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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?
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
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
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
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.
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
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
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!