Short circuit

1. Oct 18, 2015

Janiceleong26

1. The problem statement, all variables and given/known data

2. Relevant equations

3. 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?

2. Oct 18, 2015

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.

3. Oct 18, 2015

Janiceleong26

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?

4. Oct 18, 2015

showzen

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.

5. Oct 18, 2015

Janiceleong26

I see..I got it , thanks