C&S Chapter 8 Notes

 

Definition:      Electric Potential Voltage=

                       Work done by applied force to bring test charge to that location.

                                                                       Charge

 

           =          -Work done by E-Field

                                   Charge

Electric Potential has units of J/C, which is called Voltage and denoted by the symbol V

RBD1 :          Due to point change Q:V(r) =

                       Change in Potential = DV = -E  Dr In constant E-Field

                                                       = -ºE¥ dr in non constant E-Field

Comment:  Recall that work is W = F¥ Dx , in like  manner, Voltage is the dot product of E and Dx

 

RBD2:                                 

 

 

NOTE: to instructor :  Find the work done by an applied force to move  from r1 to r2

Give values to the quantities and do explicit problems.

 

 

Point out how itÕs also q1DV!

 

RBD3:                       DUE =  = potential energy of the system consisting of q1 and q2

                       a distance of r12 apart.

RBD4:                       If  a charged particle goes in the direction it wants to go WE > 0

                                                                                                  DUE < 0

                                                                                                  DV Maybe Either

 

RBD5:                       DV  when moving in the direction of E < 0.

                       DV when moving in the opposite direction of E > 0.
EXERCISE:

DO 8.2.1b

DO 8.2.1c

 

8.2.2:             Recall that moving sideways in g-field takes no work.  The same goes for  for E-fields

 

8.3:                 DV = -ºE DL

8.4  Path independence comes from fact that dr = dl cos u

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Definition:      Electron volt - work required to move an electron through potential diff of 1 Volt.

                       qeDV = 1 eV = 1.6 x 10-19J   so,   = 1 is the conversion ratio to convert from Joules to electron volts (eV) and vice versa.

 

 

RBD6:                       DVCLD path = 0

 

RBD7:                       Vinside metal = CONSTANT but not zero.

 

8.5:                 DO AT HOME WIILL BE ON TEST!!!!!!!!!  (214 only)    

8.5b  DVinside metal = 0 if charge is static,  if charge is moving, DVinside metal = -E ¥ dl

8.6:                 E =  When E constant

DO 8.6a, b, c

 

E=  When E not given.

DV = 300V

 

Do 8.7.1a:      FNCS = 2.4 x 10-15J  is the work done on each electron , so the potential difference between the plates of the battery is  DV = FNCS / e = ?

 

8.7.2:              Definition of  capacitance - Charge Q on a cap is prop to DV across capacitor.  Q = CDV

 

Note:   The units of capacitance, charge / voltage =

8.7.2a             For parallel plate capacitor 

Ds = distance between plates, Q = charge on each plate, and  is the the permitivity of free space.
DO 8.7.2b      If A = 50cm X 30cm & Ds = .25mm, C=

 

Do 8.7.2c       How much charge Q on each plate of the 1 F caps?  Q = CDV if DV = 3 V ?

                       (What charge with 2 batts?)

 

Do 8.7.3a (214 only)

 

8.7.3b              If r1 Å r2 , then  r1 r2 Å r2, etc  

 

8.7.4a             Work  done on the electrons to accelerate them is the work done by the electric field,

which is W=DK qDV = e(15000V) = .5 mv2       v=?

 

8.8.1 & 8.8.2 You need to be able to understand these in order to

 

Do 8.8.3a (214 only)

 

8.9.2              

8.9.3               q must include sign!!!!

8.10                C=kC0

DO 8.10.1b (214 only)

 

NOTE:           The easier it is to polarize a material, the greater itÕs dieletric constant.

                       With this in mind, what is the dielectric constant of metal?