If you charge a pockecekzon 9kh2-. t comb by rubbing it with a silk scarf, how can you determine if the comb is positively or negatively charged?nz-o2hck.k 9 e

Why does a shirt or blouse taken from a clothes dryer sometimes 7/ h/coxc 6mmwfx3fz*cling to your body 7x6/o /*fz xcwfhm3cm?

Explain why fog or rain droplets tend to form around ik c; htjx 55zh55*hsobons or electrons in the air.ho 5zkjs th5 b5ch*x5;

A positively charged rod is brought close to a ne *pp 1vdv.1r3 7re(l-awqvw n6i6rjdbutral piece of paper, which it attracts. Draw a diagram showing the separation of charge and wqrd wvp6 i11ld-b( rprvavj. e63*7nexplain why attraction occurs.

Why does a plastic ruler that has been rubbed with a cloth have *s5dkr egnma 0 m7soa 42s(2sithe ability to pick up small pieces 4si7aon22srg0kmdes*5(a sm of paper? Why is this difficult to do on a humid day?

Contrast the net charge on a conduc; ;amo)un40tsqggkx+m4 t l. btor to the “free charges” in the conductor.

Figures 16–7 and 16–8 show how a charged rod placed near an unchargedj+znh 5jz1*/ dvocv, o metal object can attract (or repel) electr/j+h5,*zoc voj1nv zd ons. There are a great many electrons in the metal, yet only some of them move as shown. Why not all of them?

When an electroscope is charged, its/.sg9 c tbz9vt two leaves repel each other and remain at an angle. What balances the electric force ogcvt9.9zb t/ sf repulsion so that the leaves don’t separate further?

The form of Coulomb’s law is very similarzacy+hqi5m n8ckwo8 cc;lx f8w2s7 8 5 unf5c, to that for Newton’s law of universal +ci85l5,uxcqc ww2 cnh;8f57nka8ysfz moc8gravitation. What are the differences between these two laws? Compare also gravitational mass and electric charge.

We are not normally aware of the gravitational or electric jw7v2qxfg ,,t force between two ordinary objects. What is the reason in each case? Give an example where we are aware of each one and why.g jtf,7wvx,q2

Is the electric force a ./h0c8sj6: j kr* k3kx learcdconservative force? Why or why not? (See Chapter 6.)

When a charged ruler attracts small pi;;4r qw74lv -z jmrbwe-mne- teces of paper, sometimes a piece jumps quickly a;rqw-;j- nl4wt e7 v -bm4remzway after touching the ruler. Explain.

Explain why the test charges we use when measuring electric fields must be sma4 hr+gu3 lx35mcdudn0 ll4 ud 5c0+3hxdurmgln3.

When determining an electric field, must we use a positive tesm/iu p;;l+qbxj wsm: .13xyfn ev k:e;t charge, or would a negative one do as well? Expj+e;:li./x xm:e y f3nvmb;qu k1 w;pslain.

Draw the electric field lines surrounding two negative electric chk8xuyv.bj7 p sl47 0i6rzz11l2 sgbzrarges a distance l apaz u11ir2vp4l 60l7bzbgy x.kj8ssr7 zrt.

Assume that the two opposite charges in Fig. 16–31a ar127y.4 s ujhp,c0y mgbusw(omw5qsf 7e 12.0 cm apart. Consider the magnitude of the electric field 2.5 cm from the positive charge. On which side of this charge — top, bottom, left, or right — is the electric field the strongest? The c5p7s.(2u4by1o sj gwfwm ,myhs 07q uweakest? Explain.

Consider the electric field at points Ac7x 14 pz:i1j(hyusuc , B, and C in Fig. 16–48. First draw an arrow at each point inp1yuz (u7ji4hxscc 1 :dicating the direction of the net force that a positive test charge would experience if placed at that point, then list the points in order of decreasing field strength (strongest first).

Why can electric field lines never crosse3nh /wakl 5xq m xt,6x7*jx.j?

Show, using the three rules for field lines given in Section 16–8, that the es) m kho2tr66flectric field lines starting or ending on a single point charge must be so6fk6mh) 2rt symmetrically spaced around the charge.

Given two point charges Q azlaj2 3vht: ;qnd 2Q, a distance l apart, is there a point along the straight line that passes through them where E=0 when their signs are (a) opposite, (b) the same? If yes, state roughly where this p2:;zt 3lvh jqaoint will be.

Consider a small positive test charge located ptxko -)e e1znp kas(e62y1:kon an electric field line at some point, such as point P in Fig. 16–31a. Is the direction of the velocity and/or acceleration of the test charge along this linenake )-skp zk 1t:oe2 ep6y1x(? Discuss.

Sketch the electric field lines for a uniform line (),kqhpqqcl s2 *+a vhjk3l 2lof charge which is infinitely long. 2qj,cl ql*) 3(+lpkhvkqs2ah (Hint: Use symmetry.) Is the electric field uniform in strength?

If the electric flux through a closed surface is zero, is the eleqhfl (z-r-q7a3 fg6w)m 3t :oriwz 0ufctric field necessarily zero at all points on the surface? Explain. What about the g wmqh)3qz6o zfri7w(-lr 30f:fat-u converse: If $\vec{E}\,=\,0$ at all points on the surface is the flux through the surface zero?

A point charge is surrounded by a spherical gaussitw*7 i/ -pct4nz; feq cga6s4nan surface of radius r. If the sphere is replacedg zftna c 7-cwsnqi*46te;/4p by a cube of side r, will $\Phi_E$ be larger, smaller, or the same? Explain.

  

  How many electrons make uptmg,+itk u9o- a charge of $-30.0\mu C\,?$   $ \times10^{ 14}\, electrons $

  What is the magnitude of the electric force of attraction between an )(o8o ad 6/vasxrit6siron nucleus/draa8o(vx)is6t6so $(q\,=\,+26e)$ and its innermost electron if the distance between them is $ 1.5\times10^{-12 }\,m\,?$    $ \times10^{-3 }N$

  What is the repulsive elect z7ua p1 ogoy+)whny)9ku7 gl*, k*ainrical force between two protons $ 5.0 \times10^{ -15}\,m$ apart from each other in an atomic nucleus?    N

  What is the magnitude of the forin;ncg+9v- gezu3nz5 ce a $+25\,\mu C\,C$ charge exerts on a $+\,3.0\,mC$ charge 35 cm away?    N

  Two charged dust particles exert kg+ *ks3:kmxu a force of $ 3.2\times10^{ -2}\,N$ on each other. What will be the force if they are moved so they are only one-eighth as far apart?    N

  Two charged spheres are 8.45 cm apart. They are movec9pevdb2+ 5pnd, and the force on each of them is found to have been tripled. How far apart are tvdecp5 b+2n 9phey now?    cm

  A person scuffing her feet on a wool rug on a9wk9 zh xiwl,bp;o ;2p dry day accumulates a net charge of wob2w;x 9;kpl, pi9zh $-42\,\mu C\,.$ How many excess electrons does she get,    $ \times10^{14 }\,electrons$
by how much does her mass increase?    $ \times10^{-16 }kg$

  What is the total charge of g q0kio/5dzp bo9d24qall the electrons in 1.0 kg of $H_2O\,?$    $ \times10^{ 7}\,C$

  Compare the electric force holding the electron8cvgxel,aprjq, 0n9 6 in orbit $(r\,=\,0.53 \times10^{-10 }\,m)$ around the proton nucleus of the hydrogen atom, with the gravitational force between the same electron and proton. What is the ratio of these two forces?    $ \times10^{39 }\,C$

Two positive point cha5*3un n0cq40sx- 6nzdhuwx jerges are a fixed distance apart. The sum of their charges is u*n5cxjn3 4nhx0e- us0dz qw6 What charge must each have in order to
(a) maximize the electric force between them,
(b) minimize it?

  Particles of charge 89qbw*2hf7qq l1 y(ebd t ym4o$+75$,$+48$ and $-85\,\mu C\,$ are placed in a line (Fig. 16–49). The center one is 0.35 m from each of the others. Calculate the net force on each charge due to the other two.

$F_{+75}$=   $NN$
$F_{+48}$=   $NN$
$F_{-85}$=   $NN$

  Three positive particles ofyiot:wf 6 u;)i0ie)y hs zo b8btu5)3o equal charge, $+11.0\,\mu C\,$ are located at the corners of an equilateral triangle of side 15.0 cm (Fig. 16–50). Calculate the magnitude and direction of the net force on each particle.


the force on the lower left charge is of magnitude    N, and
will point    $^{\circ}$below the-x axis. The force on the lower right charge is of magnitude    N, and will point    $^{\circ}$below the +x axis.

  A charge of 6.00 mC is placed at each corner of a sqoh xwh7;c (g tw-)fdlkl7-(wuuare 0.100 m on a side. Determine the magnitude and direction of the forcl-f u(;w(dxo7kcg) 7hl- twwhe on each charge.
$F_4$ =   $ \times10^{ 7}\,N$
$\theta$ =    $^{\circ}$

  Repeat Problem 14 for the case when two of the positiv npz9550yr, xcei 5pxohky *3:w8fpe ve charges, on opposite corners, are replaced by negative charges of the same magnitude (F rox*fieex 5vcp5p8,:39y hzw knyp50ig. 16–51).
$F_4$ =   $ \times10^{ 7}\,N$
$\theta$ =    $^{\circ}$

  At each corner of a square of side l there are point charges of magnitudu ttus:f2k+*ik1 wio 2t. i8aqe Q, 2Q, 3Q, and u* :fq is28t+i ut.oa1 ktw2ik4Q (Fig. 16–52). Determine the force on (a) the charge 2Q, and (b) the charge 3Q, due to the other three charges.
$F_{2Q}$ =    $\frac{kQ^2}{l^2}$
$\theta_{2Q}$ =    $^{\circ}$
$F_{3Q}$ =    $\frac{kQ^2}{l^2}$
$\theta_{3Q}$ =    $^{\circ}$

  Three charged particles are placed at the corners of an equilateral tsdkr6ua ft7jl(i)2ber77y2 f8o 4t sjriangle of side 1.20 m (Fig. 16–53). T7 f47 2ojrl(27st)y ksed8ruita fb j6he charges are $+4.0\,\mu C\,,$$-8.0\,\mu C\,,$ and $-6.0\,\mu C\,.$ Calculate the magnitude and direction of the net force on each due to the other two.
$F_1$ =    N
$\theta_1$ =    $^{\circ}$
$F_2$ =    N
$\theta_2$ =    $^{\circ}$
$F_3$ =    N
$\theta_3$ =    $^{\circ}$

  Two point charges have a total charge ofx y7vgh:gtp/h4ev3g 2 $560\,\mu C\,.$ When placed 1.10 m apart, the force each exerts on the other is 22.8 N and is repulsive. What is the charge on each?
q = $ 5.54\times10^{a }\,C$ a =  ,
$ 5.54\times10^{b }\,C $ b =   
Q-q = $ 5.54\times10^{a }\,C$ a =  ,
$ 5.54\times10^{b }\,C$ b =   

Two charges, $-Q_0$ and $-3Q_0$ are a distance l apart. These two charges are free to move but do not because there is a third charge nearby. What must be the charge and placement of the third charge for the first two to be in equilibrium?

  A $+4.75\,\mu C\,$ and a $-3.55\,\mu C\,$ charge are placed 18.5 cm apart. Where can a third charge be placed so that it experiences no net force?    cm

  Two small nonconducting spheres have ayh*kyb) pr:/ d total charge of $90.0\,\mu C\,.$
(a) When placed 1.06 m apart, the force each exerts on the other is 12.0 N and is repulsive. What is the charge on each?    $ \times10^{ -6}\,C$$\;\;\;\;$    $ \times10^{ -6}\,C$
(b) What if the force were attractive?    $ \times10^{ -6}\,C$$\;\;\;\;$    $ \times10^{ -6}\,C$

  A charge Q is transferred from an initially unc* )ht9ua w*obfyy:j k +mw7bx+ktor.h-- 2nvbharged plastic ball to an identical ball 12 cm away. The force of attraction is then 17 mN- fb*:7wr kb+ bhm*v.t 9oj)n-t xk2owhyy+ua . How many electrons were transferred from one ball to the other?    $ \times10^{ 12}$ electrons

  What are the magnitude p*(5dz y ex4p i(dv95ewamw+fand direction of the electric force on an electron in a unifei*e a vmx+zd w((9f4wpp55yd orm electric field of strength $2360\,N/C$ that points due east?
   $ \times10^{ -16}\,N$
 ----待选择----westeast 

  A proton is released in a uniform electric field, and it experiences an electxy/flu;2h8*gnc fuw/.ubf /g ric force uguf h2flnf/;b8c*gy/x w./ uof $ 3.75\times10^{ -14}\,N$ toward the south. What are the magnitude and direction of the electric field?
   $ \times10^{5 }\,N/C$
 ----待选择----southnorth 

  A downward force of 8.4 N/5lavz,9tksk is exerted on a $-8.8\,\mu C\,$ charge. What are the magnitude and direction of the electric field at this point?    $ \times10^{5 }\,N/C$ ----待选择----updown 

  What are the magnitude and6dv0 gosy w7s/c9gr 1walx+5z direction of the electric field 20.0 cm directly above an isos7wlz rya 6s0co+dw gg9x51/vlated $33.0 \times10^{ -6}\,C$ charge?    $ \times10^{6 }\,N/C$ ----待选择----updown 

  What is the magnitude of the acceleration experiencet;,(ft a oup:yv3(u9*:z gpcnsx er+f d by an electron in an elect*pc,: e :9;otf(nz suxr+3ftu v(ypagric field of $750\,N/C\,?$ How does the direction of the acceleration depend on the direction of the field at that point?    $ \times10^{ 14}\,m/s^2$  ----待选择----opposite to the field in line with the direction of the field 

  What are the magnitude and direction of tho3dsbn 9t 45zne electric field at a point midway bet zn3 dst4n59obween a $-8.0\,\mu C\,$ and a $+7.0\,\mu C\,$ charge 8.0 cm apart? Assume no other charges are nearby.    $ \times10^{7 }\,N/C$ ----待选择---- towards the negative charde opposite to the negative charge 

Draw, approximately, the electric fiez,tuz/4euwk: ld lines about two point charges, $+Q$ and $-3Q$ which are a distance l apart.

  What is the electric field strength anjviej;ly.: 7t a point in space where a proton $(m= 1.67\times10^{ -27}\,kg) $experiences an acceleration of 1 million “g’s”? $\approx$    N/C（Round it to one decimal place.）

  An electron is released frrbvy0/9b+ rb8 qri-mzom rest in a uniform electric field and accelerates to th/r8zrm b q+b90vyib-re north at a rate of $115\,m/s^2\,.$ What are the magnitude and direction of the electric field?    $ \times10^{-10 }\,N/C$ ----待选择----northsouth 

  The electric field midway between two1s5yzvkl.t 7 gnj xmk66yo 65b equal but opposite point charges is $745\,N/C\,,$ and the distance between the charges is 16.0 cm. What is the magnitude of the charge on each?    $ \times10^{-10 }\,C$

  Calculate the electric field at the cen2eb x3fqa(e 0b6tmdo z7,n vi5ql de5*ter of a square 52.5 cm on a side if one corneblmb oe i a fn7 zx2t(0,dqe*6e5q5vd3r is occupied by a $+45.0\,\mu C\,$ charge and the other three are occupied by $-27\,\mu C\,$ charges.    $ \times10^{6 }\,N/C$ at    $^{\circ}\,$

  Calculate the electr h)m ;nrj1-m ,a8abbfhic field at one corner of a square 1.00 m on a side if the otherm-a8b ,f)b j1hamr;hn three corners are occupied by $ 2.25\times10^{-6 }\,C$ charges.    $ \times10^{4 }\,N/C$   $^{\circ}\,$ from the x-direction.

Determine the direction and 3, 1 oun4o nm3nr;qdn+w,zrp imagnitude of the electric field at the point P in Fig. 16–54. The charges are separated by a distance 13r;dn wzru+4,n mnnqo pi,3o 2a, and point P is a distance x from the midpoint between the two charges. Express your answer in terms of Q, x, a, and k.

  Two point charges, $Q_1\,\equiv\,25\,\mu C\,$ and $Q_2\,=\,+50\mu C\,,$ are separated by a distance of 12 cm. The electric field at the point P (see Fig. 16–55) is zero. How far from $Q_1$ is P?    cm

(a) Determine the electric field 4g86,1thk0hgi ler x t$\vec{E}$ at the origin 0 in Fig. 16–56 due to the two charges at A and B. (b) Repeat, but let the charge at B be reversed in sign.

  Use Coulomb’s law to determine the magnitude and direcm*y c ;6s,eqxftion of the electric field at points A and B in Fig. 16–57, 6xq m;fsec*y due to the two positive charges $(Q\,=\,7.0\mu C\,)$ shown. Are your results consistent with Fig. 16–31b?
$E_A$ =    $ \times10^{6 }\,N/C$
$\theta_A$ =    $^{\circ}\,$
$E_B$ =    $ \times10^{7 }\,N/C$
$\theta_B$ =    $^{\circ}\,$
(16-51)
(16-31b)

  You are given two unknowx-xqa1t -1kt-/p 2 gpb 31w0rcnhl*y3zuq jhwn point charges, $Q_1$ and $Q_2$ At a point on the line joining them, one-third of the way from $Q_1$ to $Q_2$, the electric field is zero (Fig. 16–58). What is the ratio $Q_1/Q_2\,?$   

Determine the direction and magnitude of the electric field at the point P sh)q0 ti8yqt 5 )v+jqpjpv)vp a(own in Fig. 16–59. The two charges are separated by a distance of 2a.q vij qvt05pp8tp y)+))(j vqa Point P is on the perpendicular bisector of the line joining the charges, a distance x from the midpoint between them. Express your answers in terms of Q, x, a, and k.

  An electron (mass $m\,=\,9.11 \times10^{ -31}\,kg$) is accelerated in the uniform field $\overrightarrow{\mathbf{E}}\left(E=1.45 \times 10^{4} \mathrm{~N} / \mathrm{C}\right)$ between two parallel charged plates. The separation of the plates is 1.10 cm. The electron is accelerated from rest near the negative plate and passes through a tiny hole in the positive plate, Fig. 16–60.
(a) With what speed does it leave the hole?    $ \times10^{ 6}\,m/s$
(b) Show that the gravitational force can be ignored.

  An electron moving to the right at 1.0% the speed of l qq n8wgnz2hl4 :og/:pight enters a uniform electric fieldh2:n gwp4q :/8onzglq parallel to its direction of motion. If the electron is to be brought to rest in the space of 4.0 cm,
(a) what direction is required for the electric field,  ----待选择----rightleft 
(b) what is the strength of the field?    N/C

  The total electric flux fro9y l3g*n8y zxxk zu.e(m a cubical box 28.0 cm on a side is $1.45 \times10^{3 }\,N\cdot m^2/C$. What charge is enclosed by the box?    $ \times10^{ -8}\,C$

  A flat circle of radiu3xwuk r8msbjr0c)+ e;s 18 cm is placed in a uniform electric field of magnitude k ejswc +xrr83 bu)0;m$ 5.8\times10^{2 }\,N/C$ What is the electric flux through the circle when its face is
(a) perpendicular to the field lines,    $N\cdot m^2/C$
(b) at 45° to the field lines,    $N\cdot m^2/C$
(c) parallel to the field lines?   

  In Fig. 16–61, two objects, mac xo5cxx8q -cl 8 n5mk4i+m6$O_1$ and $O_2$ have charges $+1.0\mu C$ and $-2.0\mu C$ respectively, and a third object, $O_3$ is electrically neutral.
(a) What is the electric flux through the surface $A_1$ that encloses all three objects?    $ \times10^{5}\,$$N\cdot m^2/C$
(b) What is the electric flux through the surface $A_2$ that encloses the third object only?   

A cube of side l is placed puat4*byt5s5 e-r*w ein a uniform field $ E\,=\,6.50\times10^{3 }\,N/C$ with edges parallel to the field lines.
(a) What is the net flux through the cube?
(b) What is the flux through each of its six faces?

  The electric field betw))t7 -peh *w-l cqysjgn* wb+jeen two square metal plates is The plates are 1.0 m on a side and are separated by 3.0 cm. Wq) *hlg c+yp)w7jjweb-ts -n*hat is the charge on each plate (assume equal and opposite)? Neglect edge effects.    $ \times10^{-9 }\,C$

  The field just outside aoghjf(om,v: tm 1ui*c ,,j, fu 3.50-cm-radius metal ball is $ 2.75\times10^{2}\,N/C$ and points toward the ball. What charge resides on the ball?    $ \times10^{-11 }\,C$

  A solid metal sphere of radrzh6p dt +x+m.8jem/ iius 3.00 m carries a total charge of $-3.5\mu C$ What is the magnitude of the electric field at a distance from the sphere’s center of
(a) 0.15 m,   
(b) 2.90 m,   
(c) 3.10 m,    N/C
(d) 6.00 m?    N/C
(e) How would the answers differ if the sphere were a thin shell?

A point charge Q rests at the center of an uncharged thin spherical conducting s ao42xq dh5v5shell. (See Fig. 16–33.) What is the electric field E as a function of r (a) for r less than the inner radius of the shell, (b) inside the shell, and (c) beyond the shell? (d) Does the shell affect the field due to Q alone? Doeshosx45q5va2d the charge Q affect the shell?

  The two strands of the helix-shaped DNA molecule are held together by eles 1lu*,uz4n8ndjhz)v;g1tp o ctrostatic forces as shown in Fig. 16–44. Assume that the net average charge (due to electron sharing) indicated on H and N atoms is 0.2e and on the indica1psgou1 zn ,*v;htun4 )j dz8lted C and O atoms is 0.4e. Assume also that atoms on each molecule are separated by $ 1.0\times10^{ -10}\,m$. Estimate the net force between
(a) a thymine and an adenine;    $ \times10^{-10 }\,N$
(b) a cytosine and a guanine. For each bond (red dots) consider only the three atoms in a line (two atoms on one molecule, one atom on the other).    $ \times10^{-10 }\,N$
(c) Estimate the total force for a DNA molecule containing $10^5$ pairs of such molecules.    $ \times10^{-5 }\,N$

  How close must two electrons be if the electric force between tha-q w7g+:mwdg em is equal to the weight of either at the Earth’s surfaceaw7: d qg-wg+m?    m

  A 3.0-g copper penny has a positive charge of 7u: c6lw,axd b$38\mu C$. What fraction of its electrons has it lost?    $ \times10^{-10 }\,$

  A proton $(m\,=\,1.67 \times10^{-27}\,kg)$ is suspended at rest in a uniform electric field $\vec{E}$. Take into account gravity at the Earth’s surface, and determine $\vec{E}$.    $ \times10^{-7 }\,N/C$  ----待选择----updown 

  Measurements indicate that there is an electric field s*m/cnvh2o : qca*n vz0n ,,4hbx wkq9burrounding the Earth. Its qhxq, z2ocwb0achnm nk :*/9v,4*nbvmagnitude is about $150\,N/C$ at the Earth’s surface and points inward toward the Earth’s center. What is the magnitude of the electric charge on the Earth? Is it positive or negative? [Hint: the electric field outside a uniformly charged sphere is the same as if all the charge were concentrated at its center.]    $ \times10^{5 }\,C$  ----待选择----positivenegative 

  (a) Given the local electric field of what is the acceleration epdnddnzx7; q3j 6 3s4lxperienced by an electron near the surface odp3s6z j4 n3lnd7qxd;f the Earth?    $ \times10^{13 }\,m/s^2$, ----待选择----updown 
(b) What about a proton?    $ \times10^{10 }\,m/s^2$, ----待选择----updown 
(c) Calculate the ratio of each acceleration to $g\,=\,9.8\,m/s^2$ For the electron: $\approx$    $ \times10^{12 }$ For the proton: $\approx$    $ \times10^{9 }$

  A water droplet of radv 2,ts;ybm 1btius 0.018 mm remains stationary in the air. If the downward-directed electric field of the Earv,t2b ;st1yb mth is $150\,N/C$ how many excess electron charges must the water droplet have?    $ \times10^{ 6}$ electrons

  Estimate the net force betwjq x ak7,2qnrx3f fr9-een the CO group and the HN group shown in Fig. 16–62. Thef q-rk7fr92jxq3an, x C and O have charges $\pm0.40e$, and the H and N have charges $\pm0.20e$, where $e\,=\, 1.6\times10^{-19}\,C$. [Hint: Do not include the “internal” forces between C and O, or between H and N.]    $ \times10^{ -10}\,NN$

  In a simple model of the hydrogen atom, the electron revolves in a cirfaju4g *(w 9vbcular orbit around the prot9 vwu*(g4bja fon with a speed of $ 1.1\times10^{6 }\，m/s,$. Determine the radius of the electron’s orbit. [Hint: See Chapter 5 on circular motion.]    $ \times10^{-10 }\, m$

  Suppose that electrical attraction, rather than gravity, were respo h/1np(,ru jj5m4 yampnsible for holding the Moon in orbit around the Earth. If equal and opposite charges Q were placed on the Earth and the Moon, what should be the value of Q trn5ym/ uj map,4(1jhpo maintain the present orbit? Use these data: mass of $Earth\,=\, 5.98\times10^{24}\,kg$, mass of $Moon\,=\, 7.35\times10^{22}\,kg$, radius of $orbit\,=\, 3.84\times10^{8}\,m$. Treat the Earth and Moon as point particles.    $ \times10^{13 }\,C$

  An electron with spee +md3a1ciuh,dm23mpn7p+dj cd $v_0\,=\, 21.5\times10^{6}\,m/s$ is traveling parallel to an electric field of magnitude $E\,=\, 11.4\times10^{3}\,N/C$.
(a) How far will the electron travel before it stops?    m
(b) How much time will elapse before it returns to its starting point?    $ \times10^{-8 }\,s$

  A positive point cha,:b tg1z+jqu uedt 7+mrge $Q_1\,=\, 2.5\times10^{-5}\,C$. is fixed at the origin of coordinates, and a negative charge $Q_2\,=\, -5.0\times10^{-6}\,C$. is fixed to the x axis at $x\,=\,+2.0m$ Find the location of the place(s) along the x axis where the electric field due to these two charges is zero.    m from $Q_1$,   m from $Q_2$

A small lead sphere is encased in insulatirm y.25c mbb9lng plastic and suspended vertically from an ideal sprin9.b yc lm5rmb2g $(k\,=\,126N/m)$ above a lab table, Fig. 16–63. The total mass of the coated sphere is 0.800 kg, and its center lies 15.0 cm above the tabletop when in equilibrium. The sphere is pulled down 5.00 cm below equilibrium, an electric charge $Q\,=\, -3.00\times10^{-6}\,C$ is deposited on it and then it is released. Using what you know about harmonic oscillation, write an expression for the electric field strength as a function of time that would be measured at the point on the tabletop (P) directly below the sphere.

  A large electroscope is ma)m )hjn2*p bfn:fs6kw de with “leaves” that are 78-cm-long wires with tiny 24-g spheres at the ends. When bjk6m nfhfn):2 *s)wpcharged, nearly all the charge resides on the spheres. If the wires each make a $30^{\circ}$ angle with the vertical (Fig. 16–64), what total charge Q must have been applied to the electroscope? Ignore the mass of the wires.    $ \times10^{-6 }\,CC$

  Dry air will break down and generate a spark if the elejgrkso 3ggra1tpk;tn*w +f/ ) x8)mj2 ctric field exceeds about; ntgw+ar8 2 gj)*ojmfs x/t)p1r gkk3 $ 3\times10^{6 }\,N/C$. How much charge could be packed onto a green pea (diameter 0.75 cm) before the pea spontaneously discharges? [Hint: Eqs. 16–4 work outside a sphere if r is measured from its center.]    $ \times10^{-9 }\,C$

  Two point charges, $Q_1\,=\,-6.7\mu C$ and $Q_2\,=\,1.8\mu C$ are located between two oppositely charged parallel plates, as shown in Fig. 16–65. The two charges are separated by a distance of $x\,=\,0.34\,m$ Assume that the electric field produced by the charged plates is uniform and equal to $E\,=\,73,000\,N/C$ Calculate the net electrostatic force on $Q_1$ and give its direction.    N  ----待选择----right left 

  A point charge $(m\,=\,1.0\,g)$ at the end of an insulating string of length 55 cm is observed to be in equilibrium in a uniform horizontal electric field of $12,000\,N/C$ when the pendulum’s position is as shown in Fig. 16–66, with the charge 12 cm above the lowest (vertical) position. If the field points to the right in Fig. 16–66, determine the magnitude and sign of the point charge.
   $ \times10^{-7 }\,C$
 ----待选择----positivenegative 

  A point charge of mass 0.210 kg, and ne k,h0c(hn e)qxt charge $+0.340\,\mu C$, hangs at rest at the end of an insulating string above a large sheet of charge. The horizontal sheet of uniform charge creates a uniform vertical electric field in the vicinity of the point charge. The tension in the string is measured to be 5.67 N. Calculate the magnitude and direction of the electric field due to the sheet of charge (Fig. 16–67).    $ \times10^{7 }\,N/C$  ----待选择----updown 

  What is the total charve b+ 4okx3yc(/3qb vuge of all the electrons in a 15-kg bar of aluminum?    $ \times10^{8 }\,C$What is the net charge of the bar?    C (Aluminum has 13 electrons per atom and an atomic mass of 27 u.)

Two small, identical conducting spheres A and B are a distance R apart; va jq- mny8*;,dbs7jr each carries the same charge Q. (a) What is the force sphere B exerts on sphere A? (b) An identical sphere with zero charge, sphere C, makes contact with sphere B and is then moved very far away. What is the net force now acting on sphere A? (c) Sphere C next makes contact with sphere A and is then moved far away. What is the force on sphere A i8nydj*; b7samj- rq v,n this third case?

Given the two charges shown in Fig. 16–68, at 0 u7jji0ldc*v what position(s) x is the electric field zero? Is the field zero at any other points, not on the x axis00* cjdlju i7v?

Two point charges, $+Q$ and $-Q$ of mass m, are placed on the ends of a massless rod of length L, which is fixed to a table by a pin through its center. If the apparatus is then subjected to a uniform electric field E parallel to the table and perpendicular to the rod, find the net torque on the system of rod plus charges.

Four equal positive point charges, each of cj k wzgm7/4x;dharge $8.0\,\mu C$ are at the corners of a square of side 9.2 cm. What charge should be placed at the center of the square so that all charges are at equilibrium? Is this a stable or unstable equilibrium (Section 9–4) in the plane?