goal: to discover the consequences of connecting batteries to bulbs and capacitors

each group should have

capacitor (probably green, cylinder-shaped, with two prongs sticking out)

1-battery, 2-battery, and 4-battery holder with batteries

light bulb in socket with wires attached at each end

breadboard

oscilloscope

bare wire

In this lab, you are asked to make a number of predictions. "Predict" means "predict and write the prediction in your lab book."

1) Make a simple circuit with just the light bulb and the 2-battery (in the battery pack) so that the bulb lights

a) without using the breadboard

b) using the breadboard

Draw the circuit diagram that lights the bulb in your lab book.

Make a prediction about what will happen if you reverse the battery in your circuit.

2) Make a prediction about what will change if you use a 1-battery pack in place of the 2-battery pack? Then try it. Same (predict and try) for replacing the 2-battery pack by a 4-battery pack. Make a general conclusion from your observations in parts (1) and (2). Your conclusion should contain the words "voltage difference."

3) Make a simple circuit containing just the light bulb and the uncharged capacitor; make the contacts with your fingers; does the bulb light? why or why not? (your answer should again the words "voltage difference")

4) Now make a simple circuit containing the light bulb, the capacitor, and the 2-battery pack ( do not use the 4-battery pack here; look at the capacitor rating). Use the breadboard and bare wire (if necessary) to make some of the connections, but you'll have to use your fingers to connect the capacitor. Do not entwine the ends of the bare wire around anything to make connections.

What do you observe? (hint: it takes longer than 15 seconds, by which time you should have made 3 distinct observations)

5) Draw a large (half-page) diagram for the circuit used in part (4) above (i.e., while the bulb was lit), using appropriate physics symbols for the three elements in the circuit. Label appropriately the positive and negative ends of the battery. Show the correct sign of the charge on the two plates of the capacitor. Annotate your diagram with a description of how each plate become charged in the manner it did. Describe in terms of electron flow.

7) Now make a simple circuit that contains only the light bulb and the capacitor (just like in part 3). What happens? (and why was what happened different than in (3) ?) As in part (5), draw another large diagram showing the circuit used here. Again annotate it with a description of what happens in terms of electron flow at the instant you first complete the circuit. What is the charge status of capacitor plates after the circuit connection has been complete for a fairly long time? How do you know?

8) Based on your observations in parts (4) through (7) and your diagram annotations, what conditions must be present for the light bulb to light?

(What therefore was the purpose of the light bulb in the preceding experiments? Could you have charged the capacitor without the bulb?)

9) Make sure that the capacitor is uncharged. Explain how you did this and why your method worked.

10) Turn on the oscilloscope, and make the electron beam a continuous line across the center of the screen. Repeat part (4) above with the oscilloscope connected as shown below and turned on. Note the break in the circuit.

11) Now remove the break in the circuit (i.e., connect the positive end of the battery to the unconnected end of the light bulb). What happens to the bulb? What happens on the oscilloscope? Explain what you are seeing on the oscilloscope and what is happening in the capacitor. Make a connection with the explanation that you gave in part (5).

12) Now remove the battery from the circuit, leaving only the light bulb and the charged capacitor in a complete loop, but with the oscilloscope left attached as shown above (across the capacitor). What happens? Again explain what you are seeing on the oscilloscope and what is happening in the capacitor. Make a connection with the explanation that you gave in part (7).