# Parallel circuits

1. Jul 20, 2014

### iScience

my living room light switch controls two lights. one of the light bulbs just went out.
since the switch controlled two bulbs, aren't they in parallel? so then why didn't the light bulb that is still on get any brighter?

2. Jul 20, 2014

### dlgoff

Why would you expect a bulb with a given wattage get brighter?

3. Jul 21, 2014

### davenn

indeed.

consider that it still has the same voltage across it 110V 240V ( whatever in your country)

so say 100W globe, 110V what is the current flowing through it ?

2 x 100W globes in parallel with 110V across them, what is the current through each of them ?

3 100W globes in parallel with 110V across them, what is the current through each of them ?

when you have worked that out ... do you see where its leading ?

cheers
Dave

4. Jul 21, 2014

### iScience

I thought the power rating tells you what the device was made for. regardless of what current and voltage you put across the device, the device has its inherent Z value, whether or not its dynamic (a function).

if the light in parallel goes out, all the sudden there is a change in Z(eq). Remaining at the same brightness implies either...

1.) The circuit is not just a simple parallel circuit.

or..

2.) the light bulb has somehow increased its Z value.

there is definitely an initial change in Z(eq); what i'd like to know is what happened such that the current remained the same?

5. Jul 21, 2014

### iScience

$$I=\frac{P}{V}=\frac{V}{R_{eq}}=V(\frac{1}{R_1+R_2+...})$$

still dependent on the number of loads in parallel. i still don't get it

6. Jul 21, 2014

### MrSparkle

why do you think current has remained the same?

7. Jul 21, 2014

### iScience

because the second light bulb (one that's still on) didn't change in brightness

8. Jul 21, 2014

### MrSparkle

right, the current in that leg of the parallel circuit has stayed the same. But the other leg now has zero current, so the total current in the light fixture must have dropped in half.

9. Jul 21, 2014

### iScience

ohhh... i see .

okay thanks!