                   Simple experiments with the bat-light board click for large picture

If you haven't already, install the 4 batteries in the holder. When you connect the alligator clips clip them securely on to the exposed copper and not the insulation on the wire [why?].

Each battery produces a potential difference of 1.5 volts. Four in series therefore create a potential difference of 6 volts, voltages in series *add*.

In the pictures the alligator clips are shown as the colored connection. The color of the wire used makes no difference.

Parallel circuits

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

For a technical discussion: http://en.wikipedia.org/wiki/Parallel_circuit

The basic idea is that the energy from the battery (i.e. the electrons) can flow through each bulb equally so that each bulb consumes the same amount of power (which means they will have equal brightness).

### One bulb

We'll first start with one light bulb. Take one alligator clip and connect it to the switch connection, and the other end to light bulb #1. Take a second alligator clip and connect it to the *other* wire on light bulb number one and to the negative wire from the battery (as shown in the picture below). Press the switch, the bulb should light. This really isn't a parallel circuit since it only has one bulb in it.

Exercise: Draw the schematic representation of this circuit.

### Two bulbs

Take two more alligator clips and connect light bulb #3 in *parallel* with light bulb #1. Push the switch and observe the brightness of each bulb compared to the single bulb [Note: if it is noticeably dimmer then your batteries are weak]

Exercise: Draw the schematic representation of this circuit.

Now think of an alternate way to connect the two bulbs in parallel. Note that the same schematic applies to both ways to hook them up - a wire is considered a "direct" connection between points, regardless of the actual number of wires needed and the order that they are connected (at least for simple circuits like this - in a device like a computer this statement is no longer true...something not taught until college).

The two picture below illustrate two possible ways to connect the bulbs. Try them both and check that it doesn't make any difference.

###  ### Three and four bulbs

Add a 3rd bulb in parallel with the other two and compare the brightness with 3. Then repeat for four. Being careful to not accidentally short the battery (i.e. create a connection from the battery + terminal to the battery - terminal when the switch is pushed) try a couple of different ways to hook up four bulbs. How many can you come up with ? [Note: If you actually try them all it would take a long time] How many alligator clips does it take to connect all 4 bulbs?

### Notes

Assuming you have fresh batteries then you should observe that the bulbs stay the same brightness regardless of how many are connected (a very slight dimming is normal).

This demonstrates the basics of a parallel circuit; each load (light bulb) is connected to the battery, so the number of light bulbs does not affect the voltage applied to any given bulb, and it therefore each bulb puts out the same amount of light.

Also, since the wire (alligator) clips are good conductors, how things are hooked up does not make a difference, i.e. the bulbs can be connected to each other or all to the battery and the circuit functions the same.

Series Circuits

Remove all of the alligator clips from the previous connections. In a series circuit the electrons from the battery must flow through more than one circuit element in series (in this case something with resistance, i.e. the wires do not count) which means that the power is divided across each bulb.

### Hook two bulbs in series. Connect one side of bulb #1 to the switch, the other side to bulb #2. The last connection from bulb number 2 to the negative battery terminal will create a series circuit. The bulbs will shine at about half brightness.

As a simplistic explanation, you can think of the power from the battery now has to be shared by the two bulbs. In the parallel circuit case each bulb had its own direct connection to the battery so each bulb could draw the same amount of power and therefore shine at the same brightness. In the series circuit the power is shared between the two bulbs.

Another way to look at it is that the battery voltage is divided across the two bulbs, and the voltage applied to the bulbs determines how bright they are. To show this, remove two batteries (the right two in the picture) and connect one bulb up to the remaining two batteries. The brightness of one bulb connected to two batteries (3 volts total) is the same as two bulbs in series connected to all 4 batteries (6 volts).

We can therefore think of our light bulbs as somewhat crude voltmeters; if they shine at half brightness then the voltage across them is one half of the voltage when they shine at full brightness. For these bulbs that full value is 6 volts.

### Three bulbs Connect three bulbs in series. Note that they are now pretty dim. Each bulb is now "sharing" the battery's power with two other bulbs, so each bulb only has 6 volts / 3 = 2 volts available (versus the full 6 volts when only one bulb is hooked up or a parallel setup is used).

Exercise: Draw the schematic representation of this circuit.

### Four bulbs

Now hook up all four bulbs in series. Each bulb now "shares" the battery's power with 3 others, so the voltage across each bulb is 6 volts / 4 = 1.5 volts. If you connect one bulb to one battery you will observe the same brightness as that from each bulb when four are connected in series.

Go on to part II