Hands On Physics

The Great Bungee Jump Messing Around Investigation 2: Capacitors

Advice for the Lost

If you forget what the bungee jump is all about, review the story. If you find the difference between current and voltage confusing, if you are not sure about current, LEDs, and electrons, take a look at the electronic primer. If you forget why we need a timer, look at Where we are going. If you haven't read the caution about using the 1 F capacitors, do it now.

Investigation Overview

This investigtation is designed to give you a "feel" for a capacitor. You use a hand crank to generate electricity that you can pump into a capacitor. Because we use a special, huge capacitor, you can actually feel the back pressure from the capacitor as it fills up. It gets harder and harder to put more charge into the capacitor as it fills up. You can use the crank to squeeze electrical charge into the capacitor and trap it there using a diode that lets in the charge but won't let it out. Then, later, you can let the charge out and see it flow because it lights an LED. These huge capacitors have one farad capacity, much larger than ones you commonly encounter in circuits. That means you can pump an entire coloumb of charge into them before they generate one volt opposition voltage. For comparsion, we will be using a microfarad capacitor in the timing circuit, a million times smaller capacitance.


The expensive 1 F capacitors are easily ruined if you apply too much voltage or if the voltage is applied backwards. So always check and re-check your circuit before beginning an experiment. Here is what you must do:

Figure M3 b

The basic circuit used in this investigation. Be sure you know what way to turn the generator to get positive voltage. Carefully watch the resulting voltage on the meter. Keep the voltage between 0 V and +4.0 V.

Investigation Procedure

The point of this investigation is for you to get a feel for how a capacitor works by pumping current into it, storing the charge there, and letting it out later.

1. Show that your generator is just a motor. You can do this by attaching your generator to a small battery, the low-voltage power supply, any other low-voltage power supply, or another generator. Start with voltages around 1 V and go up.

Figure M4
Generator - Motor

2. Determine how to get positive voltage. Figure out which way you have to turn the crank on the generator to get a positive voltage on the meter. Look at the crank generator carefully. There are two wire leads coming from it. Attach them to a voltmeter. Note what happens to the voltage when you turn the crank one way and then another.

Figure M5
Current Direction

Draw a big arrow on the generator to remind you which way to turn the crank to produce a positive voltage. Also, mark the wire that is attached to the + (red) lead wire on the voltmeter with a big +3. Light an LED with the generator.

Attach the generator to an LED so that it makes current go through the LED. The long lead on a LED is positive and should be connected to the + side of your generator.

Figure M6

What happens when you reverse the direction of the crank? Does the crank need more force to light the LED? Why?

4. Use two LEDs to tell which way the current flows. Attach two different colored LEDs in parallel, but pointing in different directions. What do you predict will happen when you attempt to pass current in one direction through these? What will happen when the current is reversed? Try it. Explain your results in your journal.

5. Re-read the caution.

6. Create a 4 farad capacitor.

Figure M7
Capacitors in Parallel

If you are using 1 F capacitors, connect four in parallel. Use a breadboard and wire to make the connections. Make sure that all the "+" terminals are connected together, and that all the "-" terminals are connected together. Label the "+" terminals.

7. Attach the generator to the capacitors and the meter.

Figure M3a (again)

Be sure that the "+" lead on the generator is attached to the positive side of the capacitors. If you are in doubt, please ask for help before turning the crank.

8. Explore the "feel" when you pump charge into the capacitors.

Have one person assigned to watching the voltmeter.
That person must be sure the voltage never goes negative and never goes above 4 V.

Another person should turn the crank on the generator. Observe what it feels like as the voltage increases. Let go of the crank handle when there is 4 V on the meter. Describe what you think is happening. Let each member of the team turn the crank.

9. Trap charge in the capacitors. Attach one LED so that current goes into the capacitors but cannot get out. How can you do this? Try it. Trap energy in the capacitors by charging them. Can you later extract the energy to turn the crank or light an LED? Try it.

10. Use a pair of LEDs to detect the direction of the current. Figure out how to use two LEDs to determine whether the capacitors are charging or discharging. Draw the ciruit and then construct it. Now turn the crank until you get 4 V on the capacitor. What do the LEDs indicate? How can you explain your observations?

Be sure that someone always checks that
the voltage stays between 0 V and 4 V.

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