Q: Given a wire with a current $$I = 2\ \mathrm{A}$$ and a cross-sectional area $$A = 1\ \mathrm{mm}^2$$, what is the current density $$J in $$the wire?

$$J = 2 \times 10$$^{3}$$\ \mathrm{A/m}^2$$

Q: A current of $$I = 3\ \mathrm{A}$$ flows through a wire for $$t = 4\ \mathrm{s}. $$How many electrons pass through the wire in this time? ($$e = 1.6 \times 10$$^{-19}$$\ \mathrm{C})$$

$$1.2 \times 10$$^{20}$$ electrons

Q: A hospital uses a copper wire to connect medical equipment. If the wire must carry $$10\ \mathrm{A}$$ and the maximum allowed current density is $$5 \times 10$$^{6}$$\ \mathrm{A/m}^2$$, what is the minimum cross-sectional area required?

$$2 \times 10$$^{-6}$$\ \mathrm{m}^2$$

Question 1

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

Accepted Answer

The flow of electrons from high potential to low potential.

Question 2

Given a wire with a current $$I = 2\ \mathrm{A}$$ and a cross-sectional area $$A = 1\ \mathrm{mm}^2$$, what is the current density $$J in $$the wire?

Accepted Answer

$$J = 2 	imes 10$$^{3}$$\ \mathrm{A/m}^2$$

Question 3

If the drift velocity of electrons in a copper wire is doubled, what happens to the current, assuming all other factors remain constant?

Accepted Answer

The current is doubled.

Question 4

A battery provides a voltage $$V$$ across a resistor $$R. $$Which equation correctly relates the current $$I$$ through the resistor?

Accepted Answer

$$I = \dfrac{V}{R}$$

Question 5

Which of the following statements about the direction of conventional current is correct?

Accepted Answer

Conventional current flows from positive to negative terminal.

Question 6

A wire carries a current of $$I = 5\ \mathrm{A}$$ for $$t = 10\ \mathrm{s}. $$How much charge $$Q$$ passes through the wire?

Accepted Answer

$$Q = 50\ \mathrm{C}$$

Question 7

Which formula gives the current density $$J in $$terms of current $$I$$ and area $$A$$?

Accepted Answer

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

Question 8

A copper wire has a resistivity $$ho = 1.7 	imes 10$$^{-8}$$\ \Omega\cdot\mathrm{m}$$, length $$L = 2\ \mathrm{m}$$, and cross-sectional area $$A = 1\ \mathrm{mm}^2. $$What is its resistance $$R$$?

Accepted Answer

$$R = 0.034\ \Omega$$

Question 9

Which of the following best explains why metals are good conductors of electricity?

Accepted Answer

Metals have free electrons that can move easily through the lattice.

Question 10

A student connects a resistor to a battery and measures a current of $$0.5\ \mathrm{A}$$ when the voltage is $$3\ \mathrm{V}. $$What is the resistance of the resistor?

Accepted Answer

$$1.5\ \Omega$$

Question 11

If the cross-sectional area of a wire is halved while keeping the current constant, what happens to the current density?

Accepted Answer

It is doubled.

Question 12

Which equation correctly relates the drift velocity $$v_d to $$the current $$I in a $$wire with $$n$$ charge carriers per unit volume, charge $$q$$, and cross-sectional area $$A$$?

Accepted Answer

$$I = n q v_d A$$

Question 13

A wire of length $$L$$ and area $$A$$ has resistance $$R. If $$the length is doubled and the area is halved, what is the new resistance?

Accepted Answer

$$R_{new} = 4R$$

Question 14

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

Accepted Answer

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

Question 15

A current of $$I = 3\ \mathrm{A}$$ flows through a wire for $$t = 4\ \mathrm{s}. $$How many electrons pass through the wire in this time? ($$e = 1.6 	imes 10$$^{-19}$$\ \mathrm{C})$$

Accepted Answer

$$1.2 	imes 10$$^{20}$$ electrons

Question 16

Which of the following is true about the relationship between resistance and temperature for a typical metal?

Accepted Answer

Resistance increases as temperature increases.

Question 17

A wire has a resistance $$R at$$ $$20^\circ\mathrm{C}. If $$the temperature coefficient of resistance is $$\alpha$$, what is the resistance $$R_T at $$temperature $$T$$?

Accepted Answer

$$R_T = R (1 + \alpha (T - 20))$$

Question 18

Which of the following best describes the physical meaning of current density $$J$$?

Accepted Answer

The current per unit area perpendicular to the flow.

Question 19

A hospital uses a copper wire to connect medical equipment. If the wire must carry $$10\ \mathrm{A}$$ and the maximum allowed current density is $$5 	imes 10$$^{6}$$\ \mathrm{A/m}^2$$, what is the minimum cross-sectional area required?

Accepted Answer

$$2 	imes 10$$^{-6}$$\ \mathrm{m}^2$$

Electric Current, Magnetic Fields, and Electromagnetic Induction: Study Notes

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