A sphere of radius R has charge Q. The electric field strength at distance r > R is Ei. What is the ratio Ef /Ei of the final to initial electric field strengths if (a) Q is halved, (b) R is halved, and (c) r is halved (but is still > R)? Each part changes only one quantity; the other quantities have their initial values.

A. Ef/ Ei = 1/2

B. EF/ Ei = 1

C Ef / Ei = 4

Explanation:

To solve this we apply Coulomb's law which States that

E = Kq / r^2

Where

q = charge r = straight line distance from q to the point in question and

K = Coulomb's constant

Then

Ei = K Q / r^2

So

A) If Q is halved then

Ef = K Q / (2 r^2)

Ef/Ei = 1/2

B) If R is halved, the value of the E-f

at a distance r remains unchanged. So

Ef/Ei = 1

C) if r is now r/2 then

Ef = K Q / (r/2)^2 = K Q / r^2/4 = 4 K Q / r^2

Ef / Ei = 4

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