Transformer Question

[Shark 774]

From Heinemann:

A security light is operated from mains voltage (240 V RMS) through a step-down transformer with 800 turns on the primary winding. The security light operates normally on a voltage of 12 V RMS and a RMS current of 2.0 A. Assume that the light is operating normally and that there are no losses in the transformer. It is then decided that the globe in the security light needs replacing and it is removed. This results in no current flowing in the secondary circuit for 10 minutes, during which time the primary coil is still connected to the supply. How much energy is consumed by the primary circuit during this time? Justify your answer.

Heinemann's answer just makes me more confused......

Any help please?

[Lasercookie]

Hmm... Heinemann's answer is pretty confusing.
Couldn't you just apply one of the transformer equations?



(current in secondary current is 0)


(power in primary circuit is 0)

[Shark 774]

Hmm... Heinemann's answer is pretty confusing.
Couldn't you just apply one of the transformer equations?



(current in secondary current is 0)


(power in primary circuit is 0)

But that only applies if both are connected, doesn't it?

[Lasercookie]

But that only applies if both are connected, doesn't it?

Oh yeah, forgot about that.

I just noticed in the chapter - page 382, paragraph below figure 10.26 explains it pretty well. I think that's what Heinemann meant by their answer.  No load is connected to secondary, so all flux comes from primary coil. The flux induces a current in the primary. Of course, Lenz's Law - this current will oppose the change of flux. This causes current in the primary to reduce to almost 0. I think in an ideal situation, you would have no current. No current means no power and no power means no energy.