Electricity & Magnetism

Where science and magic meet

What about it?

• While the study of electricity is While the study of electricity is relatively new (less than 200 relatively new (less than 200 years), a great deal of years), a great deal of investigation has been done investigation has been done recentlyrecently

• Due to innovations in the modern Due to innovations in the modern electrical world (1920 –)electrical world (1920 –)

Wringers…Wringers…

Electricity!

• is the flow of is the flow of electronselectrons, described , described as an as an electrical currentelectrical current..

Charges

• A A chargecharge is a is a point sourcepoint source of of electrical force. electrical force.

• There are only two charges; There are only two charges; Positive and Negative.Positive and Negative.

Rule of Charge

• The Rule of Charges is stated:The Rule of Charges is stated:

““Opposites Attract, Likes Repel”Opposites Attract, Likes Repel”

Electricity

• Electrons are negative, so the Electrons are negative, so the electrons move from a electrons move from a negative negative polepole towards a towards a positive polepositive pole..

• ElectricityElectricity can be described as the can be described as the forces between charges. forces between charges.

Electrical FieldElectrical Field• Area over which an electrical Area over which an electrical

charge can apply a forcecharge can apply a force

SQUIDS?SQUIDS?

SQUIDS?SQUIDS?

Charges in the world

• Most common objects are Most common objects are neutralneutral (having the same number of + and – (having the same number of + and – charges)charges)

• Law of Conservation of ChargeLaw of Conservation of Charge• An object which has more of one An object which has more of one

charge than another is said to be charge than another is said to be chargedcharged..

Shocking!

• The buildup of charges on an The buildup of charges on an object is object is static electricitystatic electricity..

• When the static charges do move, When the static charges do move, it is called an it is called an electrical dischargeelectrical discharge (the charges move along the path (the charges move along the path of least resistance).of least resistance).

How to charge

• InductionInduction– Bringing a charged object near an Bringing a charged object near an

uncharged object causes the like uncharged object causes the like charges to run away and attracts the charges to run away and attracts the opposite chargesopposite charges

– Give the like charges an “out”, Give the like charges an “out”, leaving the oppositesleaving the opposites

Inducing

• A charged object is brought A charged object is brought near a neutral objectnear a neutral object

• The Rule of Charge causes The Rule of Charge causes like charges to “flee” from their like charges to “flee” from their equalsequals

• If the neutral object is If the neutral object is “grounded” the like charges “grounded” the like charges leaveleave

• The grounding path is severed The grounding path is severed after the like charges fleeafter the like charges flee

• The “neutral” object now has The “neutral” object now has the opposite chargethe opposite charge

How to charge

• ConductionConduction– Touching a charged object to an Touching a charged object to an

uncharged object causes the uncharged object causes the charges on the charged object to charges on the charged object to spread through the two objectsspread through the two objects

Conducting

• A charged object is A charged object is brought into contact with a brought into contact with a neutral objectneutral object

• The Rule of Charge The Rule of Charge causes like charges to causes like charges to “flee” from their equals“flee” from their equals

• The like charges spread The like charges spread equally over the two equally over the two objects, charging the objects, charging the previously neutral objectpreviously neutral object

Shazam!

• LightningLightning is a form of electrical is a form of electrical discharge.discharge.

• The static charges are formed The static charges are formed when the particles inside the storm when the particles inside the storm cloud rub against each other, cloud rub against each other, separating charges.separating charges.

Lightn

ing

Lightn

ing

Get motivated!

• The The voltage differencevoltage difference between between two points in a circuit causes the two points in a circuit causes the electrons to flowelectrons to flow

• The larger the The larger the electric potential electric potential differencedifference, or , or voltagevoltage, the more , the more “push” for the electrons to flow“push” for the electrons to flow

• Unit and formula symbol: VUnit and formula symbol: V

Get moving!

• The The electric currentelectric current is the amount is the amount or number of electrons that flow or number of electrons that flow through a point in a circuitthrough a point in a circuit

• Measured in Amperes, or Amps, Measured in Amperes, or Amps, symbolized by an “A”symbolized by an “A”

• Symbol “Symbol “II” in a formula” in a formula– Originally called “Electrical Intensity”Originally called “Electrical Intensity”

Comparing V and IComparing V and I

• Voltage and Current can be Voltage and Current can be changed in varying amounts changed in varying amounts depending on the load to be depending on the load to be accomplishedaccomplished

Get out of the way!

• The The electric resistanceelectric resistance is the is the resistance to the flow of electricity resistance to the flow of electricity due to the nature of the materialdue to the nature of the material– Conductors have lower resistance Conductors have lower resistance

than insulatorsthan insulators

• Measured in Ohms,, symbolized Measured in Ohms,, symbolized by an omega “by an omega “ΩΩ””

• Symbol “Symbol “RR” in a formula” in a formula

Get connected!Get connected!

• The wires or leads in a circuit have The wires or leads in a circuit have somesome resistance resistance– LengthLength

• Shorter wires have less resistance than Shorter wires have less resistance than longer wireslonger wires

– ThicknessThickness• Thicker wires have less resistance than Thicker wires have less resistance than

thinner wiresthinner wires

– TemperatureTemperature• Cooler wires have less resistance than Cooler wires have less resistance than

hotter wireshotter wires

Ohm’s LawOhm’s Law• Ohm’s law states that the current Ohm’s law states that the current

flowing through a wire is flowing through a wire is dependent on the ratio of voltage dependent on the ratio of voltage to resistanceto resistance– How much “push” divided by how How much “push” divided by how

much resistancemuch resistance

IIcurrentcurrent

==

VVvoltagevoltage

RRresistanceresistance

EMFEMF• Any source of voltage can be Any source of voltage can be

called an “Electromotive Force”called an “Electromotive Force”

• EMFEMF

• Portable sources are Portable sources are usuallyusually “cells” or “batteries”“cells” or “batteries”

Know the difference!

• A “A “cellcell” is a reactor designed to ” is a reactor designed to create create electric potential differenceelectric potential difference due to a chemical reactiondue to a chemical reaction

Know the difference!A “battery” is a series of cells that A “battery” is a series of cells that

work together to produce higher work together to produce higher voltagesvoltages

Wet or dry?Wet or dry?• A A “wet cell” battery“wet cell” battery uses a liquid uses a liquid

electrolytic solution to conduct electrolytic solution to conduct electricity and ionselectricity and ions– Car, motorcycle, lawnmowerCar, motorcycle, lawnmower

So dry, it’ll blow away…• A A “dry cell” battery“dry cell” battery uses an uses an

alkaline paste to conduct alkaline paste to conduct electricity and ionselectricity and ions– Alkaline batteries, certain Alkaline batteries, certain

rechargeable batteriesrechargeable batteries

Electrons Electrons freed freed from the from the Zinc Zinc casing casing flow flow through through the the circuit circuit and are and are absorbed absorbed by the by the Carbon.Carbon.

How to tell…

• An An electroscopeelectroscope is a device that is a device that can detect whether or not an can detect whether or not an object is chargedobject is charged

• As charges spread over the As charges spread over the leavesleaves (sing. leaf), (sing. leaf), likes repel likes repel and and the leaves separatethe leaves separate

Bringing a charged object near an Bringing a charged object near an electroscope causes like charges to repel, electroscope causes like charges to repel, separating the leavesseparating the leaves

Circuits

• A circuit describes the path that electrons flow through.

• If there is a break in the circuit, the electric current will not flow.

Circuits

• An incomplete or broken circuit is said to be an “open circuit”.

• A complete circuit is called a “closed circuit”.

• Switches are devices designed to make a circuit open or close.

Oops!

• A short circuit occurs when the electrical current finds a “short cut” across the circuit.

• When this happens, the electricity jumps across the path of least resistance, and the designated work, or load, is not accomplished.

A complete circuitA complete circuit

• A voltage sourceA voltage source

• Wires or leadsWires or leads

• A loadA load

• Optionally,Optionally,

• SwitchesSwitches– Multiples…Multiples…

Circuit SymbolsCircuit SymbolsWire EMF Fuse Lamp

Speaker Switch Connection Transformer

Capacitor Resistor Variable Resistor

Ammeter Voltmeter Fuse 2 Galvanometer

Ground Circuit Breaker

All in a row…All in a row…

• A A series circuit series circuit has all the parts of has all the parts of the circuit in the circuit in oneone line line

• If any break opens the circuit NO If any break opens the circuit NO CURRENT will flowCURRENT will flow

• ““If one goes out, they all go out.”If one goes out, they all go out.”

Measuring electricity

• Voltmeters must be linked across Voltmeters must be linked across a part of a circuit to measure a part of a circuit to measure voltagevoltage

• Ammeters must be linked in the Ammeters must be linked in the circuit to measure currentcircuit to measure current

EMFEMF

Resistor or LOADResistor or LOAD

AmmeterAmmeter

Which Which way does way does the circuit the circuit operate?operate?

EMFEMF

Resistor or LOADResistor or LOAD

AmmeterAmmeter

Which Which way does way does the circuit the circuit operate?operate?

VoltmeterVoltmeter

Series calcs…Series calcs…

• The resistance of any resistors or The resistance of any resistors or loads in a series circuit simply add loads in a series circuit simply add together…together…

• RRtotaltotal = R = R11 + R + R22 + R + R33 + R + R44……

All in a row…All in a row…• A A parallel circuit parallel circuit has has multiple paths multiple paths

between the EMF and the loadsbetween the EMF and the loads

• Current will flow though any part that can Current will flow though any part that can get back to the EMFget back to the EMF

• How houses and such are wiredHow houses and such are wired

• Otherwise, you would have to have Otherwise, you would have to have something in and turned on in every outletsomething in and turned on in every outlet

Parallel calcs…Parallel calcs…

• The resistance of any resistors or The resistance of any resistors or loads in a parallel circuit actually loads in a parallel circuit actually reduce reisistance…reduce reisistance…

• 1 1 1 1 11 1 1 1 1

• RRtotaltotal = R = R11 + R + R22 + R + R33 + R + R44……

• More parallel circuits reduce More parallel circuits reduce resistance, increase current…resistance, increase current…

Safety First!

• Because buildings and vehicles contain so many parallel connections, safety devices had to be designed to limit the flow of current

FusesFuses• Fuses are the simplest of electrical Fuses are the simplest of electrical

safety devices.safety devices.

• A fuse consists of a thin strip of A fuse consists of a thin strip of metal between connectionsmetal between connections

• When too much current flows, the When too much current flows, the strip melts, opening the circuitstrip melts, opening the circuit

• Once a fuse has “blown”, it can Once a fuse has “blown”, it can not be reset, and must be not be reset, and must be replacedreplaced

Breaker, breaker…• A A circuit breaker circuit breaker is a spring-loaded is a spring-loaded

switch that will “trip” and open the switch that will “trip” and open the circuit when a metallic strip inside circuit when a metallic strip inside gets hot from excess currentgets hot from excess current

• Circuit breakers are resettable Circuit breakers are resettable after they are trippedafter they are tripped

WARNINGWARNING

• Do not Do not continue resetting a circuit continue resetting a circuit breaker – find out why it is trippingbreaker – find out why it is tripping

GFCIGFCI• A A Ground-Fault Circuit Interruptor,Ground-Fault Circuit Interruptor, or GFCI, is or GFCI, is

a circuit breaker built into an outlet in your a circuit breaker built into an outlet in your home.home.

• GFCIs are required in newer homes, usually GFCIs are required in newer homes, usually in areas that experience excess moisturein areas that experience excess moisture

WARNINGWARNING

• Do not Do not continue resetting a GFCI breaker– continue resetting a GFCI breaker– find out why it is trippingfind out why it is tripping

ELECTRICITY!ELECTRICITY!• ChargeCharge• FlowFlow• Rule of ChargeRule of Charge• Electric FieldElectric Field• Static ElectricityStatic Electricity• Electric DischargeElectric Discharge• Charging by:Charging by:

– ConductionConduction– InductionInduction

• LightningLightning• VoltageVoltage• CurrentCurrent

• ResistanceResistance• Ohm’s LawOhm’s Law• CellsCells• BatteriesBatteries• ElectroscopeElectroscope• van de Graff Generatorvan de Graff Generator• CircuitCircuit• Series CircuitSeries Circuit• Parallel CircuitParallel Circuit• Safety DevicesSafety Devices