Electricity and Magnetism

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Finding the Radius of a Ring of Charge

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Figure 1 depicts a ring of charge of radius $R$ whose center lies a distance $z$ from a field point $P$.

Figure 1. Timothy Black. Created for Albert.io. Copyright 2016. All rights reserved.

If the linear charge density (charge per unit length) of the ring of charge is equal to $\lambda = 1.30 \: \rm\frac{nC}{m}$, we can plot the electric potential at point $P$ as a function of the displacement $z$, for some particular value of $R$, shown in Figure 2.

Figure 2. Timothy Black. Created for Albert.io. Copyright 2016. All rights reserved.

What is the radius $R$?

A

$R = 10.0 \text{ cm}$

B

$R = 5.45 \text{ cm}$

C

$R = 6.50 \text{ cm}$

D

$R = 18.4 \text{ cm}$