Q: A dipole of moment $$\vec{p} is $$placed in a uniform electric field $$\vec{E}. $$What is the torque $$\vec{\tau}$$ acting on the dipole?

$$\vec{\tau} = \vec{p} \times \vec{E}$$

Question 1

Which of the following statements best describes the principle of superposition for electric forces?

Accepted Answer

The force on a charge due to a group of other charges is the sum of the forces due to each charge, calculated one at a time, and is unaffected by the presence of the other charges.

Question 2

A glass rod is rubbed with silk and becomes positively charged. What happens at the atomic level?

Accepted Answer

Electrons are transferred from the glass rod to the silk, leaving the rod with a net positive charge.

Question 3

Which of the following is the correct expression for the electric field $$E at a $$distance $$r$$ from a point charge $$q in $$vacuum?

Accepted Answer

$$E = \frac{1}{4\pi\$$varepsilon_0$$} \frac{q}{r^2$}$$

Question 4

A charged comb attracts small pieces of paper even though the paper is initially uncharged. What is the best explanation for this phenomenon?

Accepted Answer

The electric field of the comb induces a dipole moment in the paper, and the non-uniform field exerts a net force on the induced dipole.

Question 5

Which of the following is NOT a property of electric field lines?

Accepted Answer

Electric field lines can cross each other.

Question 6

The SI unit of electric flux is:

Accepted Answer

$$\mathrm{N\, m^2$\, C$$^{-1}$$}$$

Question 7

A point charge $$q is $$placed at the center of a spherical Gaussian surface of radius $$r. $$What is the total electric flux through the surface?

Accepted Answer

$$\frac{q}{\$$varepsilon_0$$}$$

Question 8

Which of the following best describes the quantization of electric charge?

Accepted Answer

Electric charge exists only in integer multiples of a basic unit $$e.$$

Question 9

A thin, uniformly charged infinite plane sheet has a surface charge density $$\sigma. $$What is the magnitude of the electric field just outside the sheet?

Accepted Answer

$$E = \frac{\sigma}{2\$$varepsilon_0$$}$$

Question 10

A dipole of moment $$\vec{p} is $$placed in a uniform electric field $$\vec{E}. $$What is the torque $$\vec{	au}$$ acting on the dipole?

Accepted Answer

$$\vec{	au} = \vec{p} 	imes \vec{E}$$

Question 11

Which of the following is the correct expression for the electric field at a distance $$r$$ from an infinitely long straight wire with uniform linear charge density $$\lambda$$?

Accepted Answer

$$E = \frac{\lambda}{2\pi\$$varepsilon_0$$ r}$$

Question 12

A uniformly charged thin spherical shell of radius $$R$$ carries a total charge $$q. $$What is the electric field at a point inside the shell ($$r \u003c R$$)?

Accepted Answer

$$E = 0$$

Question 13

Which of the following best explains why the gravitational force between two protons is much weaker than the electrostatic force between them?

Accepted Answer

The ratio of the electrostatic force to the gravitational force is approximately $$10^{36}$$ for two protons.

Question 14

A cube of side $$a is $$placed in a uniform electric field $$E$$ parallel to one of its axes. What is the net electric flux through the cube?

Accepted Answer

Zero, because the number of field lines entering and leaving the cube is equal.

Question 15

Which of the following is the correct definition of surface charge density $$\sigma$$?

Accepted Answer

$$\sigma = \frac{\Delta Q}{\Delta S}$$

Question 16

If $$10^9$$ electrons move out of a body every second, how much time is required to accumulate a total charge of $$1\ \mathrm{C} on $$another body?

Accepted Answer

$$6.25 	imes 10^9$ seconds

Question 17

Which of the following is a correct statement about conductors and insulators?

Accepted Answer

Conductors have free electrons that can move throughout the material, while insulators do not.

Question 18

A point charge $$+q is $$placed at the center of a neutral conducting spherical shell. What is the net charge induced on the inner surface of the shell?

Accepted Answer

$$-q$$

Question 19

Which of the following best describes the direction of the electric field due to a negative point charge?

Accepted Answer

Radially inward toward the charge

Electric Charges and Fields: Structured Study Notes

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