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54. Two fixed charges, -4.0 C and -5.0C, areseparated by a certain distance. a.

ID: 1751546 • Letter: 5

Question

54. Two fixed charges, -4.0 C and -5.0C, areseparated by a certain distance. a. Is the net electric field at a location halfway between thetwo charges (1) directed toward the -4.0C charge, (2) zero,or (3) directed toward the -5.0C charge? Why? (b) If thecharges are separated by 20cm, calculate the magnitude of the netelectric field halfway between the charges? a. Is the net electric field at a location halfway between thetwo charges (1) directed toward the -4.0C charge, (2) zero,or (3) directed toward the -5.0C charge? Why? (b) If thecharges are separated by 20cm, calculate the magnitude of the netelectric field halfway between the charges?

Explanation / Answer

   a.   Electric field due to -4.0C   =   (1/40)* (q1 / r12) *r1^          E1   =   9.0* 109 * ( -4.0 * 10-6) * r1^ /r12   =   -(3.6 * 104 / r12) *r1^            N/C                     Electricfield due to - 5.0C   =   (1/40)* (q2 / r22) *r2^          E1   =   9.0* 109 * ( -5.0 * 10-6) * r2^ /r22   =   -(4.5 * 104 / r22) *r2^          N/C          Given   r2^   =   -   r1^         (as point lies at midpoint)          and   r2   =   r1             Netelectric field atmidpoint   E   =   E1+ E2   =   - 3.6 *104 *r1^/r2  +  (4.5 * 104 / r12) *r1^             E   =   +(4.5 * 104 / r12) *r1^                   field isdirected along    r1^ i.e. towardscharge   - 4.0 C.    b.   Putting   r1   =   20/2   =   10   cm   =   0.1   m          E   =   4.5* 104 /0.12   =   4.5 *106   N/C                Netelectric field atmidpoint   E   =   E1+ E2   =   - 3.6 *104 *r1^/r2  +  (4.5 * 104 / r12) *r1^             E   =   +(4.5 * 104 / r12) *r1^                   field isdirected along    r1^ i.e. towardscharge   - 4.0 C.    b.   Putting   r1   =   20/2   =   10   cm   =   0.1   m          E   =   4.5* 104 /0.12   =   4.5 *106   N/C   
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