2 Basics of Matter

Static Electricity and Charge

Editors’ note: This section is derived from Derived from 18.2 Static Electricity and Charge: Conservation of Charge by OpenStax, Bobby Bailey

This piece of gold-colored amber from Malaysia has been rubbed and polished to a smooth, rounded shape.
Figure 1. Borneo amber was mined in Sabah, Malaysia, from shale-sandstone-mudstone veins. When a piece of amber is rubbed with a piece of silk, the amber gains more electrons, giving it a net negative charge. At the same time, the silk, having lost electrons, becomes positively charged. (credit: Sebakoamber, Wikimedia Commons)

What makes plastic wrap cling? Static electricity. Not only are applications of static electricity common these days, its existence has been known since ancient times. The first record of its effects dates to ancient Greeks who noted more than 500 years B.C. that polishing amber temporarily enabled it to attract bits of straw (see Figure 1). The very word electric derives from the Greek word for amber (electron).

Many of the characteristics of static electricity can be explored by rubbing things together. Rubbing creates the spark you get from walking across a wool carpet, for example. Static cling generated in a clothes dryer and the attraction of straw to recently polished amber also result from rubbing. Similarly, lightning results from air movements under certain weather conditions. You can also rub a balloon on your hair, and the static electricity created can then make the balloon cling to a wall. We also have to be cautious of static electricity, especially in dry climates. When we pump gasoline, we are warned to discharge ourselves (after sliding across the seat) on a metal surface before grabbing the gas nozzle. Attendants in hospital operating rooms must wear booties with aluminum foil on the bottoms to avoid creating sparks which may ignite the oxygen being used.

How do we know there are two types of electric charge? When various materials are rubbed together in controlled ways, certain combinations of materials always produce one type of charge on one material and the opposite type on the other. By convention, we call one type of charge “positive”, and the other type “negative.” For example, when glass is rubbed with silk, the glass becomes positively charged and the silk negatively charged. Since the glass and silk have opposite charges, they attract one another like clothes that have rubbed together in a dryer. Two glass rods rubbed with silk in this manner will repel one another, since each rod has positive charge on it. Similarly, two silk cloths so rubbed will repel, since both cloths have negative charge. Figure 2 shows how these simple materials can be used to explore the nature of the force between charges.

 

(a) Negatively charged cloth is attracted to the positively charged glass rod which is hanging by the thread. (b) A positively charged glass rod is hanging with a thread. When another positively charged glass rod brought close to the first rod it deflects due to the repulsive force. (c) Two negatively charged silk cloth brought close to each other repel each other.
Figure 2: A glass rod becomes positively charged when rubbed with silk, while the silk becomes negatively charged. (a) The glass rod is attracted to the silk because their charges are opposite. (b) Two similarly charged glass rods repel. (c) Two similarly charged silk cloths repel.

More sophisticated questions arise. Where do these charges come from? Can you create or destroy charge? Is there a smallest unit of charge? Exactly how does the force depend on the amount of charge and the distance between charges? Such questions obviously occurred to Benjamin Franklin and other early researchers, and they interest us even today.

Instructor’s Note

 

The structure of the atom will be discussed in more detail in a later section.

Charge Carried by Electrons and Protons

Franklin wrote in his letters and books that he could see the effects of electric charge but did not understand what caused the phenomenon. Today we have the advantage of knowing that normal matter is made of atoms, and that atoms contain positive and negative charges, usually in equal amounts.

Figure 3 shows a simple model of an atom with negative electrons orbiting its positive nucleus. The nucleus is positive due to the presence of positively charged protons. Nearly all charge in nature is due to electrons and protons, which are two of the three building blocks of most matter. (The third is the neutron, which is neutral, carrying no charge.) Other charge-carrying particles are observed in cosmic rays and nuclear decay, and are created in particle accelerators. All but the electron and proton survive only a short time and are quite rare by comparison.

Three electrons are shown moving in different direction around the nucleus and their motion is similar to planetary motion.
Figure 3: This simplified (and not to scale) view of an atom is called the planetary model of the atom. Negative electrons orbit a much heavier positive nucleus, as the planets orbit the much heavier sun. There the similarity ends, because forces in the atom are electromagnetic, whereas those in the planetary system are gravitational. Normal macroscopic amounts of matter contain immense numbers of atoms and molecules and, hence, even greater numbers of individual negative and positive charges.

The charges of electrons and protons are identical in magnitude but opposite in sign. Furthermore, all charged objects in nature are integral multiples of this basic quantity of charge, meaning that all charges are made of combinations of a basic unit of charge. Usually, charges are formed by combinations of electrons and protons. The magnitude of this basic charge is

 |q_e| = 1.602\times 10^{-19} \mathrm{C}

The symbol q is commonly used for charge and the subscript e indicates the charge of a single electron (or proton).

The SI unit of charge is the coulomb (C). The number of protons needed to make a charge of 1.00 C is

 1.00 \, \mathrm{C} \times \frac{1 \, proton}{1.602 \times 10^{-19} \, \mathrm{C}} = 6.25 \times 10^{18} \, \mathrm{protons}

Similarly,  6.25 \times 10^{18} electrons have a combined charge of −1.00 coulomb. Just as there is a smallest bit of an element (an atom), there is a smallest bit of charge. There is no directly observed charge smaller than  |q_e| , and all observed charges are integral multiples of  |q_e| .

Figure 4 shows a person touching a Van de Graaff generator and receiving excess positive charge. The expanded view of a hair shows the existence of both types of charges but an excess of positive. The repulsion of these positive like charges causes the strands of hair to repel other strands of hair and to stand up. The further blowup shows an artist’s conception of an electron and a proton perhaps found in an atom in a strand of hair.

A girl is touching a Van de Graaff generator with her hair standing up. A magnified view of her single hair is shown which is filled with electrons and protons.
Figure 4: When this person touches a Van de Graaff generator, she receives an excess of positive charge, causing her hair to stand on end. The charges in one hair are shown. An artist’s conception of an electron and a proton illustrate the particles carrying the negative and positive charges. We cannot really see these particles with visible light because they are so small (the electron seems to be an infinitesimal point), but we know a great deal about their measurable properties, such as the charges they carry.

Key Takeaways: Things Great and Small: The Submicroscopic Origin of Charge

With the exception of exotic, short-lived particles, all charge in nature is carried by electrons and protons. Electrons carry the charge we have named negative. Protons carry an equal-magnitude charge that we call positive. (See Figure 4.) Electron and proton charges are considered fundamental building blocks, since all other charges are integral multiples of those carried by electrons and protons. Electrons and protons are also two of the three fundamental building blocks of ordinary matter. The neutron is the third and has zero total charge.

Instructor’s Note

 

This fact that there are no observed free particles with less than  |q_e| of charge is important and will be used in some of your homework problems.

Separation of Charge in Atoms

Charges in atoms and molecules can be separated—for example, by rubbing materials together. Some atoms and molecules have a greater affinity for electrons than others and will become negatively charged by close contact in rubbing, leaving the other material positively charged. (See Figure 5.) Positive charge can similarly be induced by rubbing. Methods other than rubbing can also separate charges. Batteries, for example, use combinations of substances that interact in such a way as to separate charges. Chemical interactions may transfer negative charge from one substance to the other, making one battery terminal negative and leaving the first one positive.

When materials are rubbed together, charges can be separated, particularly if one material has a greater affinity for electrons than another. (a) Both the amber and cloth are originally neutral, with equal positive and negative charges. Only a tiny fraction of the charges are involved, and only a few of them are shown here. (b) When rubbed together, some negative charge is transferred to the amber, leaving the cloth with a net positive charge. (c) When separated, the amber and cloth now have net charges, but the absolute value of the net positive and negative charges will be equal.
Figure 5: When materials are rubbed together, charges can be separated, particularly if one material has a greater affinity for electrons than another. (a) Both the amber and cloth are originally neutral, with equal positive and negative charges. Only a tiny fraction of the charges are involved, and only a few of them are shown here. (b) When rubbed together, some negative charge is transferred to the amber, leaving the cloth with a net positive charge. (c) When separated, the amber and cloth now have net charges, but the absolute value of the net positive and negative charges will be equal.

No charge is actually created or destroyed when charges are separated as we have been discussing. Rather, existing charges are moved about. In fact, in all situations the total amount of charge is always constant. This universally obeyed law of nature is called the law of conservation of charge.

Play with the Simulation

Below is a simulation of a balloon and a sweater. As you probably know, if you rub a balloon on a sweater, it will stick to a wall.

A few things to note:

  • The total number of charges is conserved – electrons move from the sweater to the balloon.
  • If you have two balloons with negative charge, they will repel, just like in real life (check it for real if you don’t believe us!)
  • When you bring the balloon near the wall, what happens to the electrons in the wall?

 

 

Law of Conservation of Charge

The total charge is constant in any process. This law is truly universal and of such importance that we will revisit it again in a later section.

In more exotic situations, such as in particle accelerators, mass, \Delta m , can be created from energy in the amount using Einstein’s famous relation:

 \Delta m = \frac{E}{c^2}.

Sometimes, the created mass is charged, such as when an electron is created. Whenever a charged particle is created, another having an opposite charge is always created along with it, so that the total charge created is zero. Usually, the two particles are “matter-” counterparts. For example, an anti-electron would usually be created at the same time as an electron. The anti-electron has a positive charge (it is called a positron), and so the total charge created is zero. (See Figure 6.) All particles have antimatter counterparts with opposite signs. When matter and antimatter counterparts are brought together, they completely annihilate one another. By annihilate, we mean that the mass of the two particles is converted to energy E, again obeying the relationship

 \Delta m = \frac{E}{c^2}.

Since the two particles have equal and opposite charge, the total charge is zero before and after the annihilation; thus, total charge is conserved.

Instructor’s Note

 

We will be occasionally dealing with antimatter in this class. You need to know that matter and antimatter are identical in mass, but opposite in charge: an anti-electron has a positive charge. You also need to know that when matter and anti-matter come together the result is pure energy.

Here energy is shown by a vector. Initially electrostatic charge q tot is equal to zero. Now energy gets converted into matter and creates one electron and antielectron pair but final electrostatic charge is equal to zero so change in mass delta m is equal to two m e, which is equal to E divided by c square. (b) In this figure, Electron and antielectron are colliding with each other. The electrostatic charge q tot before collision is zero and after collision it will remain zero.
Figure 6: a) When enough energy is present, it can be converted into matter. Here the matter created is an electron–antielectron pair. (me is the electron’s mass.) The total charge before and after this event is zero. (b) When matter and antimatter collide, they annihilate each other; the total charge is conserved at zero before and after the annihilation.

 

Making Connections: Conservation Laws

Only a limited number of physical quantities are universally conserved. Charge is one—energy, momentum, and angular momentum are others. Because they are conserved, these physical quantities are used to explain more phenomena and form more connections than other, less basic quantities. We find that conserved quantities give us great insight into the rules followed by nature and hints to the organization of nature. Discoveries of conservation laws have led to further discoveries, such as the weak nuclear force and the quark substructure of protons and other particles.

The law of conservation of charge is absolute—it has never been observed to be violated. Charge, then, is a special physical quantity, joining a very short list of other quantities in nature that are always conserved. Other conserved quantities include energy, momentum, and angular momentum.

Section Summary

  • There are only two types of charge, which we call positive and negative.
  • Like charges repel, unlike charges attract, and the force between charges decreases with the square of the distance.
  • The vast majority of positive charge in nature is carried by protons, while the vast majority of negative charge is carried by electrons.
  • The electric charge of one electron is equal in magnitude and opposite in sign to the charge of one proton.
  • An ion is an atom or molecule that has nonzero total charge due to having unequal numbers of electrons and protons.
  • The SI unit for charge is the coulomb (C), with protons and electrons having charges of opposite sign but equal magnitude; the magnitude of this basic charge  |q_e| = 1.602 \times 10^{-19} \, \mathrm{C}
  • Whenever charge is created or destroyed, equal amounts of positive and negative are involved.
  • Most often, existing charges are separated from neutral objects to obtain some net charge.
  • Both positive and negative charges exist in neutral objects and can be separated by rubbing one object with another. For macroscopic objects, negatively charged means an excess of electrons and positively charged means a depletion of electrons.
  • The law of conservation of charge ensures that whenever a charge is created, an equal charge of the opposite sign is created at the same time.

 

A Deeper Structure of the Atom

This section is available both as a video and as text. Below, you see the video as well as a text transcript. The content is the same: read or watch as is your preference.

Instructor’s Note

 

The things you need to know for your homework and quizzes are:

  • Electrons and protons are charged, neutrons are not; the size of the charge on the electron and proton is the same, but the signs are different, so same magnitude different sign
  • Opposites attract and that is what holds the atom together
  • Protons and neutrons have the same mass, and electrons are way lighter
  • Protons and neutrons made of stuff, electrons are fundamental
  • The nucleus is super tiny relative to atom

 

You should be familiar with the basic structure of the atom, but as a review, in the middle of the atom the positively-charged protons and neutrons are huddled together in the nucleus.

The basic structure of the Atom.
The basic structure of the Atom.

However, you might not be familiar with the related symbols. This symbol,

p+p+{p} ^ {+}means proton, p for proton, and then plus to remind us that it has a positive electrical charge (If you’re wondering, can you have a negatively charged proton? Yes, it’s called an proton.) You also have neutrons. Neutron has zero charge, so it’s symbol is

n0n0{n} ^ {0}Those are huddled together in the nucleus, and then surrounding the nucleus is a big cloud of negatively charged electrons, so we will use the symbol

e-e-{e} ^ {-}for electron. It’s the attraction between the positively charged protons and the negatively charged electrons that sort of hold the entire atom together, and how that all works will be the emphasis of Unit III.

Electrons are a big focus of this course, so it is worth discussing what they are made of. To our best of our knowledge they are not made up of anything. They are fundamental, we have been trying to smash them apart, but no luck. Maybe it’s possible, but no one’s been able to do it. If it is possible, we haven’t hit it hard enough. That’s very much the particle physics approach to everything- hit it harder and see if it breaks. So, electrons are fundamental building blocks- as far as we know they’re not made up of anything.

Protons and neutrons on the other hand, are a lot more fun, because they are made up of smaller pieces called quarks. There are six kinds of quarks, we have ‘up’, ‘down’, ‘strange’, ‘charm’, ‘top’, and ‘bottom’. Those are their official scientific names, I kid you not. In Figure 7 below, the sizing of the circles shows you the masses, how heavy this stuff is (but not their sizes – as far as we know the quarks have zero size!). The ‘top’ quark, the heaviest of the known quarks, actually has about the same mass as an entire atom. It’s quite a heavy little thing. Three of these quarks, ‘top’, ‘bottom’, and ‘charm’, are actually heavier than protons.

 

Quark Masses as balls
Figure 7: The masses of the quarks: u for up, d for down, c for charm, s for strange, b for bottom, and t for top. Again, the size represents the mass, NOT the size; as far as we know all of these quarks have zero size! The grey ball in the lower left is a proton for scale. The small red dot inside the grey ball is an electron. (Credit: Incnis Mrsi [CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)])

But, if charms, bottoms, and tops are all heavier than protons and neutrons, so what makes up a proton in a neutron? Protons and neutrons are made up of just these two ‘ups’ and ‘downs’. So, a proton is made up of two ‘up’ quarks and one ‘down’ quark, a neutron on the other hand is two ‘down’ quarks and an ‘up’ quark as shown below in Figure 8.

Quarks
Quarks make up both protons and neutrons. Here you can see the smaller-and-smaller steps all the way down to the two ups and a down which make up the proton. (Credit: Finches & quarks [CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)])

Now you start doing math, so you need the proton to have +1 charge, the neutron to have 0 charge, you have two ‘ups’ and a ‘down’, and two ‘downs’ and an ‘up’. If you play with those numbers, what do you get? You have that ‘up’ quarks have a charge of

2323{2} over {3}that of the proton, and down quarks are

−13−13- {1} over {3}that of the proton. And this works out: think ‘up’ ‘up’ ‘down’ so that’s

23+23−13=+123+23−13=+1{2} over {3} + {2} over {3} – {1} over {3} =+1the charge of a proton, and it works out.

Similarly, think,

−13−13+23=0−13−13+23=0-{1} over {3}- {1} over {3} + {2} over {3} =0the charge of a neutron, they work out.

Instructor’s Note

 

What’s your big takeaway for this? Electrons are fundamental to our knowledge and cannot be broken apart. Protons and neutrons are made up of smaller stuff.

The other key things to know about atoms: protons and neutrons are very very close to the same mass, but neutrons are a tiny bit heavier, but not by much. Electrons on the other hand are way lighter than protons or neutrons. In fact, the electron is the lightest known particle to have electric charge with a mass of  9.11 \times 10^{-31} \, \mathrm{kg} . Protons on the other hand, are much bigger,  1.67 \times 10^{-27} \, \mathrm{kg} .

Instructor’s Note

 

What should you take away from this? Protons are way bigger than electrons, roughly 2,000 times (1836 times to be specific).

If you prefer to think about atoms instead of protons and neutrons, you can think about a Helium atom, you know there are two protons, two neutrons, and two electrons. The electrons make up 0.03% of the mass of helium. Electrons don’t weigh squat. They don’t really matter as far as mass goes. While the nucleus has most of the mass, it doesn’t take up a lot of space. The standard analogy that people make is if you blow up the atom to the size of a large college football stadium, bigger than ours, the nucleus is roughly the size of a marble. Atoms are a whole bunch of empty nothing. The nucleus is about the size of a pea but it is 99.97% of the mass is in that marble comparatively speaking.

More on Conservation of Electric Charge

This section is also available as a video. As usual the video and transcript are both below, the content is the same.

We are going to do examples using conservation of electric charge.

Let’s say I rub a plastic rod with some fur, basic friction, and I move about  10^6 electrons from the fur to the rod. What’s the charge of everything when we’re done?

Well we have  10^6 electrons going from the fur to the rod. Now charge has to be conserved. The rod is clearly going to end up with a negative charge from the added electrons, but since charge has to be conserved, and I took those electrons from the fur, my fur has to have an equal positive charge. Charges had to come from somewhere, and they came from the fur.

Now let’s talk about how much charge. Now let’s talk about how much charge. We know it’s  10^6 electrons worth. We know it’s  1.602 \times 10^{−19} \, \mathrm{C} for each electron. So, the rod will have a charge of:

 \left( 10^{6} \, \mathrm{electrons} \right) \times \left( \frac{-1.602 \times 10^{-19} \, \mathrm{C}}{\mathrm{electron}} \right) = -1.602 \times 10^{−13} \, \mathrm{C}

The rod will thus have a total charge of  q_{\mathrm{rod}} = -1.602 \times 10^{−13} \, \mathrm{C} (remembering that  q is the symbol we use for charge).

Since charge is conserved, the fur will have the exact same positive charge.

 

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Physics 132: What is an Electron? What is Light? by Roger Hinrichs, Paul Peter Urone, Paul Flowers, Edward J. Neth, William R. Robinson, Klaus Theopold, Richard Langley, Julianne Zedalis, John Eggebrecht, and E.F. Redish is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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