Fancy experiments with the bat-light board    


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This step should be done after completing the basic series and parallel activities. As with the first part, the color of the wire used makes no difference.



Basic Series-Parallel circuit


In this experiment you'll create a simple series-parallel circuit.

First connect two light bulbs in series, as shown below. Press the switch and both bulbs will light at about half brightness.

2 bulbs in series

Leaving those two bulbs connected, wire two more bulbs in series and connect them as shown in the next picture.

2 parallel circuits with 2 series bulbs

Earlier we made parallel circuits with just one bulb in each branch. All we have done is place two bulbs in series in each branch. Each bulb will shine with equal brightness since the parallel circuits all receive the same power from the battery, regardless of how many parallel circuits there are (the battery, however, has to supply more power for each parallel branch. A weak battery will have limited power which would cause the bulbs to dim when more parallel branches are connected).

Exercise: Draw the schematic representation of this circuit.

Looking at the schematic, you can see that there's a certain symmetry to the circuit, each of the parallel branches is the same, and the upper and lower half of each branch is the same. From a purely qualitative standpoint it kind of suggests that each bulb will shine at the same brightness.

A more complete answer requires some knowledge of the relationship between voltage, current, and resistance, and a little simple math to "prove" why it works; that level a detail is beyond the scope of this experiment.

Now what...

Connect one more wire as shown in the next picture (red in the picture below) to connect the mid point of each series circuit together. What do you think will happen when you close the switch?

series parallel connection with mid points connected

Try connecting and removing the added connection (red in the picture) while applying power. Does the brightness change?

Why does adding/removing this wire make no difference? Proof of why it makes no difference requires the same analysis as discussed previously, and is outside of the scope of this exercise.



Series Parallel Circuit II



Remove all of the alligator clips from the previous connections.

Connect three bulbs as shown; the first bulb(#1) is in series with two bulbs (#2 and #4) connected in parallel. Does the brightness of the bulbs make sense?


series parallel - more complex

Exercise: Draw the schematic representation of this circuit.

Again a little math would provide the proof for what is observed. However a rough qualitative explanation could be given as first thinking of bulbs #2 and #4 as "one entity." Then this is like the 2 bulb series circuit done previously, and each bulb lights at half brightness. Except now one of the bulbs in the original series circuit is replaced by two in parallel, and they therefore have to share the available power as unlike the parallel circuits done previously (where each bulb connects to the battery) #2 and #4 bulbs are limited by #1 bulb as to the power they can use.

However that explanation somewhat falls apart as bulb #1 is *brighter* than two bulbs connected in series - disconnect bulb #4 with the switch closed and you'll see #1 get brighter.

And somewhat counter-intuitively, hooking a 3rd bulb in parallel with the #2 and #4 makes #1 brighter.

series parallel headaches

Again, the answer is easily explained with some simple math, but that's outside the scope of this exercise.