5a)
The voltmeter gives a reading of a certain sign at the instant the switch is closed.
It then returns to zero while the switch remains closed.
It shows a reading of the opposite sign when the switch is re-opened.
(It then returns to zero).
5b)
When the switch is closed, the primary coil current produces a magnetic flux in the core.
The secondary coil, since it is wrapped around the core, experiences an increase in magnetic flux linkage, hence an induced emf. (Faraday’s Law)
The direction of the induced emf is such that it attempts to produce magnetic flux in opposite direction to that in the core. (Lenz’s Law)
As the switch remains closed, the magnetic flux in the core remains constant. With zero change in flux linkage, there is no induced emf in the secondary coil.
When the switch is reopened, the magnetic flux disappears.
The secondary coil experiences a decrease in magnetic flux linkage, hence an induced emf. (Faraday’s Law)
The direction of the induced emf is such that it attempts to produce magnetic flux in the same direction as that in the core, and hence in opposite direction to what was induced when the switch was closed. (Lenz’s Law)
COMMENT: Your answer should state clearly the role played by the core of the transformer.
COMMENT: Note that there is never any current in the secondary coil since the voltmeter has infinite resistance.
xmPhysics 