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Electricity and Magnetism

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Radius of a Disk of Charge

EANDM-VPENDB

Figure 1 depicts a thin non-conducting disk of charge of radius $R$, upon which a surface charge density of $\sigma = 14.4 \: \rm \frac{nC}{m^2}$ has been placed.

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

A field point $P$ exists a distance $z$ above the center of the disk. Figure 2 shows the electric potential $V$ at the field point as a function of the displacement $z$ of the field point from the center of the disk.

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

What is the radius $R$ of the disk?

A

$R = 55.5 \text{ cm}$

B

$R = 7.41 \text{ cm}$

C

$R = 5.54 \text{ cm}$

D

$R = 6.34 \text{ cm}$