Q: A housefly (mass $$12\ \mathrm{mg}$$, charge $$+73\ \mathrm{pC}) is to be $$levitated by an electric field. What is the required field strength? ($$g = 9.8\ \mathrm{m/s^2$})$$

$$E = \frac{mg}{q} = \frac{12 \times 10$$^{-6}$$ \times 9.8}{73 \times 10$$^{-12}$$} \approx 1.6 \times 10^6$\ \mathrm{N/C}$$

Question 1

A plastic rod is rubbed with wool and then brought near a neutral metal sphere. What is the nature of the force between the rod and the sphere, and why does it occur?

Accepted Answer

There is an attractive force due to polarization of charges in the sphere.

Question 2

Which of the following statements best describes a neutral object?

Accepted Answer

A neutral object has no net charge, but contains equal amounts of positive and negative charge.

Question 3

Coulomb’s law describes the force between two point charges. Which equation correctly represents this law?

Accepted Answer

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

Question 4

A parallel-plate capacitor has a uniform electric field between its plates. Which of the following changes will increase the electric field strength between the plates?

Accepted Answer

Increase the magnitude of the charge on both plates.

Question 5

A small glass bead (mass $$15\ \mathrm{mg}$$, charge $$+10\ \mathrm{nC}) is $$placed 1.0 cm below a plastic sphere charged to $$-10\ \mathrm{nC}. $$Will the bead leap upward? (Assume $$k = 8.99 	imes 10^9$\ \mathrm{N\cdot m^2$/C^2$$}$$ and g = 9.8$$\ \mathrm{$$m/s^2$$}$)

Accepted Answer

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

Question 6

Which of the following best describes the electric field inside a conductor in electrostatic equilibrium?

Accepted Answer

It is zero.

Question 7

A dipole is placed in a uniform electric field. What happens to the dipole when released from rest?

Accepted Answer

It rotates to align with the field.

Question 8

Two identical metal spheres, initially neutral, are touching. A positively charged rod is brought near (but not touching) sphere 1. The spheres are then separated while the rod is still present, and finally the rod is removed. What are the final charges on the spheres?

Accepted Answer

Sphere 1 is positive, sphere 2 is negative.

Question 9

Which of the following is true about the electric field created by a point charge $$q at a $$distance $$r$$?

Accepted Answer

$$E = k \frac{q}{r^2$}$$, directed away from $$q if$$ $$q is $$positive

Question 10

A housefly (mass $$12\ \mathrm{mg}$$, charge $$+73\ \mathrm{pC}) is to be $$levitated by an electric field. What is the required field strength? ($$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$E = \frac{mg}{q} = \frac{12 	imes 10$$^{-6}$$ 	imes 9.8}{73 	imes 10$$^{-12}$$} \approx 1.6 	imes 10^6$\ \mathrm{N/C}$$

Question 11

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

Accepted Answer

Field lines are closer together where the field is stronger.

Question 12

A DNA fragment with negative charge is placed in a gel electrophoresis apparatus with a uniform electric field. What is the direction of the force on the DNA fragment?

Accepted Answer

Toward the positive electrode.

Question 13

Which of the following best explains why a lightning rod is pointed?

Accepted Answer

The charge density and electric field are highest at sharp points, allowing charge to dissipate into the air.

Question 14

A proton and an electron are placed in the same uniform electric field. Which statement is correct about their accelerations?

Accepted Answer

The proton has a much smaller acceleration than the electron.

Question 15

Which of the following is a correct statement about the force between two charges according to Coulomb’s law?

Accepted Answer

The force is proportional to the product of the charges and inversely proportional to the square of the distance between them.

Question 16

A negatively charged rod is brought near a neutral metal sphere. What happens to the charges in the sphere?

Accepted Answer

Negative charges move away from the rod, and positive charges move toward the rod, causing polarization.

Question 17

Which of the following best describes the electric field inside a parallel-plate capacitor?

Accepted Answer

It is uniform and directed from the positive to the negative plate.

Question 18

A point charge $$q is $$placed at the origin. At what distance $$r$$ from the charge is the electric field $$E$$ equal to $$1.0 	imes 10^4$\ \mathrm{N/C}$$? ($$k = 8.99 	imes 10^9$\ \mathrm{N\cdot m^2$/C^2$$}$$, q = 1.0$$ 	imes $$10^{-9}$$\ \mathrm{C}$)

Accepted Answer

$$r = \sqrt{\frac{kq}{E}} = \sqrt{\frac{8.99 	imes 10^9$ 	imes 1.0 	imes 10$$^{-9}$$}{1.0 	imes 10^4$}} \approx 0.95\ \mathrm{m}$$

Chapter 20: Electric Fields and Forces – Study Notes

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