Measuring voltage

An activity with potential

Introduction

Voltmeters measure the voltage difference between two points touched by their leads.

Material:

• A commercial voltmeter.
• 2, 1.5 volt cell such as AA "batteries".
• A solar cell.

To Do and Notice

Measure the voltage across one, "1.5 volt battery".

Details

What voltage does the battery claim to be? (1.5 volts)

Set the meter scale to volts, DC either 5 or 2 by turning the dial.

If you choose 5 then 5 volts will be full scale.

The 1.5 volts you expect to measure will be readable on these scales.

Read the answer from the arc of numbers which has 5 on the right side. Interpolate as needed.
Or read the number from the digital display.

Your answer is probably not 1.5 Volts but it should be close. The voltage across older batteries will be lower.

If it isn't see "Troubleshooting" below.

Measure the voltage produced by the solar cell.
How does the voltage change when the solar cell is illuminated by different amounts of light?
In general, notice that you measure slightly higher voltages when brighter light shines on the solar cell.

Measure the voltage across combinations of 2 batteries.

Each battery by itself will produce an increase in voltage of about 1.5 volts between its negative end and its positive end.

Try batteries: in series, with the positive end of one touching the negative end of the other; in opposition, with the positive end of one touching the positive end of another; and in parallel, with both positives and negatives joined.

Optional. Measure other cells such as C and D cells, notice that they produce the same voltage difference between their ends as do AA cells.

Troubleshooting

If the meter needle "goes the wrong way" then reverse the two meter leads, the red lead should go to the positive end of the cell&emdash;the end with the bump on it&emdash; while the black lead should go to the negative end. On digital meters you will get a minus sign in front of the voltage reading if the red positive lead is at a lower voltage than the black negative lead.

If your answer is not between 1.4 and 1.7 volts on a meter using a dial scale then you are probably reading the wrong arc of numbers, ask a neighbor for help. If they too get an answer far from 1.5 volts then try another battery. Test the battery first on another meter to make sure it gives the correct voltage. If the voltage is still way off try another meter.

What you have just done is called troubleshooting, in particular this is troubleshooting by substitution, and is one of the most useful skills an electronics technician can possess.

What's going on?

What is voltage?

We know from electrostatics experiments that there are positive and negative electric charges. These charges come in units equal to the charge on the proton &emdash; Milliken discovered this by watching the charge on tiny oil drops. The charge on the drops was always an integer multiple of one basic charge which was equal in magnitude to the charge on an electron or proton. The SI unit of charge is the coulomb, one coulomb, C, contains about 6.2 x 1018 proton charges. The coulomb is a collective term for charge just like a dozen or a gross are collective terms for eggs. So when I say "coulomb" picture 6 billion billion electric charges. ;-)

Like charges repel each other, so to bring one positive charge next to another I have to do work. The work I do forcing the charges together is stored as potential energy, electrical potential energy. The electrical potential energy turns into kinetic energy if I release the charge and allow it to be repelled away. Electrical potential energy, like all energy, is measured in joules. One joule is the energy needed to raise a quarter pounder hamburger patty (a mass of 0.1 kg) through a height of 1 meter near the earth.

The electrical potential energy per coulomb of charge is called the potential. Yikes! what a confusing name. Potential is not potential energy it is potential energy per charge. Another name for potential is voltage, V. As an analogy, compare the difference between the total price for the 2 kg of hamburger for tonight's dinner meatloaf and its price per kg. The total price \$5.24 corresponds to the total electrical potential energy the price per kg corresponds to the potential or the voltage. (By the way, the price per kg is \$2.62.) If I move a coulomb of electric charge to a place with a voltage that is higher by one volt then I must do one joule of work on the charges. One volt is one joule per coulomb.

A model for voltage is height. To raise a 1 kg mass to a height of 3 meters requires 30 joules of work. The mass then has 30 J of gravitational potential energy. (For those of you who like equations, U = mgh: potential energy, U, equals mass , m, times acceleration of gravity, g, times height change, h.) The gravitational potential is the energy per kilogram which is proportional to height. (U/m = gh). So in an electrical circuit you can think of the change in height of a mass as a model for voltage differences for charge.

The model continues. Just as balls with positive mass tend to roll downhill, positive electric charges move from places of high potential to places with lower potential. So positive charges move under electrical forces from the positive terminal of the battery through resistors to the negative terminal.

There are no negative masses, however, helium balloons in air behave a lot like negative masses. Balloons tend to rise when gravity pulls downward. So too, negative charges move from places with low voltage to places with higher voltage. Thus, negative electrons move outside of a battery from the negative or low voltage terminal to the positive or high voltage terminal.

How is voltage created?

The 1.5 volt batteries you are measuring are carbon-zinc cells (or similar cells). The metal case is lined with zinc which is attached to the negative terminal, there is a carbon rod in the center attached to the positive terminal. Between the carbon and the zinc there is a black paste made from manganese dioxide, carbon, and ammonium and zinc chlorides. In the cheapest batteries there is also a cylindrical flour and paste separator. Cells marked "heavy duty" have a larger amount of zinc chloride and a better mechanical design, that is they don't leak as easily. Alkaline cells on the other hand have a powdered zinc negative rod surrounded by a highly corrosive potassium hydroxide electrolyte surrounded in turn by manganese dioxide and carbon as an outer positive cathode.

You may cut open a carbon zinc cell with a hacksaw, but you must not cut open an alkaline cell due to the corrosive nature of the electrolyte. If you do cut open a carbon zinc battery, discard the paste. You may replace the paste with salt water or a lemon and re-use the carbon rod and the zinc metal in your own homemade electric cells.

Inside the battery positive charges move in the paste toward the positive electrode. They thus move against the electric force. The positive charges are moved by a chemical potential inside the cell. A battery converts chemical energy to electrical energy. In the height analogy, outside work must be done on a ball to raise it to the top of a hill. From the top of the hill the ball will roll back to the bottom without help. The outside work provided by your muscles in the case of the ball is like the chemical work done in the battery.

Etc.

The word battery, as in gun battery, really means a line of many things. The 1.5 volt cells we are using are really single cells and so are not really batteries, however, most people call them batteries. Nine-volt carbon-zinc batteries are made out of a stack of 6, 1.5 volt cells and so they really are batteries. Cut them open and see! You can even cut them open if they are alkaline cells.

 Scientific Explorations with Paul Doherty © 2000 2 August 2000