Fundamentals of Fundamentals of ElectricityElectricity

Circuits 1Circuits 1Fall 2005Fall 2005

Harding UniversityHarding University

Jonathan WhiteJonathan White

OutlineOutline Benjamin Franklin / History of ElectronicsBenjamin Franklin / History of Electronics Atoms/ElectronsAtoms/Electrons

– Electron shells and orbitsElectron shells and orbits– Valence electronsValence electrons

ChargeCharge CurrentCurrent VoltageVoltage Electrical GroundElectrical Ground ResistanceResistance Electric circuitsElectric circuits

History of ElectronicsHistory of Electronics Began with Benjamin Franklin in 1747Began with Benjamin Franklin in 1747

– He determined that electricity was a single force, with He determined that electricity was a single force, with positive and negative aspectspositive and negative aspects

– Coined over 25 new terms, including armature, battery, Coined over 25 new terms, including armature, battery, and conductorand conductor

– Famous kite-flying experiment in a thunderstorm was Famous kite-flying experiment in a thunderstorm was performed in 1752, near the end of his work in the fieldperformed in 1752, near the end of his work in the field

Ben Franklin arbitrarily assumed that the actual Ben Franklin arbitrarily assumed that the actual carriers of electrical current had a positive carriers of electrical current had a positive electrical charge.electrical charge.– While this assumption was wrong, all his experiments While this assumption was wrong, all his experiments

still worked, and this assumption is still often used still worked, and this assumption is still often used today.today.

However, the theories of electricity go back a lot However, the theories of electricity go back a lot farther than this.farther than this.

History 1History 1 600 B.C. – Thales of Miletus writes about amber 600 B.C. – Thales of Miletus writes about amber

becoming charged by rubbing.becoming charged by rubbing. 1600 – English scientist William Gilbert coins the 1600 – English scientist William Gilbert coins the

term electricity from the Greek word for amber. term electricity from the Greek word for amber. Experiments with magnets, coining the terms Experiments with magnets, coining the terms electric force, magnetic pole, and electric electric force, magnetic pole, and electric attraction.attraction.

1745 – Dutch physicist Pieter van Musschenbroek 1745 – Dutch physicist Pieter van Musschenbroek invented the “Leyden Jar”, a device that stored invented the “Leyden Jar”, a device that stored static electricity. This was the first capacitor.static electricity. This was the first capacitor.

1747 – William Watson discharged a Leyden Jar 1747 – William Watson discharged a Leyden Jar through a circuit. This begins the comprehension through a circuit. This begins the comprehension of current and circuits.of current and circuits.

1800 – Alessandro Volta invents the first electric 1800 – Alessandro Volta invents the first electric battery. He also proved that electricity could battery. He also proved that electricity could travel over wires.travel over wires.

History 2History 2 1820 – Oersted and Ampere observe that a coil of 1820 – Oersted and Ampere observe that a coil of

wires acts like a magnet when a current is passed wires acts like a magnet when a current is passed through itthrough it

1821 – Faraday invents the first electric motor1821 – Faraday invents the first electric motor 1826 – Ohm states a relationship between 1826 – Ohm states a relationship between

potential, current, and circuit resistancepotential, current, and circuit resistance 1873 – Maxwell writes equations that described 1873 – Maxwell writes equations that described

the electromagnetic fieldthe electromagnetic field 1876 – Edison Electric Light Co. founded1876 – Edison Electric Light Co. founded 1879 – First commercial power station opens in 1879 – First commercial power station opens in

San Francisco, uses a Brush generator and arc San Francisco, uses a Brush generator and arc lightslights

1883 – Transformer invented1883 – Transformer invented 1886 – Alternating current electric system 1886 – Alternating current electric system

developed by William Stanleydeveloped by William Stanley 1897 – Electron discovered by J.J. Thompson1897 – Electron discovered by J.J. Thompson

AtomsAtoms

Atom – Smallest particle of an Atom – Smallest particle of an element that retains all the element that retains all the characteristics of that element.characteristics of that element.– Consists of positively charge protons Consists of positively charge protons

and uncharged neutrons in the nucleus, and uncharged neutrons in the nucleus, and several negatively charge electrons and several negatively charge electrons that surround the nucleus.that surround the nucleus.

Electrons occupy specific energy Electrons occupy specific energy levels, or “shells” around the nucleuslevels, or “shells” around the nucleus

ElectronsElectrons Each shell is discrete and electrons will try and occupy the Each shell is discrete and electrons will try and occupy the

lowest energy level available to them. lowest energy level available to them. – Negative electrons that are close to the nucleus are attracted Negative electrons that are close to the nucleus are attracted

to the positive nucleus and are tightly bound.to the positive nucleus and are tightly bound.– Electrons farther out are loosely bound, and hence have a Electrons farther out are loosely bound, and hence have a

higher potential energyhigher potential energy However, there is a limit to the number of electrons that However, there is a limit to the number of electrons that

each shell can hold.each shell can hold.– Number of electrons possible = 2NNumber of electrons possible = 2N22, where N is the shell , where N is the shell

number.number. The outermost shell is known as the valence shell, and the The outermost shell is known as the valence shell, and the

electrons in it are called valence electrons.electrons in it are called valence electrons. The factor that becomes important is that those elements The factor that becomes important is that those elements

with only 1 or 2 electrons in their outermost shells don’t with only 1 or 2 electrons in their outermost shells don’t hold on the them very strongly. Therefore it requires little hold on the them very strongly. Therefore it requires little energy to pull these electrons from their parent atoms and energy to pull these electrons from their parent atoms and move them someplace else.move them someplace else.

The Copper AtomThe Copper Atom

29 electrons that orbit the nucleus in 4 29 electrons that orbit the nucleus in 4 shells.shells.

When a copper atom gains sufficient When a copper atom gains sufficient thermal energy, it can break away from thermal energy, it can break away from the parent and become a free electron. the parent and become a free electron. – This happens at room temperature for copper.This happens at room temperature for copper.

Categories of MaterialsCategories of Materials

Conductors: readily allow current, large Conductors: readily allow current, large number of free electrons, 1 – 3 valence number of free electrons, 1 – 3 valence electrons. What’s the best conductor? electrons. What’s the best conductor? Exp: silver, copper, gold, aluminum, iron.Exp: silver, copper, gold, aluminum, iron.

Semiconductors: 4 valence electrons in Semiconductors: 4 valence electrons in their structures. Exp: silicon, germanium.their structures. Exp: silicon, germanium.

Insulators: poor conductors of electric Insulators: poor conductors of electric current, more than 4 valence electrons.current, more than 4 valence electrons.

Electrical ChargeElectrical Charge 2 types, positive and negative. Charge of a proton is equal in 2 types, positive and negative. Charge of a proton is equal in

magnitude to the charge of an electron.magnitude to the charge of an electron.– Symbolized by letter Q.Symbolized by letter Q.

Static electricity is the presence of a net positive or negative Static electricity is the presence of a net positive or negative charge.charge.

Like charges repel, unlike charges attract.Like charges repel, unlike charges attract.– This attraction or repulsion is a force called an electromagnetic field.This attraction or repulsion is a force called an electromagnetic field.– This force and gravity are the only forces that we humans can This force and gravity are the only forces that we humans can

experience directly.experience directly. Charge is measured in coulumbsCharge is measured in coulumbs

– One coulumb is the total charge possesed by 6.25 X 10One coulumb is the total charge possesed by 6.25 X 1018 18 electronselectrons Like matter, charges follow an inverse square law. Like matter, charges follow an inverse square law.

– FF1,21,2 = q = q11 q q22/(4/(4ππEE00 r r22) ) Where q1, q2 are measured in coulumbs, r is the distance between the Where q1, q2 are measured in coulumbs, r is the distance between the

charges measured in meters, and Echarges measured in meters, and E0 0 is a fundamental constant of nature, = is a fundamental constant of nature, = ~8.885419 x 10-12 Farads/meter~8.885419 x 10-12 Farads/meter

However, charge is different than matter. Does anti-matter have However, charge is different than matter. Does anti-matter have a negative mass? Also, two masses always seem to attract each a negative mass? Also, two masses always seem to attract each other.other.

Charge 2Charge 2 The charge on an electron is always the same for every The charge on an electron is always the same for every

electron in the universe. electron in the universe. If you have one electron and one proton, then the two If you have one electron and one proton, then the two

charges cancel each other out and the atom is said to have charges cancel each other out and the atom is said to have no charge. Also, the electron and proton will cancel out no charge. Also, the electron and proton will cancel out each other exactly.each other exactly.

The charge on an electron is a fundamental quantity - a The charge on an electron is a fundamental quantity - a constant of nature. constant of nature.

Where is charge used?Where is charge used?– When you plug in a light bulb, charge flows from the socket, When you plug in a light bulb, charge flows from the socket,

through the connecting wire, and then through the bulb through the connecting wire, and then through the bulb filament, heating it up and giving off light. The electrons aren’t filament, heating it up and giving off light. The electrons aren’t destroyed, but they do lose energy.destroyed, but they do lose energy.

– Your car battery stores energy by storing charge on the Your car battery stores energy by storing charge on the battery plates. When you start your car, charge flows from battery plates. When you start your car, charge flows from your battery to the engine, providing enough energy for the your battery to the engine, providing enough energy for the vehicle to run.vehicle to run.

CurrentCurrent Current – charge in motion.Current – charge in motion.

– The “flowing” of charges through something.The “flowing” of charges through something. We typically think of charge flowing through a wire, but it We typically think of charge flowing through a wire, but it

can also flow through water, air and even vacuum.can also flow through water, air and even vacuum. You can think of current as water flowing through the You can think of current as water flowing through the

interior of a pipe, though current actually flows though the interior of a pipe, though current actually flows though the empty spaces between atoms in a wire.empty spaces between atoms in a wire.

Current is represented by the mathematical symbol Current is represented by the mathematical symbol ii..– i = Q/t, or, current is equal to the number of electrons that flow i = Q/t, or, current is equal to the number of electrons that flow

past a point in a given amount of time.past a point in a given amount of time.– Current is measured in amperes, which is equal to Current is measured in amperes, which is equal to

coulombs/sec. Amperes is abbreviated with the letter A.coulombs/sec. Amperes is abbreviated with the letter A. Current is a “through” variable, meaning that in order to Current is a “through” variable, meaning that in order to

measure it, you need the current to go through something.measure it, you need the current to go through something.

Current 2Current 2

Charge comes in discrete packets, Charge comes in discrete packets, but it is useful to assume that it can but it is useful to assume that it can take continuous values. Then, we take continuous values. Then, we can imagine making the time frame can imagine making the time frame very small and find the current at an very small and find the current at an instant. Then, instant. Then, i(t) = dQ(t)/dti(t) = dQ(t)/dt

It takes energy to make charges flow It takes energy to make charges flow through something. The energy that through something. The energy that makes current is called voltage.makes current is called voltage.

VoltageVoltage Can be thought of as the driving force behind the current Can be thought of as the driving force behind the current

(though it isn’t really a force).(though it isn’t really a force). Voltage is the energy per unit charge.Voltage is the energy per unit charge. Current flows through electrical elements when a voltage Current flows through electrical elements when a voltage

appears across the terminals of the element, similar to appears across the terminals of the element, similar to when water flows through a pipe when a pressure when water flows through a pipe when a pressure difference appears across the pipe. difference appears across the pipe. – Voltage is an “across” variable. We talk about pressure Voltage is an “across” variable. We talk about pressure

differences and voltage differences.differences and voltage differences. Voltage is related to potential energy. Voltage is defined as Voltage is related to potential energy. Voltage is defined as

the electrical potential energy that a charge has by its the electrical potential energy that a charge has by its position in spaceposition in space..– If you pull two charges apart, you put potential energy into the If you pull two charges apart, you put potential energy into the

systemsystem– That potential energy can be converted into other forms of That potential energy can be converted into other forms of

energyenergy– Energy can neither be created or destroyed, only transferredEnergy can neither be created or destroyed, only transferred

Voltage 2Voltage 2 A mass m, h meters above the Earth has potential A mass m, h meters above the Earth has potential

energy mgh. A mass at h=0 has 0 potential energy mgh. A mass at h=0 has 0 potential energy. A charge at electrical ground has 0 energy. A charge at electrical ground has 0 potential engergy. Voltage is the potential potential engergy. Voltage is the potential energy per unit charge, or energy per unit charge, or V= W/QV= W/Q..

If we move a charge from point A to point B, and If we move a charge from point A to point B, and put a given number of joules of work into the put a given number of joules of work into the charge, we will recover exactly the same number charge, we will recover exactly the same number of joules from the charge if it moves back from of joules from the charge if it moves back from point B to point A.  If we move the charge through point B to point A.  If we move the charge through any closed path or circuit, there will be no net any closed path or circuit, there will be no net energy input to the system and no net energy energy input to the system and no net energy recovered from the charge. recovered from the charge.

Voltage and BatteriesVoltage and Batteries Batteries are voltage sources.Batteries are voltage sources. Batteries can be thought of as charge pumps.Batteries can be thought of as charge pumps.

– They take a charge and though chemical reactions pump They take a charge and though chemical reactions pump them up to a certain voltage, or potential energy level.them up to a certain voltage, or potential energy level.

As the charge flows through the circuit, this As the charge flows through the circuit, this potential energy can be used by the circuit to do potential energy can be used by the circuit to do work. The charge loses energy as it goes through work. The charge loses energy as it goes through the loads.the loads.– Heat up a filamentHeat up a filament– Make a motor turn.Make a motor turn.

Energy gained from the battery = energy lost by Energy gained from the battery = energy lost by the loads.the loads.– Law of conservation of energyLaw of conservation of energy

GroundGround Reference voltage from which all other measurements are Reference voltage from which all other measurements are

measured – the potential of the Earth.measured – the potential of the Earth.– Defined as having 0 V potential energy with respect to the rest Defined as having 0 V potential energy with respect to the rest

of the circuit.of the circuit. In physics equations, ground level is used as the point of 0 In physics equations, ground level is used as the point of 0

potential energy when lifting a weight, another thing potential energy when lifting a weight, another thing electrical systems have in common with mechanical electrical systems have in common with mechanical systems.systems.

In wiring for houses, the ground is physically connected to In wiring for houses, the ground is physically connected to the Earth – a place of 0 potential energy when compared to the Earth – a place of 0 potential energy when compared to the rest of the wiring.the rest of the wiring.

Ground provides a return path for the current back to the Ground provides a return path for the current back to the source because all the ground points are electrically the source because all the ground points are electrically the same point and provide a zero resistance pathsame point and provide a zero resistance path

ResistanceResistance

Opposition to the flow of current.Opposition to the flow of current. When there is current through any When there is current through any

material that has resistance, heat is material that has resistance, heat is produced by the collisions of produced by the collisions of electrons and atoms.electrons and atoms.

Can be thought of as partially closed Can be thought of as partially closed valve in our pipe system – it restricts valve in our pipe system – it restricts the flow of water.the flow of water.

CircuitsCircuits

Consists of a voltage source, a load, Consists of a voltage source, a load, and a path for current between the and a path for current between the source and the load.source and the load.– A load is a device on which work is done A load is a device on which work is done

by the current through it.by the current through it. Open circuits versus closed circuits.Open circuits versus closed circuits.

ReviewReview

ChargeCharge– Definition: Fundamental property of Definition: Fundamental property of

matter based on the absence or excess matter based on the absence or excess of electrons.of electrons.

– Symbol: QSymbol: Q– Measured in Coulombs.Measured in Coulombs.– 1C = total charge of 6.25 X 101C = total charge of 6.25 X 101818

electronselectrons– 2 types, positive and negative2 types, positive and negative

ReviewReview

CurrentCurrent– Definition: Charge in motionDefinition: Charge in motion– Symbol: ISymbol: I– Measured in amperes (A).Measured in amperes (A).– I = Q/T, current is the amount of charge I = Q/T, current is the amount of charge

that passes a point in a given amount of that passes a point in a given amount of time. Or, i(t) = dq / dttime. Or, i(t) = dq / dt

– Current is a “through variable”.Current is a “through variable”.– The water that’s flowing through a pipe.The water that’s flowing through a pipe.

ReviewReview

VoltageVoltage– Definition: Energy per unit charge.Definition: Energy per unit charge.– Symbol: VSymbol: V– V= W/Q , voltage is the energy per unit V= W/Q , voltage is the energy per unit

charge. Where, W is the energy charge. Where, W is the energy expressed in joules, Q is in Coulombs.expressed in joules, Q is in Coulombs.

– Voltage is an across variable.Voltage is an across variable.– The pressure that pushes the water The pressure that pushes the water

through the pipe.through the pipe.

Ground and ResistanceGround and Resistance

GroundGround– Definition: Reference point that is at 0 Definition: Reference point that is at 0

volts with respect to all other points in volts with respect to all other points in the circuit.the circuit.

ResistanceResistance– Opposition to the flow of electrons.Opposition to the flow of electrons.– The resistance caused by the collision of The resistance caused by the collision of

electrons and atoms causes heat to be electrons and atoms causes heat to be given off.given off.