A transformer with a large light bulb in its primary circuit, as seen in the pho

Question

A transformer with a large light bulb in its primary circuit, as seen in the photograph below, has an open secondary - that is, nothing is connected across the secondary coil of the transformer. The primary coil of the transformer is on the right, and the secondary is on the left in the photograph. Pushing the switch in the lower left corner of the photograph connects the small light bulb across the secondary coil of the transformer. The variable resistor at the right in the photograph is used to adjust the secondary voltage to the proper amount for the light bulb that will be connected.

With no load on the secondary coil, the light bulb in series with the primary coil glows. The coil has a relatively small impedance so it allows a significant amount of current to flow.
When a light bulb is connected across the secondary coil of the transformer, the light bulb in series with the transformer primary coil will
(a) get brighter.
(b) get dimmer.
(c) remain the same.

Explanation / Answer

I just discussed this with a real live E.E. guy. The bulb on the primary should get brighter. Power on both sides of the transformer will be equal when the circuits are both closed.

Switch open: The primary only has it's own power, current and voltage (the primary light it on)...the secondary P = 0 and the light is off b/c the switch is open.

Switch closed: The primary light will still be on but this time there will be more current through the primary because there is now the P=IE for the primary side. The primary must also account for the additional P=IE for the secondary side, as the secondary light is lit by inductive coupling coming from the primary.

Please rate if this helps, thanks. :)

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