Chapter 5

PSC 4011Electricity: Whats the connection?

PSC 4011: Static Electricity & MagnetismElectricity describes all of the phenomena caused by positive charges (protons) and negative charges (electrons)If a body (atom/molecule or object) has MORE PROTONS than electrons, it will be positively charged.If a body has MORE ELECTRONS than protons, it will be negatively charged.PSC 4011: Static Electricity & MagnetismElectrically charged objects, which have gained or lost ELECTRONS, tend to exert a force on other objects nearby.Like charges (+ + or - - ) will tend to REPEL each other.Opposite charges (+ - ) will tend to ATTRACT each other.Charges, are conserved.PSC 4011: Static Electricity & MagnetismWhen an object has the exact same number of protons and electrons it is called NEUTRAL.

Charging an object consists in creating an imbalance in the electrical charge of that object.

Materials can be divided in three groups, according to their reaction to the process of charging:

ConductorsSemiconductorsInsulators

PSC 4011: Static Electricity & MagnetismEvery object fits somewhere along this conducting continuum

Where do you think the human body would fit in the above diagram?

PSC 4011: Static Electricity & MagnetismConductors : permit the free flow of electrical charges.

When charged, its electrons move in one general direction, pushing one another and making the charge move along the circuit.Metals and electrolytic solutions are conductors (ions).

PSC 4011: Static Electricity & MagnetismInsulators: impede the free flow of electrical charges.

When charged, its electrons are strongly held by the atomic nuclei, so charges do not move.Wood, glass, plastic, ceramics, paper, rubber, silk and air are also insulators.

PSC 4011: Static Electricity & MagnetismStatic electricity (electrostatics) describes all the phenomena related to electrical charges at rest.

Charges are at rest when they are INSULATED.Static charge is often detected by a device known as an electroscope.There are three methods for charging objects.

PSC 4011: Static Electricity & MagnetismDateEvent6th century BC_Thales discovers the electrical attraction of rubbed amber1600_Gilbert discovers that many materials can be electrified_Bodies are divided into 2 categories: conductors & insulators_The human body is discovered to be a conductor1746_Musschenbroek makes the first Leyden jar1760_Franklin invents the lightning rod1785_Coulomb establishes his law (Coulombs law)~ 1790_Galvani believes to have demonstrated that animals body is an organic Leyden jar1800_Volta invents the first cell (birth of electrodynamics)PSC 4011: Static Electricity & MagnetismCharging by friction:

Two neutral bodies rubbed against each other.Some electrons are pulled from one of them.Resulting: two opposite charged bodies (attraction!)

PSC 4011: Static Electricity & MagnetismCharging by friction:

PSC 4011: Static Electricity & MagnetismCharging by conduction:

Two solid bodies in contact with each other.The charge is transferred from the charged object to the second object.Resulting: two equally charged bodies, each with less charge than initially

PSC 4011: Static Electricity & Magnetism

PSC 4011: Static Electricity & MagnetismCharging by induction:

PSC 4011: Static Electricity & MagnetismCharging by induction:

PSC 4011: Static Electricity & MagnetismCharging by induction:

PSC 4011: Static Electricity & MagnetismPrecautions (Static Electricity):Objects must be grounded in order to eliminate static charge built on them

PSC 4011: Static Electricity & MagnetismApplications (Static Electricity):Objects are previously electrified, so their charge is kept, and attraction is guaranteed when in use

PSC 4011: Static Electricity & MagnetismPSC 4011: Static Electricity & MagnetismExample 1:For two positively charged particles, each with a charge of 5x10-8C and placed 1cm apart. Calculate the electrical force between the two particles:

PSC 4011: Static Electricity & MagnetismFe = k(q1)(q2)/r2Fe = (9 x 109 Nm2/C2)(5 x 10-8 C)(5 x 10-8C)/(0.01m)2Fe = (9 x 5 x 5) x 10 [9+(-8)+(-8)] / (0.0001) (Nm2/C2)(C)(C) / (m2)Fe = (225) x 10-7 / 1 x 10-4 NFe = [(225)/1] x 10[(-7)-(-4)] NFe = 225x 10-3 NFe = 0.225 N

PSC 4011: Static Electricity & MagnetismMagnetism:

Physical phenomenon produced by the motion of charge, resulting in attractive and repulsive forces between objects

PSC 4011: Static Electricity & MagnetismDateEventBC_Scientists observed an attraction between magnetite and iron11th century_Chinese and Arab sailors used compasses~1270_Compass appears in Western Europe_Maricourt distinguishes North and South poles 1600_Gilbert fails to relate electricity and magnetismPSC 4011: Static Electricity & MagnetismMagnets:Objects upon which magnetism is observed.

Magnets ALWAYS have two poles: North and South (if you break a magnet, you create two smaller magnets, each with its north and south poles)PSC 4011: Static Electricity & MagnetismMagnetic field:Magnetic influence of a magnet on its surroundings.

Magnetic field lines:Representation of the magnetic field of a magnet. Lines exit North poleLines enter South polePSC 4011: Static Electricity & MagnetismAttraction Repulsion

PSC 4011: Static Electricity & MagnetismEarth is a gigantic magnet

PSC 4011: Static Electricity & MagnetismElectric forceMagnetic force_It acts on two types of charges (positive and negative)_Magnets have two types of poles (North and South)_Like charges: repulsion_Different charges: attraction_Like poles: repulsion_Different poles: attraction_Objects CAN have ONE type of charge (positively or negatively charged)_Magnets CANNOT have ONE type of pole (North or South)_It is inversely proportional to the square of the distance between the charges_It is inversely proportional to the square of the distance between the objects