Basic Electricity All material is made of atoms (we think) All material is made of atoms (we think) –Atoms are comprised of  Nucleus: protons (+) and neutrons –Fixed  Electrons (-) orbit outside the nucleus

Basic Electricity –Electrons and protons are charged  Opposite charges attract  Like charges repel  In other words, charges desire to be “zeroed” or “neutralized” or “balanced”

Basic Electricity How tightly electrons are bound depends on material How tightly electrons are bound depends on material –Tightly bound electrons: insulator  Does not conduct electricity (or heat) well  glass, plastic, rubber –Loosely bound (“free”) electrons: conductor  Conducts electricity (and heat) well  metals –In between: semiconductors  Important for computer chip production  silicon, germanium, carbon

Basic 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.

Voltage What causes electrons to flow? What causes electrons to flow? Protons fixed; Electrons “free” Protons fixed; Electrons “free” Electrons move! Electrons move!          battery Charge imbalance caused by chemical reactions  Charges desire to be neutralized  Charges flow!

There must be a complete path between excess + and excess – charges There must be a complete path between excess + and excess – charges Otherwise + and – charges don’t “see” each other Otherwise + and – charges don’t “see” each other Voltage          battery

Voltage A battery has regions of excess negative and positive charges caused by chemical reactions A battery has regions of excess negative and positive charges caused by chemical reactions The amount of “push” exerted on the electrons is called “electromotive force” The amount of “push” exerted on the electrons is called “electromotive force” –unit: volt (V) –Voltage is relative, only is meaningful when talking about voltage difference between 2 points

Voltage Do batteries run out of charge? Do batteries run out of charge? What happens to total # of electrons? Nothing! Electrons do not get used up – energy does!! What happens to total # of electrons? Nothing! Electrons do not get used up – energy does!!          battery

Current Number of electrons that pass a certain point in a certain time: current Number of electrons that pass a certain point in a certain time: currentwire –Unit: Ampere (amp) = 6.25  electrons/s –Or Amp = 1 Coulomb/second           Count ‘um passing here!

Current Historically, current discussed in terms of positive charge flow: “conventional” current flow – actually it’s wrong! Historically, current discussed in terms of positive charge flow: “conventional” 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? Does it take time for electrons to flow from a switch to a bulb? Does it take time for electrons to flow from a switch to a bulb? Light Bulb miles

Time Delay? Does it take time for electrons to flow from a switch to a bulb? Does it take time for electrons to flow from a switch to a bulb? NO!! Wire is already full of electrons! NO!! Wire is already full of electrons! Light Bulb miles

Current How much current will flow? How much current will flow? –Wire is broken? Zero (need a complete circuit) –Large voltage – large current –Small voltage – small current –All other things being equal! For a given V, current depends on how much of the electron’s energy is used up: For a given V, current depends on how much of the electron’s energy is used up: –Wire: electron’s energy  overcomes friction –Light bulb: electron’s energy  light and heat –Motor: electron’s energy  turns motor

Resistance Measure of energy “used up” Measure of energy “used up” –Depends on the material (and temperature) –Resistance: opposition to electron movement –Unit: Ohm (  ); –Wires: very low resistance (often neglected) –Insulators: very high resistance (often assumed to be infinite) –As resistance increases, it takes more “push” (voltage) to cause a current Ohm’s Law I = V / R

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

AC/DC Batteries produce “direct current” (DC) Batteries produce “direct current” (DC) –Current is steady –One direction “Wall outlets” provide “alternating current” (AC) “Wall outlets” provide “alternating current” (AC) –Current oscillates back and forth More on this later…(old movie) More on this later…(old movie)

Power Law Power is the rate at which energy is being delivered or consumed Power is the rate at which energy is being delivered or consumed Power = (Current)(Voltage) P = IV –Units: Watt (W) –So if 2 A of current is flowing through a load at 120 V, the Power used by the load is P = IV = (2A)(120V) = 240 W

Power Law How much current flows through a household (120 V) 60 W light bulb? How much current flows through a household (120 V) 60 W light bulb? P = IV I = P/V = 60 W / 120 V = 0.5 A

Power vs. Energy What’s the difference? What’s the difference? –Power (Watts): Rate at which energy is used or delivered. Doesn’t accumulate, and cannot be stored in a bucket –Energy (kWhrs): A quantity that accumulates, or can be stored in a bucket Energy = Power x Time

How to measure voltage and current Think “voltage across” and “current through” Think “voltage across” and “current through” When measuring voltage, + and - probes are inserted in the circuit where you want to measure the voltage difference. When measuring voltage, + and - probes are inserted in the circuit where you want to measure the voltage difference. V

How to measure voltage and current When measuring current, the circuit must be broken so the current flows through the meter When measuring current, the circuit must be broken so the current flows through the meter A

In Summary Electricity is the flow of electrons through a conductor driven by an electromotive force Electricity is the flow of electrons through a conductor driven by an electromotive force V = IR (Ohms Law) V = IR (Ohms Law) P = IV (Power Law) P = IV (Power Law) Energy = Power x Time Energy = Power x Time