Electricity and Magnetism

Though electronic devices weren't common until the $20^\text{th}$ century, people were aware of basic electric phenomena for much longer. The ancient Greeks traded goods for a kind of precious stone they called elektron — a fossilized pine resin that we now call amber.

Thales of Miletus, a Greek philosopher, documented a peculiar property of amber: when rubbed with animal fur, it attracts light objects like bits of leaf and sand. Much later, when $17^\text{th}\text{-}$century scientists experimented with the same phenomenon, they used a word with the same root to describe the clingy result: electricity.

In the day to day, electric forces are mostly hidden from your senses because electrons and protons usually remain evenly mixed within matter.

But sometimes this balance is disrupted and we can see electric forces at work before our very eyes!

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When a piece of amber is rubbed with fur, we say that it becomes electrically charged. Amber's clingy behavior didn't make much sense to Thales of Miletus, or even to the electric pioneers of the $17^\text{th}$ century.

They had no knowledge of subatomic particles like electrons and protons, and materials that behave this way were uncommon, as they are today.

In this quiz, we'll consider an effect similar to what Thales observed. In the following video, we're going to walk you through charging a piece of adhesive tape:

To answer some of the questions in this quiz, you can either build your own setup using the common materials listed in the video, or you can watch the embedded video clips of demonstrations.

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Now that we've prepared a tape strip, how can you tell that it's electrically charged?

Watch what happens as we slowly move a piece of aluminum foil toward the piece of tape.

You'll see the tape move as the foil gets close to it. This is an example of an electric force. Electric forces may be either attractive (like Earth's gravity) or repulsive (like two north poles of a magnet). How would you characterize the force between the charged tape and the foil?

Do a test yourself, or you can watch this video:

Note: if the tape-strip experiment doesn't work for you, try cleaning your surface before you try again. If it still doesn't work, try moving the experiment outdoors.

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Have you ever had a mishap with clingy laundry? Been shocked by a doorknob? Had trouble getting your hair to stay down?

If so, then you've fallen victim to the buildup of static charge. The force between the tape and the foil, and Thales' charged amber, are intentional demonstrations of the same principle.

Electrons and protons are packed into the matter around us — like the evenly mixed grains of red and yellow sand in the previous quiz. But sometimes this balance can be disrupted.

Rubbing a piece of amber with wool leads to a buildup of electrons on the amber, just as adding yellow grains to an orange sand pile makes the pile more yellow.

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We can test whether it's true that the charging of a tape strip, a piece of amber, or anything else amounts to a transfer of subatomic charged particles:

• Charge a tape strip by ripping it off of a clean surface (you can even use the same piece of tape) and check that it's charged.
• Then drag your fingers over the surface of the charged tape strip.

What does touching the charged strip with your fingers do to the charge on it?

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Repeated contact with your fingers restores the balance between the electrons and protons within the strip, bringing it back to a neutral configuration. In other words, the tape gets discharged.

But you might be wondering, "Did the charged strip have more electrons or more protons?"

Based on the tape and foil observations you made in this quiz (and only these observations), can you tell if the charged tape has more protons than electrons, or more electrons than protons?

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Some materials are more reliable than others when it comes to holding onto electric charge. Since the mid-1700's chemists and physicists have organized various materials' ability to become charged in what is known as the triboelectric series.

Some materials on the triboelectric series

The triboelectric series shows how pairs of materials exchange electrons when they are in contact. Materials on the left (yellow) side of the series tend to gain extra electrons after they are rubbed by materials on the right (red) side.

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In the video below, we take two materials from the series — PVC (vinyl) and wool — and rub them together to move some foil strips and paper pieces with the electric force.

Based on the triboelectric series, what would happen if you held the charged PVC to the charged tape?

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When the triboelectric series was introduced, it was a purely empirical result based on observing electrical interactions between objects like charged tape or amber. Until recently the triboelectric series has had no theoretical basis, but with cutting-edge microscopy techniques, scientists are investigating correlations between the molecular structures of different materials and their position on the series.

The triboelectric series isn't without applications. Buildup of static charges in certain situations can be dangerous. For example, air rushing over a vehicle as it speeds down the highway can lead to charge buildup, and if sudden discharge (in the form of an arc or spark) were to ignite gasoline vapors during refueling, a life-threatening explosion may occur.

In this quiz, we've looked at some static electrical effects in which charges accumulate on the surface of materials. We've implicated electrons in these phenomena, but a material can become charged even when no electrons are transferred to it.

In the next quiz, we'll explore how this is possible.

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