s$$^{-1}$$

Question 15

Which equation correctly expresses Ohm’s law for a conductor?

Accepted Answer

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

Question 16

Which of the following devices is nonohmic?

Accepted Answer

A diode

Question 17

If the electric field in a wire is doubled, what happens to the electron current, assuming all other properties remain constant?

Accepted Answer

It doubles.

Question 18

A 1.0 A current passes through a 1.0-mm-diameter copper wire. If the conduction-electron density is $$8.5 	imes 10$$^{28}$$ m$$^{-3}$$, what is the electron drift speed v_d$?

Accepted Answer

$$9.6 	imes 10$$^{-5}$$ m/s

Question 19

Which of the following best describes the current density $$J in a $$wire?

Accepted Answer

$$J = \frac{I}{A}$$

Current and Resistance: Study Notes for Physics Students

Study Guide - Practice Questions

Study Guide - Flashcards