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Home , Electrostatics

In this lecture

Electricity (Part I)   Electrostatics Electrification Electrostatic Charge Electric Electrostatic Lines of

Electricity Matter

• X-ray tube converts electric energy into electromagnetic energy • Matter has mass, form and energy equivalence. • Other devices convert electric energy into other forms of energy – Kinetic energy: • Matter may also have electric charge – Heat energy: – Chemical energy:

Electric Charge Electric Charge

• Comes in discrete quanta • are often free – Negative or positive to move form the outer – Electrons & shell of one to – Equal in magnitude, opposite in sign another • Protons are fixed in the • In most cases the net charge of matter nucleus and are not free is neutral to move – Total negative charge balances total positive charge

1 Electrification Electrification

• Material takes on net charge (+ve or –ve) • Electrification causes static (stationary) charge • Outer-shell electrons of some types of are loosely bound and can easily be removed • Electrostatics is the study of • Removal of these electrons leaves the stationary electric charge material with a net positive charge • Created by contact, friction or induction

Electrification Electrostatic Charge • Smallest unit of charge () far Electrification occurs due to movement • Electrification occurs due to movement too small to be useful of negative charge • Units of electric charge: (C) • Positive charge remains in material • – Charles (1736-1806) – 1 Coulomb = 6.24 X 1018 electron charges • Transfer of electrons from one object • -16 to another causes the first object to – Electron charge = 1.6 X 10 C become positively and the to become negatively charged

Electrostatic Charge

Example: An electrostatic charge is transferred • Unlike charges attract between two people after one has scuffed their feet across a nylon rug is of the order of one microcoulomb. • Like Charges repel How many electrons are transferred?

2 Electric Field

• What happens when we • An electric field is associated bring charges close with each electric charge together?

• Field radiates outwards from • Similar charges +ve charges and into –ve charge • Fields in opposite direction

• Charges repel each other

Electric Field Electric Field

• What happens when we • What happens when we bring charges close bring charges close together? together?

• Like charges • Neutral objects N N

• Fields in same direction • No Fields

• Charges attract each • No attraction or repulsion other

Q2 Electrostatic Force Coulomb’’s Law r • Governs magnitude of • Force between charges is due to electrostatic force electric field Q Q • Repulsive or attractive force known as = 1 2 F K 2 F = electrostatic force (Newtons) electrostatic force r Q = electrostatic charge (Coulombs) R = distance between charges 1 K = constant K = = 8.988 ×109 m 2C −2 • Uncharged particles have no field are 4πεo not affected by electric field • Force is directly proportional to product of electrostatic charges and inversely proportional to the square of the distance between them

3 Electric Field Point, p r Electric Field and Force • Charge Q placed in an electric field E • Magnitude of Electric Field • experiences the following force: at point, p experiences the following force: Q 1 E = electric field = × E = K Q = electrostatic charge (Coulombs) F Q E 2 R = distance between charges r K = constant • What are the units of Electric field? • Electric Field is directly proportional to the electrostatic charges and inversely proportional to the square of the distance between them

Electric Potential • is done by the electric field if the • What is potential energy? electric force acting on the charge causes it to move from one point to another. – These two points differ in their electric potential. • Electric charges possess potential energy when placed in an electric field W V = Q • Units of electric potential • A charge of one Coulomb in a potential difference of 1 is equivalent to 1

Summary Practice Questions

Electricity PAM2011: Lecture 5 Problem Sheet Solutions

Electric Charge 1. If the distance between two charged bodies is doubled, what will be the change in Electric Charge electrostatic force?

Electrification 2. A lightening bolt caries 50 Coulombs of charge. How many electrons is this?

3. What is the electrostatic charge of one electron? Electrostatic Charge 4. Two electrons are separated by 100 nm. What is the electrostatic force between them? Electric Field Is the force attractive or repulsive? Electric Field Electrostatic Force 5. mAs is a measure of what quantity? Lines of Equipotential

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