Question 1

Which of the following best describes the drift velocity $$v_d of $$electrons in a conductor when a steady current is established?

Accepted Answer

It is the average velocity of free charges due to the applied electric field.

Question 2

Given a copper wire with cross-sectional area $$A$$, number density of charge carriers $$n$$, and each carrier with charge $$q$$, which equation correctly relates the current $$I to $$the drift velocity $$v_d$$?

Accepted Answer

$$I = nqAv_d$$

Question 3

In a conductor, why is the direction of conventional current opposite to the direction of electron flow?

Accepted Answer

Because current is defined as the direction positive charges would move.

Question 4

A wire carries a steady current $$I. If $$the number density of charge carriers is doubled while keeping the cross-sectional area and drift velocity constant, what happens to the current?

Accepted Answer

The current doubles.

Question 5

Which of the following equations correctly expresses Ohm’s Law for a conductor of length $$L$$, cross-sectional area $$A$$, and resistivity $$\rho$$?

Accepted Answer

$$V = IR$$

Question 6

A copper wire of length $$L$$ and area $$A$$ has resistance $$R. If $$the wire is stretched to twice its original length without changing its volume, what happens to its resistance?

Accepted Answer

The resistance increases by a factor of four.

Question 7

Which of the following best describes the current density $$\vec{J} in a $$conductor?

Accepted Answer

It is a vector quantity whose direction is the same as the conventional current.

Question 8

If the cross-sectional area of a wire is reduced while keeping the current constant, what happens to the current density $$J$$?

Accepted Answer

It increases.

Question 9

A steady current flows through a conductor with varying cross-sectional area, as shown in the images. What can be said about the current at different cross-sections?

Accepted Answer

The current is the same at all cross-sections.

Question 10

Which of the following statements about resistance is correct?

Accepted Answer

Resistance is directly proportional to the resistivity and length, and inversely proportional to the cross-sectional area.

Question 11

A wire has a resistance $$R. If $$both its length and cross-sectional area are doubled, what happens to its resistance?

Accepted Answer

It remains the same.

Question 12

Which of the following best explains why the drift velocity of electrons in a conductor is much smaller than the speed at which the electric signal propagates?

Accepted Answer

Because the drift velocity is the average net velocity due to the electric field, while the signal propagates at nearly the speed of light.

Question 13

If the resistivity $$\rho of a $$material increases, what happens to the resistance of a wire made from that material, assuming length and area are constant?

Accepted Answer

The resistance increases.

Question 14

A current of $$2\ \mathrm{A}$$ flows through a wire for $$10\ \mathrm{s}. $$How much charge passes through the wire in this time?

Accepted Answer

$$20\ \mathrm{C}$$

Question 15

Which of the following is the correct SI unit for current density $$J$$?

Accepted Answer

$$\mathrm{A/m^2$}$$

Question 16

A wire of length $$L$$ and area $$A$$ has resistance $$R. If $$the wire is cut into two equal lengths and the two pieces are connected in parallel, what is the total resistance?

Accepted Answer

$$R/4$$

Question 17

Which of the following best describes the analogy between water flow in a pipe and electric current in a wire?

Accepted Answer

Current is analogous to the rate of water flow.

Question 18

If the electric field inside a conductor is zero, what can be said about the motion of electrons?

Accepted Answer

They move randomly with no net displacement.

Question 19

A wire has a current density $$J$$ and cross-sectional area $$A. $$What is the total current $$I$$ through the wire?

Accepted Answer

$$I = JA$$

Week 4 Lec. 1

Study Guide - Practice Questions

Study Guide - Flashcards