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Question
A cylindrical bar magnet is rotated about its axis (Figure). A wire is connected from the axis and is made to touch the cylindrical surface through a contact. Then
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a direct current flows in the ammeter A.
no current flows through the ammeter A.
an alternating sinusoidal current flows through the ammeter A with a time period T = 2π/ω.
a time varying non-sinosoidal current flows through the ammeter A.
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Solution
A cylindrical bar magnet is rotated about its axis (Figure). A wire is connected from the axis and is made to touch the cylindrical surface through a contact. Then no current flows through the ammeter A.
Explanation:
The phenomenon of electromagnetic induction is used in this problem. Whenever the number of magnetic lines of force (magnetic flux) passing through a circuit changes (or a moving conductor cuts the magnetic flux) an emf is produced in the circuit (or emf induces across the ends of the conductor) is called induced emf. The induced emf persists only as long as there is a change or cutting of flux.
When cylindrical bar magnet is rotated about its axis, no change in flux linked with the circuit takes place, consequently no emf is induced and hence, no current flows through the ammeter A. Hence the ammeter shows no deflection.
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