Faulty lamp because it short circuit

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Homework Help Overview

The discussion revolves around a physics problem involving a circuit with lamps and their resistances. The original poster questions how to determine which lamp is faulty based on the resistance measurements provided in a specific scenario.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants explore the readings from an ohmmeter and discuss the configuration of lamps in series and parallel. Questions are raised about the resistance of individual lamps and how they relate to the overall circuit behavior.

Discussion Status

Participants are actively engaging with the problem, clarifying the connections between the lamps and the implications of resistance readings. Some guidance has been offered regarding the nature of faulty lamps and the conditions under which they can be considered shorted.

Contextual Notes

There is an ongoing examination of the assumptions regarding the resistance values and the configuration of the lamps, particularly in relation to the readings obtained from the ohmmeter.

thoradicus
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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 :/
 
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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
 


ehild said:
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?>
 


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
 


ehild said:
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?
 


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
 


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?
 


thoradicus said:
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
 

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