# Physics GRE GR 1777, Problem 020 Solution

## 20. Electrodynamics (Magnetic field)

##### Solution
The magnitude of the magnetic field can be obtained from the Biot-Savart law.

$B = \frac{ \mu_0 I }{ 4\pi R^2} \oint dL$$B = \frac{ \mu_0 I }{ 4\pi R^2} \oint dL$
where $R$$R$ is the radius of the loop and $I$$I$ is the current.

In this problem, the radius of the circular loop is $a$$a$, the magnitude of the magnetic field at the center of a circular loop is

$B = \frac{ \mu_0 I }{ 4\pi a^2} \cdot (2\pi a) = \frac{ \mu_0 I }{ 2a }$$B = \frac{ \mu_0 I }{ 4\pi a^2} \cdot (2\pi a) = \frac{ \mu_0 I }{ 2a }$

(D) $\frac{ \mu_0 I }{ 2a }$$\frac{ \mu_0 I }{ 2a }$
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