What is needed to light a bulb?   -- Basic circuits




Objective:  Students will construct a simple circuit, and see that nothing will happen if the loop (circuit) is not closed.   Once they see this, allow them to start using the term "circuit" to describe a closed loop.  Define the term, give it visual meaning with analogies to racetracks, etc.

Notes:  Attempt to connect the necessity of the closed loop to the previous lesson of flow.  If we assume that something is moving around in the conductors, why would a break in the loop cause the flow to stop?  Why does it stop so quickly?


1.  Your first goal is to use the Snap-Circuit kit to light two light bulbs.  You will need the battery holder, two AA batteries, at least two of the blue snap-wire pieces, two identical light bulbs (with a pink marking on the base), and two light bulb holders.  Use snap-wires to connect one end of each bulb holder to one of the ends of the battery holder, then connect the two bulb holders to each other, and insert the bulbs into their spots.  Sketch what you made below -- you should be able to identify a "closed loop" made by the elements, and you should have seen both bulbs light up when you connected them.  If this did not happen, stop to figure out how to get the bulbs to light.








2.  "Break" the loop by disconnecting one of the snap-wires from the battery holder.  Did the two bulbs go out at the same time?  Now reconnect the wire -- do the two bulbs relight at exactly the same time?  Do you believe that they go out at the same time?  Turn on at the same time?  Why?






3.  With the loop connected as before, disconnect one of the other connections in the loop.  What happens?  Is there anyplace you can disconnect which allows either bulb to stay lit?





4.  Replace one of the snap-wires with an alligator clip wire.  Disconnect one end, and bring it up to the connection slowly, watching carefully what happens.  Do the two bulbs light?  Do you actually need to make contact in order for the bulbs to stay lit?






5.  What seems to be the basic requirement for the bulb to light?






6.  Can you explain why a circuit (loop) is necessary?  Any idea what the battery does?





7.  What is happening to cause the bulb to light?