What quantity is measured by 79lf x7os-gcsa battery rating given in ampere-hours $(A\cdot H)$ ?

When an electric cell is xw.a mi1gy:u+ake 4l+ connected to a circuit, electrons flow away from the negative terminal in the circuit. But wk+4. ge:aaxiw1 ly+umithin the cell, electrons flow to the negative terminal. Explain.

When a flashlight is op ;./oajjb se8nerated, what is being used up: battery current, battery voltag jbaj;o/se .8ne, battery energy, battery power, or battery resistance? Explain.

One terminal of a car battery is said to be connected to “ground- uq62tegv,4opmyy1 + ws;wul.” Since it is not really connected to the ground, what is meant buy;+,vme qw s4t 2g1lwp-6y ouy this expression?

When you turn on a water faucet, the water usually flows immediately7o+are k 3j+6nwpjcy0b, pr; m*ttq, i. You don’t have to wait for water to flow from the faucet valve to the spout. Why not? Is the same thing true when you connect a wire to the terminals of a battery y;ab+erct+q0, 3tro nwp,6jmkpj7i*?

Can a copper wire and an aluminum wire of the same length a4 us841uid unyn08 nfb0+ekehave the same resistance? E4+bu4 00a 8f18yueensin udnkxplain.

If the resistance of a small immersi1x5re 7 t*hyqxon heater (to heat water for tea or soup, Fig. 18–32) was increased, would it speed up or slow down the heatixeq*x75 y t1rhng process? Explain.

If a rectangular solid made of car51j z5 kktcn9d;g )d0gny7dowbon has sides of lengths a, 2a, and 3a, how would you connect the wires from a battery so as to obtaitkogw 1dd9ky;n7 zd 550nc)j gn (a) the least resistance, (b) the greatest resistance?

The equation $P\,=\,V^2/R$ indicates that the power dissipated in a resistor decreases if the resistance is increased, whereas the equation $P\,=\,I^2/R$ implies the opposite. Is there a contradiction here? Explain.

What happens when a ligh 3+j1z d6ncxrwdj7tr7tbulb burns out?

Explain why lightbulbs almost always burn out just as they are 42h.p ycuu oe4 vm)3vqturned on and not after they have been on for some tim v2q. eo)m4 3uvyhup4ce.

Which draws more currenyg2(kym t2o j6xk 7m8at, a 100-W lightbulb or a 75-W bulb? Which has the higher resistaamkytmg2x8o2 ( y67 jknce?

Electric power is transfetv;ev t0kuhw8vyg 7.1rred over large distances at very high voltages. Explain how the high voltage reduces power losses in the transhy7g18.wk;t0 vevvtumission lines.

A 15-A fuse blows repeatedly. Why is it da .o6o .rfutc*rxoj:0mngerous to replace this fuse with a 25-A fusej r.u0.crfo*otxm:6 o ?

When electric lights are operated on low-frequ; 2ut ws 9b)n-bwjfe-dency ac (say, 5 Hz), they flicker noticesb;j w-)wf-be unt2 d9ably. Why?

Driven by ac power, the same electroniml rm32.so v)s pass back and forth through your reading lamp over and over again. Explain why the light stays lit inst2r m3vos)i m.lead of going out after the first pass of electrons.

The heating element in a toaster is made kh3s 4b th-l:eof Nichrome wire. Immediately after the toaster is turned on, is the currents- le3t h:4bhk $(I_{rms})$ in the wire increasing, decreasing, or staying constant? Explain.

Is current used up in a res7 n 6y kw qkgqn)olp(c54*4odzdz n,r5istor? Explain.

Different lamps might have batteries conne5fh1bo 7g8mm, gndx rtk7q+5 d z9fv0wcted in either of the two arrangements shown ikg mqx tb m o817gr+hd9d zfv0fnw575,n
Fig. 18–33. What would be the advantages of each scheme?

  A current of 1.30 A flows in a wire. How many electrons are flowing past m*8r fe d0ta*aztyjg 8 (,cbs7any pointt s8,8(b t*07jaea*gc yrzmfd in the wire per second?    $ \times10^{18 }\;electrons/s$

  A service station charges a battery using a current of 6.7 A for ;n3jo/ a*2ouq6kgzb/dyv) hp 5.0 h. How much charge passes through the batter2nq 6b gv;h z/p )d*ajyu3/ooky?    $ \times10^{ 5}\;C$

  What is the current in amperesyuj 32b+lqy m4q9hc(ea.g 0aa if $1200\,Na^+$ ions flow across a cell membrane in $3.5\mu s$? The charge on the sodium is the same as on an electron, but positive.    $ \times10^{-11 }\;A$

  What is the resistance of a toastek. e ecc:s pbw65c:xl* nwb9+fr if 120 V produces a current of 4.2 A?    $\Omega$

  What voltage will prh+ a- f4pix* 9ri3h: kzayzfn6oduce 0.25 A of current through a $3800-\Omega$ resistor?    V

  A hair dryer draws 7.5 A when plugged into a 120-V lin3;jueejsq* )cbpntb/ i+xog5.g 53fhe.
(a) What is its resistance?    $\Omega$
(b) How much charge passes through it in 15 min? (Assume direct current.)    $ \times10^{ 3}\;C$

  An electric clothes dryer has a heating element with a resi9 vrtg :kb25j.2zfna mstance of $9.6-\Omega$.
(a) What is the current in the element when it is connected to 240 V?    A
(b) How much charge passes through the element in 50 min?    $ \times10^{ 4}\;C$

  A 9.0-V battery is connected to a bulb whosekiz(c x8mxuvjn/-u, ; q hiatmj7u*7+ resistance is $1.6\Omega$. How many electrons leave the battery per minute?    $ \times10^{21 }\;electrons/min$

  A bird stands on a dc electric transo:gqua :ls4ht /kg(;r mission line carrying 2800 A (Fig. 18–34). The line hahl k /;sr:ggt4o uqa(:s $2.5\times10^{ -5}\;\Omega$ resistance per meter, and the bird’s feet are 4.0 cm apart. What is the potential difference between the bird’s feet?    $ \times10^{-3 }\;V$

  An electric device draws 6.50 A at 2408. zigfurw :d9 V.
(a) If the voltage drops by 15%, what will be the current, assuming nothing else changes? $\approx$    A (retain one decimal places)
(b) If the resistance of the device were reduced by 15%, what current would be drawn at 240 V? $\approx$    A (retain one decimal places)

  A 12-V battery causes a 5j voszbt )5us*ylf cam,)3:d current of 0.60 A through a resistor.
(a) What is its resistance, $\approx$    $\Omega$
(b) how many joules of energy does the battery lose in a minute?    J

  What is the diameter of a 1.00-m length a8tc;zzh1 )se7i4p* (vsqo +n9 ihjweof tungsten wire whose resistance is $0.32\Omega$?    $ \times10^{-4 }\;m$

  What is the resistance of a 3.5-m length of copper wire 1.5 mm i 5zoipl*w9m6wa6o 2aq a)3nyqn diameter?    $ \times10^{ -2}\;\Omega$

  Calculate the ratio of the resistancehdk 70v0 x o3hl3sr* : /vihf lc7aw:88heyadd of 10.0 m of aluminum wire 2.0 mm in diameter, to 20.0 m of copper wire 2.5 mm in diame:dlr0 7ef*vdwv :3sh y8o8hca ikh d/hal0x73ter.   

  Can a 2.5-mm-diameter copper wire have the same resistance as a tungsten werq l:uhyv.y08jothy a m:92q )iq+u(ire of the same l q)h ju9ay.+q yetmi :0uh: vro(l2qy8ength? Give numerical details.    mm

  A certain copper wire has a resistance ofu*mq(*b wh9(ao fy:bs $10.0\Omega$ At what point along its length must the wire be cut so that the resistance of one piece is 4.0 times the resistance of the other? What is the resistance of each piece?
$R_{short }$   $\Omega$
$R_{long }$    $\Omega$

  How much would you have to raise thef3h 2 wuy7b.t2ix hg6jm t4x/f temperature of a copper wire (originally at f.ytji/hx h22f 7u4bwgtm3x 6$20^{\circ}\,C$) to increase its resistance by 15%?    $C^{\circ}\,$

  Estimate at what temperature copper will have the same resistivity rd+ec ick++av27i ;wg as tungsten doec wg+2+ vcik;+dei r7as at $20^{\circ}\,C$.$\approx$    $^{\circ}\,C$ (Round to the nearest integer)

  A 100-W lightbulb has a resista /xbm9 f4mijeuh; s43yyz15utnce of about $12\Omega$ when cold ($20^{\circ}\,C$) and $140\Omega$ when on (hot). Estimate the temperature of the filament when hot assuming an average temperature coefficient of resistivity $\alpha\,=\,0.0060(C?)^{-1}$ .    $^{\circ}\,C$

  Compute the voltage drop along a 26-m length of household no. 14 coppehssb3,b ,nh3b r wire (used in 15-A circuits). The wirbshn3,3, hbsbe has diameter 1.628 mm and carries a 12-A current.    V

  A rectangular solid made of carbon has sides of lengths 1.0 cm, 2.0 cm, an1x4ojl,g ty7wqyhws kz7 8z27 fb r4b,d 4.0 cm, lying along the x, y, and z axes, respectively (Fig. 18–35). Determine the resistance for cb7 7s7zwrqj8l y gybf ,4 1xk,4hot2wzurrent that passes through the solid in
(a) the x direction,    $ \times10^{ -4}\;\Omega$
(b) the y direction,    $ \times10^{ -3}\;\Omega$
(c) the z direction. Assume the resistivity is $\rho\,=\,3.0\times10^{ -5}\;\Omega\cdot m$.
   $ \times10^{ -3}\;\Omega$

  Two aluminum wires have the same resistance. If one has twice the2*muz *-rp -ij yzizgc3)g3tj length of the other, what is the ratio of the di 3jtgciy2mug-3)i-rz**zj p zameter of the longer wire to the diameter of the shorter wire? $\approx$    (retain three decimal places)

  A length of aluminum wire : +xgtyqy( -gv:ipfj7)vb*t w is connected to a precision 10.00-V power supply, and a current of 0.4212 v 7gy:j+v:ybt wtgpi *q)x-(f A is precisely measured at 20.0$^{\circ}\,C$. The wire is placed in a new environment of unknown temperature where the measured current is 0.3618 A. What is the unknown temperature?    $^{\circ}\,C$

  A 10.0-m length of wire con2it* (h*4tl t;-bo izm(mxdg7-j. egwflz ag5sists of 5.0 m of copper followed by 5.0 m of aluminum, bg2tez4g-*d5f*7 .m tl; ti azw- oh(bm(glxjioth of diameter 1.0 mm. A voltage difference of 85 mV is placed across the composite wire.
(a) What is the total resistance (sum) of the two wires? $\approx$    $\Omega$(retain two decimal places)
(b) What is the current through the wire?    A(retain two decimal places)
(c) What are the voltages across the aluminum part and across the copper part?
$V_{Cu}$    V(retain three decimal places)
$V_{Al}$    V(retain three decimal places)

  For some applications, it is important that the value of a resistance not ckfeygm 2u.o j5dl;(; a8auvd8 hange with temperature. For example, supposdkj 588g ;ya2.;veualfo m( ude you made a $4.70-k\Omega$ resistor from a carbon resistor and a Nichrome wire-wound resistor connected together so the total resistance is the sum of their separate resistances. What value should each of these resistors have (at 0$^{\circ}\,C$) so that the combination is temperature independent?
$R_{0C}$   $\Omega$
$R_{0N}$   $\Omega$

  The heating element of an electrih vnh: rgjy)qn- +b;va (el/+ic oven is designed to produce 3.3 kW of heat whl/ v)(v reb- y;g+n hqnaij:+hen connected to a 240-V source. What must be the resistance of the element?    $\Omega$

  What is the maximum power consumption of a 3.0-V portable CD player that r++edi :60 pu: woqmy2zfx8ztdraws a maximum of 320 mA of :20 qodyp +zzeutx m6f+r :8wicurrent?    W

  What is the maximum voltagec4 mf,*e+hzjz8sex0 7 s 5htma that can be applied across a $2.70-k\Omega$ resistor rated at $\frac{1}{4}$ watt?    V

  (a) Determine the resistance of, and currencn-v*a o, 4widcefy 4)z-ijc ,t through, a 75-W lightbulb connected to its proper source voltage of 120 Vjvc4cdz,*o,any 4 i-f wei c-).
   $\Omega$
   A
(b) Repeat for a 440-W bulb.
   $\Omega$
$\approx$    A(retain one decimal places)

  A 115-V fish-tank heater is rated at 110 W. Calkrvm (fz((1l.yyktq1 4nxb qi 0x)7pyculate
(a) the current through the heater when it is operating,    A
(b) its resistance?    $\Omega$(Round to the nearest integer)

  A 120-V hair dryer has two g+ay *gl)/)gopad +g 0eh6sgwsettings: 850 W and 1250 W.
(a) At which setting do you expect the resistance to be higher? After making a guess, determine the resistance at
(b) the lower setting;    $\Omega$
(c) the higher setting.    $\Omega$

  You buy a 75-W lightbulb int.w;pi)bwftb(, k 0:a(cpbm /7 qvyvt Europe, where electricity is delivered to homes at 240 V. If you use the lightbulbcbk;aqf. y0 iwbv, tv7)p(/b( mtt pw: in the United States at 120 V (assume its resistance does not change), how bright will it be relative to 75-W 120-V bulbs? [Hint: assume roughly that brightness is proportional to power consumed.]    as bright

  How many kWh of energy i jhdy;z g:y00b: u:rrc+x lh.n+;eyrdoes a 550-W toaster use in the morning if it is in operation fgu zi cx . y0+:hnr0+y:dryr;;j :belhor a total of 15 min? At a cost of $9.0\,cents/kWh$, estimate how much this would add to your monthly electric energy bill if you made toast four mornings per week.
energy $\approx$    kWh
cost $\approx$    cents/mouth

  At 0.095 dollars per kWh, what does it cost to leave a 25-W porch kmbo; rpe 0 :t4* hy+bb*9t+)ura fsn8dmb)cp light on day and night for a y: ar ;9+e8tdt)rm+)b*bn*f u ypbkhb0cpsm4oear? $\approx$    dollars

  An ordinary flashlight uses two D-cell 1.5-V batteriku* *:3rdeks7 *w;zxnmkaay a 1rl(0dx u tm4;es connected in series as in Fig. 18–4b (Fig. 18–36). The bulb dradnzk:aa mutdsmyk**7r4e*;1xr kwl a3 x0(u ; ws 450 mA when turned on.
(a) Calculate the resistance of the bulb and the power dissipated.
$\approx$    $\Omega$(retain one decimal places)
   W
(b) By what factor would the power increase if four D-cells in series were used with the same bulb? (Neglect heating effects of the filament.) Why shouldn’t you try this?   

  What is the total amount of energy stored in a a 2zma -abgg9b 78,luq12-V, $85-A\cdot h$ car battery when it is fully charged?    $ \times10^{6 }\;J$

  How many 100-W lightbulbs, connected to 120 V as in Fig. 18–20)g w,q7x-:b zaasgv: y, can be used without blw): y q-a,az: xsbv7ggowing a 15-A fuse?    bulbs

  An extension cord made of two wires of diameter 0.129 cm (no. 16 c9*wgw9qh f 88p a- znfbwek4c2opper wire) and of length 2.7 m (9 ft) is connected to an elep 8gnhw 8cw4ffqek9w9 2*ab-zctric heater which draws 15.0 A on a 120-V line. How much power is dissipated in the cord?    W

  A power station delivers 620 kW of power at 12,000 V to a nmecjy n/j ,5,ftkx44;jny ji0 v4g+dfactory through wires with total resistaytm5+4c0jfx,4;je/n d j n4kji,y vgnnce $3.0\Omega$ How much less power is wasted if the electricity is delivered at 50,000 V rather than 12,000 V?    $ \times10^{ 3}\;W$

  The current in an electromagnet connected to a p so(*r1.*az.h 9:hrz: cwemxui3c sh240-V line is 17.5 A. At what rate must cooling water pass over the coils if the water temperature is to rise by no more th3(9*s:cxhsermz1*awhhczu: oir. p .an 7.50 $C^{\circ}\,$?    kg/s

  A small immersion heagcec*q0oy k:zkq 1x 6-ter can be used in a car to heat a cup of water for coffee or tea. If the heac ezcqy6k* - qg:ox0k1ter can heat 120 mL of water from 25$^{\circ}\,C$ to 95$^{\circ}\,C$ in 8.0 min,
(a) approximately how much current does it draw from the car’s 12-V battery, $\approx$    A(Round to the nearest integer)
(b) what is its resistance? Assume the manufacturer’s claim of 60% efficiency.    $\Omega$

  Calculate the peak current in i s85kai9w2 /x 5skfqup/am/9n-opox a $2.2-k\Omega$ resistor connected to a 220-V rms ac source.    A

  An ac voltage, whose peak value is r yb/40dzadn,180 V, is across a $330-\Omega$ resistor. What are the rms and peak currents in the resistor?
$I_{peak}$$\approx$    A(retain two decimal places)
$I_{rms}$ =    A

  Estimate the resistance .uq) zv2qo// otsi bw8 9e8lxzof the $120-V_{rms}$ circuits in your house as seen by the power company, when
(a) everything electrical is unplugged,
(b) there is a lone 75-W lightbulb burning.    $\Omega$

  The peak value of an alternating current in a 1500-W device is 5.4w:fs c9ljhw 8 d.w5 1cu5,kzn ;c* zbjbb65ubu A. What is the rms wnu s.bu5b cuc bj1zlww5*9ck:bfd6zj ,8 h5;voltage across it?    V (Round to the nearest integer)

  An 1800-W arc welder is connected tr 1;bx8)nfe jeo a $660-V_{rms}$ ac line. Calculate
(a) the peak voltage$\approx$    V(retain one decimal places)
(b) the peak current.    A

  (a) What is the maximum instantaneous power dissipated by a 3.0- nig. pyu9jvi-r q857zehmi r75o oc3-hp pump connecte7ruiq8z-yoim .r35oh g-7e9jpic5vn d to a $240-V_{rms}$ ac power source?    W
(b) What is the maximum current passing through the pump?    A

  A heater coil connected to a mhz1s;x* yqge0w57 ( lxkodhro/ ujq f)-m.n*$240-V_{rms}$ ac line has a resistance of $34\Omega$.
(a) What is the average power used?    W
(b) What are the maximum and minimum values of the instantaneous power?
$P_{max}$    W
$P_{min}$    W

  A 0.65-mm-diameter copper wire carries a tiny curre.kc hr3 8g b53(vy.jgupc4nd dnt of $2.3\mu A$. What is the electron drift speed in the wire?    $ \times10^{-10 }\;m/s$

  A 5.80-m length of 2.0-mm-diameter wire carries a 750-mA current w *i fx63wnm3a*p- /q0;sc87n)vjy zbig mmepqhen 22.0 mV is applied to its ends. If3-mi0/3 fmanmgq )icsz pnq7b p*vw8jx6y;e * the drift speed is $ 1.7\times10^{-5 }\;m/s$, determine
(a) the resistance R of the wire, $\approx$    $\Omega$(retain three decimal places)
(b) the resistivity $\rho$.    $ \times10^{-8 }\;\Omega\cdot m$
(c) the number n of free electrons per unit volume.    $ \times10^{28 }\;e^-/m^3$

  At a point high in the Earth’smrkqzn0jpp)(53 y5frx t;;0si;uz oh atmosphere, $He^{2+}$ ions in a concentration of $ 2.8\times10^{12 }\;/m^3$ are moving due north at a speed of $ 2.0\times10^{6 }\;m/s$. Also, a $ 7.0\times10^{11 }\;/m^3$ concentration of ${O_2}^-$ ions is moving due south at a speed of $ 7.2\times10^{6 }\;m/s$. Determine the magnitude and direction of the net current passing through unit area $(A/m^2)$.    $A/m^2$  ----待选择----NorthSouth 

  At a point high in the Earth’s ai3o +sz ndp7bf )k d+u/x(16ccr5bodp tmosphere, $He^{2+}$ ions in a concentration of $ 2.8\times10^{12 }\;/m^3$ are moving due north at a speed of $ 2.0\times10^{6 }\;m/s$. Also, a $ 7.0\times10^{11 }\;/m^3$ concentration of ${O_2}^-$ ions is moving due south at a speed of $ 7.2\times10^{6 }\;m/s$. Determine the magnitude and direction of the net current passing through unit area $(A/m^2)$.    $A/m^2$  ----待选择----NorthSouth 

  What is the magnitudsh8podd)u2-v jyjo 2(e:i6dva)u: p ee of the electric field across an axon membrane $ 1.0\times10^{ -8}\;$. thick if the resting potential is $-70 Mv$   $ \times10^{6 }\;V/m$

  A neuron is stimulated with an electric pulse. The action potential is fq t ije0rh urv0)u+5d;b-kc. detected at a point 3.40 cm down the axon 0.0052 s later. When the action potential is detected 7.20 cm from the point of stimulation, the time required is 0.0063 s. What is the speed of the electric pulse cqt0 uir+djf) bv 5.e;u khr0-along the axon? (Why are two measurements needed instead of only one?)    m/s

  Estimate how much energy is required to transmit one action potentialro7k*g z. tn(k along the axon of Example 18–15. [Hint: the energy to transmit one pulse is equivalent to the energy stored by charging the axon capacitance; see Section 17–9]. What minimum average power is requ o7.gknk( *rztired for $10^4$ neurons each transmitting 100 pulses per second?
E =    $ \times10^{-11 }\;J$
P =    $ \times10^{-5 }\;W$

  During an action potentialhqz8* tqeai30, $Na^+$ ions move into the cell at a rate of about $3 \times10^{-7 }\;mol/m^2\cdot s$. How much power must be produced by the “active $Na^+$ pumping” system to produce this flow against a $+30-Mv$ potential difference? Assume that the axon is 10 cm long and $20\mu m$ in diameter.    $ \times10^{-9 }\;W$

  How many coulombs argq-8 6l z8+oqvek7 h iwhz)yt7e there in 1.00 ampere-hour?    C

  What is the average current drawn by 1 xa9 aipkeu5*gc 2ld2a 1.0-hp 120-V motor? $(1 hp\,=\,746\,W.)$    A

  A person accidentally leaves a car with the lights r8xyoei7 dc)fdjf90f 46 e8j lon. If each of the two headlights uses 40 W and each of the two taillights 6 W, f8ryd ie6ff98 ) ejo4lxc7fjd0or a total of 92 W, how long will a fresh 12-V battery last if it is rated at $95A\cdot h$? Assume the full 12 V appears across each bulb.    h

  The heating element of a 110-V, 1500-W heater is 5.4 m lon4v3 re5jq:pes3 tqhgim,ibp n.fgt856zj)x8 g. If it is made of iron, what must its,pq)rxg4q3 ji8bhv5ie6e:njzmf g5 p t.83 ts diameter be?    $ \times10^{ -4}\;m$

  The conductance G of an object is defined as the recip3qtq9w0dq+ e l:wwl17,ilee m v*5 mmgrocal of the resistance R;eeq7 *w:v9 3tllgdewm lm, 5qiq1 m+0w that is, $G\,=\,1/R$. The unit of conductance is a mho $(=ohm^{-1})$, which is also called the siemens (S). What is the conductance (in siemens) of an object that draws 730 mA of current at 3.0 V?    S

  A small city requires about 10+hkcg8r0thx,x.r f8 cqmq6ebwg 3g-3 MW of power. Suppose that instead of using high-voltage lines to supply the power, the power is delivered at 120 V. Assuming a two-wire linek8e0q-qw3 .3c rbr68,m ch xgf+g ghtx of 0.50-cm-diameter copper wire, estimate the cost of the energy lost to heat per hour per meter. Assume the cost of electricity is about 10 cents per kWh.    dollars per hour per meter

  (a) A particular household usesb la/b;1 ilqoran9 k0, a 1.8-kW heater $3.0 h/day$ (“on” time), four 100-W lightbulbs $6.0 h/day$, a 3.0-kW electric stove element for a total of $1.4 h/day$ and miscellaneous power amounting to $2.0 kWh/day$. If electricity costs $0.105 per kWh, what will be their monthly bill (30 d)?    dollars per month
(b) How much coal (which produces $7000 kcal/kg$) must be burned by a 35%-efficient power plant to provide the yearly needs of this household?    kg of coal

A length of wire is cut in half and the two lengths ar m o0i2:wp9a s,no0z+frjr9s-juz )pve wrapped together side by side to make a thicker wire. How does the re jj sw2 oin9:pzov +rfzau)9r, 0ps0m-sistance of this new combination compare to the resistance of the original wire?

  A 1200-W hair dryer is des7p ;hss u 9sr-vfyfl)rh i:ce( ui1v,7igned for 117 V.
(a) What will be the percentage change in power output if the voltage drops to 105 V? Assume no change in resistance.    % decrease
(b) How would the actual change in resistivity with temperature affect your answer?

  The wiring in a house must be thick enough so it does not become soyq x:zrphq,;j37t a bv8:mi 2h hot as to start a fire. What diameter must a copper wire be if itbqtz2:,3:;ia7 hq8 vpmj y xhr is to carry a maximum current of 35 A and produce no more than 1.8 W of heat per meter of length?    $ \times10^{ -3}\;m$

  Suppose a current is given by the equatiw eend+3v3g(.;ekcuj m7w 1limm h5 .von $I\,=\,1.80sin210t$, where I is in amperes and t in seconds.
(a) What is the frequency?    Hz
(b) What is the rms value of the current?    A
(c) If this is the current through a $42-\Omega$ resistor, write the equation that describes the voltage as a function of time. (  sin 210 t)V

  A microwave oven running at 65% efficiency delivers 950 W of energy per secondc(h7ilvz4 fy*uu (*vt to the interi4uuv7l v*y( hi cz(f*tor. Find
(a) the power drawn from the source,    W
(b) the current drawn. Assume a source voltage of 120 V.    A

  A $1.00-\Omega$ wire is stretched uniformly to 3.00 times its original length. What is its resistance now?    $\Omega$

  220 V is applied to two dif mxi5ji.- xm7lww2 86bb0cv b3r/rhn jferent conductors made of the same material. One conductor isxlxrji8rb 3nb/6v7-c0jm .ih 5wmw2b twice as long and twice the diameter of the second. What is the ratio of the power transformed in the first relative to the second? $P_1$ : $P_2$ =    : 1

  An electric heater is used to heat ac0l)e,qy tc,qq5a 0td room of volume $62\,m^3$ Air is brought into the room at 5$^{\circ}\,C$ and is completely replaced twice per hour. Heat loss through the walls amounts to approximately $850 kcal/h$. If the air is to be maintained at 20$^{\circ}\,C$, what minimum wattage must the heater have? (The specific heat of air is about $0.17 kcal/kg\cdot C?$)    W

  A 2200-W oven is hooked to a 240-V source)3 ml4 sesgq) 3kq0z(vnc( t1kgevrk1.
(a) What is the resistance of the oven?    $\Omega$
(b) How long will it take to bring 120 mL of 15$^{\circ}\,C$ water to 100$^{\circ}\,C$ assuming 75% efficiency?    s
(c) How much will this cost at $11 cents/kWh?$    cents

  A projected electric vehicle makes use of storage batteries as its sourcn8ozv5yxjk; 81m umr osq;m5 *e of energy. Its mass is 1560 kg and it is powered by 24 batteries, eac*rj8s k mm vn55o;oy8 qzm;x1uh 12 V, $95A\cdot h$. Assume that the car is driven on level roads at an average speed of $45 km/h$ and the average friction force is 240 N. Assume 100% efficiency and neglect energy used for acceleration. No energy is consumed when the vehicle is stopped, since the engine doesn’t need to idle.
(a) Determine the horsepower required.    hp
(b) After approximately how many kilometers must the batteries be recharged?    km

  A $12.5-\Omega$ resistor is made from a coil of copper wire whose total mass is 18.0 g. What is the diameter of the wire, and how long is it?
L =    m
A =    $ \times10^{-4 }\;m$

  A 100-W, 120-V lightbuvv 79/9l ypw;xq/pfgrz4i:nx lb has a resistance of $12\Omega$ when cold (20$^{\circ}\,C$) and $140\Omega$ when on (hot). Calculate its power consumption at
(a) the instant it is turned on,    W
(b) after a few moments when it is hot.    W

  The Tevatron accelerator at Fermilan394ch+b*+l* +dojwoj wt orj b (Illinois) is designed to carry an 11-mA beww ndo++3jro+ *b hljtoj *49cam of protons traveling at very nearly the speed of light ($ 3.0\times10^{8 }\;m/s$) around a ring 6300 m in circumference. How many protons are stored in the beam?    $ \times10^{12 }\;protons$

  An air conditioner draws 12 A at 220-V acs )tp :h5ku(ys. The connecting cord is copper wire with a diameter of 1.62sktys) (:5 uph8 mm.
(a) How much power does the air conditioner draw?    W
(b) If the total length of wire is 15 m, how much power is dissipated in the wiring? $\approx$   W(Round to the nearest integer)
(c) If no. 12 wire, with a diameter of 2.053 mm, was used instead, how much power would be dissipated? $\approx$   W(Round to the nearest integer)
(d) Assuming that the air conditioner is run 12 h per day, how much money per month (30 days) would be saved by using no. 12 wire? Assume that the cost of electricity is 12 cents per kWh. $\approx$   cents per month (Round to the nearest integer)

  A fish-tank heater is rated at 99dow )k2i2qfb ;otbu.5 W when connected to 120 V. The heating element is a coil of Nichrome wire. When uncoiled, the wire 2u.; bo9)tbkqdo2w if has a total length of 3.8 m. What is the diameter of the wire?    $ \times10^{ -4}\;m$

  In an automobile, the system voltage varies from aboutbg o wfjk;5kdc:h -fn*y ,44yx 12 V when the car is off to about 13.8 V when the car is on and the charging system is in operation, a difference of 15%. By what percentage does the power delivered to the headlights vary as the voltage changes from 12 V to 13.8 V? Assume the headlight *kfjg 4 ;,:bd4hcxofn yy5wk- resistance remains constant.    % increase

  Lightbulb A is rated at 120 V and 40 W for hxm0doe: 4z.n wousehold applications. Lightbulb B is rated at 12 V and 40 W for automotivz4mwn.:doe 0 xe applications.
(a) What is the current through each bulb? $I_A$ =    A $I_B$ =    A
(b) What is the resistance of each bulb? $R_A$ =  $\Omega$ $R_B$ =  $\Omega$
(c) In one hour, how much charge passes through each bulb? $Q_A$ =    C$Q_B$ =    C
(d) In one hour, how much energy does each bulb use?    $ \times10^{5 }\;J$
(e) Which bulb requires larger diameter wires to connect its power source and the bulb?  ----待选择----Bulb BBulb A 

  Copper wire of diameter 0.259 cm is used totym1 a zzv1v/3 b)e-xd connect a set of appliances at 120 V, which draw 2250 W of power total. x/a bvz)zm-e y1d13vt
(a) What power is wasted in 25.0 m of this wire?    W
(b) What is your answer if wire of diameter 0.412 cm is used?    W

  A copper pipe has an inside diameter ofpq;,m6s0bde bv llr55 3.00 cm and an outside diameter of 5.00 cm (Fig. 18–37qrd, eb; lp6ml5bs v50). What is the resistance of 10.0 m of this pipe?    $ \times10^{-4 }\;\Omega$

If a wire of resistance R is stretched ev3o n-w;0:a b4 /ru:bwtuw:x u c:btusca/l5uniformly so that its length doubles, by what factor does the power dissipatube::/r450- tcb;lus xoa3cw:vaut nwbu :/wed in the wire change, assuming it remains hooked up to the same voltage source?

  A tungsten filament used in a flashlight bulb opdxm6h.qd u8v(erates at 0.20 A and 3.2 V. If its resishx6 dvd8 muq(.tance at 20$^{\circ}\,C$ is $1.5\Omega$ what is the temperature of the filament when the flashlight is on?    $^{\circ}\,C$