In figure 4, what is the net electric potential at point A, i.e. V_A? b). In fig
ID: 1419014 • Letter: I
Question
In figure 4, what is the net electric potential at point A, i.e. V_A? b). In figure 4, as we move from point A to point B does the electric potential increase, decrease, or stay the same? c). In figure 4, how is the electric potential at point B related to the electric potential energy of a charge that is placed at point B? d) In figure 5, circle or otherwise indicate the correct depiction of the electric field lines and equipotential surfaces around a negatively charged point particle. e). Explain why excess charge in a conductor is found on the surface of the conductor at electrostatic equilibrium.Explanation / Answer
The potential at A is the sum of the potential at A due to the positive and the negative charges
The potential at A due to the positive charge is
VA+ = k q / r
Where the subscript A+ refers to the potential at A due to the positive charge,k is a constant and its value is 9 x 109 N m2/ C2 , r is the distance between the charge and the point
VA+ = 9 x 109 x 20 x 10-9 C / 2 cm
VA+ = 9000 V
The potential at A due to the negative charge is
VA- = - 9 x 109 x 10 x 10-9 C / 10 cm
VA- = - 900 V
The net potential at A is
V = VA+ + VA- = 9000 V - 900 V = 8100 V
b) The electric potential at the point B can be found in the same way mentioned above
VB- = - 9 x 109 x 10 x10-9 C / 2 cm
VB- = - 4500 V
VB+ = 9 x 109 x 20 x 10-9 C / 10 cm
VB+ = 1800 V
The total potential at B is
V = VB+ + VB- = 1800 V - 4500 V = - 2700 V
Thus the potential at A is greater than the potential at B . So the electric potential decreases from point A to point B
c) The electric potential energy of a charge q at a point is related to the potential at the same point as
U = W = q V
Thus if q is the charge placed at B is then the potential energy of the charge is
U = q VB
d) The second figure from the left,correctly represents the field and the potential of a negative charge
The electric field lines emerge from positive charge and terminate at the negative charge
The equipotential surface of a negative charge is spherical and surrounds the negative charge