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

$$1.6 \times 10$$^{-12}$$\ \mathrm{N}$$

Question 1

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

Accepted Answer

Electrons are transferred from the wool to the plastic rod, giving the rod a net negative charge.

Question 2

A glass rod is rubbed with silk and then brought near a hanging plastic rod that has been rubbed with wool. What is the observed interaction and why?

Accepted Answer

The rods attract because they 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 a distance $$r$$?

Accepted Answer

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

Question 4

A neutral metal sphere is touched by a negatively charged rod. What happens to the sphere?

Accepted Answer

The sphere becomes negatively charged due to the transfer of electrons.

Question 5

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

Accepted Answer

In conductors, electrons are free to move throughout the material.

Question 6

A charged rod is brought near, but does not touch, a neutral metal sphere. The sphere is then grounded while the rod is still nearby. What is the final charge on the sphere after the rod is removed and the ground connection is broken?

Accepted Answer

The sphere acquires a net charge opposite to that of the rod (charging by induction).

Question 7

Which of the following best describes charge quantization?

Accepted Answer

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

Question 8

A small glass bead (mass $$15\ \mathrm{mg}$$, charge $$+10\ \mathrm{nC}) $$rests on a table. A plastic sphere with charge $$-10\ \mathrm{nC} is $$held $$1.0\ \mathrm{cm}$$ above it. Will the bead leap up to the sphere? (Use $$K = 9.0 	imes 10^9$\ \mathrm{N\,m^2$/C^2$$}$$ and g = 9.8$$\ \mathrm{$$m/s^2$$}$)

Accepted Answer

Yes, because the electric force exceeds the gravitational force by a large factor.

Question 9

Which of the following is the correct expression for the electric field $$\vec{E} at a $$distance $$r$$ from a point charge $$q$$?

Accepted Answer

$$\vec{E} = \frac{1}{4\pi\$$epsilon_0$$} \frac{q}{r^2$} \hat{r}$$

Question 10

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

Accepted Answer

$$1.6 	imes 10$$^{-12}$$\ \mathrm{N}$$

Question 11

Which of the following is true about the force between two charges according to Coulomb’s Law?

Accepted Answer

The force is attractive if the charges are of opposite sign and repulsive if they are of the same sign.

Question 12

If the distance between two point charges is halved, how does the magnitude of the electrostatic force between them change?

Accepted Answer

It increases by a factor of 4.

Question 13

Which of the following best describes the concept of charge polarization in a neutral object?

Accepted Answer

A slight separation of positive and negative charges within the object, resulting in a net force toward an external charge.

Question 14

A neutral atom becomes a negative ion when:

Accepted Answer

It gains an extra electron.

Question 15

Which of the following is the correct value for the permittivity constant $$\$$epsilon_0$$?

Accepted Answer

$$8.85 	imes 10$$^{-12}$$\ \mathrm{C^2$/(N\,m^2$)}$$

Question 16

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

Accepted Answer

Electric charge can be created or destroyed in isolation.

Question 17

A unit vector $$\hat{r}$$ used in the electric field equation has what units?

Accepted Answer

No units (it is dimensionless).

Question 18

Which of the following real-world scenarios is best explained by charge polarization?

Accepted Answer

A charged balloon sticking to a neutral wall.

Electric Charges and Forces – Study Notes

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