Q: A copper wire of diameter 2 mm carries a current of 1.3 A. What is the current density $$J in $$the wire?

$$J = 2 \times 10^6$\ \mathrm{A/m^2$}$$

Question 1

A particle of mass $$m$$ and charge $$q is $$placed in a uniform electric field $$E. $$What is the acceleration $$a of $$the particle?

Accepted Answer

$$a = \frac{q}{m} E$$

Question 2

If a conductor is placed in an external electric field, what happens to the electric field inside the conductor after equilibrium is reached?

Accepted Answer

The electric field inside becomes zero due to redistribution of charges.

Question 3

Which statement best describes the motion of conduction electrons in a metal when no electric field is present?

Accepted Answer

They move randomly due to thermal energy, exhibiting Brownian motion.

Question 4

When an electric field $$E is $$applied to a conductor, how do the electrons move?

Accepted Answer

They drift opposite to the direction of the electric field, superimposed on their random motion.

Question 5

According to the Drude model, what is the average drift velocity $$v_d of $$electrons in a conductor under an electric field $$E$$?

Accepted Answer

$$v_d = \frac{e 	au}{m} E$$

Question 6

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

Accepted Answer

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

Question 7

Every minute, 120 C of charge flow through a wire. What is the current in the wire?

Accepted Answer

$$I = 2\ \mathrm{A}$$

Question 8

A copper wire of diameter 2 mm carries a current of 1.3 A. What is the current density $$J in $$the wire?

Accepted Answer

$$J = 2 	imes 10^6$\ \mathrm{A/m^2$}$$

Question 9

Two segments of a wire made of the same material carry a current $$I_1$$ into segment 1 and $$I_2$$ into segment 2. How does $$I_1$$ compare to $$I_2$$?

Accepted Answer

$$I_1$$ = $$I_2$$

Question 10

If two segments of a wire have different cross-sectional areas but carry the same current, how does the current density $$J_1 in $$the smaller segment compare to $$J_2 in $$the larger segment?

Accepted Answer

$$J_1$$ \u003e $$J_2$$

Question 11

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

Accepted Answer

It is the direction in which positive charges would move.

Question 12

The microscopic form of Ohm's Law relates current density $$J$$ and electric field $$E as$$:

Accepted Answer

$$J = \sigma E$$

Question 13

A metal rod of length $$L$$, cross-sectional area $$A$$, and resistivity $$\rho$$ has resistance $$R. $$Which equation gives $$R$$?

Accepted Answer

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

Question 14

A Nichrome wire of radius 2.00 mm is used to wind a heating coil. If the coil must draw a current of 8.50 A when a voltage of 120 V is applied, what resistance must the coil have?

Accepted Answer

$$R = 14.1\ \Omega$$

Question 15

Which property of a material determines how easily it conducts electricity?

Accepted Answer

Conductivity $$\sigma$$

Question 16

If the resistivity $$\rho of a $$wire increases, what happens to its resistance $$R ($$assuming length and area are constant)?

Accepted Answer

Resistance increases.

Question 17

Which equation correctly relates voltage $$V$$, current $$I$$, and resistance $$R in a $$circuit?

Accepted Answer

$$V = I R$$

Question 18

In the Drude model, what physical quantity does $$	au$$ represent?

Accepted Answer

The mean time between collisions of electrons.

Question 19

Why does the light turn on instantly when you flip a switch, even though the drift velocity of electrons is very slow?

Accepted Answer

The electric field propagates rapidly through the circuit, causing electrons everywhere to start moving almost simultaneously.

Electric Current, Conductors, and Ohm's Law

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