Science help!

[laushik]

1. Question:  In a series circuit that has one cell, a switch and two bulbs. Describe the brightness of each bulb. 

 

I don't really get this question, but I know the bulb closest to the negative side of the cell will be the brightest, but I don't understand why.

 

2. I also have another question:  in a parallel circuit that has two cells, a switch and three bulbs (each on their own branch of the circuit).  Describe the brightness of each bulb. 

 

Now in this question I have no idea which bulb would be brighter, but in a parallel circuit how would a switch work? wouldn't it have multiple paths to flow from if the switch was open, would the switch be pointless?

 

 Pls help and thanks in advance!

 

 

[Mihle]

1. As  the bulbs is in series they will both get half the voltage from the cell, and will therefore have the same brightness as eachother, but less brightness that if it was one or they was in parallel.

That is if the bulbs are the same.

 

2. When the bulbs are in parallel, they will all get the same voltage as the cells send. So all would have the same brightness if they are all the same.

 

The switch would be before the wires split in to the different bulbs, so it break the connection of all of them.

[mariushm]

 

An incandescent (or halogen) bulb can be replaced in a circuit with a resistor.   So basically the problem is translated into something like this :

 

--- -  [ battery ] +  ---- switch ------  [resistor ] ------ [ resistor ] --- to battery [-]

 

Series and parallel resistors : http://physics.bu.edu/py106/notes/Circuits.html

 

Basic laws of physics ... V = I * R  so the voltage drop on each resistor will be the same, the current consumption will be the same. Each resistor drops some voltage, so each resistor gets 1/2 of cell voltage, so in theory each bulb is half brightness compared to having just one bulb in circuit.

 

For two, when the bulbs are in parallel, in an ideal world the bulbs will also have same brightness, because each bulb is across the poles of the battery. 

 

In the not ideal world we live in, the more bulbs you're going to connect in parallel, they'll go down in brightness a bit, because the battery itself has some amount of internal resistance, it can only output a limited amount of current.

So for example if the battery can only output 2.5A of current, you put one 1A bulb and it will get 1A, you put a second bulb and that one will also get 1A but if you connect a third bulb the 3A load on the battery will be too much and the bulbs will not get 1A each, the voltage of the battery will go down so the three bulbs will be less bright.

Also, in real world, with 3 bulbs in parallel, the wire lengths to the bulbs will be different. The wires are not ideal, they have some resistance, so the bulb with longest wire will be ever so slightly less bright because some power is lost on the wire.  The resistance of the wire is very small typically so this is not a problem you think of, but it is there. 

 

Again, most of these types of questions assume ideal scenarios like ideal battery with no internal resistance, ideal wires with no internal resistance, even the bulbs themselves as the filament heats up their resistance changes slightly...