Understanding the Effects of Shorted Lamp A on Power Supply

[Janiceleong26]

1. Homework Statement

Homework Equations

The Attempt at a Solution

My answer to (b) was that power supply would have an internal resistance which is not negligible and will affect results. The correct answer is that shorted lamp A would cause damage to the lamps/supply.
How is lamp A shorted? I don't see how there is a short cut way for the current to flow through to short lamp A. And how can we measure the resistance of the lamps by using an ohm meter without any power supply?

 

[showzen]

We don't know which bulb is faulty or the nature of the fault. A normally operating lamp should have some measurable resistance, where a faulty one may have 0 (short circuit) or infinity (open circuit). The switches are a convenience, because they let us test the circuit components without moving the meter around.
Now, what could happen in the case where we close switch 1 and are testing bulb A individually?

As for the ohmmeter, they usually use a battery to source the current to measure resistance. This is typically much smaller that the current sourced by a power supply.

 

[Janiceleong26]

We don't know which bulb is faulty or the nature of the fault. A normally operating lamp should have some measurable resistance, where a faulty one may have 0 (short circuit) or infinity (open circuit). The switches are a convenience, because they let us test the circuit components without moving the meter around.
Now, what could happen in the case where we close switch 1 and are testing bulb A individually?

As for the ohmmeter, they usually use a battery to source the current to measure resistance. This is typically much smaller that the current sourced by a power supply.

The ohm meter will measure the resistance of bulb A only
Oh I see, so does it mean that the ohm meter has a power source inside but much smaller than the power supply?

 

[showzen]

The ohm meter will measure the resistance of bulb A only
Oh I see, so does it mean that the ohm meter has a power source inside but much smaller than the power supply?

Your power supply is set to supply constant voltage, so in the case of a short circuit you get current approaching infinity until a fuse blows.

Your ohmmeter supply's a constant current.

 

[Janiceleong26]

Your power supply is set to supply constant voltage, so in the case of a short circuit you get current approaching infinity until a fuse blows.

Your ohmmeter supply's a constant current.

I see..I got it , thanks