Following figure shows a metal rod PQ resting on the smooth ralls AH and positioned between the poles of a permanent magnet. The rails, the rod and the magnetic field are in three mutual perpendicular directions. A galvanometer G connects the rails through a switch K. Length of the rod 15 cm, B 0.50 T, resistance of the closed loop containing the rod `9.0 mOmega`. Assume the field to be uniform. How much power is required (by an external agent) to keep the rod moving at the same speed `(=12" cm s"^(-1))` when K is closed ? How much power is required when K is open?

Following figure shows a metal rod PQ resting on the smooth ralls AH a

1.	Following figure shows a metal rod PQ resting on the smooth ralls AH and positioned between the poles of a permanent magnet. The rails, the rod and the magnetic field are in three mutual perpendicular directions. A galvanometer G connects the rails through a switch K. Length of the rod 15 cm, B 0.50 T, resistance of the closed loop containing the rod `9.0 mOmega`. Assume the field to be uniform. How much power is required (by an external agent) to keep the rod moving at the same speed `(=12" cm s"^(-1))` when K is closed ? How much power is required when K is open?
Answer» Given, length of the rod `l=15cm=15xx10^(-2)m` Magnetic field `B=0.50T` Resistance of the closed - loop containing the rod, `R=9mOmega=9xx10^(-3)Omega` Velocity of rod `V=12" cm/s "12 xx 10^(-2) m//s.` To keep the rod moving at the same speed the required power `="retarding force "xx" velocity "=7.5xx10^(-2)xx12xx10^(-2)=9xx10^(-3)W`

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