If you charge a pocket comb by rubbing j6f: joxm y8pbzgs.v :/9;ff(k aoil-it with a silk scarf, how can you determine if the comb is positively or negatively chakbfvjia(-8;lf:sx m gzoo . y9f6jp/:rged?

Why does a shirt or blouse taken from a clothes dxb-ft f6 kyqkmx88e hmol- )n;m m+);sryer sometimes cling to your bo8yb6f l; f)hm8n; -- omtmksx exmk)q+dy?

Explain why fog or rain droplets tend to form around ions or electrons imzn,tgpcl 0o1/p)q5. o xv ;uzn the air.z.no/1m0vt qpcz u5pl),gx o;

A positively charged rod is brought close to a neutral piece of pa nyvu4p n 27gzhbjvgr4n1+w84per, which it attracts. Draw a diagram showing the separation of charge an br12vgnnjp w48 7zgy4+h un4vd explain why attraction occurs.

Why does a plastic ruler that has been rubbed with a cloth:0rhtd5r-w(mder m/xt inc05 have the ability to pick up small pieces of paper? Why is this difficult to d/drt i -crt:m(e dxw005rnh5 mo on a humid day?

Contrast the net charge on a conductor to the “free charges” in the conductu 0jcdrxby5 e2u(.5dd+tmp*xwb,n t q 4ev)3yor.

Figures 16–7 and 16–8 show how a charged rod placed near an uncharged met5edfw 7t 7oaxxpp+3 ;xal object can attract (or repel) electrons. There are a great many electrons in the metal, yet only some of them move as shown. Why not all o3pt5x+7 xxfwdap o;e 7f them?

When an electroscope is chaf6q3,iio1 htarged, its two leaves repel each other and remain at an angle. What balances the electric force ofiho 16tf ,3qai repulsion so that the leaves don’t separate further?

The form of Coulomb’s l .iby26 vst *7tzvcrg3l 9jw-5n6prs+xcaxp3aw is very similar to that for Newton’s law of universal gravitation. What are the differences between these two laws? Compare also gravitational mass and el+bpr579vayl3 pv c6r snj-z2*gw3itt.xs xc6 ectric charge.

We are not normally aware of the gravitational or electric force between two ora vcnm s:g7/4tg jhxmpr,/xg 16901 fnil-fyzdinary objects. What is the reason in each case? Give an example where we are aware of each one and whszpc ,1mg gnlh ntm0-ja r6xvg 4y17 :ifx/9f/y.

Is the electric force a conservative forcea i5drr/ h8r.oq edn.*? Why or why not? (See Chapter 6.)

When a charged ruler attracts small pieces of paper, sometimes a piece jumps ntf l* c*r+i-z7x8grg quickly away after touching the rux+*t frzi-8 * g7lgcnrler. Explain.

Explain why the test chargeka gtt/ g 77)a)ceoy4ys we use when measuring electric fields must be small.77y)t/a4c )tkag oegy

When determining an bw 7i+w.k p)nmnj* -htelectric field, must we use a positive test charge, or would a negative on+n7t-nji*bm.w w)h pke do as well? Explain.

Draw the electric field lines surrounding two +lmxh(cdwf i x(8j- t*negative electric charges a distance l+x -tm( 8*fwlc(ijhx d apart.

Assume that the two opposite charges in Fig. 16–31a are 9fbu3( 4ha 4t 5 y3v3hdhswn3di+laec12.0 cm apart. Consider the magnitude of the electric field 2.5 cm from the positive charge. On which side of thih talv 3 u 35y3wbd+4c4shh3iaf 9ned(s charge — top, bottom, left, or right — is the electric field the strongest? The weakest? Explain.

Consider the electric field at points A,6 gv* 8ikepj:g B, and C in Fig. 16–48. First draw an arrow at eac8g keigj 6pv:*h point indicating 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 linesz 6wvgw)xj uw55-*epl never cross?

Show, using the three u4,su *5i)l*kr+(jwpudx bli 2wmtf-rules for field lines given in Section 16–8, that the electric field lines starti t 4 xupwik(u-r)s,wmj +5*b*lu dl2fing or ending on a single point charge must be symmetrically spaced around the charge.

Given two point charges Q and 2Q, a distance l apart, is there a point alo 1o7p jkti8b*wng the straight line that passes through them where E=0 when their signs are (a) opposite, (b) the same? If yes, p7*jo18ikt w bstate roughly where this point will be.

Consider a small positive test charge located on an electric field line at somedj n:7ldxf p11 poinn:d p1 fx7lj1dt, such as point P in Fig. 16–31a. Is the direction of the velocity and/or acceleration of the test charge along this line? Discuss.

Sketch the electric field lines for a unifokfp6jdz i(s d5*vv)9trm line of charge which is infinitely long. (Hint: Use symmetry.) Is the electric *dz6 disktfvjp)9 v5(field uniform in strength?

If the electric flux through a close3qexk n yg)8dd:;0+j og0yncybv jq .:d surface is zero, is the electric field necessarily zero at all points on the surface? Explain. What about the c dn q+nvb:go x8 yy:dj0;ye0jqkcg. )3onverse: If $\vec{E}\,=\,0$ at all points on the surface is the flux through the surface zero?

A point charge is surrounded by a sphericalqxizcd 2k0sd.,tx3u,3kt7a e gaussian surface of radius r. If the sphere is replac,0udeqxi2 txa3c7 s t.k kd,3zed by a cube of side r, will $\Phi_E$ be larger, smaller, or the same? Explain.

  

  How many electrons make up a chargeb6 cxi4o;0c mvsu( h4b of $-30.0\mu C\,?$   $ \times10^{ 14}\, electrons $

  What is the magnitude of the electric ft sw vy(dsj4+3orce of attraction between an iron nucleuj(syts w43d+v s $(q\,=\,+26e)$ and its innermost electron if the distance between them is $ 1.5\times10^{-12 }\,m\,?$    $ \times10^{-3 }N$

  What is the repulsive electrical force between gindeak3047;x( vh ptmv70)ry vb+nxtwo protons $ 5.0 \times10^{ -15}\,m$ apart from each other in an atomic nucleus?    N

  What is the magnitude of the for/atr;t;7ejxd2c zs7t d xm49zce a $+25\,\mu C\,C$ charge exerts on a $+\,3.0\,mC$ charge 35 cm away?    N

  Two charged dust para5zt w at4p8h(ticles exert 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 moved, a ehh/d0m1;m wq4caw6+i(mg1 vmn zw 8s-uxze0nd the force on each of them is found to have bee mmaw w zs m1/1u;g0+qwzi8n-0xh6dec( h4me vn tripled. How far apart are they now?    cm

  A person scuffing her feet on a wool rug on a dry day accumulates a net charge ot9olnv-3akn 9-tako3 n9v l9nf $-42\,\mu C\,.$ 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 all t*- 3d olkt klc5atcn08b70v lzhe electrons in 1.0 kg of $H_2O\,?$    $ \times10^{ 7}\,C$

  Compare the electric force holding the electron inj id+st+i 7u22a ulm 6fw,;,8uyokalw 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 charges are a fixed8uvzh38(.cfvl3o xr lgj 7s6c distance apart. The sum of their charges is What charolg(jrv6us f837cl 3z xvch.8ge must each have in order to
(a) maximize the electric force between them,
(b) minimize it?

  Particles of charge 2my:9ot3qw*x 9e r ztt/rbz .u$+75$,$+48$ 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 of equa8* k tg .8+f9q casue2brtf1yul 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 ti s y8(gzknj(zl;i96square 0.100 m on a side. Determine jty;98szn(i6 glzk( ithe magnitude and direction of the force on each charge.
$F_4$ =   $ \times10^{ 7}\,N$
$\theta$ =    $^{\circ}$

  Repeat Problem 14 for the case when two of the positive charges, on opposite cor(v9w-v x xvu-smi*jz8ners, are replaced by negative charges of the same magnitude w9vi*uv(v- 8 sm- jzxx(Fig. 16–51).
$F_4$ =   $ \times10^{ 7}\,N$
$\theta$ =    $^{\circ}$

  At each corner of a square of side l there are point charges of magnipjvjxw*u0j9w+8 r:s;9 ws: e,qj odrltude Q, 2Q, 3Q, and 4Q (Fig. 16–52). Determine the force on (a) theww9+ldj:u9 rjo,j*qsv: je x0p rw8;s 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 9.,yfcsh0cwypne;o cqgyu78 x6 (;tpan equilateral triangle of side 1.20 m (Fig.y7c0 h;c,wxpu9nt.yg(c6;8o p sqefy 16–53). The 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 tg*:u,yedd ;xc otal charge of $560\,\mu C\,.$ 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 hbs qgpb( ; fuf(tln772q5f 0fwnm70ueave a 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 f9 j)ukab ..i3lcy0nw nrom an initially uncharged plastic ball to an identical ball 12 cm away. The force of attraction is then 17 mN. How many electrons were tcl3jun. )wna.9kbi0y ransferred from one ball to the other?    $ \times10^{ 12}$ electrons

  What are the magnitude and direction of the electric force oe :al1/3tw,jy .xf 4i ks:jpfpn an electron in a uniform elecjwx: :e ytfplfi ,k/3.1jasp4tric 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 experienceiyxd1:7fs, 0stnf cv :kyjz*.s an electric force of 0 *t,yf1jyd.xcskvz 7n is:f:$ 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 is exer5o h1 8/ ,hwyjem.xrb .f-: fgfm,u*ekyjm)ntted 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 and direction of the electric field 2a 3 fsh6lx 6/.spbc mxqql/39-g9iy rz9a4lee0.0 cm directly above a6hsae39 zaf ix /c9s.l -lrgx3 ybq/9 m4pe6qln isolated $33.0 \times10^{ -6}\,C$ charge?    $ \times10^{6 }\,N/C$ ----待选择----updown 

  What is the magnitude of the acceleration +9 wi v72xwbccj7z* qeexperienced by an electron in an electric9qjvbwce7 2 *ixwz +7c 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 the ele2 zrajt9bqm4 -m/g3mictric field at a point midway between a mm4 3mr ta-j9/2 igbzq$-8.0\,\mu C\,$ 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 field lines about twith.r): i c*pcl:z . ax5g3sndo point charges, $+Q$ and $-3Q$ which are a distance l apart.

  What is the electric field st 8hmdihynz: np:0w/ eg2q,wff *f7cz- rength at 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 from rest in a uniform electric field -)i:ld1b awcxpp:bxt 9vp m90/ mkk2xand accelerates to the north at a ratec9ap2 txdml:bwx9 k:k)i mp/v1 b-x0p 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 two equal but opposite p 8nyj2nspz)evlh49( ooint 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 center of a square 52.5 cm on ha-ka2/ry(* fsn11x r(y v dwia side if one co2nvy( dr1 wr(f*k1 /h-ixasayrner 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 electric fiel0vvbw97b1 r1qi:m bcg cds-*zd at one corner of a square 1.00 m on a side if the other three corncc1s0*9bv b-7iq1 gv:zrwmd b ers are occupied by $ 2.25\times10^{-6 }\,C$ charges.    $ \times10^{4 }\,N/C$   $^{\circ}\,$ from the x-direction.

Determine the direction and magnitude of the electric w1i4fyu4+.55 xgc,ge s rw nswj3m(eqfield at the point P in Fig. 16–54. The charges are separated by a distance 2a, and point P is a distance x from the midpoint between the two charges. Express your answer in tm,jiw1e3 xg yw+fgr(5 e4n4.us swq5 cerms 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 el* ij xcazwx n:ar;.5;dectric field $\vec{E}$ 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 direction o44 g 9bkrhp /lt.b7xkof the electric field at points A and B in Fig. 16–57 due to the two positive charges bpr/kbhxlt4 47 o .9gk$(Q\,=\,7.0\mu C\,)$ 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 unknown point cha m x** *7httt549fw vdgrpp 5./alxoyxrges, $Q_1$ 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 poinhaf 1:l 6;fbyq caeg9:de/p5vt P shown in Fig. 16–59. The two charges are separated by a distance of 2a. Point P is on the perpendicular bisector of the line joining the charges, a distance x from the midpoint between them. Exp /:qe p 6y:b1faagfhd9ve5lc; ress 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 tjujc,360hb bd o the right at 1.0% the speed of light enters a uniform electric fie3 dj 0hbcub,j6ld 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 from a c ed8,xri;hm9 tubical 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 radius 18 cm is placed in a uniform electric field of mx9erl1yp3kgy-j np (1agnitude (yny1gkjx3p le19rp- $ 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 o +rk7xkm rukc0,m(s3khzrkt h9*m .: hbjects, $O_1$ 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 in t) wlgjn+*ir* j-w q),7wmxjea 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 between two square metal plates is The plates are 1.0 m oiui bmk. 66 82hc5zkxyn a side and are separated by 3.0 cm. What is the charge on each plate (assume equal and opposite)? Neglect edge effects. k mk8i5.6 czb yxu2i6h   $ \times10^{-9 }\,C$

  The field just outside a 3.50-cm-radius metal ball is0s l(wbaughy*,mr.k959ryj p $ 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 radr9t y * daait7*50lhpxx-k y1eius 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 a9+0slp 5 vhj7( (bb:ormwftrspherical conducting shell. (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, avba :(9 r(pfjs+lo5 tw7hrm0bnd (c) beyond the shell? (d) Does the shell affect the field due to Q alone? Does the charge Q affect the shell?

  The two strands of theaerb0szf9u / 9 tv..ij helix-shaped DNA molecule are held together by electrostatic 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 indicated C and O atoms is 0.4e. Assume also that atoms on each molecule are st.90jsz a9.e vi fu/breparated 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 bezmf )n4*y yveg yte6qh/*emppj;.( j6 if the electric force between them is equal to the weight of .(gzth;ej) 6mp pyj6 neef/m*yyq* v4either at the Earth’s surface?    m

  A 3.0-g copper penny has axj6df4 g +emjchx.zo d w5)(,,l7 vigv positive charge of $38\mu C$. 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 surrounding 4.8n8 lbbiv fhmrg(a u8)nu l1the Earth. Its magnitude.n a m8u8vlbfr4) ihub81ng (l 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 thq du(j xz-7 fsowgyt2r- h)kru8k+;1ie acceleration experienced by an ele8gsf7hdz2y+)(ikuq;r x 1wo j t- k-urctron near the surface of 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 radius 0.018 mm remains sdnrf /4;pvcu5tationary in the air. If the downward-directed electric field of the Ear;vrdc p/ fu54nth is $150\,N/C$ how many excess electron charges must the water droplet have?    $ \times10^{ 6}$ electrons

  Estimate the net force between the CO group and thf) ajfbo:n5hydz-1p5 e HN group shown in Fig. 16–62. The C and O have c o yn:)b5dfpza5-j1fhharges $\pm0.40e$, 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 circular o 3zg3tfuf*w8brf:* /wskcwukqh0(s 0rbit around the proton wfu t*r c fkw*08qb(3z whk/s0 swfu3:gith 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, razoela y .i-/4tther than gravity, were responsible for holding the Moon in orbit around the Earth. If equal and opposite charges Q were placed o-e az y4o/.itln the Earth and the Moon, what should be the value of Q to 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 speed *pws)k(.vgi pi 9sm flu7,/-2got iij $v_0\,=\, 21.5\times10^{6}\,m/s$ 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 charv-i,sdy7n 5 wage $Q_1\,=\, 2.5\times10^{-5}\,C$. 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 insulating plak-nk :cji665 ekk h3yastic and suspended vertically from an ideal spri6:ce3yajki-k5 khk n6ng $(k\,=\,126N/m)$ 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 made with “leaves” that are 78-cm-long wirw x5+,(uvkoqp 4ken a0es with tiny 24-g se45 p on +0k,wax(qukvpheres at the ends. When charged, 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 thm pny85 ysk+e cu6+//vhni y bp4wb/+re electric field exceeds about8/hnrv cpysu y/54+k yb+w+pe/inmb6 $ 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 dd/ (o2ydqkx-, and net 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 charge of apo:3vzncl8-8vvgck) ll 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 apar+xvm 3 3;*fwpgda xmaxi,c9z )t; each carries the same charge Q. (a) What is the fo,+)miz d f3;xx 3cap9a v*xgwmrce 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 in this third case?

Given the two charges shown i2z,lppo1 kj; rn Fig. 16–68, at what position(s) x is the electric field zero? Is the fiel,1;rpzo jlp k2d zero at any other points, not on the x axis?

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, eaqjyr, :;,npig0h a/df8ds-tjur3j z+ch of charge $8.0\,\mu C$ 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?