Q: A copper wire has a cross-sectional area of $$1.00 \times 10$$^{-6}$$\ \mathrm{m}^2$$ and carries a current of $$2.0\ \mathrm{A}. If $$the number density of conduction electrons in copper is $$1.10 \times 10$$^{29}$$\ \mathrm{m}^{-3}$$, what is the drift speed of the electrons? (Assume the charge of an electron is $$1.60 \times 10$$^{-19}$$\ \mathrm{C}.)$$

$$1.1 \times 10$$^{-4}$$\ \mathrm{m/s}$$

Q: A resistor has a resistance of $$19.8\ \Omega at$$ $$15.0\ ^\circ\mathrm{C}$$ and $$25.0\ \Omega at$$ $$85.0\ ^\circ\mathrm{C}. $$What is the temperature coefficient of resistivity $$\alpha$$? (Give your answer in $$\mathrm{C}^{-1}.)$$

$$3.75 \times 10$$^{-3}$$\ \mathrm{C}^{-1}$$

Q: If a current of $$80.0\ \mathrm{mA}$$ flows through a wire for $$10.0$$ minutes, how many electrons pass through a cross-section of the wire? (Charge of electron $$= 1.60 \times 10$$^{-19}$$\ \mathrm{C})$$

$$3.00 \times 10$$^{20}$$ electrons

Question 1

A copper wire has a cross-sectional area of $$1.00 	imes 10$$^{-6}$$\ \mathrm{m}^2$$ and carries a current of $$2.0\ \mathrm{A}. If $$the number density of conduction electrons in copper is $$1.10 	imes 10$$^{29}$$\ \mathrm{m}^{-3}$$, what is the drift speed of the electrons? (Assume the charge of an electron is $$1.60 	imes 10$$^{-19}$$\ \mathrm{C}.)$$

Accepted Answer

$$1.1 	imes 10$$^{-4}$$\ \mathrm{m/s}$$

Question 2

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

Accepted Answer

It is opposite to the direction of the flow of electrons.

Question 3

A resistor has a resistance of $$19.8\ \Omega at$$ $$15.0\ ^\circ\mathrm{C}$$ and $$25.0\ \Omega at$$ $$85.0\ ^\circ\mathrm{C}. $$What is the temperature coefficient of resistivity $$\alpha$$? (Give your answer in $$\mathrm{C}^{-1}.)$$

Accepted Answer

$$3.75 	imes 10$$^{-3}$$\ \mathrm{C}^{-1}$$

Question 4

Which equation correctly relates the resistance $$R of a $$uniform cylindrical conductor to its resistivity $$\rho$$, length $$L$$, and cross-sectional area $$A$$?

Accepted Answer

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

Question 5

A resistor with a $$15\ \mathrm{V}$$ potential difference across its ends dissipates thermal energy at a rate of $$327\ \mathrm{W}. $$What is the resistance of the resistor?

Accepted Answer

$$0.688\ \Omega$$

Question 6

Which of the following statements about ammeters and voltmeters is correct?

Accepted Answer

Ammeters are connected in series and have low internal resistance.

Question 7

The current density $$J in a $$conductor is given by which of the following expressions?

Accepted Answer

$$J = nqv_d$$

Question 8

A material is considered ohmic if which of the following relationships holds?

Accepted Answer

$$\Delta V \propto I$$

Question 9

The resistivity of a material as a function of temperature is given by $$\rho(T) = \$$rho_0$$ (1 + \alpha (T - T_0$)). $$What does $$\alpha$$ represent?

Accepted Answer

The temperature coefficient of resistivity

Question 10

Which of the following best describes a superconductor?

Accepted Answer

A material with zero resistivity below a critical temperature

Question 11

If a current of $$80.0\ \mathrm{mA}$$ flows through a wire for $$10.0$$ minutes, how many electrons pass through a cross-section of the wire? (Charge of electron $$= 1.60 	imes 10$$^{-19}$$\ \mathrm{C})$$

Accepted Answer

$$3.00 	imes 10$$^{20}$$ electrons

Question 12

Which of the following is NOT a correct way to increase the resistance of a cylindrical conductor?

Accepted Answer

Decrease its length

Question 13

A resistor is tested and the plot of $$V$$ versus $$I is $$not a straight line. What does this indicate about the resistor?

Accepted Answer

It is non-ohmic

Question 14

Which of the following equations gives the power dissipated by a resistor in terms of current $$I$$ and resistance $$R$$?

Accepted Answer

$$P = I^2$ R$$

Question 15

A voltmeter is used to measure the potential difference across a resistor. How should it be connected in the circuit?

Accepted Answer

In parallel with the resistor

Question 16

Which of the following best explains why the drift speed of electrons in a conductor is much less than the average speed of electrons?

Accepted Answer

The random motion of electrons cancels out, so only a small net drift occurs due to the electric field.

Question 17

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

Accepted Answer

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

Question 18

If the resistivity of a material increases with temperature, what type of temperature coefficient of resistivity $$\alpha$$ does it have?

Accepted Answer

Positive

Question 19

A cylindrical wire is stretched to double its original length without changing its volume. What happens to its resistance?

Accepted Answer

It increases by a factor of 4

Question 20

Which of the following is a correct expression for Ohm's law in terms of current density $$J$$ and electric field $$E$$?

Accepted Answer

$$J = \sigma E$$

Current, Resistance, and Resistivity: PHY 131 Study Notes

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