The Science Notebook
Current Electricity and Simple Circuits - Pt. 2

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 On this page...
Making and Using a Resistor 
Making a Capacitor 
Examining  Batteries
Examining How Cells and Batteries Work
Making a "Short" Circuit
Making a Lemon Cell
Combining Lemon Cells to Make a Battery
Identifying the Terminals of a Cell or Battery Using a Potato

Making and Using a Resistor

Sometimes you want to add something to a circuit to limit the amount of electricity moving through a circuit without blocking it completely.  A device that does this is called a “resistor”.  Let's see how one works.

Materials Needed:  A mechanical pencil lead or lead from a regular pencil;  homemade battery holder with two batteries; light bulb holder and light; wire; connectors of your choice (See this experiment in Part 1).  (Clothespin 
clamps work well for this experiment.)

CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

Procedure:   If you don't have a lead from a mechanical pencil get an adult to help you split a wooden pencil to expose the lead.  You will first need to pull the eraser and metal holder from the pencil using a pair of pliers.  The lead is sandwiched in between two layers of wood.  If you look carefully at the trimmed end of the pencil, you can see the two layers.  Carefully split the pencil and separate the two pieces of wood.  The lead may then be removed, but care must be taken not to break it, or better still, it may be left in the wood on one side of the pencil.  Note:  Some less expensive pencils do not have the lead sandwiched in between two layers of wood, so you need to look for one that has two clear layers.

Next, assemble the following circuit. 


One end of the pencil lead should be clipped to the bulb holder wire using foil and a clothespin or other connector.  Be careful not to break the lead!  Now firmly press the wire from the other end of the battery holder on the lead near - but not touching - the bulb holder wire.  The bulb should light.  How bright is it?  Now move the wire farther down the lead and again firmly press down.  Is there any change?
What To Look For: You should notice a difference in the brightness of the light as you move the wire along the pencil lead.

What Happened:  The pencil lead allows electricity to move through the circuit, but some of the electrical energy is being lost to the pencil lead in the form of heat.  Thus, the bulb is not able to burn as brightly when the electricity moves through the pencil lead.  In fact, the more lead the electricity has to flow through, the dimmer the bulb gets.

You probably already know that a "lead" pencil is not really made of lead.  Instead, it is made from graphite, a form of carbon.  Carbon is one of a class of substances that will conduct some electricity, like a conductor, but will also block some of the current.  Any substance that restricts the flow of electricity is called a resistor.

Resistors are used in almost all electronic circuits, and you have just made a model of one such circuit.  A dimmer switch is actually a combination of a switch and a variable resistor (a resistor that may be adjusted).  By moving your wire along the pencil lead, you actually used the pencil lead as a variable resistor in a dimmer circuit. 

The volume on many radios, televisions and other similar devices is also controlled by a resistor.  

Making a Capacitor

Many electronic circuits use devices called capacitors. These devices perform a number of useful functions. While a study of how they work is beyond the scope of this site, we can make a capacitor and study at least one feature of these devices.

Materials Needed:   Two 15 cm (6 in) square pieces of aluminum foil; paper towel, plate; water; 9 volt battery; old speaker; voltmeter (optional).

Procedure:   Place one square of aluminum foil inside of a folded paper towel with one end of the foil sticking out as shown in the illustration.  Now place the other piece of foil on top of the folded paper towel.  It is very important that the two pieces of foil are completely separated by the towel, and that they do not touch. However, one edge of the top layer of foil should be near the edge of the paper towel and about 1 cm (1/4 in) from the bottom piece of foil.

Place the foil sheets and folded towel into the plate.  Soak the paper towel by pouring just enough water onto the towel in the plate.  Pour off any excess water.  Press the sheets of foil and paper towel together and again pour off any excess water. 

Next, press the two terminals of the 9 volt battery onto the two sheets of foil.  One terminal should touch one piece of foil, and the other terminal should touch the other sheet of foil, but the two pieces of foil should not touch each other. Hold the battery in place for a few seconds and then remove it.   This will "charge" the capacitor with electricity, and you should hear a slight sizzle as your capacitor charges.

Touch one of the speaker wires to one of the foil sheets and, at the same time, touch the other wire to the other sheet.  Do you hear anything?  You should be able to hear a distinct crackle or static in the speaker or earphones.  This indicates the presence of electric current.  To prove that it does, touch the two speaker wires to the ends of a battery.  The static will probably be much louder, since there is far more energy stored in the battery than in this capacitor.

If you have a voltmeter, have an adult to help you measure the voltage between the two pieces of foil. You should be able to measure about a volt between the two pieces of foil. If you hold the probes in place for a while, you will see the voltage begin to drop as the capacitor loses its charge, or discharges.

What Happened:  When you placed the battery terminals against the foil, some of the electricity generated by the battery charged the capacitor.  By placing the meter on the capacitor, you were not only able to see that it was charged, but you could also see it slowly discharging as the voltage dropped.

Going Further:  All capacitors consist of two or more metal "plates" separated by an insulator called a "dielectric".  In your capacitor, the sheets of aluminum foil were the plates, and the paper towel soaked in water was the “dielectric”.  Under ordinary circumstances, clean water does not conduct electricity and is an insulator.  However, it can be made to conduct electricity if the voltage or current is high.  It will also conduct electricity if it contains any impurities.  The water is not a conductor here, but it is used to hold the towel paper close to the two layers of foil. 

In capacitors used in electronic circuits, the plates are usually metal.  The dielectric may be air, paper, mica, or some type of plastic.  There are many different kinds of capacitors, but all will hold a charge in much the same way as yours did.

Examining  Batteries

Materials Needed:    Homemade battery holder with two AA, AAA, C or D batteries; bulb and holder; connectors of your choice; foil; 9 volt battery.

Procedure:   First examine one of your AA, AAA, C or D batteries carefully.  You will notice that it has a "+" at the top end and a "-" at the bottom. 

Strictly speaking, this is not a battery.  Instead, it should be called a cell.  AA, AAA, C and D cells are all 1.5 volt cells.  The difference between each is obviously the size.  Generally, the larger sizes will last longer in any given circuit, but the voltage in all is the same - 1 1/2 volts. When you place the two cells in the battery holder,  you make a "battery".  A battery is made of two or more cells connected together.  In your homemade battery holder, two cells are connected “+” end to “-“ end in series to make a 3 volt battery.  With a wider strip of paper, you could combine three cells end to end to make a 4.5 volt battery.

On the other hand, the 9 volt battery really is a battery.  If you could see inside of it, you would see that it is made of 6 very small 1.5 volt cells connected in series.   (6 x 1.5 = 9)  Each terminal of the 9 volt battery is also marked "+" or "_".

What To Look For:  We say that the "+" side of the battery or cell is the positive terminal, while the "-" side or end is the negative terminal.  You'll see why this is important in the next experiment. As you have you have already seen, there is a difference between a cell and a battery.  However, most people will call it a battery, regardless of whether it really is a battery (made of two or more cells), or just a single cell. 

But now you know better. 

Examining How Cells and Batteries Work

Materials Needed:   Homemade battery holder with two AA, AAA, C or D cells;  light bulb with holder; 9 volt battery;  connectors of your choice.

Procedure:   Touch the end of one of the cells to one of the wires from the lamp holder, and the other wire to the other end of the cell.  Observe the brightness of the light.  Next, place two cells together in the homemade battery holder so that the “+” end of one cell touches the “-“ of the other cell and hook up the light.  Is there any change in the brightness?  Finally, touch the ends of the bulb holder to the terminals of the 9 volt battery. Now what do you see?
What Happened:  When you hooked up the light to a single 1.5 volt cell, the light lit.  When you combined the two cells 1.5 volt cells, you created a 3 volt battery and the light was brighter.  Finally, the light was brightest with the 9 volt battery.
Electric current is actually a stream of negative electric charges called electrons moving through the parts of a circuit.  A cell or battery uses chemicals to generate a concentration of electrons at it's negative terminal.  When you make a circuit by connecting the ends with wire, electrons begin moving through the wire and bulb, and they end up at the positive terminal, which has a shortage of electrons.  We say that electricity flows through the circuit.  When all of the excess electrons have moved from the negative terminal through the wire to the positive terminal, the cell or battery is said to be discharged.  When this happens, we commonly say the battery is dead.

Voltage is actually a measure of the pressure with which the electrons are being "pushed" through the circuit.  The greater the pressure (voltage), harder the electrons are being pushed, and the brighter the light will burn.

Going Further: Can you use several AA, AAA, C or D cells to make a 4.5, 6 or 7.5 volt battery?

Making a "Short" Circuit

Materials Needed:   AA, AAA, C or D cell; 9 volt battery; a 15 cm (6 in) piece of wire with 1 cm (1/4 in) stripped from each end.

Procedure:   Touch the two ends of the wire to the two cell terminals.  Feel the wire as you do.  Do you notice anything? Repeat with the 9 volt battery.  CAUTION!  Do not hold the wire in place for more than a couple of seconds, or you will run the cell or battery down.

What To Look For:  The wire should very quickly begin to feel warm.

What Happened:  You just created a circuit with nothing but a wire path.  There was nothing such as a light bulb or resistor, to offer any resistance to the flow of electric current. A large number of electrons moving through the wire with little or no resistance created heat.  Because the voltage was greater with the 9 volt battery, you may have noticed that it heated up more quickly.
If you were to leave the wire in place for long, the wire would get very hot, and quickly run the battery down.  A short circuit can be very dangerous because the heat generated may be enough to start a fire.  In your home, electric circuits are protected from "shorts" by fuses or circuit breakers.

Making a Lemon Cell

CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

Materials Needed:  Fresh lemon; knife; small piece of copper; small piece of zinc (A piece of copper wire or brass hardware such as a screw may be used for the copper, and any galvanized hardware such as a nail may be used for the zinc); speaker from an old radio; voltmeter (optional) connector of your choice.

Procedure:  With a sharp knife, carefully cut two slits into the lemon.  Stick the piece of zinc into one of the slits and the piece of copper into the other slit.  Make sure the slits do not cross each other and that the two metal pieces do not touch.  Connect one of the speaker wires to one of the pieces of metal using a connector of your choice. (One of the clothespin connectors works well.)  Touch the other wire from the speaker to the other piece of metal.  Rub the bare end of the wire against the metal as you listen to the speaker.  Do you hear anything?

If you have a voltmeter, see whether you can read a voltage between the two pieces of metal.

What To Look For:  What do you hear when you touch the wires to the metal?  How much voltage do you read using the voltmeter? 

What Happened: Sound is created in a speaker by an electric current. The static you heard when you touched the wires to the lemon cell  was caused by a weak electric current. (Remember the capacitor?) If you had a voltmeter, you should have been able to read a little less than 1 volt.

You have just made a very simple, but very weak, cell.  Although a voltmeter will measure about 1 volt, the amount of moving electrons, or current, this cell can produce is very small. It is not enough to do any useful work - to light up a light bulb, for example. 

All electric cells consist of two different kinds of metal, or metal and a carbon rod, separated by some chemical.  The chemical is usually an acid.  If the chemical is a liquid, the cell is known as a wet cell, but if it is in the form of a chemical paste (usually a liquid mixed with a dry material), it is known as a dry cell.  The cells you have been using in the previous experiments are dry cells. 

By connecting several of these lemon cells together, it is possible to make a battery that will produce enough electricity to do some useful work, but even to light your small Christmas light would require quite a few lemons!

Combining Lemon Cells to Make a Battery

This experiment will use a battery made from three lemon cells to actually light an electronic device called an LED (light emitting diode).  It may take a little patience, but the results are well worth the trouble.

CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

Materials Needed:   Very low current light emitting diode (LED).   (These are available from Radio Shack®.  One that definitely works is part number 276-310, but most any small LED will probably be OK.); 3 lemons; 3 small pieces of copper; 3 small pieces of zinc (You can use brass screws for the copper and galvanized nails for the zinc.); several  connectors of your choice.  (Clothespin connectors with foil work well to connect the lemon cells, and alligator clips or leads work well to connect the diode.); 4 15 cm (6 in) pieces of insulated wire with 1 cm (1/4 in) insulation removed from each end.

Procedure:   A light emitting diode is an electronic component that produces light when an electric current passes through it in one direction.  Although it looks like a small light bulb when lit, it does not work the same as a light bulb.  For one thing, it requires far less energy to light up, and for another, it will light up only when hooked up one way.

Prepare 3 lemon cells as you did in the last experiment and line them up so that the copper from one lemon is lined up with the zinc in the lemon next to it. Connect the three cells together with the wire, and clothespins (or other connectors). Because the clothes pins are so large, you may want to use small alligator clips if you have them in your school science lab. If not, you can get them from Radio Shack® (Part Number 270-374).  They can be used wherever clothes pins are called for. 

The wire should be connected so that the copper from one lemon is connected to the zinc from the next. There will be a free wire coming from the copper from the lemon at one end and from the zinc from the lemon on the other end.  You have connected three lemon cells in series to form a lemon battery.

Most LED’s have one wire that is longer than the other.  Many also have a flat edge close to one of the leads.  The side with the shorter wire and/or flat side is the negative side, and is known as the cathode.  The other side is the positive side and is called the anode. Carefully bend the wire leads of the LED away from each other.  Connect the free wire coming from the zinc piece of the lemon on one end to the cathode side of the LED,  and connect the free end of the wire from the copper side of the lemon on the other end to the anode side of the LED.  The LED should light faintly.  You may need to darken the room or cup your hands around the LED to see this.

If it does not light, make sure that all of your connections are tight.  If it still doesn’t light, try reversing the leads on the LED. 

Now, reverse the two leads on the LED.   What happens?

What Happened:   The three cells were combined together in series to make a battery. When cells are combined together with the “+” of one cell connected to the “-“ side of the next cell, they are said to be wired in series, and the total voltage is the sum of the voltages of each cell.  This combination of lemon cells is a battery. Since each lemon cell has a voltage of about 1 volt, the total voltage of the three lemon cells is about 3 volts, and this is just enough to light this LED, even with the weak current the lemon cells produce.

When these leads are reversed, the LED will not light.  This is because current will flow through an LED in one direction only. 

Some books will tell you that you can hook up a small bulb to a lemon cell and it will glow faintly, but this simply is not the case. 

Going Further: There are several investigations you can try.  First, see if you can observe any light from the LED with just two lemons, and then with one.  You may need to do this in a dark room.  If you have a voltmeter, you may want to measure the voltage of each cell and the total voltage.  You may need an adult to help you do this.  Many voltmeters will also measure current, and you might want to get an adult to show you how to do this as well.

Also, try making cells using oranges, grapefruit, or other citrus fruit.  You may also want to try making cells using small cups filled with vinegar, soft drink mix, or carbonated beverages.  If you do, make sure that the copper and zinc pieces in each cup are not allowed to touch each other. 

You may want to learn how voltage, current and resistance are related.  This is explained by a principle known as “Ohm’s Law”.  By learning to understand Ohm's Law, and using what you have already learned about electricity, you should be able to design one or more experiments for a science project that will increase your understanding of electricity.

You may also want to learn more about how an LED produces light.

Identifying the Terminals of a Cell or Battery Using a Potato

CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

Materials Needed:  Homemade battery holder with two AA, AAA, C or D cells; a raw potato.

Procedure:   Cut the potato into halves.  Make two slits in one of the potato halves about 2 cm (˝ in) apart and stick the two wires from the battery into the two slits.  Wait a few minutes.
What To Look For:  Do you see any change in the potato at either of the wires?  If so, which terminal is the wire connected to?

What Happened:  The copper wire from the positive  terminal  reacted with the starch in the potato, causing it to produce a greenish  color.

Going Further:  Try this with another type of fruit or vegetable?  Is there a color change?

You might not realize it, but electricity is very closely related to magnetism.  If you want to learn how, visit the Magnetism, Magnets and Electricity - Pt. 1 page.

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