Q: A copper wire carries a current of 2 A. If the cross-sectional area is $$1 \times 10$$^{-6}$$ \ \mathrm{m}^2$$, what is the current density?

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

Q: A 1.0 mm diameter aluminum wire carries a current of 1.0 A. If the conduction electron density is $$6.0 \times 10$$^{28}$$ \ \mathrm{m}^{-3}$$, what is the electron drift speed $$v_d$$? (Use $$e = 1.6 \times 10$$^{-19}$$ \ \mathrm{C})$$

$$1.3 \times 10$$^{-4}$$ \ \mathrm{m/s}$$

Question 1

Which of the following best describes electric current in a conductor?

Accepted Answer

The flow of charge through a conductor, measured in amperes.

Question 2

A wire is connected to a battery, causing a nonuniform surface charge distribution. What is the immediate effect inside the wire?

Accepted Answer

An electric field is established inside the wire.

Question 3

Which law states that the sum of currents entering a junction equals the sum of currents leaving?

Accepted Answer

Kirchhoff's Junction Law

Question 4

What is the main difference between resistivity and resistance?

Accepted Answer

Resistivity is a property of the material, resistance depends on both material and geometry.

Question 5

According to Ohm’s law, what is the relationship between current $$I$$, potential difference $$\Delta V$$, and resistance $$R$$?

Accepted Answer

$$I = \Delta V / R$$

Question 6

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

Accepted Answer

4

Question 7

Which of the following is a direct indicator that current is flowing in a wire?

Accepted Answer

A nearby compass needle is deflected.

Question 8

The current density $$J in a $$wire is defined as:

Accepted Answer

$$J = I/A$$

Question 9

A copper wire carries a current of 2 A. If the cross-sectional area is $$1 	imes 10$$^{-6}$$ \ \mathrm{m}^2$$, what is the current density?

Accepted Answer

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

Question 10

If 120 C of charge flows through a wire in one minute, what is the current?

Accepted Answer

2 A

Question 11

In a metal, the direction of conventional current $$I is$$:

Accepted Answer

Opposite to the direction of electron flow.

Question 12

A circuit has two identical lightbulbs in series with a battery. Which bulb is brighter?

Accepted Answer

The bulbs are equally bright.

Question 13

A junction has three wires with currents of 5 A in, 2 A in, and 9 A out. What is the current in the fourth wire, and in which direction?

Accepted Answer

2 A to the right

Question 14

The resistivity $$\rho of a $$material is given by:

Accepted Answer

$$\rho = 1/\sigma$$

Question 15

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

Question 16

Which of the following materials is considered ohmic?

Accepted Answer

A metal wire

Question 17

A device sends a small current through a person’s body to measure body fat percentage. This works because:

Accepted Answer

Fat and muscle have different resistivities.

Question 18

If the electric field in a wire segment is doubled, what happens to the current density $$J ($$assuming the material is ohmic)?

Accepted Answer

It doubles.

Question 19

Which of the following best explains why a wire gets warm when current flows through it?

Accepted Answer

Collisions of electrons with atoms transfer energy to the lattice.

Question 20

A 1.0 mm diameter aluminum wire carries a current of 1.0 A. If the conduction electron density is $$6.0 	imes 10$$^{28}$$ \ \mathrm{m}^{-3}$$, what is the electron drift speed $$v_d$$? (Use $$e = 1.6 	imes 10$$^{-19}$$ \ \mathrm{C})$$

Accepted Answer

$$1.3 	imes 10$$^{-4}$$ \ \mathrm{m/s}$$

Chapter 27: Current and Resistance – Study Notes

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