Question 1

A point charge $Q$ is placed at the origin. What is the magnitude of the electric field at a distance $r$ from the charge?

Accepted Answer

$E = \dfrac{kQ}{r^2}$

Question 2

Which of the following best describes the direction of the electric field created by a positive point charge?

Accepted Answer

Radially outward from the charge

Question 3

A proton is at the center of a hydrogen atom. The electron orbits at a radius of $0.053$ nm. What is the electric field at the position of the electron due to the proton? (Use $k = 8.99 	imes 10^9$ N·m$^2$/C$^2$, $e = 1.60 	imes 10^{-19}$ C)

Accepted Answer

$E = \dfrac{k|e|}{r^2}$

Question 4

Which of the following statements about electric field lines is FALSE?

Accepted Answer

Field lines can cross each other.

Question 5

A parallel-plate capacitor has plates of area $A$ and charge $Q$. What is the magnitude of the uniform electric field between the plates?

Accepted Answer

$E = \dfrac{Q}{\varepsilon_0 A}$

Question 6

Two charges, $Q_1 = +7.00\ \mu	ext{C}$ and $Q_2 = -5.00\ \mu	ext{C}$, are placed $0.300$ m apart. What is the direction of the electric field at a point $0.400$ m from $Q_1$ and $0.300$ m from $Q_2$?

Accepted Answer

Toward $Q_2$ (the negative charge)

Question 7

A small bead with charge $+10$ nC is at the origin. What is the magnitude of the electric field at the point $(3.0\ 	ext{cm}, 4.0\ 	ext{cm})$?

Accepted Answer

$E = \dfrac{kQ}{r^2}$, where $r = 0.05$ m

Question 8

Which of the following is a correct statement about the electric field inside a parallel-plate capacitor?

Accepted Answer

The field is uniform in both magnitude and direction.

Question 9

A droplet of oil with mass $m = 0.01$ g and charge $q = -2.0\ \mu$C is suspended in air. What electric field $E$ is required to keep it floating?

Accepted Answer

$E = \dfrac{mg}{|q|}$

Question 10

Which of the following factors does NOT affect the strength of the electric field inside a parallel-plate capacitor?

Accepted Answer

The shape of the electrodes (as long as they are close together)

Question 11

What is the direction of the electric field at the center of a square with charges $+Q$ and $-Q$ at opposite corners, measured from the $+x$ direction?

Accepted Answer

45°

Question 12

Which of the following best describes the electric field lines for a dipole?

Accepted Answer

They start on the positive charge and end on the negative charge.

Question 13

If the charge density on the plates of a parallel-plate capacitor increases, what happens to the electric field between the plates?

Accepted Answer

It increases.

Question 14

Which of the following is the correct expression for the permittivity of free space $\varepsilon_0$?

Accepted Answer

$\varepsilon_0 = \dfrac{1}{4\pi k}$

Question 15

A charged disk and a charged ring are shown. The charges on the disk are packed more closely than on the ring. Where is the electric field stronger?

Accepted Answer

Near the disk

Question 16

Which of the following is a correct application of the superposition principle for electric fields?

Accepted Answer

The net electric field is the vector sum of the fields due to each charge.

Question 17

Which of the following best describes the electric field inside an atom?

Accepted Answer

It is extremely strong, on the order of $10^{11}$ N/C.

Question 18

Which of the following is true about electric field lines for a negative point charge?

Accepted Answer

They point radially inward toward the charge.

Question 19

A test charge $q'$ is placed in an electric field $\vec{E}$. What is the force $\vec{F}$ on the test charge?

Accepted Answer

$\vec{F} = q' \vec{E}$

1/20 Electric Fields: Point Charges, Distributions, and Field Lines

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