Q: Estimate the number of electrons in 1 liter of water, given that each water molecule ($$\mathrm{H}_2\mathrm{O}) $$has 10 electrons and there are $$3 \times 10$$^{25}$$ molecules in 1 liter.

$$3 \times 10$$^{26}$$ electrons

Question 1

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

Accepted Answer

Electric charge exists in discrete units, specifically integer multiples of the elementary charge $$e.$$

Question 2

A neutral lithium atom contains 3 protons, 4 neutrons, and 3 electrons. What is the net electric charge of this atom?

Accepted Answer

$$0$$

Question 3

Which of the following best explains why metals are good electrical conductors?

Accepted Answer

Metals have a 'sea of electrons' where valence electrons are only weakly bound and can move freely.

Question 4

Estimate the number of electrons in 1 liter of water, given that each water molecule ($$\mathrm{H}_2\mathrm{O}) $$has 10 electrons and there are $$3 	imes 10$$^{25}$$ molecules in 1 liter.

Accepted Answer

$$3 	imes 10$$^{26}$$ electrons

Question 5

Which statement about electric dipoles is correct?

Accepted Answer

An electric dipole consists of two charges of opposite sign separated by a distance.

Question 6

A red blood cell carries a net negative charge of $$2.5 	imes 10$$^{-16}$$ C. If two such cells are 2.0$$ 	imes $$10^{-6} m $$apart, what is the magnitude of the electric force between them? (Use $$k = 8.99 	imes 10^9$ N·m^2$/C^2$)

Accepted Answer

$$1.4 	imes 10$$^{-14}$$ N

Question 7

Which of the following is the correct vector form of Coulomb's Law for the force between two point charges $$q_1$$ and $$q_2$$ separated by vector $$\vec{r}$$?

Accepted Answer

$$\vec{F} = k \dfrac{q_1$ q_2$}{r^2$} \hat{r}$$

Question 8

A small glass bead of mass $$15 mg $$and charge $$+10 nC is $$held $$1.0 cm $$above a plastic sphere charged to $$-10 nC. $$What is the magnitude of the electric force between them? (Use $$k = 8.99 	imes 10^9$ N·m^2$/C^2$)

Accepted Answer

$$9.0 	imes 10$$^{-2}$$ N

Question 9

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

Accepted Answer

Radially outward from the charge

Question 10

A charge $$q = -8.0 nC is $$located at the origin. What is the electric field at the point $$\vec{r} = 1.2\hat{i} + 1.6\hat{j} m$$? (Use $$k = 8.99 	imes 10^9$ N·m^2$/C^2$)

Accepted Answer

$$(-11\hat{i} + 14\hat{j}) N/C$$

Question 11

Where is the electric field zero between two point charges $$Q_1 = +4$ C and Q_2$ = +9 C $$separated by $$0.5 m$$?

Accepted Answer

Between the two charges, closer to $$Q_1$$

Question 12

A particle of mass $$6.64 	imes 10$$^{-27}$$ kg travels 2.00$ cm between two parallel plates in 1.07$$ 	imes $$10^{-7} s $$under a uniform electric field of $$1.00 	imes 10^6$ N/C. What is the magnitude of the charge of the particle?

Accepted Answer

$$3.29 	imes 10$$^{-19}$$ C

Question 13

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

Accepted Answer

Electric field lines start on positive charges and end on negative charges.

Question 14

What is the direction of the force on a negative test charge placed in an electric field?

Accepted Answer

Opposite to the direction of the electric field

Question 15

The electric field inside a conductor in electrostatic equilibrium is:

Accepted Answer

Zero

Question 16

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

Accepted Answer

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

Question 17

In gel electrophoresis, why do shorter DNA fragments move faster through the gel under an electric field?

Accepted Answer

Shorter fragments experience less viscous drag and thus higher mobility.

Question 18

For an infinite plane of charge with surface charge density $$\sigma$$, what is the magnitude of the electric field just above the surface?

Accepted Answer

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

Electric Charge and Electric Field: Physics for Bioscience and Bioengineering

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