Physics 241 Exam 1 February 19, 2004

Transcription

1 Phsics 241 Em 1 Februr 19, 24 One (both sides) 8 1/2 11 crib sheet is llowed. It must be of our own cretion. k = 1 = N m2 4p 2 2 = N m 2 e = c = m/s (speed of light) N Avogdro = (number of toms in 12 g of 12 ) m fi 1-3 m fi 1-6 n fi 1-9 p fi 1-12 f fi 1-15 k fi 1 3 M fi 1 6 G fi 1 9 T fi 1 12 P fi 1 15 For 2 b c = = -b ± b2-4c 2 1. Plese sign the opscn sheet nd print our nme on it. 2. Use #2 pencil to fill in our full nme, our student identifiction number (old one), nd finll the nswers for problems Plese be prepred to show our Purdue ID when ou hnd in our opscn sheet. 1. onsider the three infinite chrge sheets shown on edge below. I 2s Wht is vlue of the electric field in region III? mgnitude direction ) s b) 3s 2 c) 3s 2 d) 2s e) II s III 3s IV 14 pges totl 1 14 pges totl 2 2. onsider the point chrges q i ( i, i ) shown below: q 2 (, 2) = 2 m 3. onsider two concentric conducting spheres s shown below. The outer sphere is hollow nd hs totl chrge of 5 m chrge on it; its inner rdius is R 1 = 9 cm nd its outer rdius is R 2 = 1 cm. The inner sphere hs rdius of 1 cm, is solid, nd hs chrge 3 m on it. Wht is the potentil of the inner surfce of the sphericl shell? I.e., wht is V(R 1 = 9 cm)? Assume tht the potentl t infinite distnce is zero. P (1,1) q 1 (2, ) = 1 m q 3 (5, ) = 1 m R 2 R 1 3 m i nd i re in meters. Wht is the electric potentil t point P, which is t (, ) = (1,1)? 49 kv 15 kv 134 kv 178 kv None of the bove 18 kv 2 kv 3 kv 72 kv 14 pges totl 3 14 pges totl 4 1

2 4. How much work must be done to plce the three chrges in the configurtion shown below, in which q 1 = 6 m is t (, ) = ( 1 m, ), q 2 = 12 m is t (, ), nd q 3 = 24 m is t ( 1 m, )? Assume tht the three chrges were initill infinitel fr w. 5. Suppose potentil is described b V(,,z) = ( z 3 2-2z 2 ) volts where,, nd z re in meters. Wht is E, the - component of the electric field t =1 m, = 2 m, nd z = 3 m? 1 V/m 4 V/m 1 V/m 4 V/m q 1 q2 q (m).65 J 1.6 J 1.3 J 3.9 J 14 pges totl 5 14 pges totl 6 6. Two chrges of equl mgnitude but opposite sign re seprted b.1 nm, thus forming n electric dipole. If the re oriented in uniform electric field of 3 V/m s shown below, wht is the mgnitude of the torque on the electric dipole? The mgnitude of ech of the chrges is Wht is the mgnitude nd direction of the electric field needed to cncel the erth's grvittionl force on n electron (e - )? The mss of n electron is kg nd g r erth = -9.8 ˆ j m/s r E ˆ j N/ ˆ j N/ ˆ j N/ ˆ j N/ N m N m N m N m 14 pges totl 7 14 pges totl 8 2

3 8. Point chrges q 1 = 1. m nd q 2 = 3. m re 1 cm prt s shown below. If q 1 is t =, where would ou plce n electron (mss = kg, chrge = e = ) so tht no electrosttic force cts on it? 9. The cube shown below hs edge lengths of 1.4 m nd is plced such tht the hidden corner is t the origin of the coordinte sstem. If E r = 3. ˆ j, where E r is in units of newtons/coulomb nd is in meters, wht is f net = Ú S E n da? q 1 q 2 1 cm = 23.7 cm = 13.7 cm = 3.7 cm = 13.7 cm 3. N m N m N m2 14 pges totl 9 14 pges totl 1 1. hrges q 1 nd q 2 in the figure below re seprted b 1 m. If q 1 hs chrge of 1 m nd q 2 hs chrge of 1m, which of the plots below best describes E()? The origin is denoted b in ll figures. E() q 1 1 m q 2 1 m.5.5 E() (m) 11. A uniform line chrge is bent into semi-circle of rdius. chrge densit l P E() E() Wht is the mgnitude of the electric field t point P? kpl kl 2 2kl kl 14 pges totl pges totl 12 3

4 12. A point chrge Q = 1 n is t the origin [(, ) = (, )] of the coordinte sstem shown below. A proton (mss = kg, chrge e = ) is initill t point P 1 ( = 1 m, = ), moved to point P 2 ( = 1 m, = 1 m), nd finll to point P 3 ( = m, = 1 m). Wht is the mount of work needed to ccomplish this move (P 1 to P 2 to P 3 )? (m) 13. The clinder of 5 cm rdius nd length 2 cm shown below hs uniform chrge densit throughout its volume. Its totl chrge is Q = 12 m. Wht is the electric field t distnce of 3 cm from the is of the clinder? onsider the clinder s one smll segment of n infinitel long chrged clinder. 5 cm 1 P 3 P 2 L 12 MV/m Q P 1 1 (m) 36 MV/m 22 MV/m 13 MV/m J J J J 14 pges totl pges totl 14 4

5 Phsics 241 Answer ke for blue Em 1 2/19/4 1. 2s, kv kv J 5. 4 V/m N m ˆ j N/ cm N m kl 12. J MV/m