Q: A wire carries a current of $$2\ \mathrm{A}$$ and has a cross-sectional area of $$0.5\ \mathrm{mm}^2. $$What is the current density $$J in$$ $$\mathrm{A/m}^2$$?

$$J = 4 \times 10^6$\ \mathrm{A/m}^2$$

Question 1

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

Accepted Answer

A nearby compass needle is deflected.

Question 2

What is the SI unit of electric current, and how is it defined?

Accepted Answer

The ampere, defined as one coulomb per second: $$1\ \mathrm{A} = 1\ \mathrm{C/s}$$

Question 3

A copper wire is connected to a battery. What causes the electrons to move and create a current?

Accepted Answer

A nonuniform surface charge distribution creates an internal electric field.

Question 4

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

Accepted Answer

Kirchhoff's junction law

Question 5

What is the difference between resistivity and resistance?

Accepted Answer

Resistivity is a material property; resistance depends on material, size, and shape.

Question 6

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

Accepted Answer

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

Question 7

A wire carries a current of $$2\ \mathrm{A}$$ and has a cross-sectional area of $$0.5\ \mathrm{mm}^2. $$What is the current density $$J in$$ $$\mathrm{A/m}^2$$?

Accepted Answer

$$J = 4 	imes 10^6$\ \mathrm{A/m}^2$$

Question 8

If 120 C of charge flows through a wire in 1 minute, what is the current?

Accepted Answer

$$2\ \mathrm{A}$$

Question 9

In metals, which particles are the charge carriers responsible for electric current?

Accepted Answer

Electrons

Question 10

A wire is already full of electrons. Why does a capacitor discharge almost instantaneously when connected by a wire?

Accepted Answer

Only a slight rearrangement of charges is needed, not the movement of electrons all the way through the wire.

Question 11

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

Accepted Answer

It is opposite to the direction of electron flow.

Question 12

A 2.0-mm-diameter aluminum wire carries a current of $$800\ \mathrm{mA}. If $$the resistivity of aluminum is $$2.8 	imes 10$$^{-8}$$\ \Omega\cdot\mathrm{m}$$, what is the electric field strength inside the wire?

Accepted Answer

$$E = \frac{J}{\sigma}$$,\ where\ J = \frac{I}{A}$$ and $$\sigma = 1/\rho$$;\ so E$$ \approx 0.023\ \mathrm{V/m}$

Question 13

Which material property determines how reluctantly electrons move in response to an electric field?

Accepted Answer

Resistivity

Question 14

A wire has twice the length and twice the diameter of another wire made of the same material. What is the ratio of their resistances $$R_2$$/$$R_1$$?

Accepted Answer

$$0.5$$

Question 15

Which of the following devices is considered nonohmic?

Accepted Answer

A diode

Question 16

A battery is connected to a resistor with ideal wires. Where does the voltage drop occur?

Accepted Answer

Across the resistor only

Question 17

If the current through a resistor is $$15\ \mathrm{mA}$$ when connected to a $$9.0\ \mathrm{V}$$ battery, what is the resistance?

Accepted Answer

$$R = \frac{9.0\ \mathrm{V}}{0.015\ \mathrm{A}} = 600\ \Omega$$

Question 18

Which of the following best describes the behavior of a superconductor below its critical temperature?

Accepted Answer

It exhibits zero electrical resistance.

Question 19

A device measures body fat by sending a small electric current through the body. Why does this work?

Accepted Answer

Muscle and fat have different resistivities, affecting the current.

Electric Current, Current Density, and Ohm’s Law: Chapter 27 Study Notes

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