Lecture 1Introduction. Electrical Forces.

Chapter 15.1 15.3

Outline

• Electric Charge• Conductors and Insulators• Coulomb’s Law

The Electrical Phenomenon

The phenomenon occurs from rubbing of one material (e.g., amber electron in Greek) with another material (e.g., fur).

There are 2 types of charge: positive and negative.Like charges repel, unlike charges attract.

Discovered in ancient Greece more than 2500 years ago

It manifests itself by attraction or repellence of ‘charged’ materials.

The Nature of Charge

Charge is not produced by rubbing.Uncharged objects contain equal amounts of negative and positive charge. They are electrically neutral.

The nature of charge can be traced to the composition of matter, to atoms.

Electric charge is always conserved.In the process of rubbing, charge is not created.Negative charge is transferred from one object to the other.

Atomic Structure

92 chemical elements have been identified in the Universe.Nearly 20 more have been created artificially.

Each chemical element is made from a different type of atom.Atoms are made from particles called protons, neutrons, and electrons.Protons and neutrons form the nucleus in the center of the atom.Electrons surround the nucleus.Electrons are responsible for the charge transfer.

Atomic Structure

Positively charged protons are held together by the strong force, which overcomes electrical repulsion.

Negatively charged electrons are attracted to the nucleus.

Proton has a mass 1.673 1027 kg and is positively chargedNeutron has a mass 1.675 1027 kg and is unchargedElectron has a mass 9.11 1031 kg and is negatively charged

Proton and neutron are ~2000 times heavier than electron

Conduction and Induction

Charge can be transferred onto an object by:

Conduction objects are in physical contactResult: the object being charged gets the same sign charge as the object doing the charging.

Induction no contact between the objectsResult: the object doing the charging is left with the same charge; the object being charged get an opposite charge.

Polarization: charging an insulator by induction.

The CoulombThe unit of electric charge is the coulomb (C).The proton has a charge of + 1.6 1019 C.The electron has a charge of 1.6 1019 C.The quantity of charge is abbreviated e.

Electric charge occurs only in multiples of e.It is said to be quantized.

Coulomb’s Law:

|q1| |q2|F = ke ---------- r2

ke = 8.99 109 N m2/C2

Applying Coulomb’s Law

Object A has a charge of +5mC, and object B has a charge of 10mC. Which statement is true:

a) FAB = 2 FBA b) FAB = FBA

c) FAB= FBA d) FAB = 2 FBA

Calculate the magnitude of the Coulomb force between the 2 charges of 1mC each separated by 1m.

F=8.99 109Nm2/C2 (103 C)(103 C)/(1m)2 = 8990 NThis is equal to the gravitational attraction force between the 2 masses of 10Mkg separated by 1m.

Electricity and Gravity

The law of gravity and Coulomb’s law have the same form.

It is harder to collect a large electric charge of either sign than a large mass of matter.

Gravitational force is always attractive.Electric force can be either attractive or repulsive.Forces have both magnitude and direction.

Thus, gravity is more significant on a large scale (cosmic size), while electric forces are more significant on a small scale (atomic size)

Conductors and Insulators

A conductor is a substance through which electric charge flows readily. Example, metals.

In an insulator charge flows with great difficulty.

The difference between the 2 types of substances is in the strength of connection between outer electrons and nuclei.

Conductivity in fluids and gases involves ions.

Summary

•Electrical forces are different from gravity.They are more influential on small scales.

• Most properties of ordinary matter can be traced to electrical forces

• Electric properties of matter allow us to transmit and store energy.