# 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.

#### Key Points

• Electric charge is a physical property of matter created by an imbalance in the number of protons and electrons in a substance.

• Charge can be created or destroyed. However, any creation or elimination of charge occurs at a ratio of 1:1 between positive and negative charges.

• Static electricity is when an excess of electric charge collects on an object's surface.

#### Terms

• the act of releasing an accumulated charge

• A quantum number that determines the electromagnetic interactions of some subatomic particles; by convention, the electron has an electric charge of -1 and the proton +1, and quarks have fractional charge.

• an electric charge that has built up on an insulated body, often due to friction

#### Figures

1. ##### 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.

2. ##### Static Electricity in a Slide

Friction between the girl's hair and the slide results in a transfer of electrons, which causes the hair and slide to be attracted to one another.

3. ##### Lightning

Lightning is a dramatic natural example of static discharge.

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 Figure 1.

The SI unit for charge is the Coulomb (C), which is approximately equal to 6.24 · 1018 elementary charges. (An elementary charge is the charge of a proton or electron.)

## Conservation of Charge

Charge, like matter, is essentially constant throughout the universe and over time. In physics, charge conservation is the principle that electric charge can neither be created nor destroyed. The net quantity of electric charge, the amount of positive charge minus the amount of negative charge in the universe, is always conserved.

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

$Q(t_2)=Q(t_1)+Q_{in}-Q_{out}$

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.

This does not mean that individual positive and negative charges cannot be created or destroyed. Electric charge is carried by subatomic particles such as electrons and protons, which can be created and destroyed. For example, when particles are destroyed, equal numbers of positive and negative charges are destroyed, keeping the net amount of charge unchanged.

## Static Electricity

Static electricity is when an excess of electric charge collects on an object's surface. It can be created through contact between materials, a buildup of pressure or heat, or the presence of a charge. Static electricity can also be created through friction between a balloon (or another object) and human hair (see Figure 2). It can be observed in storm clouds as a result of pressure buildup; lightning (see Figure 3) is the discharge that occurs after the charge exceeds a critical concentration.

#### Key Term Glossary

concentration
The proportion of a substance in a mixture.
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conservation
A particular measurable property of an isolated physical system does not change as the system evolves.
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continuity
Lack of interruption or disconnection; the quality of being continuous in space or time.
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coulomb
In the International System of Units, the derived unit of electric charge; the amount of electric charge carried by a current of 1 ampere flowing for 1 second. Symbol: C
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discharge
the act of releasing an accumulated charge
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electric charge
A quantum number that determines the electromagnetic interactions of some subatomic particles; by convention, the electron has an electric charge of -1 and the proton +1, and quarks have fractional charge.
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elementary charge
The electric charge on a single proton.
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equation
An assertion that two expressions are equal, expressed by writing the two expressions separated by an equal sign; from which one is to determine a particular quantity.
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force
A physical quantity that denotes ability to push, pull, twist or accelerate a body which is measured in a unit dimensioned in mass × distance/time² (ML/T²): SI: newton (N); CGS: dyne (dyn)
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Force
A force is any influence that causes an object to undergo a certain change, either concerning its movement, direction or geometrical construction.
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friction
A force that resists the relative motion or tendency to such motion of two bodies in contact.
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heat
energy transferred from one body to another by thermal interactions
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Law
A concise description, usually in the form of a mathematical equation, used to describe a pattern in nature
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matter
The basic structural component of the universe. Matter usually has mass and volume.
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particle
A very small piece of matter, a fragment; especially, the smallest possible part of something.
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pressure
the amount of force that is applied over a given area divided by the size of that area
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quantity
A fundamental, generic term used when referring to the measurement (count, amount) of a scalar, vector, number of items or to some other way of denominating the value of a collection or group of items.
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SI units
International System of Units (abbreviated SI from French: Le Système international d'unités). It is the modern form of the metric system.
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static
Fixed in place; having no motion.
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static electricity
an electric charge that has built up on an insulated body, often due to friction