Electric Charge in the Atom

Atoms contain negatively charged electrons and positively charged protons; the number of each determines the atom’s net charge.


Key Takeaways

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Overview of Atomic Electrical Charges

All molecules contain three types of particles: protons, neutrons, and electrons.Protons and electrons have a net electric charge, while neutrons are neutral and do not have a charge.

Both protons and electrons have charge that is quantized. That is, the magnitude of their respective charges, which are equal each other, is 1. This standard value is equal to approximately 1.6×10-19 Coulombs.

Protons

Protons are found in the center of the atom; they, with neutrons, make up the nucleus. Protons have a charge of +1 and a mass of 1 atomic mass unit, which is approximately equal to 1.66×10-24 grams. The number of protons in an atom defines the identity of the element (an atom with 1 proton is hydrogen, for example, and an atom with two protons is helium). As such, protons are relatively stable; their number rarely changes, only in the instance of radioactive decay.

Electrons

Electrons are found in the periphery of the atom and have a charge of -1. They are much smaller than protons; their mass is \frac{1}{1836} amu. Typically in modeling atoms, protons and neutrons are regarded as stationary, while electrons move about in the space outside the nucleus like a cloud. The negatively charged electronic cloud indicates the regions of the space where electrons are likely to be found. The electrons cloud patterns are extremely complex and is of no importance to the discussion of electric charge in the atom. More important is the fact that electrons are labile; that is, they can be transferred from one atom to the next. It is through electronic transfer that atoms become charged.

Ions

In the ground state, an atom will have an equal number of protons and electrons, and thus will have a net charge of 0. However, because electrons can be transferred from one atom to another, it is possible for atoms to become charged. Atoms in such a state are known as ions.

If a neutral atom gains an electron, it becomes negative. This kind of ion is called an anion.

If a neutral atom loses an electron, it becomes positive. This kind of ion is called a cation.

The steady flow of electrons is called current. Current is what flows through electrical wires and powers electronics items, from light bulbs to televisions.


Planetary Model of an Atom: Small electrons orbit the large and relatively fixed nucleus of protons and neutrons.


Key Takeaways

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Properties of Electric Charge

Electric charge, like mass and volume, is a physical property of matter. Its SI unit is known as the Coulomb (C), which represents 6.242×1018e, where e is the charge of a proton. Charges can be positive or negative; a singular proton has a charge of 1.602×10−19 C, while an electron has a charge of -1.602×10−19 C.

Invariance

Like mass, electric charge in a closed system is conserved. As long as a system is impermeable, the amount of charge inside it will neither increase nor decrease; it can only be transferred. However, electric charge differs from other properties—like mass—in that it is a relativistic invariant. That is, charge is independent of speed. The mass of a particle will rise exponentially as its speed approaches that of light, its charge, however, will remain constant.

The independence of electric charge from speed was proven through an experiment in which one fast-moving helium nucleus (two protons and two neutrons bound together) was proven to have the same charge as two separate, slow-moving deuterium nuclei (one proton and one neutron bound together in each nucleus).

Attraction and Repulsion

When electric charge is present, it produces forces that can attract or repel matter.While mass also produces forces that can attract and repel matter, it only produces forces that attract.Nonetheless, the formula for describing the interactions between charges and masses is strikingly similar.When electric fields are involved, the force (F) is proportional to q1, q2, and r between the charges:

\text{F}=\frac{1}{4\pi \epsilon_0}\frac {\text{q}_1\text{q}_2}{\text{r}^2}

This is a constant equation where * and epsilon_0 represent constants.It is also known as Coulomb's law.


*

Coulomb’s Law: The forces (F1 and F2) sum to produce the total force, which is calculated by Coulomb’s Law and is proportional to the product of the charges q1 and q2, and inversely proportional to the square of the distance (r21) between them.


It takes the same form as Coulomb's Law, but is based on the product of two masses (rather than the charge) and uses a different constant.Both act in a vacuum and are central (dependent only on distance between forces) and conservative (unaffected by path taken).If one compares similar terms, it should be noted that charge-based interaction is substantially greater than mass-based interaction.Electric repulsion between two electrons, for example, is about 1004 times greater than their gravitational attraction.


Charge Separation

Charge separation, often referred to as static electricity, is the building of space between particles of opposite charges.


Key Takeaways

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Atoms are composed of protons and electrons, and are negative in charge.An atom is in its ground state when its protons and electrons are equal in number, and it has no permanent dipole.Since electrons are labile (i.e., can be transferred from atom to atom), it is possible for the phenomenon of "charge separation" (often referred to as static electricity) to occur.


Static Electricity: Due to friction between her hair and the plastic slide, the girl on the left has created charge separation, resulting in her hair being attracted to the slide.


In chemistry, this charge separation is illustrated simply by the transfer of an electron from one atom to another as an ionic bond is formed. In physics, there are many other instances of charge separation that cannot be written as formal chemical reactions. Consider, for example, rubbing a balloon on your hair. Once you pull the balloon away, your hair will stand on end and “reach” towards the balloon. This is because electrons from one have transferred to the other, causing one to be positive and the other to be negative. Thus, the opposite charges attract. A similar example can be seen in playground slides (as shown in ).

Charge separation can be created not only by friction, but by pressure, heat, and other charges. Both pressure and heat increase the energy of a material and can cause electrons to break free and separate from their nuclei. Charge, meanwhile, can attract electrons to or repel them from a nucleus. For example, a nearby negative charge can “push” electrons away from the nucleus around which they typically orbit. Charge separation occurs often in the natural world. It can have an extreme effect if it reaches a critical level, whereat it becomes discharged. Lightning is a common example.


Polarization

Dielectric polarization is the phenomenon that arises when positive and negative charges in a material are separated.


Key Takeaways

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The concept of polarity is very broad and can be applied to molecules, light, and electric fields. For the purposes of this atom, we focus on its meaning in the context of what is known as dielectric polarization—the separation of charges in materials.

Dielectrics

A dielectric is an insulator that can be polarized by an electric field, meaning that it is a material in which charge does not flow freely, but in the presence of an electric field it can shift its charge distribution. Positive charge in a dielectric will migrate towards the applied field, while negative charges will shift away. This creates a weak local field within the material that opposes the applied field.

Depending on their dielectric constant, different materials will respond differently to an induced field.The extent to which they become polarized (this constant determines their polarizability).

Atomic Model

An understanding of dielectrics is based on their charged components: protons and electrons.When an electric field is applied to an atom, the electrons will migrate away from the field.Protons remain exposed to the applied field.They create a dipole moment, as represented in .


*

The electrons in an atom drift away from an electric field when E is applied.Their average position is displaced by a distance d from the average position of the protons.This is represented by M, the dipole moment.


Dipole Polarization

At the molecular level, both dipoles and ions can undergo polarization.Electrons are more attracted to one nucleus than to the other in polar bonds.


Water Molecule: Water is an example of a dipole molecule, which has a bent shape (the H-O-H angle is 104.45°) and in which the oxygen pulls electron density away from the H atoms, leaving the H relatively positive and the O relatively negative.


When a dipolar molecule is exposed to an electric field, the molecule will align itself with the field, with the positive end towards the electric field and the negative end away from it.

Ionic Polarization

Ionic compounds are those that are formed from permanently charge-separated ions. For example, table salt (NaCl) is formed from Na+ and Cl– ions that are not formally bound to one another through a chemical bond, but interact very strongly due to their opposite charges.

Ions are still free from one another and will naturally move at random. If they happen to move in a way that is asymmetrical, and results in a greater concentration of positive ions in one area and a greater concentration of negative ions in another, the sample of ionic compound will be polarized—a phenomenon is known as ionic polarization.


Static Electricity, Charge, and the Conservation of Charge

Electric charge is a physical property that is perpetually conserved in amount; it can build up in matter, which creates static electricity.


Learning Objectives

Formulate rules that apply to the creation and the destruction of electric charge


Key Takeaways

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Electric charge is a physical property of matter. It is created by an imbalance in a substance’s number of protons and electrons. The matter is positively charged if it contains more protons than electrons, and it is negatively charged if it contains more electrons than protons. In both instances, charged particles will experience a force when in the presence of other charged matter.

Charges of like sign (positive and positive, or negative and negative) will repel each other, whereas charges of opposite sign (positive and negative) will attract each another, as shown in.


*

Charge Repulsion and Attraction: Charges of like sign (positive and positive, or negative and negative) will repel each other, whereas charges of opposite sign (positive and negative) will attract each other.


Approximately 6.24 times 10*[18] elementary charges are equivalent to one Coulomb (C), which is the SI unit for charge.The charge of a proton or electron is called an elementary charge.

Conservation of Charge

The charge, like the matter, is essentially constant within the universe and over time.Electric charge cannot be created or destroyed according to the charge conservation principle in physics.Electricity has always been conserved because it is the net amount of positive charge minus the net amount of negative charge in the universe.

For any finite volume, the law of conservation of charge (Q) can be written as a continuity equation:

\text{Q}(\text{t}_2)=\text{Q}(\text{t}_1)+\text{Q}_{\text{in}}-\text{Q}_{\text{out}}

where Q(t1) is the charge in the system at a given time, Q(t2) is the charge in the same system at a later time, Qin is the charge that has entered the system between the two times, and Qout is the amount of charge that has left the system between the two times.

Positive and negative charges can be created and destroyed individually, however.The electric charge is carried by subatomic particles such as electrons and protons. These particles can be created and destroyed.For example, when particles are destroyed, there is an equal amount of positive and negative charges destroyed, keeping the net charge the same.

Static Electricity

If there is an excess of electric charge on an object's surface, static electricity will occur.In friction, materials are in contact with each other, heat or pressure is built up, or a charge is present.You can also create static electricity through friction between a balloon and hair (see ).As a result of increased pressure in storm clouds, lightning (see ) is the discharge that occurs when the charge exceeds a critical concentration.


Static Electricity: Due to friction between her hair and the plastic slide, the girl on the left has created charge separation, resulting in her hair being attracted to the slide.


Lightning: Lightning is a dramatic natural example of static discharge.


Key Takeaways

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Overview

All materials can be categorized as either insulators or conductors based on a physical property known as resistivity.

An insulator is a material in which, when exposed to an electric field, the electric charges do not flow freely—it has a high resistivity. Conversely, a conductor is a material that permits the flow of electric charges in one or more directions—its resistivity is low.

Conductors

All conductors contain electric charges that, when exposed to a potential difference, move towards one pole or the other. The positive charges in a conductor will migrate towards the negative end of the potential difference; the negative charges in the material will move towards the positive end of the potential difference. This flow of charge is electric current.

Ionic substances and solutions can conduct electricity, but the most common and effective conductors are metals. Copper is commonly used in wires due to its high conductivity and relatively inexpensive price. However, gold-plated wires are sometimes used in instances in which especially high conductivity is necessary.

Every conductor has a limit to its ampacity, or amount of current it can carry. This usually is the current at which the heat released due to resistance melts the material.

Insulators

Insulators are materials in which the internal charge cannot flow freely, and thus cannot conduct electric current to an appreciable degree when exposed to an electric field.

While there is no perfect insulator with infinite resistivity, materials like glass, paper and Teflon have very high resistivity and can effectively serve as insulators in most instances.

Just as conductors are used to carry electrical current through wires, insulators are commonly used as coating for the wires.

Insulators, like conductors, have their physical limits. When exposed to enough voltage, an insulator will experience what is known as electrical breakdown, in which current suddenly spikes through the material as it becomes a conductor.


Conductor and Insulator in a Wire: This wire has a copper core (an insulator) and a polyethylene coating (a conductor).Copper allows current to flow through the wire, while polyethylene prevents it from escaping.


Key Takeaways

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The Oil-Drop Experiment

The Oil-Drop Experiment, otherwise known as the Millikan Oil-Drop Experiment, is one of the most influential studies in the history of physical science.

Performed by Robert Millikan and Harvey Fletcher in 1911, the experiment was designed to determine the charge of a single electron, otherwise known as the elementary electric charge.

Millikan designed his experiment to measure the force on oil droplets between two electrodes.

He used an atomizer to spray a mist of tiny oil droplets into a chamber, which included a hole. Some droplets would fall through this hole and into a chamber, where he measured their terminal velocity and calculated their mass.

Millikan then exposed the oil droplets to X-rays, which ionized molecules in the air and caused electrons to attach to the oil droplets, making the droplets charged.The top and bottom of the chamber were connected to a battery, which produced an electric field that acted on the charged oil drops.

Adjusting the voltage perfectly, Millikan was able to balance the force of gravity (which was exerted downward) with the force of the electric field on the charged particles (which was exerted upward), causing the oil droplets to be suspended in mid-air.


Fig. 1. Millikan's oil drop experiment: two parallel metal plates are used here.They are connected by an electric field.Each hole on the ring allows for illumination, and one hole permits microscopical viewing.Spraying special oil into the chamber, which then becomes electrically charged, gives droplets of the oil electrical charge.Droplets form between the plates when the voltage is switched between them.


Next, Millikan calculated a particle's charge in midair.He assumed the force of gravity, which is the product of mass (m) and gravitational acceleration (g), was equal to the force of the electric field (the product of charge (q) and electric field (E)):

\text{q}\cdot \text{E}=\text{m}\cdot \text{g}

\text{q}=\frac {\text{m}\cdot \text{g}}{\text{E}}

Since he already knew the mass of the oil droplets and the acceleration due to gravity (9.81 m/s^2), as well as the energy of the x-rays he was using, he was able to calculate the charge.

While Millikan was unsure of the charge of each oil droplet, he adjusted the power of the X-rays ionizing the air and measured many values of (q) for many different types of oil drops.The smallest electric charge measured in each case was 1.5924(17)×10−19 C.This suggests the elementary electric charge is 1.5924(17)×10−19 C.

We received very accurate results.The Oil-Drop Experiment calculates a value which differs from the accepted value of 1.602176487(40)*10*19 C by less than one percent.

Oil-Drop Experiment was immensely influential at that time, not only for determining the charge of an electron, but for showing that particles smaller than atoms exist.Back then, protons, neutrons, and electrons were still not widely accepted.