DC Circuit Demos

These demos are mounted on yellow wooden boards. There are two different ones. The first has one large light bulb and a variable resistor, and the second has three light bulbs and three switches. Be careful with both of them because the 120V connections are exposed. You should handle the switches only by their insulated handles, and un-plug the board when you are finished.

Demo 1: Resistance and current.

This board is a trivial circuit with an AC source in series with a large wire-wound variable resistor and a light bulb. The resistor is simple and all of the students can see how it works: it has a helical coil of resistive wire, a contact fixed at one end, and a sliding contact. There is not too much to this demo: you slide the contact, and the light gets brighter and dimmer. It shows the qualitative relation between resistance and current, and it shows that the resistance of an object increases with its length. Applications: Variable resistors (potentiometers) are ubiquitous in analog electronics.

Demo 2: Parallel and Series resistances:

The three bulbs are identical. Each is rated at 40W, which means it should have a resistance around 360 Ohms when they have 120V across them. (Light bulbs are not really ohmic, as the students see for themselves when they do the "Circuit Analysis" experiment in the IPL, but you can neglect this for the purposes of this demonstration). The first bulb is always in the circuit, and the second two can be taken in and out with their individual SPST switches. The DPDT switch is used to change the circuit from "parallel" mode to "series/parallel" mode.

These demonstrations are very simple, and they offer a nice opportunity to work out quantitative examples.

"Parallel Mode" (with the DPDT switch thrown to the outside of the board). By varying the SPST switches, you can have 1, 2, or 3 bulbs in parallel. You can calculate the total current, total resistance, and total power in each case.

"Series/parallel Mode" (with the DPDT switch thrown to the inside of the board). In this case, the first bulb is in series with the two other bulbs who are in parallel with each other. Again, the 2nd or 3rd bulb can be removed from the circuit with its own switch. For each case, you can calculate the voltage drop across each bulb, the current through each, the power in each, and the total of these.

S3 open:

S3 closed: