Faulty lamp because it short circuit

[thoradicus]

Homework Statement

http://www.xtremepapers.com/papers/CIE/Cambridge%20International%20A%20and%20AS%20Level/Physics%20%289702%29/9702_s06_qp_2.pdf

number 7a

Homework Equations

None really

The Attempt at a Solution

The answer is that C is the faulty lamp because it short circuit. But how do I know it is faulty?

Im quite sure it has something to do with the last row: open closed closed 15 ohm

Just realized it was a 1 word title :/

 

[ehild]

The ohmmeter reads the resultant resistance between its terminals. Which lamp does it measure in the first case? So what is the resistance of one lamp? Which lamps are connected in series in the third case?

ehild

 

[thoradicus]

The ohmmeter reads the resultant resistance between its terminals. Which lamp does it measure in the first case? So what is the resistance of one lamp? Which lamps are connected in series in the third case?

ehild

For the 1st case it measures no lamps?
The resistance for 1 lamp is 15ohm.
A and B?>

 

[ehild]

Sorry, I wanted to ask the first case when S1 is closed. But it is right, the resistance of one lamp is 15 ohm. In the third case, A and B are connected in series, the resistances add up, and the meter measures 30 ohm, which is the resultant of two lamps in series. So lamp B is not faulty.

How are the lamps connected in the fourth case? What is the resistance of C when the resultant is 15 ohm?
ehild

 

[thoradicus]

Sorry, I wanted to ask the first case when S1 is closed. But it is right, the resistance of one lamp is 15 ohm. In the third case, A and B are connected in series, the resistances add up, and the meter measures 30 ohm, which is the resultant of two lamps in series. So lamp B is not faulty.

How are the lamps connected in the fourth case? What is the resistance of C when the resultant is 15 ohm?



ehild

ummm in parallel? I am not sure
Is it 0 for C?

 

[ehild]

B and C are connected in parallel, and A is in series with the resultant of B and C. Yes, the resistance of C has to be zero.

ehild

 

[thoradicus]

Ok, so let me get this right

the current passes through A but also passes through C only, because it is faulty so it has 0 resistance, so it was shorted?

 

[ehild]

Ok, so let me get this right

the current passes through A but also passes through C only, because it is faulty so it has 0 resistance, so it was shorted?

Yes. But a faulty lamp can have either infinite or zero resistance. If it is not shorted, the meter would read a resistance higher than 15 ohm. The measured resistance is 15 ohm, so leaving lamp A, the current goes through a short. Or: The resultant resistance of B and C in parallel has to be zero. That can happen only when C is a short (as B is not faulty).


ehild