Static Electricity

Topics Covered

• The Nature of Electricity

• Electric Charges

• Static Electricity Laws

• Neutralizing & Grounding

• Insulators & Conductors

• Charging: By Induction & By Conduction (or contact)

• The Electroscope

• The Quantity of Charge

The Nature of Electricity

(Also see Atomic Theory  and  Atomic Models)

Anything around us that has mass and occupies volume is known as matter.  Matter is made up of tiny particles called atoms.  Atoms join together to form molecules.

Atoms are made up of  three fundamental particles: neutrons, protons, and electrons.

According to Rutherford’s model of the atom, there is a dense, positively charged nucleus, in which all the mass is concentrated, surrounded by a region of negatively charged electrons. 

Electrons are negatively charged particles that move in circular orbits around the nucleus of an atom.
Electrons are the particles of the atom that are responsible for bonding (the electro-chemical process of combining two ore more atoms into molecules).  Electricity can be explained by investigating the properties and behaviour of electrons.

Electric Charges

Neutral object have no charge.  This means that the number of positive charges on the object equals the number of negative charges. We can charge an object by rubbing it with another object.   (physical change) is a process that will either take away negative charges from an object or add negative charges to it.

An object that has more negative charges than positive charges is called a NEGATIVELY CHARGED object.
An object that has LESS negative charges than positive charges is called a POSITIVELY CHARGED object.

• A loss of electrons = a positive charge

• A gain of electrons = a negative charge

Neutral Objects can be attracted by either positively or negatively charged objects.

The Laws of Electrostatics Charges:

• Like Charges Repel

• Unlike Charges Attract

Conservation of Charge:
This law is really another way to state the Law of Conservation of Energy. 
During the process of charging, the total number of charges exchanged between objects remains constant.
The Algebraic sum of all electrostatic charges produced during a charging process is always zero.

Neutralizing a Charged Object
Neutralizing means removing a charge from a charged object.  This can happen in several different ways.

1. Charge "Leak Off":
Charged objects can hold on to their charge for a limited period of time only.  After a while their charge "leaks off".  The loss of charge may be due to the presence of another nearby charged object or to the presence of humidity (water vapour) around the charged object. Water is a polar molecule (the hydrogen side is slightly more positive than the oxygen side of the molecule).  Because of this polarity, water can neutralize either type of charge.

2. Grounding:
Connecting a charged object to the ground by means of a conductor, will neutralize the charged object.
The ground (the earth) is so big that can be either a reservoir of electrons or a receptor of electrons.  Positively charged object will be neutralized when grounded because electrons will flow from the Earth (the ground) into the object.  Negatively charged objects will be neutralized when grounded because electrons will flow from the charged objects into the ground (the earth).

Insulators & Conductors

Static electricity and current electricity depend on the accumulation or movement of electrons within an object.

Conductors are materials (typically metals) which have mobile electrons.  In these materials electrons are free to move within the material.  In metals electrons form a "cloud of free electrons" on the surface of the material.  The movement of these electrons accounts for the charge on the material.

In materials called Insulators electrons are held tightly in place and are not allowed to move.  These materials do not easily conduct electricity -- for example: air, vacuum, glass.

There are two ways to electrically charge an object:

Conduction: free electrons are physically transmitted from a charged object to a neutral object or vice versa.  When an object is charged by conduction (or contact) both the "charging" object and the object being charged will acquire the same charge.

Induction: free electrons are either attracted towards or repelled away from the charging object.  The shift in electrons within the object polarizes the object being charged whereby one end will be positive and the opposite end will be negative (or vice versa).  When an object is charged by induction  the "charging" object and the object being charged will momentarily acquire the opposite charge.

The Electroscope

The electroscope is a simple instrument that can give us a qualitative measurement of the amount of charge on an object.
It can also be used to predict the type of charge on an object.  Note that the positive charges (neutrons) remain always stationary.  Only the electrons are responsible fro the movement of charges.

Calculating the quantity of Charge on a charged object.

The electron is the particle that accounts for the charge on an object.   Movement of electrons form and to the object will establish the type of charge the object will have.  The object will be positively charged where their is a  loss of electrons, negatively charged where there is a gain or surplus of electrons and neutral where the positive and negative charges are equal.

The total amount of electrons deposited or removed from the object will determine the quantity (the amount) of charge on the object.

The charge on one single electron is therefore defined as  the fundamental unit of charge.   Charge is a fundamental measurement which cannot be derived from any other measurements.  Charge is measured in units of Coulombs

The symbol for charge is "Q", and the symbol for Coulomb is "C".

The amount of charge on one single electron can be mathematically determined to equal a very small amount of charge.

This small charge is 1.6 X 10^-19 C  this small amount of charge is known as the fundamental charge and its symbol is e (to remind us that it is the charge on one single electron).  This is a constant value.

Therefore  e = 1.6 X 10^-19 C/e     [Coulombs per electron]

To calculate the total charge on an object we multiply the constant value of e by the number of electrons deposited  on (or removed from) an object.

A simple formula is used to summarize this important fact:

Q = N x e

Where:  
Q is the total charge on an object, N is the number of electrons involved, and e is the fundamental; charge on one single electron (1.6 X 10^-19 C/e)

Example:
If you walk across a carpet with your socks on and then touch a metal door knob you will feel a shock.  The amount of charge transferred in the shock between your fingers and the door knob (to ground) is about 3.0 X 10²⁰ C.  How many electrons where transferred from the carpet to ground through your finger during the shock?

Solution: