What quantity is measured by a battek ;m cwdpisc1-lf qa,ze(50s/ ry rating given in ampere-hours $(A\cdot H)$ ?

When an electric cell is connected to a circuit, electrons flow away rym(8 l omp32ffrom the negative terminal in the circuit. But within the cell, electrons flow to the negative terminal. (2o8m3 mpflyr Explain.

When a flashlight is operated, what is b ts:s+xjkni-u 3;w4boeing used up: battery current, battery voltage, bbxjins-s ow+ 4;3 k:utattery energy, battery power, or battery resistance? Explain.

One terminal of a car battery is said to be connected to “ground.” Since jjkev3s;4 fok.p j a::rkm/k4 it is not realvjka:ojksf:3 k4e4mp;. rk/jly connected to the ground, what is meant by this expression?

When you turn on a water faucet, the water usually flows immediately. Ye-5u mznm8)svou 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 t e zmvnsm5u8)-o the terminals of a battery?

Can a copper wire and an aluminum wire of the same length have the same resistbk7nrjsp +,j 8o:(ustancobnss (, 7kuj 8pjrt:+e? Explain.

If the resistance of a small immersion heater (to heat water for tea )hgnt84fkd y,or soup, Fig. 18–32) was increased, would it speed up or slow down the heating) ,hdy8f4 nktg process? Explain.

If a rectangular solid made of carbon has sides of lengths a, t-b pta- da. dmwr+gzo(0k*ze2- gx r*2a, and 3a, how would you connect the wires from a battery so as to obtain (a) the least resistance, (b) the greatest gok-a.btg*- rxap0* rz +z( dm -wdet2resistance?

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 lightbulb burns o;i h6qba /uv7out?

Explain why lightbulbs almosclo i)rsf sr- 1yb0echky-)6,t always burn out just as they are turned on and not after they have beskc-l)bhy ), crieor01 sf- y6en on for some time.

Which draws more current, a 100-W lightbulb or zs :i4tq7s6 bua 75-W bulb? Which has the higher resistait:buq4zss67 nce?

Electric power is transferred over large distances at very high voltages. Explat-qh)jiiik az( 8y: ,min how the high voltage reduces p 8iha j),(mkqy:i-zti ower losses in the transmission lines.

A 15-A fuse blows repeatedly. Why is it dangerous to replace this fuse with *c 1(n blp64eh f. ,uc/btexgqa 25-A fuse? te4qnc 6fcp.(1 ugbe*hbx /l,

When electric lights are operated on low-frequencyy9 dq :42(efcngn6r vy ac (say, 5 Hz), they flicker noticeably. Wh 6vc4q(r9 gf :2nyynedy?

Driven by ac power, the same electrons pass back and forth through yt0a4/ l asb fvn3sj4.zal q:6four reading lamp over and over again. Explain why the light stays lit instead of going out after tnffzt4q:b/alv3a ls aj60s 4.he first pass of electrons.

The heating element in a toaster is made of Nichkv9zg9y8 w gt4rome wire. Immediately after the toaster isyt8gz k9 wv4g9 turned on, is the current $(I_{rms})$ in the wire increasing, decreasing, or staying constant? Explain.

Is current used up inzsfouy,0.r4y a resistor? Explain.

Different lamps might have batteries connectedorak ::w z/8qm in either of the two arrangements shown i zr a:w:/mko8qn
Fig. 18–33. What would be the advantages of each scheme?

  A current of 1.30 A flows in a wire. How many e, 8)ws ww(rf s06zrdjslectrons are flowing past any point in the wire per second?d wz0sw fsrs,)rj6w(8    $ \times10^{18 }\;electrons/s$

  A service station charges a battery using a current of 6.7 A for 5.0863-sj .nkc-mowypqv h. How much charge passes through the battevmno q6pyc8s. -wk 3j-ry?    $ \times10^{ 5}\;C$

  What is the current in am jzcsw; ror2;gaufz8rn+9xs -+zhz9 : peres 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 toaster if 120 V produces a current of 4.2 5i3ve(s96dy n/ dij:q(huq mrh nnl56A?    $\Omega$

  What voltage will producl uyp*2ien mbxmh 8+ z2y(ojsdb8(1 ,we 0.25 A of current through a $3800-\Omega$ resistor?    V

  A hair dryer draws 7.5 A when plugged *lox(a1k0 y4yd0s)cd)mchr t into a 120-V line.
(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 h: u(bbnm3h :-qq z+0xbm(esp cas a heating element with a resistance 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 jf3u54/r h bt0euubc,azx40/,uv nij a bulb whose resistance is $1.6\Omega$. How many electrons leave the battery per minute?    $ \times10^{21 }\;electrons/min$

  A bird stands on a dc electric transmission line carryingxua6(9) /rabd0tzfgx : qlfsox: 8cm( 2800 A (Fig. 18–34). The lls:(tofq0bdc)rf a /(6a9gxxzx u8: mine has $2.5\times10^{ -5}\;\Omega$ 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 240 V.mf; o 6tsfp8 :1yp*q.pjuov8k
(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 currcy 3 .9o do*.* sx)zdivgw(zjsent 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 of tungsten wire whose resistanl;1y6) iowf:m jm h8qnce is $0.32\Omega$?    $ \times10^{-4 }\;m$

  What is the resistance of a 3.5-m le6;1ux:ngos5 p3gz4 j9 epasyqngth of copper wire 1.5 mm in diameter?    $ \times10^{ -2}\;\Omega$

  Calculate the ratio of the js+k2u.p8-giaa0 sez resistance of 10.0 m of aluminum wire 2.0 mm in diameter, to 20.0 m of ap0 jsa+ ek.s8 -zgiu2copper wire 2.5 mm in diameter.   

  Can a 2.5-mm-diameter copper wire have the samevjrqe3r6s hf zr6 81tp1ip-0 j resistance as a tungsten wire of the same length? 3p0qh8 z-fvpjtrrrj1s 6 i6e1Give numerical details.    mm

  A certain copper wire has a rm9zz k m:xwe-y9 afpb5c*e3s) esistance of $10.0\Omega$ 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 the temperature ofka 0,boy9nv2 y2mv85 0bqyy3gr+z uer a copper wire (originally at 2kz5n0vg,vyy2rubber09y oq 3 8m+ ay$20^{\circ}\,C$) to increase its resistance by 15%?    $C^{\circ}\,$

  Estimate at what temperature copper will have the same resistivity as 2d*54w uchcmpo4 sd 3nq5. ndxtungsten does at 5d3wuqnxdsm4cn* .2h5o dcp 4 $20^{\circ}\,C$.$\approx$    $^{\circ}\,C$ (Round to the nearest integer)

  A 100-W lightbulb has a resistance ox m)igvjp9au,pg)59k drhm5sqf4m 8:f 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 ;weho1box4)/ 8icha aalong a 26-m length of household no. 14 copper wire (used in 15-A h ihx;c8o/eo w4aa)b1circuits). The wire has diameter 1.628 mm and carries a 12-A current.    V

  A rectangular solid made of carbon has sides of lengtv8f/b /zyfy-djft)q;hs 1.0 cm, 2.0 cm, and 4.0 cm, lying along the x, y, and z axes, respectively (Fig. 18–35). Determine the resistance for current that passefj8z y-yb)ff ; dv/q/ts 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 thean iqc*d0g 5a2 length of the other, what is the ratio of the diameter of the longer wire to the diameter of a gacq50idn*2 the shorter wire? $\approx$    (retain three decimal places)

  A length of aluminum wire is connected to a precision 10.00-V power supplys 5tmor 72psxivz*9j; , and a current of 0.4212 ;rj5p v2i*smx o9stz7A 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 consists of 5.0 m of copper followed by 5 d v2inm/s55aq.0 m of aluminum, both of diameter 1.0 mm. A voltage differea52nm5/ vsqdince 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 impor4l26k1dxfu j 0relhi( tant that the value of a resistance not change with temperf2( lr d1iu40ejh6lkxature. For example, suppose 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 electric o:ruit ga-rs:*;gen c* ven is designed to produce 3.3 kW of heat when connected to a 240-V source. What must be the resistanc;nsu-*:gc*arr: te gie of the element?    $\Omega$

  What is the maximum power consumption of a 3.0-V portable CD z8 l/p2a 5igs1akiiu sw)6g+lplayer that draws a maximum of 320 mA) sk6i8gza2l1ilis a+u/pw g5 of current?    W

  What is the maximum voltage that can be applied across 4j*x tq s 30ore wxkxzcs;;;6qa $2.70-k\Omega$ resistor rated at $\frac{1}{4}$ watt?    V

  (a) Determine the resistance of, and current thr8h2wzh pgg 12i ubce;5ough, a 75-W lightbulb connected to its proper sourc5ugzig 2hwe;28h p cb1e voltage of 120 V.
   $\Omega$
   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 W2w achyo6l-:c . Calculate
(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 settings: 850pkyboa; i: 4i3 hvp)0g/. ibxo /i/cxd 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 in Europ 8a.nbqe 8xwg)wx.mbd)q1 mb2e, where electricity is delivered to homes at 240 V. If you use the lightbulb in the United States at 120 V (assdn)mw.m 88).2 bqxwxb aqbe1g ume 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 does r c0 vves(-:g* lxjsx4a 550-W toaster use in the morning if it is in operation for a total of 15 min? At a cost of svlvr(gsj c 04e-*x:x $9.0\,cents/kWh$, 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 ceh6td8kt,,ziuwi w km:4 l s..ost to leave a 25-W porch light on day and n hk t,tuiw.dzeksm:l8i.6 w4,ight for a year? $\approx$    dollars

  An ordinary flashlighapqwep chv,*wbx 1+2 1(ctkp* t uses two D-cell 1.5-V batteries connected in series as in Fig. 18–4b (Fig. 18–36). The bulb draws 450 mA whe*w1e2 pbvcpwk,aq* p h +cxt(1n 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 o9h5 2bmklyt/hfr43 ttf energy stored in a 12-V, $85-A\cdot h$ car battery when it is fully charged?    $ \times10^{6 }\;J$

  How many 100-W lightbulbs, conne4tm ax46rk r c(cnki:xsse845o:s 1evcted to 120 V as in Fig. 18–20, can be used without blowing a 158is4m45ktkxc1cv ars 6 :oerne4:(xs-A fuse?    bulbs

  An extension cord made of two wires of diameter 0.129 cm (no. 16p un5,7xlt8sey x6bq , copper wire) and of length 2.7 m (9 ft) is connected to an electric heater which tu7s8xp5,,qxynl e b6 draws 15.0 A on a 120-V line. How much power is dissipated in the cord?    W

  A power station delivers r7 6i.eus*w i0vq *pq2zqzeh;620 kW of power at 12,000 V to a factory through wires with total resistancive67.0puqisrz2z qe*wq *; he $3.0\Omega$ 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 electrojdb r219rsmq5t0q ac 4magnet connected to a 240-V line is 17.5 A. At what rate must cooling water pass over the coils if the water temperature is to rise492b t1ar qj5rdc smq0 by no more than 7.50 $C^{\circ}\,$?    kg/s

  A small immersion heater can be used in a car to heat a cup vu:; ws;wrhd 3.xds5j of water for coffee or tea. If the heater can heat 120 mL 5;v.rx; uwdj:hwssd3of 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 a7nv w+ d2)gzvj skn*y5 $2.2-k\Omega$ resistor connected to a 220-V rms ac source.    A

  An ac voltage, whose peak value is 180 V, is acrosr m c2veqc4,,b9v v/crpa+e4vu: eo o*s 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 resistancvh9y f- nupr -tz7d()je of 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 alterot3kjrf)31p483: t gsc - bhre(kohoxnating current in a 1500-W device is 5.4 A. What is the rms voltage across it? k3t8(thbr cgpr -4 :x s33o1ef ohk)jo    V (Round to the nearest integer)

  An 1800-W arc welder is connected to 7cwnepe9gz aw xr+44 0a $660-V_{rms}$ 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 yye17ru ji5 e2a 3.0-hp pump connected to ayy r5eu712 jie $240-V_{rms}$ ac power source?    W
(b) What is the maximum current passing through the pump?    A

  A heater coil connected to a hx;dfmsyh9 l:pat 11vpabg+ l: 18in8s *mcr8$240-V_{rms}$ 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 cuz(5v lbtwq9ya+i )t5csc e7a8 rrent 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-diamete(7n(6 m rfsaiieb3(vpwv )7wqr wire carries a 750-mA current when 22.0 mV is applied to its ends. If the drift speed3w a(qbe)f(ip(ws6nmvv77ri 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’s atmosphere,2dg. s gh .1zkr3l vl/kri9iv lj*od e-d6vf1. $He^{2+}$ 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 atmospv3i6bq; rso.:/cby3k9eiq j ihere, $He^{2+}$ 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 magnitude of the electx9*:w fp r40xa0fftepric 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. Thpossk4*yd1i / e action potential is 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 14/psk*sd oi ytime required is 0.0063 s. What is the speed of the electric pulse along the axon? (Why are two measurements needed instead of only one?)    m/s

  Estimate how much energy is required to transmit one acx9)dzp9 pgo .w.x *yvytion potential 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 averaw.o)ygxpvp9*x.9dz yge power is required for $10^4$ neurons each transmitting 100 pulses per second?
E =    $ \times10^{-11 }\;J$
P =    $ \times10^{-5 }\;W$

  During an action potenjgchy3j 84qe 1tial, $Na^+$ 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 are l.zf56h2x. dnqtb;,aa ,z gchthere in 1.00 ampere-hour?    C

  What is the average curren. ax0u ps( j,dfd )vrbhq-9af0t drawn by a 1.0-hp 120-V motor? $(1 hp\,=\,746\,W.)$    A

  A person accidentally leaves a mf10fi2 uud03r a(qhccar with the lights on. If each of the two headlights uses 4ff0uu mq1a d2hi(3c0r0 W and each of the two taillights 6 W, for 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 h8yee0x3 hym vwwv6j; 1)l,bpeeater is 5.4 m long. If it is made of iron, what must10jhe) w38bme6pyvye, x l vw; its diameter be?    $ \times10^{ -4}\;m$

  The conductance G of an object is defined as the u ln8 7ongpv out n9(t6t1*p-xlmy8)areciprocal of the resistance R; that1(*6 l)mvgnployxn-npt8t uot8 u 79a 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 abou94el49 g y)jjj(:grsaiw v3kt t 10 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 line of 0.50-cm-934rja l(kgviy94sge:wjj t) 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 uses a 1.8-kW heaterbwy35x4czgayd * 9 nj4 $3.0 h/day$ (“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 are wrappti aa/f,p7x0g060 i b ce5fpaded together side by side to make a thicker wire. How does the resistance of this new combination compare to the resistancep/b, paf7x0 ectg i afai600d5 of the original wire?

  A 1200-W hair dryer is design4unu: v6m* ath5hodd 0 eimu*7ed 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 9 oz(f)oogo)wenough so it does not become so hot as to start a fire. What diameter must a copper wire be if it is to carry a maximum current of 35 A and produce no more than 1.8 W of heat per meter of len(g zo 9o)foo)wgth?    $ \times10^{ -3}\;m$

  Suppose a current is givenbmm0g 0tgz 78y by the equation $I\,=\,1.80sin210t$, 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.ltlixmlwb:z01k12 u 65% efficiency delivers 950 W of energy per second to the intertlml01:1 wkuxb i. l2zior. 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 di 8 jfi;0n8tv tx/xm7tkfferent conductors made of the same material. One conductor is twice as long and twice the diameter of the second. Wht / 8x7 0x8mivjntt;kfat 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 a room of volr0ijexokc90d g 4,6 egume $62\,m^3$ 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 4mi :a3(jwcu mditid a04-je. source.
(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 4e8- mvhd8p; flzssa0;eex +n+8lbnh vehicle makes use of storage batteries as its source of energy. Its mass is 1560p+ ml hbl+av-8; 8hn4dfszx n;8e0 ees kg and it is powered by 24 batteries, each 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 lightbulb has a resistance o:p :i r3z-qut-dtyj ,if $12\Omega$ 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 Fermilab (Illinois) is desiazfq 3bo,*j hbn9 8zw*gned to carry an 11-mA beam of protons traveling at very nearly the spee*q*nbjh8zf3, z9o b awd 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 zu;39q;eudbA at 220-V ac. The connecting cord is copper wire with a diameter of 1.628 mu9b3 ezau;;dqm.
(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 95 ig: 2h5yozp/mW when connected to 120 V. The heating element is a coil of Nichrome wire. When uncoiledi5: opz/hgmy2, the wire 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 about 12 V whe6 gajq 6z. q6wjdsb0k1f:gj s5n the car is off to about 13.8 V when thej0f bsd166 .jqg 6w:sjg5qazk 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 resistance remains constant.    % increase

  Lightbulb A is rated at 120 V and 40 W for household applications. Lightbulb B xnih30(vw64gz00n;s zp rd oyis rated at 12 V and 40 W for automotive appl4 (znid3gxys6w0 pzrh; 0o0 nvications.
(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 to connect a set of appliances at 126bj9e ox)+ m-jnue +jezhol/9 0 V, which draw 2250 W of powej o/zemh-u9nj9e +o b6e)j l+xr total.
(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 diameteptcn/g;r:iz). f16b1qohrx8bu5 hr j r of 3.00 cm and an outside diameter of 5.00 cm (Fig. h :xog1n.f8tpr/ )rhiur 65bcz1qb;j 18–37). What is the resistance of 10.0 m of this pipe?    $ \times10^{-4 }\;\Omega$

If a wire of resistance R is stretched uniformly so that its length doubles, byu++w4svv:u6tg doz1 q what factor does the power dissipated in the wire change, assuming it remains hoo +: wz6uvou4tq+ vs1dgked up to the same voltage source?

  A tungsten filament used in a flasn* y pv** tn n)bo1xv .ip(q3-( k1e9aluiblkjhlight bulb operates at 0.20 A and 3.2 V. If its resistance at l*vy9-inp tni aev**.jb()okkbl1 q1(3pun x20$^{\circ}\,C$ is $1.5\Omega$ what is the temperature of the filament when the flashlight is on?    $^{\circ}\,C$