Q: A singly charged calcium ion ($$q = +e) is in a $$cell membrane with an electric field strength of $$1.0 \times 10^7$\ \text{N/C}. $$What is the magnitude of the electric force on the ion? ($$e = 1.60 \times 10$$^{-19}$$\ \text{C})$$

$$F = 1.6 \times 10$$^{-12}$$\ \text{N}$$

Question 1

Which of the following statements best describes the process of charging a plastic rod by rubbing it with wool?

Accepted Answer

Electrons are transferred from the wool to the rod, making the rod negatively charged.

Question 2

A glass rod is rubbed with silk and then brought near a hanging plastic rod that was rubbed with wool. What type of force will act between the two rods?

Accepted Answer

An attractive force, because the rods have opposite types of charge.

Question 3

Which equation correctly expresses Coulomb’s law for the magnitude of the force between two point charges $$q_1$$ and $$q_2$$ separated by distance $$r$$?

Accepted Answer

$$F = \frac{K |q_1$ q_2$|}{r^2$}$$

Question 4

A small glass bead of mass $$15\ 	ext{mg}$$ and charge $$+10\ 	ext{nC} is $$resting on a table. A plastic sphere charged to $$-10\ 	ext{nC} is $$held $$1.0\ 	ext{cm}$$ above it. Will the bead leap up to the sphere? (Assume $$K = 8.99 	imes 10^9$\ 	ext{N}\cdot	ext{m}^2/	ext{C}^2$$ and $$g = 9.8\ 	ext{m}/	ext{s}^2)$$

Accepted Answer

Yes, because the electric force is much greater than the gravitational force.

Question 5

Which of the following best explains why a charged rod attracts small pieces of paper, even though the paper is neutral?

Accepted Answer

The rod polarizes the paper, creating a separation of charge that results in an attractive force.

Question 6

Which material property allows charge to move freely through it, enabling phenomena such as charging by contact and induction?

Accepted Answer

Conductivity

Question 7

A metal sphere is charged by touching it with a charged plastic rod. What happens to the charge after the sphere is touched?

Accepted Answer

The charge spreads out over the sphere due to repulsive forces between like charges.

Question 8

Which of the following is the SI unit of electric charge?

Accepted Answer

Coulomb

Question 9

A neutral metal ball is attracted to a negatively charged rod. Which phenomenon is responsible for this attraction?

Accepted Answer

Charge polarization in the metal ball

Question 10

Which equation gives the electric field $$E at a $$distance $$r$$ from a point charge $$q$$?

Accepted Answer

$$E = \frac{K |q|}{r^2$}$$

Question 11

A singly charged calcium ion ($$q = +e) is in a $$cell membrane with an electric field strength of $$1.0 	imes 10^7$\ 	ext{N/C}. $$What is the magnitude of the electric force on the ion? ($$e = 1.60 	imes 10$$^{-19}$$\ 	ext{C})$$

Accepted Answer

$$F = 1.6 	imes 10$$^{-12}$$\ 	ext{N}$$

Question 12

If two identical metal spheres are initially neutral and are charged by induction using a negatively charged rod, then separated while the rod is still nearby, what will be the net charge on each sphere after the rod is removed?

Accepted Answer

One sphere will be positively charged and the other negatively charged.

Question 13

Which of the following statements about charge quantization is correct?

Accepted Answer

The net charge of any object is always an integer multiple of $$e.$$

Question 14

A proton and an electron are separated by a distance $$r. $$What is the direction of the electric force on the electron?

Accepted Answer

Toward the proton, because opposite charges attract.

Question 15

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

Accepted Answer

The field vectors point radially outward from the charge.

Question 16

A plastic rod is charged by rubbing and then touched to a metal sphere. The sphere is then touched by a human hand. What happens to the charge on the sphere?

Accepted Answer

The sphere is discharged as excess charge spreads to the human conductor.

Question 17

Which of the following is NOT a property of electric fields?

Accepted Answer

Electric fields exist only at the location of the source charge.

Question 18

Three point charges are placed at the corners of a rectangle. To find the net force on one charge, which procedure should be followed?

Accepted Answer

Calculate the individual force vectors using Coulomb’s law, decompose them into components, add the components, and use the Pythagorean theorem to find the magnitude and direction.

Question 19

Which of the following statements about conductors and insulators is correct?

Accepted Answer

In conductors, electrons are free to move; in insulators, electrons are tightly bound and immobile.

Electric Charges and Forces: Study Notes

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