A conducting bar slides without friction on two parallel horizontal rails $50$ cm apart. The resistance of the bar and the rails is assumed to be constant and equal to $0\mathrm{.}1.$ A uniform magnetic field is perpendicular to the plane of the rails. A force $F=0\mathrm{.}08N$ is required to keep the bar moving at a constant speed of $0\mathrm{.}5\frac{m}{s}.$ What is the magnitude of the magnetic field?

$0\mathrm{.}1$ T

$0\mathrm{.}25$ T

$0\mathrm{.}36$ T

$0\mathrm{.}54$ T

$0\mathrm{.}93$ T