Materials Included In Kit

Additional Materials Required

Prelab Preparation

Experiment 2: Pith Ball Electroscope

Assembly

1. Hold the two pith balls together and pull both pieces of string taut.
2. With the balls next to each other and the string pulled tightly, tie both pieces of string together approximately 10 cm from the pith balls.
3. Screw the metal stand into the base.
4. Hang the string over the horizontal arm so that the knot rests on the arm and the pith balls hang next to each other (see Figure 12).
   {13479_Preparation_Figure 12}
5. Clip the excess string with scissors, if necessary.
6. Repeat steps 1–5 for the second pith ball electroscope.

Experiment 3: Measuring Cell Potentials

1. Use sandpaper or steel wool to buff the metal before cutting.
2. Use scissors to cut the metal strips into 1-cm² pieces. Each group should receive approximately 5 pieces of each metal.

Experiment 4: Resistance in Wires

1. Use a meter stick and scissors or wire cutters to measure and cut the wire, respectively.
2. Cut two 2-m long 28-gauge pieces, two 2-m long 14-gauge pieces and two 1-m long 28-gauge pieces.

Safety Precautions

Be cautious of the ends of the wires. Wear safety glasses when performing these experiments. Please follow all normal laboratory safety guidelines. For Experiment 3: Copper(II) sulfate solution is toxic by ingestion. Zinc sulfate solution is slightly toxic. Magnesium metal is a flammable solid; avoid contact with flames and heat. Metal pieces may have sharp edges—handle with care. Avoid contact of all chemicals with eyes and skin. Please review current Safety Data Sheets for additional safety, handling and disposal information. Students should wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash hands thoroughly with soap and water before leaving the lab. Please follow all normal laboratory safety guidelines.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Experiment 3: Save the metal ion solutions in the properly labeled bottles for future use. The solutions may be disposed of down the drain with excess water according to Flinn Suggested Disposal Method 26b. With the exception of magnesium, the metal pieces may be reused from lab to lab and from year to year. Rinse the metal pieces with distilled water, dry them thoroughly on paper towels, and store them in properly labeled zipper-lock plastic bags for future use. Magnesium metal ribbon should be freshly cut each year. Dispose of used magnesium pieces in the solid trash according to Flinn Suggested Disposal Method #26a. The materials from each lab should be saved and stored in their original containers for future use.

Lab Hints

Experiment 1: Series and Parallel Circuits

• Enough materials are provided in this kit for three student groups to work at the same lab station. This laboratory activity can reasonably be completed in one 50-minute class period. Extra lightbulbs are included.
• Students should refer to their textbooks for further information regarding electric circuits.
• To further extend the level of instruction, voltmeters and ammeters can be used on each circuit to determine the voltage drop and current flowing through each lightbulb for each circuit setup.
• To extend the lifetime of the lightbulbs, make sure students connect the circuits for short (less than 15 seconds) time increments. This is especially important during the parallel circuit experiments since more current will travel through the lightbulbs.

Experiment 2: Pith Ball Electroscope

• Enough materials are provided in this kit for two student groups of students to work at the same lab station. This laboratory activity can reasonably be completed in one 25-minute class period.
• Static electricity experiments always work best on a dry day. Lower humidity days are better than high humidity days. Air-conditioned air, or heated winter air tends to be drier, and more conducive for electrostatic demonstrations.
• Be sure to rub the friction rods with the wool sheet rapidly for at least 15 seconds in order to obtain a good charge on the rod.
• After continuous use, the wool sheet and friction rods may become permanently charged. It may be necessary to ground the wool piece or the friction rods occasionally in order to return them to a neutral state. Rubbing them on a grounded metal table or metal table leg is a good way to remove any accumulated charge.
• Typically, it is difficult to positively charge the electroscope by conduction. The electrons do not readily leave the pith balls unless there is a large “reservoir” for the electrons to go to, such as the “ground.” They do not readily flow into a positively charged friction rod.
• Determine the polarity of an unknown charge: Charge up the electroscope with a known charge (positive or negative). Then, bring a charged object near the charged electroscope. If the unknown charge from the object causes the pith balls to diverge further, then the unknown charge has the same polarity as the electroscope. If the unknown charge causes the pith balls to collapse, then the unknown charge has the opposite polarity.
• Recommended negatively charged test objects: plastic Beral-type pipets, plastic straws, rubber balloons, and PVC pipes make for excellent negatively charged rods when rubbed with wool, flannel or fur.
• Recommended positively charged test objects: Lucite^® friction rods, glass friction rods, glass stirring rods and curled-up overhead transparency sheets (acetate) make for fair positively charged rods when rubbed with wool or silk. (Thicker and longer materials will sustain a positive charge better.)

Experiement 3: Measuring Cell Potentials

• Enough materials are provided in this kit for four groups of students to work at the same lab station. Student groups should share the solutions in the dropper bottles. This laboratory activity can reasonably be completed in one 25-minute class period.
• The laboratory work and calculations for this microscale experiment can be completed in a typical 50-minute lab period. Beginning students often forget to record which metal is connected to the positive or negative lead and then struggle with identifying the anode and cathode. Fortunately, the micro-voltaic cells are endlessly reusable and the measurements are quick and easy. Encourage students to repeat their measurements if they are unsure of their results.
• Perform the following simple demonstration to show the relationship between a spontaneous redox reaction and the identity of the cathode and the anode in a voltaic cell. Place a strip of copper metal in a solution of zinc sulfate in one beaker, and a strip of zinc metal in a solution of copper sulfate in a second beaker. Only one reaction is spontaneous—the reaction of zinc metal with Cu(II) ions. A positive cell voltage will be obtained when copper is the cathode (attached to the positive lead on the voltmeter) and zinc is the anode (attached to the negative lead on the voltmeter).
• It may save valuable lab time for the teacher to sand larger sheets or strips of metal prior to class. With the exception of magnesium, however, most of the metal pieces may be stored and reused from year to year. Students should re-polish these metal pieces each year. To avoid mixing different metals that look similar, cut these metals into different shapes.
• Assuming the concentration of metal ions in each microcell is equal, the actual concentration (e.g., 0.1 M, 0.5 M, 1 M) should not affect the measured cell potentials. We have found, however, that the most consistent results were obtained using 1 M solutions of the metal sulfates.
• We tested several variations of the procedure for measuring cell potentials. Half-cells were constructed in 50- or 100-mL beakers and in microscale reaction plates, with salt bridges provided by strips of filter paper soaked in salt solution. The micro-voltaic cell procedure recommended in this write-up gave the most consistent results and was easiest to work with. Microscale reaction plates gave good results, but it was more difficult to hold the metals pieces stable in the wells when touching them with the instrument leads (especially with the voltage probe). Larger half-cells in 50-mL beakers generally gave less accurate results.
• Many textbooks use the equation Eº_cell = Eº_red (cathode) + Eº_ox (anode) to calculate cell potentials. This equation may foster the misconception that half-cell potentials are additive. This is not the case. Standard reduction potentials for individual half-cell reactions have no intrinsic or absolute meaning. Cell potentials are only defined by difference. That is why it is better to express the cell potential as the difference between the standard reduction potential of the cathode versus the anode (see Equation 1 in the Background section).

Experiement 4: Resistance in Wires

• Enough materials are provided in this kit for two student groups of students to work at the same lab station. This is a quick experiment and can reasonably be completed in approximately 15 minutes. The long wires make it difficult for more than two groups at one lab station. Extra lightbulbs and connector cords are provided.
• Make sure students do not cross the wires, hold the wires or allow them to touch anything metal.
• The 14-gauge wire is rigid. Bend into a large loop before class so students will not need to fuss with it during the experiment.

Teacher Tips

• Set up each lab station accordingly before class. Students should leave the stations as they find them before they move on to the next lab station.
• Before class, prepare copies of the student worksheets for each student. The Background information for each experiment can also be copied at the instructor’s discretion.

Sample Data

Experiment 1: Series and Parallel Circuits

Series Circuit
One Lightbulb

Bulb glows brightly.

Two Lightbulbs

Both lightbulbs glow, but less brightly than a single lightbulb connected to the battery.

Three Lightbulbs

All three lightbulbs glow very dimly. Can only see a tiny glow from each lightbulb filament.

Open Circuit

Disconnecting one lightbulb results in all three lightbulbs turning off.

Parallel Circuit
Two Lightbulbs

Each lightbulb glows nearly as brightly as a single lightbulb connected to the battery.

Three Lightbulbs

Each lightbulb glows slightly less brightly than two lightbulbs connected in parallel.

Open Circuit

When one lightbulb is disconnected from the circuit, the other two glow slightly brighter. It does not matter which lightbulb is disconnected. When two lightbulbs are disconnected, the third lightbulb glows slightly brighter; the brightness is close to that of a single bulb connected to the battery.

Two Lightbulbs and One Short Circuit

When the circuit is shorted, the two remaining lightbulbs glow very dimly or not at all.

Experiment 2: Pith Ball Electroscope

Charge by Induction

The pith balls move away from each other when the negatively charged rod is brought near them. Pith balls also become attracted to the charged rod and move towards it. The pith balls fall when the rod is removed. When the positively charged rod is brought near the pith balls, they diverge and move in the same manner.

Charge by Conduction

When the negatively charged rod is brought near the pith balls, the pith balls are initially attracted to the charged rod. When the charged rod touches the pith balls, they immediately “fly” away from the rod. Then the pith balls are no longer attracted to the charged rod, and they diverge from each other as they hang.
When the positively charged rod is brought near the charged pith balls, the pith balls are attracted to the positive rod. They remain diverged themselves, however.
When the pith balls are touched by a finger, the pith balls fall (they are now uncharged).

Experiement 3: Measuring Cell Potentials Experiment 4: Resistance in Wires

“Simple Circuit” Lightbulb Brightness

The lightbulb glows at a medium intensity. It does not appear to be glowing as brightly as it could. (Note: This experiment uses a 1.5-V battery with a 3.7-V lightbulb so the lightbulb will not burn out, or glow as brightly as it possibly can.)

Thick Wire Lightbulb Brightness

The lightbulb appears to glow about as brightly as it did in the “simple circuit.” It may be slightly dimmer.

Thin Wire Lightbulb Brightness

The lightbulb glows with about half the brightness as the “simple circuit.”

Thin Wire (0.5 m) Lightbulb Brightness

The lightbulb glows brighter than it did with the 1-m long thin wire, but not as brightly as it did with the 1-m long thick wire. The brightness is midway between the 1-m long thick and thin wires.

Answers to Questions

Experiment 1: Series and Parallel Circuits 

1. Which circuit design produced the brightest lightbulbs? Relate this to the amount of current flowing through each lightbulb.

   The single lightbulb connected to the battery glowed the brightest. This lightbulb had the most current flowing through it.

2. What happened when all three lightbulbs were connected in series? Why did this occur?

   When the three lightbulbs were connected in series, they were barely glowing. They did not glow brightly because the current traveling through them was very low. The resistance of the three lightbulbs in series was high and this lowered the total amount of current in the circuit and the lightbulbs did not receive the energy to glow brightly.

3. Is there more resistance in the series circuit or the parallel circuit? How can you tell?

   There is more resistance in a series circuit than a parallel circuit. The parallel circuit lightbulbs glowed brighter than the lightbulbs in series meaning more current traveled through them. The amount of current that travels through the lightbulbs is inversely related to the total resistance. The less resistance, the more current and the brighter the lightbulbs.

4. What is one advantage of a series circuit? What is one disadvantage?

   One advantage of a series circuit is that the circuit uses less current to light the lightbulbs because of the higher resistance. Less current means the circuit is safer, the battery will last longer (draining current reduces the life of the battery), and the circuit will not blow a fuse as readily. The circuit can be easily turned off by disconnecting any part of it. One disadvantage of a series circuit is that when one lightbulb goes out, they all go out. Another disadvantage is that adding more loads lowers the current in all the connected loads and this can decrease their performance.

5. What is one advantage of a parallel circuit? What is one disadvantage?

   One advantage of a parallel circuit is that when one lightbulb goes out, the others remain lit. Another advantage is that there is continued performance of appliances when one is added to or removed from the circuit. A disadvantage is that more current is drained from the power source with the addition of a new load. This current drain could cause a fuse to blow if there is enough heat generated. Another disadvantage is that the voltage rating on the appliances has to be able to handle the voltage of the power source.

6. What happened with the two parallel-connected lightbulbs and the short circuit?

   The lights dimmed because resistance in the shorted wire was very small so most of the current traveled through this branch, instead of through the more resistant lightbulb branches. Since most of the current was taken away from the lightbulb branches, the lightbulbs became much dimmer. [A short can cause a problem in a circuit because it can drain all the power from a power supply. Since all the current flows through the short, it can also cause a large amount of heat which can lead to a fire.]

7. Is it better to have a string of lights, such as Christmas-tree lights, connected in series, or parallel? Explain.

   A string of lights in parallel would have the advantage of not going out when one lightbulb goes out in the series. However, a string of lights in parallel would also draw a large amount of current that may blow a fuse. Also, the voltage rating on each lightbulb would have to match the voltage rating of the power supply (120 V for a lightbulb connected to an ordinary wall outlet). A series circuit would draw less current and would be safer. [Generally, a string of lights are connected in series and parallel. Lights connected in series are combined in parallel with another string of lights in series. This decreases the amount of current needed, and also prevents a search through all the lightbulbs if one lightbulb burns out. Only one section of the whole string of lights would need to be checked if one lightbulb is out.]

Experiment 2: Pith Ball Electroscope

1. What did the pith balls do when the positive and negative charges were brought near them? Did the pith balls respond differently to the positive and negative charges?

   The pith balls moved away from each other and towards the charged rod. No, the pith balls responded the same to the positively and negatively charged rods.

2. What did charging the electroscope by conduction do? (Optional) Draw a picture showing the charged electroscope.

   Charging the electroscope by conduction permanently charged the pith balls with the same charge that was on the charged rod. The pith balls repelled each other.

3. After the electroscope was charged by conduction, what charge (positive or negative) did the electroscope carry? How do you know?

   The charge on the electroscope, after it was charged by conduction, was negative (the same as the charged rod). This was verified when the positively charged rod was brought near the charged pith balls and the pith balls fell, indicating the electroscope had zero, or little, net charge.

4. Explain why the pith balls immediately “fly away” from the charged rod after making contact.

   When the pith balls make contact with the charged rod, they immediately attain the same charge (positive or negative) as the charged rod. Like charges repel so the same-charged pith ball is forced away from the charged rod.

Experiment 3: Measuring Cell Potentials

1. Which metal was most easily oxidized (it always appeared as the anode)? Which metal ions were most easily reduced (the corresponding metal always appeared as the cathode)?

   Magnesium was the most easily oxidized metal tested in this experiment (magnesium always acted as the anode).

2. Rank the three metals tested (including zinc) from most positive to most negative standard reduction potential. Write a general statement describing the relationship between the standard reduction potential of a metal and metal activity.

   From most positive to most negative reduction potential:
   Cu > Zn > Mg
   Metals with more negative standard reduction potentials are more active (more easily oxidized) than metals with more positive standard reduction potentials. Magnesium was the most active metal tested, and it had the most negative standard reduction potential.

3. Look up the literature values of the standard reduction potentials for Cu and Mg, and calculate the percent error for each. Hint: Note the symbol for “absolute value.”
   {13479_Answers_Equation_6}
   {13479_Answers_Table_4}
   Note: Cells featuring magnesium consistently showed voltages significantly less positive than their maximum calculated values. Metals closer together in the electrochemical series tended to give the most accurate results.

Experiment 4: Resistance in Wires

1. How does the brightness of the lightbulb relate to the resistance in the circuit?

   A brighter lightbulb indicates less resistance in the circuit. The brightness is proportional to the amount of current flowing through the lightbulb. According to Ohm’s law, when the voltage in the circuit is constant, a higher current (brighter lightbulb) means there is less resistance.

2. How did the length of the short wire affect the brightness of the lightbulb? What does this mean in terms of the resistance in the circuit?

   The shorter thin wire produced a brighter lightbulb. This means the resistance is lower when the shorter thin wire is used compared to the longer thin wire.

3. How did the thickness of the wire affect the brightness of the lightbulb? What does this mean in terms of the resistance in the circuit?

   The thicker wire produced a brighter lightbulb, indicating more current flowed through the lightbulb. Therefore, a thicker wire will have less resistance compared to a thin wire of the same length (and composition).

4. What are the two physical dimensions that affect the resistance in a wire? How do these physical dimensions affect the resistance in a wire?

   The length and the diameter of the wire affect its total resistance. The longer the wire, the more resistance is has (there is more material). However, a wire with a larger diameter will have less resistance compared to a wire of the same composition with a smaller diameter.

Introduction

This all-in-one Electricity Kit is designed to give students the opportunity to explore the fundamental properties of electricity, voltage and current. Four hands-on lab stations can be arranged so student groups can experiment with different aspects of simple circuits, batteries, resistance and static electricity.

Concepts

• Series circuits
• Parallel circuits
• Interpreting circuit diagrams
• Static electricity
• Conduction
• Induction
• Charge distribution
• Electron affinity
• Oxidation–reduction
• Standard reduction potential
• Voltaic cell
• Activity series of metals
• Electrical resistance
• Ohm’s law
• Wire gauge

Background

Experiment 1: Series and Parallel Circuits
Work in an electrical system is done by moving negatively charged particles called electrons. The movement of electrons in an electrical system is called electric current. Electric current cannot be seen because electrons are too small to be viewed, but its effect can be observed and measured. The motion of electrons traveling down a wire can be compared to the movement of water in a hose. Just like with water flowing through a hose, energy must be supplied to the electrons before they will move in a wire and provide energy to do work. The energy can be supplied by chemical means, such as with a battery, or by mechanical means, such as with a waterwheel in a river turning a generator. The amount of energy supplied to each electron passing through the electrical system is called voltage. Voltage can be compared to the potential energy (stored energy) of water that is contained in a water tower. Work is done on water to lift it into a water tower, giving water potential energy. When the water is released from the tower, it will provide the same amount of energy that was initially put into it. The water cannot provide additional energy above its initial potential energy. The potential energy is directly related to the height of the water tower. The taller the water tower, the more energy the running water can supply at the bottom. Voltage in an electrical system is similar to the height of the water tower. The negative terminal of a battery can be considered the top of a water tower where all the electrons have accumulated and are ready to flow down a wire. The positive terminal can be considered the bottom of the water tower. The negative electrons are attracted to the positive terminal, according to the fundamental principle that unlike electric charges attract each other. In order for the electrons to move from the high point (negative terminal) to the low point (positive terminal) and do useful work, there must be an unbroken path between the terminals of the power supply that will allow the electrons to flow. This unbroken path is called a circuit. When the path is broken, the circuit is open, and no electricity will flow.

In a simple direct current (DC) circuit, a load (also called appliances or resistors—e.g., lightbulbs, motors, clocks), is connected between the terminals of a power supply with conductive wires. The electrons travel from the negative terminal through the load, providing energy to operate it, and stop at the positive terminal. For an incandescent lightbulb, the energy from the flowing electrons causes the tungsten filament to heat up and produce visible light. The amount of work done on each load is determined by the voltage drop across it. The voltage drop is the energy removed from the electrical system per unit of charge passing through the load. The total voltage drop of all the loads in an electrical circuit will always be equal to the total voltage provided by the electrical power source. If a 9-V battery is connected to a circuit, the voltage drop through the entire circuit will always be 9 volts—no more, no less. For this to occur, it means that one load in a multiple load circuit cannot consume all the energy from the power source. The energy distributes itself throughout all the loads depending upon how many loads there are and how they are connected in the circuit. The voltage drop across an individual load in a circuit depends on its resistance and the amount of current that travels through it. Resistance is a measure of how difficult it is for the electrons (current) to travel through a load.

Generally speaking, the resistance of load is constant. Therefore, since the total voltage and resistance of each load are constant in a simple DC circuit, the total current through each load (and therefore the voltage drop through each load) will depend on how the loads are connected in the circuit. There are two ways to connect loads in simple DC circuits—in series and in parallel. Table 1 shows common symbols used in circuit diagrams to represent components in a circuit.

In a series circuit (see Figure 1), all of the loads are connected together in a line from the negative terminal to the positive terminal of the electric power supply. There is only one path for the current to travel and therefore the current is the same through each load. The total current in the circuit, and therefore the current flowing through each load, depends on the total resistance of the entire series circuit. The more loads that are connected in series, the higher the total resistance. The higher the resistance, the lower the total current traveling through the circuit and through each load. Since every load in a series circuit will receive the same current, the voltage drop across each load in a series circuit depends on its resistance. Appliances connected in parallel are coupled by separate wire branches that connect each appliance directly to the terminals of the power source (see Figure 2). Since each load is connected directly between the terminals of the power supply, the voltage drop through the load will equal the total voltage from the power supply. This is true for all the loads that are connected in parallel. Each load produces the same voltage drop, equal to the total voltage from the power supply, and independent of its resistance. However, the current in a parallel circuit can vary through each load. Since there are multiple pathways for the current to travel in a parallel circuit, the current that flows through each load will vary depending on its resistance. The higher the resistance, the lower the current that will travel through it. As a result of having multiple current pathways, the effective resistance of a complete parallel circuit is lower than the individual load resistances. The resistance of the loads has not changed, but the arrangement of the loads allows the current to travel more efficiently, and thereby decreases the effective resistance of the entire parallel circuit.

The effective resistance of a parallel circuit is always lower than the lowest-resistant load in the circuit. Since the effective resistance is lower, the total current that is drawn from a power supply, or battery, and flows through a parallel circuit will increase as more loads are added in parallel.

Experiment 2: Pith Ball Electroscope
Static electricity is a stationary electric charge. Atoms are composed of electrically charged particles: positively charged protons, negatively charged electrons and neutrons, which carry no charge. The positive and negative charges of protons and electrons, respectively, are equal in magnitude, so the combination of one proton and one electron results in an electrically neutral atom (a hydrogen atom). Generally speaking, most objects have an equal number of protons and electrons and are therefore considered electrically neutral. Since protons form the dense inner core of atoms, they are not able to move about freely within an object. Therefore, the positive charge in an object remains reasonably constant. Electrons, on the other hand, are not held in place by rigid bonds. The electrostatic attraction between electrons and protons keep the electrons moving closely around the protons, but the electrons are generally not “locked” into position. Electrons have the ability to migrate throughout a material, and therefore are referred to as being delocalized. Electrons can also be removed from an object leaving the object positively charged, or added to an object to give the object an excess negative charge. The ease with which the electrons in a material can do this depends on the atomic composition of the material.

An electroscope works well as a detector and storage unit of static electric charge because the electrons surrounding the pith balls are highly delocalized and easily influenced. This makes the electroscope a great conductor of electric charge. Electrons in the pith balls will readily migrate to different regions in response to an external static electric charge. The fundamental principle of electric charge is that like charges repel and unlike charges attract. Two positive or two negative charges will move away from one another, whereas a positive charge and a negative charge will move towards one another.

Therefore, if an external negative charge is brought toward the pith balls of the electroscope, the negatively charged electrons in the pith balls will be repelled and migrate away from the external negative charge. The electrons will accumulate on opposite sides of the two pith balls, leaving the sides in contact with an induced positive charge. The above process of charging the electroscope is called charging by induction. The positive and negative charges remain in the electroscope so it maintains a net charge of zero, and remains neutral. No electrons are actually transferred into or out of the electroscope. However, the unbalanced charge distribution causes the electroscope to be temporarily polarized. When the external charge is removed, the charges in the electroscope will once again become evenly distributed.

The electroscope can also gain or lose electrons to become permanently charged. This can occur when the electroscope is charged by conduction. When the electroscope is charged by conduction, a charged rod is brought into direct contact with the uncharged pith balls. The charge then redistributes throughout the rod and the pith balls as if they were one object. The pith balls will diverge because they both attain the same charge and repel one another. When the charged rod is removed, the pith balls will carry a charge of the same polarity as the charged rod. The charged rod has donated some of its charge to the pith balls, and therefore has lost some of its initial charge when it is removed by the electroscope.

A substance may acquire static-electric charge through contact with a different type of substance. When two different substances are rubbed across each other, frictional energy may be enough to remove a few electrons from an “electron-releasing” material and transfer them to an “electron-holding” material. When this happens, both substances become static-electrically charged. The material that loses electrons becomes positively charged and the material that gains electrons becomes negatively charged. The ability of one substance to hold onto electrons better than another when two different objects are rubbed together reflects differences in the atomic composition of different materials. Certain atoms give up electrons easily, while other substances hold onto electrons tightly. Typically, in the electrostatic sense, metals tend to hold onto their electrons more tightly than nonmetals. A list of the relative electron “holding” and “releasing” abilities of different common materials is shown in Table 2. If any two substances in Table 2 are rubbed together, the substance that is higher in the table will become negatively charged, while the material lower in the table will become positively charged. As an example, when rubber-soled shoes (Ebonite—a form of hard rubber) are rubbed along the carpet (wool), the rubber-soled shoes will retain and collect excess electrons from the carpet. As a result, the shoes (and you) become negatively charged and the carpet becomes positively charged. The electric shock you then receive when you grab a doorknob is the result of the surplus of electrons that have accumulated and redistributed throughout your body that “jump” toward the positively grounded doorknob, and thus reestablish a charge balance.

Objects do not always carry away a charge when they move past each other. Static charges continuously transfer between objects. Static charge may or may not accumulate depending on the conditions of the materials and the surrounding environment. In the shoe and carpet example above, the charge that transfers between the shoes and carpet can easily dissipate into the surrounding air, especially humid air, without the actual “feeling” of a shock. Also, electrons readily dissipate into the air at sharp points. The more curved an object is, the less likely the static charge will dissipate. This is why the electroscope has a large metal ball as the terminal for static charge transfer. The air inside the flask of the electroscope is a closed system and is stagnant, so any charge that dissipates off the foil leaves inside the flask will stay in the flask and maintain the overall charge of the electroscope. Experiment 3: Measuring Cell Potentials
In an oxidation–reduction reaction, electrons flow from the substance that is oxidized, which loses electrons, to the substance that is reduced, which gains electrons. In a voltaic cell, the flow of electrons accompanying a spontaneous oxidation–reduction reaction occurs via an external pathway, and an electric current is produced. What factors determine the ability of a voltaic cell to produce electricity?

The basic design of a voltaic cell is shown in Figure 4 for the net reaction of zinc and hydrochloric acid. The substances involved in each half-reaction are separated into two compartments connected by an external wire and a salt bridge.

Net Reaction: Zn(s) + 2H⁺(aq) → Zn²⁺(aq) + H₂(g)

Each half-reaction takes place at the surface of a metal plate or wire called an electrode. The electrode at which oxidation occurs is called the anode while the electrode at which reduction occurs is called the cathode. Electrons flow spontaneously from the anode (the negative electrode) to the cathode (the positive electrode). Charge buildup at the electrodes is neutralized by connecting the half-cells internally by means of a salt bridge, a porous barrier containing sodium nitrate or another electrolyte. Dissolved ions flow through the salt bridge to either electrode, thus completing the electrical circuit.

The ability of a voltaic cell to produce an electric current is called the cell potential and is measured in volts. If the cell potential is large, there is a large “electromotive force” pushing or pulling electrons through the circuit from the anode to the cathode. The cell potential for a spontaneous chemical reaction in a voltaic cell is always positive. The standard cell potential (Eº_cell) is defined as the maximum potential difference between the electrodes of an electrochemical cell under standard conditions—25 ºC, 1 M concentrations of ions, and 1 atm pressure (for gases). It is impossible to directly measure the potential for a single electrode. The overall cell potential for an electrochemical cell may be expressed, however, as the difference between the standard reduction potentials (Eº_red) for the reactions at the cathode and at the anode (Equation 1). The standard reduction potential is defined as the voltage that a reduction half-cell will develop under standard conditions when it is combined with the standard hydrogen electrode (SHE), which is arbitrarily assigned a potential of zero volts (Equation 2).

Eº_red (SHE) = 0

For the zinc/hydrochloric acid voltaic cell shown in Figure 1, the measured cell potential is equal to 0.76 V. Substituting this value and the zero potential for the SHE into Equation 1 gives a value of –0.76 V for the standard reduction potential of the Zn²⁺/Zn half-cell.

Eº_red (cathode) – Eº_red (anode) = Eº_cell
Eº_red (SHE) – Eº_red (Zn²⁺/Zn) = 0.76 V
0 – Eº_red (Zn²⁺/Zn) = 0.76 V
Eº_red (Zn²⁺/Zn) = –0.76 V

When two half-cells are combined in a voltaic cell, the reaction that has a more positive standard reduction potential will occur as a reduction, while the reaction that has a less positive (or negative) standard reduction potential will be reversed and will take place as an oxidation. In this experiment, electrochemical cells consisting of different metal ion/metal half-cells (e.g., copper(II) sulfate/copper metal versus zinc sulfate/zinc metal) will be tested. The “direction” of each reaction—the identity of the anode and the cathode—will be determined when a positive voltage is observed. A positive voltage means that the half-cells have been properly connected to the positive and negative leads on the voltmeter so that a spontaneous reaction will occur. Recall that in a voltaic cell, the positive electrode is the cathode (the site of reduction) and the negative electrode is the anode (the site of oxidation).

Experiment 4: Resistance in Wires
In a simple circuit, a load is connected between the terminals of a power supply with conductive wires. The materials that compose the load and wires are not perfect conductors and therefore the movement of the electrons between the terminals of the power supply is impeded. This impedance of electron movement is known as electrical resistance. The voltage drop between the terminals of the power supply provides the energy to move the electrons. The movement of the electrons (the current) is hindered by the resistance in the conductive wires and the load. In general, the resistance in an electrical circuit is constant. Therefore, the resistance in the electrical circuit can be written in the form of Equation 3.

ΔV = Potential difference
I = Current
R = Resistance

Equation 3 can be expressed in its familiar form, known as Ohm’s law: Electrical resistance uses the SI unit known as the ohm (Ω, the Greek letter omega), which is equal to a volt per ampere. Electrical resistance depends on the type of material, the length, the diameter, as well as the temperature of the material. Each type of material (e.g., aluminum, steel, graphite) affects the movement of electrons in its own unique way. This is known as resistivity (ρ). A longer piece of material will have more material for the electrons to travel through, and therefore the electrical resistance will increase. A larger diameter conductive wire will provide more pathways for the electrons to travel, and therefore will result in lower electrical resistance. In general, colder temperatures slow down the movement of the internal electrons of the materials and this helps to reduce the electrical resistance. Higher temperatures cause an increase in the internal vibrations, which results in more electron movement impedance. Semiconductors, however are an exception to this rule. Many semiconductors are more conductive (have less resistance) at higher temperatures. The resistance of a material can be expressed in the following equation:

ρ = resistivity
L = length
A = cross-sectional area

Experiment Overview

Experiment 1: Series and Parallel Circuits
The ability to use electrical energy to do work has significantly changed the way we live. The first step to understanding electricity is by studying the behavior of simple circuits. In the following experiments on series and parallel circuits, observe how the brightness of the lightbulbs varies as the number of lightbulbs increases, and as the connections with the batteries change. The brightness of the lightbulb is a quantitative measure of the amount of current traveling through the lightbulb.

Experiment 2: Pith Ball Electroscope
Examine the existence of static-electric charges. Charge the pith ball electroscope using friction rods and friction pads to produce positively or negatively charged rods. Then use the charged electroscope to test other objects and to determine their charge polarity after being rubbed with different materials.

Experiment 3: Measuring Cell Potentials
The purpose of this experiment is to measure cell potentials for a series of micro-voltaic cells. Individual half-cells will be constructed by placing a small piece of metal onto 1–2 drops of its metal ion solution on a piece of filter paper. A “salt bridge” between the half-cells will be provided by placing a few drops of aqueous sodium nitrate on the filter paper along the path connecting the half-cells. Voltages will be measured using a multimeter.

Experiment 4: Resistance in Wires
How does the length and thickness of a wire affect an electrical circuit? Learn about some of the properties of the electrical resistance in wires.

Materials

Safety Precautions

Wear safety glasses when performing this experiment. Be cautious of the ends of the wires. Copper(II) sulfate solution is toxic by ingestion. Zinc sulfate solution is slightly toxic. Magnesium metal is a flammable solid; avoid contact with flames and heat. Metal pieces may have sharp edges—handle with care. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the lab. Please follow all normal laboratory safety guidelines.

Procedure

Experiment 1: Series and Parallel Circuits

Series Circuits

1. Refer to the circuit diagrams shown in Figure 4.
   {13479_Procedure_Figure_4_Lightbulbs in series}
2. Connect the connector cords, lightbulb and batteries together according to Figure 4a. The alligator clips connect directly to the lamp receptacle terminals and the terminals of the battery holder.
3. Observe the lightbulb. Does it glow? How brightly? Record your observations in the Series and Parallel Circuits Worksheet. Note: In order to improve the lifetime of the lightbulbs, do not connect circuit for more than 15-second intervals.
4. Open the circuit by disconnecting one clip from a battery terminal.
5. Add another lightbulb to the circuit according to Figure 4b. Use another connector cord to connect the two lamp receptacles.
6. Reconnect the battery and observe both lightbulbs light up. Does either one glow as brightly as the original single lightbulb? Record your observations in the worksheet.
7. Open the circuit by disconnecting one clip from a battery terminal.
8. Add a third lightbulb to the circuit according to Figure 4c.
9. Reconnect the battery and observe the three lightbulbs. Do the lightbulbs light up? How bright are the bulbs compared to the first experiment (one bulb) and second experiment (two bulbs in series)? Record your observations in the worksheet.
10. Disconnect one alligator clip from the second lightbulb to create an open circuit. What happens to the lightbulbs? Record your observations in the worksheet.

Parallel Circuits

11. Refer to the circuit diagrams shown in Figures 5 and 6.
    {13479_Procedure_Figure_5_Lightbulbs in parallel}
    {13479_Procedure_Figure_6_Pin coupler arrangement for parallel circuit connection}
12. Connect the connector cords, lightbulbs, battery and couplers together according to Figures 5a and 6.
13. Observe the lightbulbs. Do they glow? How brightly compared to the original single lightbulb? Record your observations in the Series and Parallel Circuits Worksheet.
14. Open the circuit by disconnecting one clip from a battery terminal.
15. Add another lightbulb to the circuit according to Figure 5b. Two more connector cords will be necessary.
16. Reconnect the battery and observe the three lightbulbs. Do they glow as brightly as the original lightbulb? How does the brightness compare to two lightbulbs connected in parallel? Record your observations in the worksheet.
17. Disconnect one alligator clip from one of the lightbulb receptacles. Do the lightbulbs turn off? Do any lightbulbs remain on? If so, has their brightness changed? Record your observations in the worksheet.
18. Reconnect the clip, and then disconnect a clip from a different lightbulb. What happens? Record your observations in the worksheet.
19. Reconnect all the lightbulbs in parallel again so that all three are glowing (Figure 5b).
20. Disconnect both leads connected to one lightbulb. Touch these two leads together to create a short circuit. See Figure 5c for a diagram of the circuit. What happens to the other two lightbulbs? Record your observations in the worksheet.
21. Answer the Post-Lab Questions on the Series and Parallel Circuits Worksheet.

Experiment 2: Pith Ball Electroscope

Charge by Induction

1. Negatively charge the PVC rod by rapidly rubbing it with the piece of wool.
2. Bring the negatively charged PVC rod near the hanging pith balls (see Figure 7).
   {13479_Procedure_Figure_7}
   Do not allow the pith balls to come in contact with the charged rod (see Figure 8). Record observations in the Pith Ball Electroscope Worksheet. Move the charged rod away from the pith balls. Record observations in the Pith Ball Electroscope Worksheet.
   {13479_Procedure_Figure_8}
3. Positively charge the test tube by rapidly rubbing it with a piece of wool.
4. Repeat step 2 using the positively charged test tube.

Charge by Conduction

5. Recharge the PVC rod following step 2.
6. Bring the negatively charged PVC rod near the pith balls and allow them to make contact with the charged rod. Record observations in the Pith Ball Electroscope Worksheet.
7. Remove the PVC rod from the ball. What happens? Record observations in the Pith Ball Electroscope Worksheet.
8. Positively charge the test tube by rapidly rubbing it with a piece of wool.
9. Move the positively charged test tube close to, but not touching, the charged electroscope. Record observations in the Pith Ball Electroscope Worksheet.
10. Touch the pith balls with a free hand to discharge them.

Experiment 3: Measuring Cell Potentials

Part A. Cell Potentials versus Zinc as the Reference Electrode

1. Label a sheet of paper with the names of the three metals to be tested (copper, magnesium and zinc). Obtain one small piece of each metal and place it on the paper.
2. Polish the metal pieces with sandpaper or steel wool, if necessary, to obtain fresh, shiny surfaces. Wipe the metal strips clean with paper towels to remove any bits of steel wool adhering to the metal.
3. Obtain a piece of filter paper. Using a pencil, draw four small circles in a symmetrical pattern on the filter paper, and connect the circles by means of a path of dots, as shown in Figure 9.

4. Using scissors, cut wedges between the circles and remove the wedges. Label the circles Cu, Mg and Zn (see Figure 10).
   {13479_Procedure_Figure_10}
5. Place the labeled filter paper in a Petri dish.
6. From the corresponding dropper bottle, place 1–2 drops of metal ion solution onto its corresponding circle on the filter paper (e.g., copper(II) sulfate on Cu). Note: Add more drops of metal ion solution to each circle as needed during the course of the experiment when the paper dries.
7. Using forceps, place each metal on the “wet spot” in the appropriate circle on the filter paper. Wipe the forceps clean with a paper towel for each metal to avoid contaminating the metals with other metal ions.
8. From the dropper bottle, add several drops of sodium nitrate solution all along the path of dots connecting the metals. Be sure there is a continuous trail of sodium nitrate solution between each circle and the center. Note: It may be necessary to add more sodium nitrate along the path of dots as the paper dries during the course of the experiment.
9. Zinc metal (Eº_red = –0.76 V) will be used as the “reference electrode” in Part A. Place the positive lead from the multimeter or voltage probe on the piece of zinc and the negative lead on the piece of copper metal. Read the live voltage. If the voltage drops to 0.00 V or if a negative voltage is displayed, reverse the leads—place the negative lead on the zinc and the positive lead on the copper.
10. In the data table, record which metal is the positive electrode (cathode) and which metal is the negative electrode (anode) when a positive voltage is obtained.
11. When the voltage reading stabilizes, record the positive voltage in the data table.
12. Repeat steps 9–11 to measure the cell potentials for the other metal (magnesium) versus zinc as the reference electrode. Remember to record which metal is the (+) electrode and which metal is the (–) electrode when a positive voltage is obtained. Replenish the metal ion solutions or the salt bridge (sodium nitrate) solution as needed during the course of the experiment if the paper dries out.
13. Using Equation 1 in the Background section, calculate the experimental value of the standard reduction potential (Eº_red) for each metal and record the result in the data table. Recall that E°_red for zinc is equal to –0.76 V.

Part B. Predicted and Measured Cell Potentials

14. Use the experimental Eº_red values of copper and magnesium to predict the cell potential for a voltaic cell made up of these two half-cells. Show the calculation and record the predicted value of Eº_cell in the data table. Note which metal should be the anode and the cathode.
15. Measure the cell potential for the voltaic cell between copper and magnesium and record the value in the data table.
16. Dispose of the material according to your instructor’s direction.

Experiment 4: Resistance in Wires

1. Obtain the two different gauge wires.
2. Assemble the lamp receptacle, lightbulb, battery, connector cords and wires as shown in Figure 11.
   {13479_Procedure_Figure_11}
3. First, complete a “simple circuit” between the battery and the lightbulb. Record the brightness of the lightbulb in the Resistance Worksheet. Note: In order to improve the lifetime of the lightbulb, do not connect the circuit for more than 15-second intervals.
4. Next, connect the 2 m long thicker wire between the lightbulb and the battery. Note: Make sure the wire does not make contact with itself if the wires cross, do not hold onto the wire, or allow the wire to touch other metal objects. Record the brightness, relative to the “simple circuit” from step 3, in the Resistance Worksheet.
5. Then connect the 2 m long thinner wire between the lightbulb and the battery. Note: Make sure the wire does not make contact with itself if the wires cross, do not hold onto the wire, or allow the wire to touch other metal objects. Record the brightness, relative to the “simple circuit” from step 3, in the Resistance Worksheet.
6. Finally, connect the 1 m long thinner wire between the lightbulb and the battery. Record the brightness, relative to the “simple circuit” from step 3, in the Resistance Worksheet.

Student Worksheet PDF

13479_Student1.pdf