# GRE Physics GR 1777 Problem Solution

## 020. Electrodynamics (Magnetic field)

### Solution

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

$B = \frac{ \mu_0 I }{ 4\pi R^2} \oint dL$
where $R$ is the radius of the loop and $I$ is the current. In this problem, the radius of the circular loop is $a$, the magnitude of the magnetic field at the center of a circular loop is

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

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