Basic ElectricityBasic Electricity

All material is made of All material is made of atoms (we think) atoms (we think) – Atoms are comprised ofAtoms are comprised of

Nucleus: protons (+) and Nucleus: protons (+) and neutronsneutrons

– FixedFixed Electrons (-) orbit outside the Electrons (-) orbit outside the

nucleusnucleus

Basic ElectricityBasic Electricity

– Electrons and protons Electrons and protons are chargedare charged Opposite charges attractOpposite charges attract Like charges repelLike charges repel In other words, In other words, charges charges

desire to be “zeroed” desire to be “zeroed” or “neutralized” or or “neutralized” or “balanced”“balanced”

Basic ElectricityBasic Electricity How tightly electrons are bound How tightly electrons are bound

depends on materialdepends on material– Tightly bound electrons: insulatorTightly bound electrons: insulator

Does not conduct electricity (or heat) wellDoes not conduct electricity (or heat) well glass, plastic, rubberglass, plastic, rubber

– Loosely bound (“free”) electrons: Loosely bound (“free”) electrons: conductorconductor Conducts electricity (and heat) wellConducts electricity (and heat) well metalsmetals

– In between: semiconductorsIn between: semiconductors Important for computer chip productionImportant for computer chip production silicon, germanium, carbonsilicon, germanium, carbon

Basic ElectricityBasic Electricity

The copper atom has one lonely electron in the valence shell, just hanging out, waiting to be stripped away. It’s a good conductor.

VoltageVoltage What causes electrons to flow?What causes electrons to flow?

Protons fixed; Electrons “free”Protons fixed; Electrons “free” Electrons move!Electrons move!

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battery

• Charge imbalancecaused by chemicalreactions

• Charges desire tobe neutralized

• Charges flow!

There must be a complete path between excess + There must be a complete path between excess + and excess – chargesand excess – charges

Otherwise + and – charges don’t “see” each otherOtherwise + and – charges don’t “see” each other

VoltageVoltage

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battery

VoltageVoltage A battery has regions of excess A battery has regions of excess

negative and positive charges negative and positive charges caused by chemical reactionscaused by chemical reactions

The amount of “push” exerted on the The amount of “push” exerted on the electrons is called “electromotive electrons is called “electromotive force”force”– unit: volt (V) unit: volt (V) – Voltage is Voltage is relativerelative, only is meaningful , only is meaningful

when talking about voltage when talking about voltage differencedifference between 2 pointsbetween 2 points

VoltageVoltage Do batteries run out of charge?Do batteries run out of charge?

What happens to total # of electrons? What happens to total # of electrons? Nothing! Electrons do not get used up – Nothing! Electrons do not get used up – energy does!!energy does!!

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battery

CurrentCurrent Number of electrons that pass a Number of electrons that pass a

certain point in a certain time: currentcertain point in a certain time: currentwirewire

– Unit: Ampere (amp) = 6.25 Unit: Ampere (amp) = 6.25 10 101818 electrons/selectrons/s

– Or Amp = 1 Coulomb/secondOr Amp = 1 Coulomb/second

Count ‘um passing here!

CurrentCurrent Historically, current discussed in terms Historically, current discussed in terms

of positive charge flow: “conventional” of positive charge flow: “conventional” current flow – actually it’s wrong!current flow – actually it’s wrong!

Electrons (-) actually flow to the (+)Electrons (-) actually flow to the (+) Symbol for battery:Symbol for battery:

Equivalent Equivalent

Time Delay?Time Delay? Does it take time for electrons to flow Does it take time for electrons to flow

from a switch to a bulb?from a switch to a bulb?

Light Bulb

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10 miles

Time Delay?Time Delay? Does it take time for electrons to flow Does it take time for electrons to flow

from a switch to a bulb?from a switch to a bulb?

NO!! Wire is already full of electrons!NO!! Wire is already full of electrons!

Light Bulb

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- - - - - - - - - - - - -

Light Bulb

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10 miles

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CurrentCurrent How much current will flow?How much current will flow?

– Wire is broken? Zero (need a complete circuit)Wire is broken? Zero (need a complete circuit)– Large voltage – large currentLarge voltage – large current– Small voltage – small currentSmall voltage – small current– All other things being equal!All other things being equal!

For a given V, current depends on how For a given V, current depends on how much of the electron’s energy is used up:much of the electron’s energy is used up:– Wire: electron’s energy Wire: electron’s energy overcomes friction overcomes friction– Light bulb: electron’s energy Light bulb: electron’s energy light and heat light and heat– Motor: electron’s energy Motor: electron’s energy turns motor turns motor

ResistanceResistance Measure of energy “used up”Measure of energy “used up”

– Depends on the material (and temperature)Depends on the material (and temperature)– Resistance: opposition to electron movementResistance: opposition to electron movement– Unit: Ohm (Unit: Ohm (););– Wires: very low resistance (often neglected)Wires: very low resistance (often neglected)– Insulators: very high resistance (often assumed to Insulators: very high resistance (often assumed to

be infinite)be infinite)– As resistance increases, it takes more “push” As resistance increases, it takes more “push”

(voltage) to cause a current(voltage) to cause a current

Ohm’s Ohm’s LawLawI = V / R

SPEED of electron’s is NOT effected by SPEED of electron’s is NOT effected by voltage! Currents transfer energy voltage! Currents transfer energy essentially instantaneously! essentially instantaneously! – Why? Wires are already full of “free” electrons!!Why? Wires are already full of “free” electrons!!– First electron into wire causes one to pop outFirst electron into wire causes one to pop out

Symbol of resistor:Symbol of resistor: Often called “loads”Often called “loads”

ResistanceResistance

AC/DCAC/DC Batteries produce “direct current” Batteries produce “direct current”

(DC)(DC)– Current is steadyCurrent is steady– One directionOne direction

““Wall outlets” provide “alternating Wall outlets” provide “alternating current” (AC)current” (AC)– Current oscillates back and forthCurrent oscillates back and forth

More on this later…(old movie)More on this later…(old movie)

Power LawPower Law Power is the rate at which energy is Power is the rate at which energy is

being delivered or consumed being delivered or consumed

Power = (Current)(Voltage)Power = (Current)(Voltage)P = IVP = IV

– Units: Watt (W)Units: Watt (W)

– So if 2 A of current is flowing through a So if 2 A of current is flowing through a load at 120 V, the Power used by the load load at 120 V, the Power used by the load isis

P = IV = (2A)(120V) = 240 WP = IV = (2A)(120V) = 240 W

Power LawPower Law How much current flows through a How much current flows through a

household (120 V) 60 W light bulb? household (120 V) 60 W light bulb?

P = IVP = IV

I = P/V = 60 W / 120 V = 0.5 A I = P/V = 60 W / 120 V = 0.5 A

Power vs. EnergyPower vs. Energy What’s the difference?What’s the difference?

– Power (Watts): Rate at which energy is Power (Watts): Rate at which energy is used or delivered. Doesn’t accumulate, used or delivered. Doesn’t accumulate, and cannot be stored in a bucketand cannot be stored in a bucket

– Energy (kWhrs): A quantity that Energy (kWhrs): A quantity that accumulates, or can be stored in a accumulates, or can be stored in a bucketbucket

Energy = Power x TimeEnergy = Power x Time

How to measure voltage and How to measure voltage and current current

Think “voltage across” and “current Think “voltage across” and “current through”through”

When measuring voltage, + and - When measuring voltage, + and - probes are inserted in the circuit where probes are inserted in the circuit where you want to measure the voltage you want to measure the voltage difference.difference.

V

How to measure voltage and How to measure voltage and current current

When measuring current, the circuit When measuring current, the circuit must be broken so the current flows must be broken so the current flows through the meterthrough the meter

A

In SummaryIn Summary

Electricity is the flow of electrons Electricity is the flow of electrons through a conductor driven by an through a conductor driven by an electromotive forceelectromotive force

V = IR (Ohms Law)V = IR (Ohms Law) P = IV (Power Law)P = IV (Power Law) Energy = Power x TimeEnergy = Power x Time