Question

Find out velocity of projectile of charge particle so that it reaches to center of ring and cross it.
a) ring fixed
b) ring free to move

Answer 1

Answer:

Suppose we have a point charge q0 located at r and a set of external charges conspire so as

to exert a force F on this charge. We can define the electric field at the point r by:

E =

F

q0

(2.1)

The (vector) value of the E field depends only on the values and locations of the external

charges, because from Coulomb’s law the force on any “test charge” q0 is proportional to the

value of the charge. However to make this definition really kosher we have to stipulate that

the test charge q0 is “small”; otherwise its presence will significantly influence the locations

of the external charges.

Turning Eq. 2.1 around, we can say that if the electric field at some point r has the value

E then a small charge placed at r will experience a force

F = q0E (2.2)

The electric field is a vector . From Eq. 2.1 we can see that its SI units must be N

C

.

It follows from Coulomb’s law that the electric field at point r due to a charge q located

at the origin is given by

E = k

q

r

2

ˆr (2.3)

where ˆr is the unit vector which points in the same direction as r.

2.1.2 Electric Fields from Particular Charge Distributions

• Electric Dipole

An electric dipole is a pair of charges of opposite sign (±q) separated by a distance