Question 1

Which of the following best describes the motion of electrons in a conductor at electrostatic equilibrium?

Accepted Answer

Electrons bounce back and forth between collisions, but their average velocity is zero.

Question 2

In the presence of an electric field, what is the net effect on electron motion within a conductor?

Accepted Answer

Electrons move in parabolic trajectories between collisions, resulting in a slow net motion opposite to the electric field.

Question 3

A copper wire with a diameter of 1.0 mm carries a current with a drift velocity $$v_d = 9.6 	imes 10$$^{-5}$$ m/s. If the electron density is n_e$, what is the formula for the current I$?

Accepted Answer

$$I = n_e e A v_d$$

Question 4

Which statement about current in a circuit is correct?

Accepted Answer

Current is a scalar, but its direction is conventionally taken as the direction positive charges would move.

Question 5

Given three branches at a junction with currents $$I_1 = 2$ A (in), I_2$ = 5 A (in$$), and $$I_3 = 9$ A (out), what is the current in the fourth branch?

Accepted Answer

2 A to the right

Question 6

What is the formula for current density $$J in $$terms of current $$I$$ and cross-sectional area $$A$$?

Accepted Answer

$$J = I / A$$

Question 7

The resistance $$R of a $$wire depends on its length $$L$$, cross-sectional area $$A$$, and resistivity $$\rho. $$Which equation correctly expresses this relationship?

Accepted Answer

$$R = \rho L / A$$

Question 8

If two resistors $$R_1$$ and $$R_2$$ are connected in series, what is the equivalent resistance $$R_{eq}$$?

Accepted Answer

$$R_{eq} = R_1$ + R_2$

Question 9

If two resistors $$R_1$$ and $$R_2$$ are connected in parallel, what is the equivalent resistance $$R_{eq}$$?

Accepted Answer

$$R_{eq} = 1 / (1/R_1$ + 1/R_2$)$$

Question 10

A space heater is operated at 120 V. Heater 1 has twice the resistance of Heater 2. Which heater gives off more heat?

Accepted Answer

Heater 2

Question 11

Which formula gives the power $$P$$ dissipated by a resistor $$R$$ with a potential difference $$\Delta V$$ across it?

Accepted Answer

$$P = \Delta V^2$ / R$$

Question 12

In a parallel circuit with a 9 V battery, a 9 $$\Omega$$ resistor, and a 1 $$\Omega$$ resistor, which resistor dissipates more power?

Accepted Answer

The 1 $$\Omega$$ resistor.

Question 13

What is the potential difference across the 10 $$\Omega$$ resistor in a series circuit with a 30 V battery and resistors of 10 $$\Omega$$ and 5 $$\Omega$$?

Accepted Answer

20 V

Question 14

A battery is connected to a circuit with two resistors in parallel: 12 $$\Omega$$ and 6 $$\Omega. $$What is the equivalent resistance?

Accepted Answer

4 $$\Omega$$

Question 15

A battery is connected to a circuit with two resistors in series: 12 $$\Omega$$ and 6 $$\Omega. $$What is the equivalent resistance?

Accepted Answer

18 $$\Omega$$

Question 16

Both segments of a wire are made of the same metal. Segment 1 is thinner than segment 2. If the same current flows through both, how does the current density $$J_1 in $$segment 1 compare to $$J_2 in $$segment 2?

Accepted Answer

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

Question 17

Both segments of a wire are made of the same metal. Segment 1 is thinner than segment 2. If the same current flows through both, how does the electric field $$E_1 in $$segment 1 compare to $$E_2 in $$segment 2?

Accepted Answer

$$E_1$$ \u003e $$E_2$$

Question 18

Which material from the following table has the lowest resistivity and highest conductivity?

Accepted Answer

Silver

Question 19

A light bulb is rated at 60 W when powered by a 120 V source. What is its resistance?

Accepted Answer

$$R = 120^2$ / 60$$

Current Density, Resistance, and Ohm’s Law: Study Notes for Physics 212

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