Sci-101Sci-101Introduction to PhysicsIntroduction to Physics

Dr. Alexander Dzyubenko

Chap. 9 Static and Current Chap. 9 Static and Current ElectricityElectricity

http://www.csub.edu/~ adzyubenko

© A.B. Dzyubenko 2003, 2004

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Electric Force and ChargeElectric Force and Charge

• A universal force, like gravity• Varies inversely as the square of the distance, like gravity• Billions and billions time stronger than gravity

• There are two types of charges “+” and “-”, unlike gravity

repel

attract

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Atomic structureAtomic structure

Every atom is composed of a positively charged nucleus surrounded by negatively charged electrons

The electrons of all atoms are identical: same charge, same mass, same everything

Protons and neutrons compose the nucleus

Protons (+) are about 2000 more massive than electrons (-)

Neutrons have slightly more mass than protons

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Atomic structure contAtomic structure cont

Normally an atom has many electrons and equal number of protons => neutral as a whole

Hydrogen atom: one proton, one electron

When an atom loses one or several electrons, it has a positive net charge; called a positive ion

An atom gains one or several electrons, it has a negative net charge; called a negative ion

-

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Making neutral objects chargedMaking neutral objects charged Material objects are

made of atoms Ordinarily have equal

numbers of electrons and protons => neutral as a whole

The electrons farthest from the nucleus can be stripped away

Easily? It depends on a substance!

Electrons are transferred from the fur to the rod

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Conservation of ChargeConservation of Charge

Whenever something is charged, no electrons are created or destroyed

The conservation of charge is a significant principle in physics

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Coulomb’s LawCoulomb’s LawThe force between two charges varies

directly as the product of their charges and inversely as the square of the

distance between them

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Coulomb’s Law contCoulomb’s Law cont

The unit of charge, Coulomb (C) Equals the amount of charge that passes

through a 100 W bulb in about 1 s The proportionality constant, k, is similar

to the gravitational constant G k = 9x109 Nm2/C2

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Gravitational vs Electric Forces Gravitational vs Electric Forces

The gravitational force holds the satellite in orbit

The electric force holds the electron in orbit

Qualitatively, very similar

Quantitatively?

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http://www.people.vcu.edu/~rgowdy/mod/077/xmp.htm

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Charge polarizationCharge polarizationCharge an inflated balloon by rubbing it

on your hair Place it again a wall and it sticks …

Negatively charged balloon polarizes atoms in the wall

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Electric FieldsElectric FieldsElectric (like gravitational)

forces act between charged objects that are not in contact

There exists a force field that influences distant charges (masses)

Electric field, E, is a vectorField (or force) lines are used

to represent E

Electric field near a negative charge

Direction? That of the force on a positive charge

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Electric field near a negative charge.

Electric field of a dipole, a pair of “+” and “–” charges. Net charge=0.

Electric field near two charged metallic plates. A capacitor.

ExamplesExamples

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Electrical Potential EnergyElectrical Potential Energy

A charged object has electrical potential energy by virtue of its location in an electric field

(a) gravitational PE (b) PE of charged particles

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Electric Potential Energy, contElectric Potential Energy, cont

Work done on the spring goes to the increase of its mechanical PE

Work done on the charge goes to its electrical PE

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Electric Potential Electric Potential

Unit of energy: Joule (J)Unit of charge: Coulomb (C)Unit of Electric Potential:

Joule/Coulomb = Volt 1 V = (1 J)/(1 C)Terms electric potential and voltage are

used

charge ofamount

energypotentialElectricpotentialElectric

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Electric Potential and PEElectric Potential and PE

Two different charges at same distance from the charged dome

Whose energy is larger?

Whose potential is larger?

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Voltage SourcesVoltage SourcesThe ends of an electrical

circuit are at different electric potentials

(+) Charges in the conductor flow from high to low potentials

To attain a sustained flow of charge, we need a battery or generator

Electrical “pumps”

the flow ceases when the difference in pressures ceases

The difference in pressures is maintained with the pump

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BatteriesBatteries

A common automobile battery provides 12 Volts

We say there is a 12-volt voltage drop in the circuit

12 Joules of energy are supplied to each 1 Coulomb of charge that is made to flow in the current

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Electric currentElectric current Is the flow of charged particles In metals, electrons are flowing

charged particles These are called conduction

electrons Positively charged nuclei

(protons+ neutrons) are locked in fixed positions

The unit of electric current is Ampere (A): it is a rate of flow of 1 Coulomb per second through a wire

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Electric current, contElectric current, contThe current-carrying wire is not

electrically chargedThe charges flow through a circuit when

voltage is applied across a circuit

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Direct and Alternating CurrentsDirect and Alternating Currents DC current: charges flow

in one direction AC current may be

accomplished by periodically switching the sign at the terminals

Commercial US standard: 60 cycles per second or 60 Hertz (Hz)

European standard: 50 Hz

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Electrical ResistanceElectrical Resistance

The longer the wire, the larger the resistance

The longer the pipeline, the larger the resistance

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Current = Voltage / Resistance For a given circuit (Resistance), current is

linearly proportional to voltage Good conductors (like metals) obey Ohm’s

Law Not all substances obey Ohm’s Law Units for Resistance: Ohms, Amperes = Volts / Ohms 1 A = 1 V/ 1

Ohm’s LawOhm’s Law

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Quick QuizQuick Quiz

How much current flows through a lamp that has a resistance of 60 when 12 Volts is impressed across it?

Current = Voltage / ResistanceAmperes = Volts / OhmsCurrent = (12 Volts) / (60 ) = 5 A

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Electric ShockElectric Shock

Which causes electric shock in human body – current or voltage?

Current! The resistance of the human body ranges from

about 100 (when soaked with salty water) to 500,000 (when the skin is very dry)

0.001 A can be felt 0.05 A is painful 0.01 A causes involuntary muscle contractions 0.07 A probably fatal if lasts > 1 s

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Electric CircuitsElectric Circuits

A circuit, any path through which electrons can flow

Usually contains more than just one device

Devices connected to each other

Connections in series Connections in parallel

Conduction electrons surge to and fro in the filament: AC voltage => AC current

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Connection in SeriesConnection in Series

Electric current has just one path

Resistance of the circuit = SUM of all resistances

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Connection in ParallelConnection in Parallel

Each device operates independently but at same voltage

The total current= SUM of all currents

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Appliances are connected in Appliances are connected in parallel parallel

Circuits that carry more that a safe amount of current are said to be overload

A fuse is connected in series to prevent a possible overload

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Electric PowerElectric Power

The moving charges do work (electric heaters and ovens, electric motors, …)

The rate at which work is done is called electric power

Power = Current x Voltage 1 Watt = 1 Ampere x 1 Volt Quiz: A 60 W lamp operates on a 120-V

line. Current?Answer: Current = 60 W / 120 V = 0.5 A