INTRODUCTORY ELECTRICITY AND MAGNETISM

PHY 104

In the 17th and 18th centuries, scientific and quantitative exploration ofelectric and magnetic phenomena started (Gilbert, 1600, Guericke, 1660,Dufay, 1733, Franklin, 1752, Galvani, 1771,Cavendish, 1775, Coulomb,1785, Volta, 1800).

Electrostatics and magnetostatics were formulated and modeledmathematically.

In the nineteenth century, the study of the interrelationship betweenelectric and magnetic fields and their time varying behavior progressed(Oersted, 1820 and 1826, Ampere, 1820, Faraday, 1831, Henry, 1831,Maxwell, 1856 and 1873, Hertz, 1893).

Brief History on Early Advances in Electricity and Magnetism

Oersted observed that an electric current produces a magnetic field.

Faraday verified that a time-varying magnetic field produces an electric field.

Henry constructed electromagnets and discovered self-inductance.

Maxwell, by introducing the concept of the displacement current, developed amathematical foundation for electromagnetic fields and waves currentlyknown as Maxwell’s equations.

Hertz verified, experimentally, propagation of electromagnetic wavespredicted by Maxwell’s equations.

Maxwell’s equations are comprehensive in that they account for all classicalelectromagnetic phenomena, from static fields to electromagnetic inductionand wave propagation.

Brief History on Early Advances in Electricity and Magnetism

28 October 2019

ELECTRIC CHARGES AND METHODS OF CHARGING

O. M. Oshakuade

Department of Physics,University of Ibadan, Ibadan,

Nigeria.

PHY 104 Lecture Note

In our day-to-day activities, we may experience the effect ofelectrostatic charge (static electricity) in instances such as:• Clothes tumble in the dryer cling together• Feeling a shock at the door knob after walking on the carpet• Spark of electricity seen, when you pull off a wool sweater or

blanket.

These implies that charges are all around us.

Electric Charges

Atoms are basically composed of protons, neutrons and electrons.

Proton: positively chargedresides in the nucleushas a mass of about 1.672 × 10−27𝑘𝑔

Neutron: has no chargeresides in the nucleushas a mass of about 1.674 × 10−27𝑘𝑔

Electron: negatively chargedresides outside the nucleushas a mass of about 9.11 × 10−31𝑘𝑔

The magnitude of charge of a proton is equal to that of an electron, that is, 1.6 × 10−19𝐶 - symbol: e

Electric Charges

When is an atom said to be electrically neutral?

What are ions?

What are cations?

What are anions?

Charge (Electric charge, Electrostatic charge)SI unit: Coulomb (C)symbol: q

Electric Charges

Like charges repel while unlike charges attract, with forces described by Coulomb’s law.

Charge is quantized. Any charge q is ne. Where n is an integer.

Charge is conserved.

Basic Properties of Charges

The Electroscope

Helps to determine thepresence and nature ofelectrification.

Credit: (Farai and Oni, 2013)

Charge and Charge Interactions

Credit: (Young and Freedman, 2012)

Classification of Materials

Credit: (Farai and Oni, 2013)

Class of materials Basic properties Examples

Resistivity(Ω cm )

InsulatorsThey allow littleelectrical charges andheat energy to flow.

GlassMicaPVC

1010 – 1014

1011 – 1015

1012 - 1013

Conductors

They allow free flow ofelectrical charges andheat energy.

SilverCopperManganeseConstantanNichrome

1.5 x 10-8

1.6 x 10-8

2.5 x 10-8

4 x 10-8

4 x 10-8

Semiconductors They allow electricalcharges to flow betterthan insulators but lessthen conductors

GermaniumSilicon

0.92000

This is the process ofseparating opposite chargeswithin an object. Thisusually involve the use ofcharged object to induceelectron movement orrearrangement.

Polarization

Unpolarized sample

Sample polarised – due to the presence of electric field

1. By Conduction

Methods of Charging

This involves contact of a charged object with a neutral object, it is useful for charging conductors.

2. By Friction/Rubbing

Methods of Charging

This involves rubbing of materials (with different electron affinities)against each other.

In the rubbing process the atoms of the different materials are forcedinto close proximity, and the electron clouds of lower electronaffinity atoms interact with the nuclei of the higher electron affinityatoms, causing transfer of electrons from the lower electron affinitymaterial to the higher one.

The electron transfer causes both materials to be charged.

This method is useful for charging insulators.

3. By Induction

Methods of Charging

This involves charging a relatively neutral object by a charged object, without any physical contact between the two objects. The object being charged is touched by a ground; electrons are transferred between the ground and the object being charged (either into the object or out of it).

• The charged object serves to polarize the object being charged. • The charged object does not transfer electrons to or receive

electrons from the object being charged. • The object being charged ultimately receives a charge that is

opposite that of the charged object which is used to polarize it.

Methods of Charging

Grounding

This is the process of removing the excess charge on an object by meansof the transfer of electrons between it and another object of substantialsize.

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31 October 2019

COULOMB’S LAW

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