L12.  Batteries, Bulbs & Capacitors

About working together:  You may work with one other student in doing this lab, but every student must participate in constructing the circuits.

About your report: Write your answers to the numbered items on notebook paper for faxing.  Provide the usual heading for your report.

Goal:  To investigate simple circuits containing batteries, a bulb, and a capacitor

Equipment

1-fd capacitor (green, cylinder-shaped, with two prongs sticking out)
4 batteries
Battery holder or tape to hold batteries together
Light bulb (screw base type)
Light bulb holder
Alligator clip leads (3)

  1. For this part, use a fresh battery, the bulb, a battery holder if you have one, and alligator leads.  Do not use the light bulb holder, as that will defeat the purpose of this exercise.  You may need another person to help you hold some of the parts.  The goal is to light the bulb.  This amounts to figuring out where on the bulb the contacts are.  Simply touch the alligator clips to those contacts wherever you think they are.  If the bulb doesn't light but you feel the battery getting warm, you've created what's called a short circuit.  That means you're connecting the battery to the same point on the bulb.  This effectively bypasses the bulb.  Once you're successful in lighting the bulb, sketch a diagram showing your circuit.  Clearly show the two points on the bulb that you touched with the alligator clips.  (If no matter what you do, you can't get the bulb to light, your battery may be too weak.  In that case, try using two batteries in series.)

  2. Having determined how to light a bulb, do so again, but reverse the positions of the alligator clips where they touch the bulb.  The point is to make the current flow the opposite direction.  Do you see a difference from the results of step 1?  Did you expect to?

Maybe you thought the above exercises were a bit too simple for high-school students.  However, studies show that many college physics students don't know how to light a bulb when it's not in a socket.

  1. Screw the light bulb into its holder now.  There are two clip contacts on the holder.  Connect an alligator clip to each one.  Touch the other ends of the alligator wires to the ends of a battery.  If your battery is fresh, the bulb should light, although it will be weak.  If the bulb doesn't light, make sure it's screwed completely into the holder.  The circuit is show in Figure 1 below.

Figure 1 Figure 2

  1. For your circuit of step 3, add a second battery in series with the first as shown in Figure 2.  Repeat with 3 batteries and then again with 4 batteries.  Don't use more than 4 batteries, as that may burn out the bulb.  Describe what you observed as you added batteries. 

Now for some theory: The power provided by a battery is Pbatt = VbattI, where Vbatt is the potential difference across the battery terminals, and I is the current in the circuit.  The power dissipated in the bulb is Pbulb = IČR, where R is the resistance of the bulb.  An alternative formula is Pbulb = VbulbČ/R, where Vbulb is the potential difference across the bulb.  (See section 21-3 of your text as needed to review this.)  The same symbol I is used in both cases, because the current is the same in the battery and the bulb.  In a given amount of time, Dt, the energy provided by the battery is Ebatt = PbattDt, and the energy converted by the bulb to heat and light is Ebulb = PbulbDt.  If we assume that the wires have negligible resistance (which they do) and therefore negligible power dissipation, then for energy to be conserved, Ebatt = Ebulb.  (Since the time interval is the same, Pbatt = Pbulb also.)

  1.  When you changed the number of batteries in step 4, the power dissipated by the bulb obviously changed.  Explain this with reference to the concepts of the previous paragraph.  You must use standard physics vocabulary such as potential difference, current, resistance, power and energy in your answer.  When you talk about potential difference, you must say "potential difference across such-and-such", where such-and-such is a circuit component.  Similarly, you would speak of "current in such-and-such" and "resistance of such-and-such."  The goal is to explain clearly without making nonsensical or ambiguous statements about electrical circuits.  Don't rely on equations to make your argument for you. Give your argument in paragraph form with reference to relationships such as those in the previous paragraph and others that you may need.

  2. Now we'll bring a capacitor into the experiment. Connect the following items in a circuit:  capacitor, bulb (in its holder). That is, connect a clip lead to each of the pins on the capacitor.  Then connect the other ends of the leads to the contacts on the bulb holder.  What do you observe?  Based on your observation, is there a potential difference across the capacitor?  Based on what you know about capacitors, could there be a potential difference across the capacitor without there being a battery in the circuit? Explain.

  3. Read the following completely before you connect the circuit:  Make a single loop circuit containing the light bulb in its holder, the capacitor, and 2 batteries (do not use more than 2 batteries).  Single loop means that there is only one path for the current to follow. There are no branches in the path. So you would connect, say, from a capacitor pin to a bulb contact, then from the other bulb contact to a battery terminal, and then from the other battery terminal to the second capacitor pin. A circuit diagram would look like the one in Figure 3. Now connect the circuit and observe.  Don't record any observations yet.

Figure 3 Figure 4

  1. Now disconnect the two wires to the battery and touch them together.  The circuit will be like the one in Figure 4.  Hold the two wires together until there is nothing interesting left to observe.

  2. Now go back to step 7 and repeat it.  Describe how the intensity of the bulb changes with time by sketching a graph of intensity vs. time.  While you won't have a scale for intensity, you can include a time scale in seconds.  If the rate at which the brightness changes has different values at different times, represent that as faithfully as possible in your sketch.  (Light intensity is defined as the radiant energy per unit area per unit time.  You can think of it as a concentration of light.  If you always observe the bulb from about the same distance, then the intensity is directly related to the radiant energy output of the bulb.)

  3. Repeat step 8. Provide a description and graph as you did in step 9.  (Repeat steps 7 and 8 as many times as needed in order to be confident of your observations.)

Explanation for step 7 observations:  You were charging the capacitor in step 7.  When you closed the circuit, the current in the circuit jumped to its maximum value and the charge on the capacitor began building up.  (The intensity of the bulb didn't immediately reach its maximum, because it takes time for the temperature of the filament to rise.)  As the charge built up, the potential difference across the capacitor increased.  (Recall that Q = CV for a capacitor.)  When the potential difference across the capacitor reached the same value as that across the batteries, current ceased. 

  1. For your step 7 observations, sketch a graph of current in the capacitor vs. time.

  2. Also for step 7, sketch a graph of potential difference across the capacitor vs. time.  Use the same time scale as for the previous graph.

  3. Redraw Figure 3.  On your drawing, label the + and - terminals of the batteries.  Indicate which plate of the capacitor is positive and which is negative when the capacitor has reached maximum charge and the current is 0.  Also label where in the circuit the potential is highest and where it is lowest.

  4. Now consider your observations in step 8.  Write a paragraph providing an explanation.  Use Explanation for step 7 observations above as a model. 

  5. Redraw Figure 4.  On your drawing, indicate the direction of electron current as the capacitor is discharging.

You need not write a conclusion for this lab.