# Why does the potential drop in this case?

1. Apr 4, 2006

### rachael

8 A voltage of 3.0 V is required to operate an alarm
circuit. Only a 12 V supply is available, together with a
number of resistors. The resistance of the alarm circuit is
about 1000 Ω. A student selects four resistors of value
10 kΩ and sets them up in a series circuit. The voltmeter
indicates a potential drop across each resistor of 3.0 V.
However, as soon as the alarm circuit is connected, the
potential drops to near zero.
a Comment on why this happens.
b Check the student’s calculations.
c Should the student have selected resistors of higher
or lower value?

2. Apr 4, 2006

### Hootenanny

Staff Emeritus
You should really show some working and thoughts

3. Apr 4, 2006

### Andrew Mason

I think you mean that the four resistors have a value of 10 Ω, not kΩ.
a. With just the 4 10 Ω resistors, what is the current? When you add the 1000 Ω load, what is the current? Since voltage drop, V = IR what is the voltage across the 1000 Ω alarm?

AM