Electrical FieldsElectrical Fields

• The space around any charged object is The space around any charged object is filled with an electrical field.filled with an electrical field.

• This field extends through space, much This field extends through space, much like a gravitational fieldlike a gravitational field

• Electric fields have both magnitude and Electric fields have both magnitude and direction, and so can be shown as direction, and so can be shown as vectors.vectors.

Electric PotentialElectric Potential

• Electric charges have Electric charges have potential energypotential energy– By moving two By moving two

positive charges positive charges closer together they closer together they gain electrical gain electrical potential energypotential energy

• They want to release They want to release this energy much like this energy much like a compressed spring.a compressed spring.

• Electrical Potential = Electrical Potential = Electrical PE/ChargeElectrical PE/Charge

• Units: joule/coulomb Units: joule/coulomb = volt (V)= volt (V)

• Since electric Since electric potential is potential is measured in volts, it measured in volts, it is called voltageis called voltage

Energy StorageEnergy Storage

• Electrical energy can be stored in a Electrical energy can be stored in a capacitor.capacitor.

• Capacitors store energy slowly and Capacitors store energy slowly and release it rapidlyrelease it rapidly

• Used inUsed in– ComputersComputers– FlashesFlashes– TelevisionsTelevisions

Flow of ChargeFlow of Charge

• Charge flows when there is a potential Charge flows when there is a potential difference, or difference in voltage, difference, or difference in voltage, between ends of a conductor.between ends of a conductor.

• The flow will continue until both ends The flow will continue until both ends reach a common potential.reach a common potential.

• A flow of electric charge is called an A flow of electric charge is called an electric current.electric current.

• Bird on a wire…Bird on a wire…

Electric CurrentElectric Current

• Current is measured in amperesCurrent is measured in amperes– The label is A, or amps.The label is A, or amps.

• 1A = the flow of 1 coulomb per 1A = the flow of 1 coulomb per secondsecond

• When electrons flow through a When electrons flow through a wire, the number entering one end wire, the number entering one end equals the number exiting the equals the number exiting the other endother end

Voltage SourceVoltage Source

• In order for charge to flow, a In order for charge to flow, a voltage source must be used to voltage source must be used to maintain a potential difference.maintain a potential difference.

• BatteriesBatteries– Wet cellWet cell– Dry cellDry cell

• Generators Generators

ResistanceResistance

• The flow of current depends on the The flow of current depends on the electric resistance a conductor electric resistance a conductor offersoffers

• For most conductors, increased For most conductors, increased temperature means increased temperature means increased resistance.resistance.

• Resistance is measured in ohmsResistance is measured in ohms• Symbol: Symbol:

Ohm’s LawOhm’s Law

• Discovered by George OhmDiscovered by George Ohm• Current is directly proportional to Current is directly proportional to

the voltage across the circuit and the voltage across the circuit and inversely proportional to the inversely proportional to the resistance in the circuit.resistance in the circuit.

• Current = voltage/resistanceCurrent = voltage/resistance• I=V/RI=V/R• Units: volt/ohms = ampereUnits: volt/ohms = ampere

Ohm’s LawOhm’s Law

• What is the What is the resistance of a resistance of a toaster that pulls 12 toaster that pulls 12 amps from a 120 volt amps from a 120 volt circuit?circuit?

• R=V/IR=V/I• 120V/12amps = 10 120V/12amps = 10

ohmsohms

• How much current is How much current is drawn by a lamp that drawn by a lamp that has a resistance of has a resistance of 100 ohms when it 100 ohms when it has a power source has a power source of 50V?of 50V?

• I=V/RI=V/R• 50V/100ohms = .5 50V/100ohms = .5

ampsamps

Effects of current on the bodyEffects of current on the body

• Current in ampsCurrent in amps• .001 .001 • .005.005• .010.010• .015.015• .070.070

• EffectEffect• Can be feltCan be felt• PainfulPainful• Involuntary muscle Involuntary muscle

contractionscontractions• Loss of muscle Loss of muscle

controlcontrol• Probably fatal if it Probably fatal if it

lasts for more than 1 lasts for more than 1 sec.sec.

DC vs. ACDC vs. AC

• Direct Current – Direct Current – Charge always Charge always flows in one flows in one direction.direction.– Electrons flow Electrons flow

from negative to from negative to positivepositive

– Ex: BatteryEx: Battery

• Alternating Alternating CurrentCurrent–Electrons Electrons move first in one move first in one direction, then in direction, then in the other.the other.– This is caused by This is caused by

alternating polarity.alternating polarity.– Wall sockets Wall sockets

provide ACprovide AC

Using ACUsing AC

• Most of the US uses AC voltage of Most of the US uses AC voltage of 110-120V. 110-120V.

• Europe uses 220-240VEurope uses 220-240V• Some appliances, like electric Some appliances, like electric

stoves, use 220-240V.stoves, use 220-240V.• How can this be done in the US?How can this be done in the US?

– Think about the number of holes in a Think about the number of holes in a wall socket.wall socket.

ConversionsConversions

• Is it possible to Is it possible to operate a DC operate a DC device on AC device on AC power?power?

• Sure, you just Sure, you just need an AC/DC need an AC/DC converter.converter.

• These step down These step down the voltage and the voltage and contain a contain a DiodeDiode

• A diode only A diode only allows electrons allows electrons to flow in one to flow in one direction. direction.

• This gives choppy This gives choppy DC.DC.

• Together with a Together with a capacitor, it capacitor, it provides good DCprovides good DC

PowerPower

• Power is the rate at which electrical Power is the rate at which electrical energy is converted to another energy is converted to another form, such as heat, light, or form, such as heat, light, or mechanical energy.mechanical energy.

• Electric power = current x voltageElectric power = current x voltage• P=IVP=IV• 1 watt = 1A x 1V1 watt = 1A x 1V