To detect whether current is flowing in a wire or not, the wire is brought near a magnetic needle but the needle shows no deflection. But then the wire is immersed in water kept in a calorimeter, the water gets heated. How could you explain this ?

Explanation

A current carrying wire does not produce any delfection on a megnetic needle placed in its vicinity in the following two cases-
(i) If the wire and the needleare placeed perpendicular to each other in the same plane,
(ii) if the needle is along the same line as the wire.
So, as per the problem, the wire carrying current may not produce any deflection on the magnetic needle.
But a current in a wire always produces heat due to Joule heating effect. thsi heat produced is independent of the direction of current and hence the temperature of water increases when the wire is immersed in water in kept in a calorimeter.