Why is current 12V÷2Ω instead of 12V÷(bulb resistance + 2Ω) in series?

[Lim Y K]

Take this question as an example

They asked for the current flowing through the bulb and the answer is 6A
I know that it's 12V divided by 2 ohms(2 ohm is the resistance of resistor X) , but why? I thought the current is supposed to be 12V divided by the resistance of bulb and resistor X(total resistance of resistors) since its arranged in series?

 

[olivermsun]

As stated, the voltage across the resistor X is measured at 12 V, so the other parts of the circuit shouldn't matter.

 

[NascentOxygen]

Yes, current would be 12/(r_total), where r_total is the sum of the series resistances.

I'm not able to agree or disagree with you on the correctness of the textbook's answer, because you haven't disclosed the exact wording of the question. The figure you selected to use as an example is not a particularly good choice---because it examplifies a fundamental wiring "mistake".

 

[davenn]

As stated, the voltage across the resistor X is measured at 12 V, so the other parts of the circuit shouldn't matter.

for a idea circuit, yes you could say that. But if it was a real world circuit, that is no longer true. The light globe will have significant resistance
which will change as it heats up

The figure you selected to use as an example is not a particularly good choice---because it examplifies a fundamental wiring "mistake".

indeed

@Lim Y K, can you recognise the problem ?Dave

 

[olivermsun]

for a idea circuit, yes you could say that. But if it was a real world circuit, that is no longer true. The light globe will have significant resistance
which will change as it heats up

I realize that is the case for a real incandescent lightbulb, but since the heated filament resistance, wasn't given in the problem, I'm assuming that the student isn't expected to incorporate that information in the solution.

 

[davenn]

maybe but they need to be informed that it isn't like that in the real world and you didn't give that clarification

the poor guy is already getting confused enough with incorrectly drawn circuits ...

 

[NascentOxygen]

As stated, the voltage across the resistor X is measured at 12 V, so the other parts of the circuit shouldn't matter.

This is not the case in the figure I'm seeing.