A $60 \: \mathrm{V}$ DC source with an internal resistance $R_{\text{i n t}}=0.5 \: \Omega$ is connected through a switch to a pair of infinitely long rails separated by $l=1 \mathrm{~m}$ as shown in the figure. The rails are placed in a constant, uniform magnetic field of flux density $B=0.5 \mathrm{~T}$, directed into the page. A conducting bar placed on these rails is free to move. At the instant of closing the switch, the force induced on the bar is
Note: Assume there is no friction between the bar and the rails. The resistances of the conducting bar and the rails are zero.
- $60 \: \mathrm{N}$ in the direction of $\hat{x}$
- $60\: \mathrm{N}$ opposite to the direction of $\hat{x}$
- $120\: \mathrm{N}$ in the direction of $\hat{x}$
- $120\: \mathrm{N}$ opposite to the direction of $\hat{x}$
