Electric CircuitsElectric CircuitsUnit 5A

Light the Bulb! What needs to happen to get the bulb to

light???

Two Requirements for electricity to “flow”1. A potential difference - Voltage2. A closed loop - Circuit

Potential Difference

AKA: VOLTAGERemember when we studied Electric Fields,

and compared to Gravitational Fields

Using a + test charge:Using a mass:

Potential in Circuits

• Potential lines point away from positive terminal of battery

• Going with the lines: Decreasing in potential

• Going against the lines: Increasing in potential

Potential DifferenceFalling objects have more potential energy

when they are “HIGHER” above the groundWater stored high in a tower will want to

flow downhillThe direction of movement of the water

naturally follows the gravitational field linesWork must be done to lift it

Potential DifferenceCharge also naturally “flows” in the

direction of the field linesLike the water pressure, this electrical

“pressure” is due to the potential difference

So how do we “pump” the charge?

Galvani vs. Volta

Luigi Galvani Alessandro Volta (1791) (1800)

Biology vs. ChemistryGalvani’s frog dissectionUsing two different metals, touching one to

spinal cord other to muscle:Frog leg jerked!Animal Electricity

Biology vs. ChemistryVolta found, however that

animals weren’t necessary…It was the two different metals

that were importantHe made a pile of copper and

zinc plates separated by thin paper soaked in an electrolyte

Voltaic Pile = Battery

VoltageThe chemical reaction Volta created

allowed electrons to move from one metal plate to the next

Because the unit of Potential Difference is named after Volta (the Volt), it is often called VOLTAGE

To measure: Use a Voltmeter2 probes (1 for reference point) Why is a bird on a wire safe?

Circuit

The 2nd requirement for charges to flow:There must be a closed loop from + to –In order for there to be a potential

difference, the charges have to have the electric field set up with a high potential (+ anode) and low potential (- cathode)

Circuit

The 2nd requirement for charges to flow:There must be a closed loop from + to –In order for there to be a potential

difference, the charges have to have the electric field set up with a high potential (+ anode) and low potential (- cathode)

CircuitWhich charges are moving?

Positive or negative?And how do they move?

Fast or slow?Instantaneously, or is there a delay?

Is a battery a source of charges that move all the way through the wires to the other side?

Do the charges get “used up” when the batteries die?

Electron Drift VelocityWhen the switch is open:Free electrons (conducting electrons) are always

moving randomly, colliding with atoms.

The random speeds are at an order of 106 m/s.BUT… there is no net movement of charge

across a cross section of a wire.

Electron Drift VelocityBut when the switch is closed:Electrons start to “drift” gradually in the

direction of the electric fieldAverage Velocity = about 0.01 cm/sSo how do lights turn on instantly when

you flip a switch?

Electron Drift VelocityAn electric field is established almost

instantly (at the speed of light, 3x108 m/s).Free electrons, while still randomly moving,

immediately begin drifting due to the electric field, resulting in a net flow of charge.

Electron Drift VelocityWhen the electric field is established, all

the charges in the circuit moveCharges don’t come out of the battery and

go all they way around the circuit…Charges in the wires are set in motionThey don’t get “used up”Conservation of charge! What does get “used up” when batteries

die? Does it really get used up? What’s a better way to explain this?

Conventional CurrentEven though we know the smaller electrons

are the ones doing the moving…..“By convention” we say that current flows

from the + terminal of the battery to the –The flow of + charge is called current (I)

and is measured in Amperes (A) using an Ammeter

1 Amp = 1 C/s QI = ---- t

AC/DCAlternating current vs. Direct currentBatteries provide Direct current

DC – Charges travel in ONE directionGenerators (electrical plants) provide AC

Charges oscillate back and forthCompare to a longitudinal waveIn US – AC frequency = 60 Hz, in Europe =50 Hz

Household circuits (plugs in walls)Battery-operated equipment needs a

transformer to plug into the wall

Factors Affecting CurrentPotential Difference

Electrical Resistance

ResistanceResistance is a measure

of opposition to the flow of charge…

Causing the electrical energy to be converted to thermal energy or light.

Studied by Georg Ohm Unit = Ohm ()

Measured with an Ohmmeter

Resistance

Electrical current acts like water currents:

Factors Affecting ResistanceResistivity

Conductor vs. InsulatorTemperature – Metals: Resist more when HOT,

can be superconductors when very cool; Semiconductors: Opposite

Length of wireLong – more collisions, Short – fewer collisions

Cross-sectional AreaWide vs. Thin

Number of Paths

Resistivity of Materials

Ohm’s LawVoltage is directly related to the currentResistance is inversely related to the

currentV = I RMost resistors follow Ohm’s Law under

normal circumstancesSome resistors are “non-Ohmic” due to

conductivity or temperature

Ohm’s LawAs current flows through a circuit, the

energy is dissipated in the resistors and the potential drops

Ohm’s LawA hair dryer operates on 110 V and

draws 1100 mA. What is the resistance of the hair dryer?

V = I RDon’t forget to convert milliAmps!110 V = (1.1 A) RR = 100 Ω

Schematic DiagramsCircuits typically contain a voltage source,

wire conductors, and one or more devices which use the electrical energy.

Schematic DiagramsHow would you draw the following circuit?

Series CircuitsResistors are said to be connected in Series

when there is only ONE pathway for charges.

If the circuit breaks, current will stop and all devices will go off

Current in Series CircuitsBecause there is only ONE path, all the

current coming out of the battery goes through each resistor

I1 = I2 = I3 … Same in all!Each resistor causes the voltage to drop,

the amount of current depends on the Equivalent Resistance of the whole circuit

Ibattery= I1 = I2 = I3 = Vbattery/ Req

Equivalent Resistance in Series

The amount of resistance that a single resistor would need in order to equal the overall affect of the collection of resistors that are present in the circuit.

For a series circuit: Req = R1+ R2 + R3 ….

Voltage Drop in SeriesTotal Voltage drop around a circuit (+ to –

terminals of battery) = Voltage of batterySum of voltage drops across each resistor

must add up to the Voltage of the batteryΔVbattery = ΔV1+ ΔV2 + ΔV3 ….

ΔV1 = I • R1 ΔV2 = I • R2 ΔV3 = I • R3

Series Circuit ProblemCalculate the following quantities:

• Req = R1 + R2 + R3 = 17Ω + 12Ω + 11Ω = 40Ω

• Itot = Vbattery / Req = (60V) / (40Ω) = 1.5 A

• V1 = I1R1 V2 = I2R2 V3 = I3R3

• V1 =(1.5A)(17Ω) V2 =(1.5A)(12Ω) V3 =(1.5A)(11Ω)

• V1 = 25.5 V V2 = 18 V V3 = 16.5 V

Parallel CircuitsHave multiple pathways for the current to

flow.

If the circuit is broken the current may pass through other pathways and other devices will continue to work.

Voltage Drops in Parallel Circuits

Each resistor has a direct path, connected to both terminals of battery

Vbattery = V1 = V2 = V3 = ... Same in all!

So the amount of current passing through each can be calculated using Ohm’s Law

I1 = V1 / R1 I2 = V2 / R2 I3 = V3 / R3

Eq. Resistance in ParallelMultiple pathways = Less Resistance

Eq. Resistance in ParallelMultiple pathways = Less Resistance1/Req = 1/R1 + 1/R2 + 1/R3 + ...

Current in Parallel CircuitsMultiple pathways = More current can flowItot = I1 + I2 + I3 … = ΔV / Req

Multiple pathways = More current can flowItot = I1 + I2 + I3 … = ΔV / Req

More current will take the path of least resistance

So the amount of current passing through each can be calculated using Ohm’s Law

I1 = V1 / R1 I2 = V2 / R2 I3 = V3 / R3

Current in Parallel Circuits

Parallel Circuit ProblemCalculate the following quantities:

• 1/Req = 1/R1 + 1/R2 + 1/R3 = 4.29 Ω (rounded)

• Itot = Vbattery / Req = (60V) / (4.29 Ω) = 14 A (rounded)

• I1= V1/R1 I2 = V2/R2 I3 = V3/R3

• I1 =(60V)/(17Ω) I2 =(60V)/(12Ω) I3 =(60V)/(11Ω)

• I1 = 3.53 A I2 = 5 A I3 = 5.45 A

Series vs. Parallel CircuitsSERIES

SAME IN ALL: CurrentAdd more, Resistance

goes up, so Current goes down (bulbs dim)

Req = R1+ R2 + R3

Itot = ΔV / Req

ΔV1 = IR1, ΔV2 = IR2

ΔVtot = ΔV1+ ΔV2 + ΔV3

PARALLELSAME IN ALL: VoltageAdd more resistors,

Resistance goes down so Current goes up (brighter)

1/Req= 1/R1+ 1/R2 + 1/R3

Itot = ΔV / Req

I1 = V1 /R1, I2 = V2/R2

Itot = I1 + I2 + I3 …

Combination CircuitsWhen circuits contain Series and Parallel

segmentsFIRST: Find Req for Parallel

THEN: Find Req for all in Series

Household CircuitsWhich circuit type would work best in your

home?

Measuring Voltage

VOLTMETERTo find a potential

difference, you need 2 probes: black is reference point ( -#, switch!)

Connect in PARALLELVoltmeters must have

HIGH Resistance so as not to affect the current

Measuring Current

AMMETERTo measure current, you

want all current to flow through but not be reduced

Connect in SERIESAmmeters must have

VERY LOW Resistance so as not to affect the current

Measuring Resistance

OHMMETERTo measure resistance, you

want the meter to send current through at a calibrated voltage

Connect in PARALLELMake sure to check the

scale and where the probes are plugged in every time you measure!

Short CircuitA short circuit is a parallel path in a circuit with

zero or very low resistance. Short circuits can be made accidentally by

connecting a wire between two other wires at different voltages.

Short circuits are dangerous because they can draw huge amounts of current.

Electrical PowerRemember: Power is the rate at which

Energy is transferredP = E / tP = (Electrial PE) /tP = (qV) / tAlso remember…. q / t = I P = I V(Pigs In Venus!)Unit: Watt (W) = J/s = AV = CV/s

Watt’s LawP = I V is called Watt’s LawCan be written other ways, depending on

what variables are known, along with Ohm’s Law

P = I V . . . And I = V/R so…P = (V/R)V = V2/ROR… Using V = IR..P = I (IR) = I2RP = I V = V2/R = I2R

Electrical Power ProblemsHow much power is dissipated by a 100-Ω

resistor if 50 mA of current flow through it?P = I V = V2/R = I2RP = I2RP = (0.050 A)2 (100 Ω) = 0.25 WIf the circuit above is left close for 10 s, how

many Joules of energy are used?P = E / tE = P t = (0.25 W) (10 s)= 2.5 J

Electrical PowerElectric companies charge for the number of

kilowatt-hours used during a set period of time, often a month.

One kilowatt-hour (kWh) means that a kilowatt of power has been used for one hour.

Since power multiplied by time is energy, a kilowatt-hour is a unit of energy.

One kilowatt-hour is 3.6 x 106 J.

Typical Power RatingsAppliance Power (W)

Stove 12,000Clothes Dryer 5,000Heater 2,000Dishwasher 1,600Photocopier 1,400Iron 1,000Vacuum Cleaner 750Coffee Maker 700Refrigerator 400Portable Sander 200Fan 150Personal Computer 150TV Receiver 120Fax Transmitter/Receiver 65Charger for Electric Toothbrush 1

Electrical Power - TransmissionElectric companies need to send energy

long distancesWires have resistance and dissipate energy

as heat – rate of Ediss = PowerWhich has best results in reducing Power?

More or less Voltage?More or less Current?

Answer: Reducing CurrentWhy? How?

Electrical Power - TransmissionP = I V can be rewritten: I = P / V

For a given amount of Power needed, increasing the Voltage will decrease the Current (and therefore the # of collisions)

P = I2RSo if V increases 10x, I decreases 10x, and

Power lost decreases 100xPower lines typically transmit at very high

voltages to reduce heat losses

Human Body and ElectricityResistance: Dry, clean skin – up to 500,000

Ω; Wet, broken, burned skin – 500-1000 ΩWhich is more lethal: High Voltage or

Current?Answer: CURRENT - 100-200 mA ACMost dangerous if through the heartVERY high voltages and currents not as

lethal, heart/lungs clamp but don’t fibrillateAC more lethal than DC

Human Body and ElectricityIf even low currents happen for a

prolonged time, skin will burnBurnt skin reduces resistance, allows more

current!

Summary

Electricity is wonderful….

But must be treated with respect!

Good luck finishing the packet!