Chapter- 12Chapter- 12Current Current ElectricityElectricity

How is current How is current produced?produced? When a high potential is When a high potential is

connected by a conductive connected by a conductive material to a low potential. material to a low potential.

– Flow of electrons is called electric Flow of electrons is called electric current.current.

– Units: C/s = Amperes (Amps)Units: C/s = Amperes (Amps) t

qI

High Potential

Low Potential Equal

PotentialEqual Potential

When the potentials are equal, When the potentials are equal, the current stops flowing the current stops flowing – To increase the potential of the To increase the potential of the

electrons an electron pump must electrons an electron pump must convert (do work) another form of convert (do work) another form of energy into electrical potential energy into electrical potential energy.energy. Electron pump examples: Electron pump examples:

– Voltaic or galvanic cell (dry cell)Voltaic or galvanic cell (dry cell)– Several cells connected (Battery)Several cells connected (Battery)– Photovoltaic cell (solar cell)Photovoltaic cell (solar cell)– GeneratorGenerator

High Potential

Low Potential

Electron Pump

Two types of current Two types of current

Direct Current – (D.C.) Flow of Direct Current – (D.C.) Flow of electrons in only one directionelectrons in only one direction

– BatteryBattery Alternating Current – (A.C.) Flow of Alternating Current – (A.C.) Flow of

electrons at first in one direction and electrons at first in one direction and then the other direction. then the other direction.

– GeneratorGenerator– In your HouseIn your House– 60 Hertz60 Hertz

What affects current What affects current flow?flow? Potential Difference – VoltagePotential Difference – Voltage Resistance - slowing or Resistance - slowing or

reduction of the flow of current reduction of the flow of current – Resistance depends on the Resistance depends on the

material, length of material, and material, length of material, and the cross sectional area of the the cross sectional area of the material through which the current material through which the current is moving.is moving.

A

LR R is resistance, is resistivity,

L is length, A is the cross sectional area

Ohm’s LawOhm’s Law

Ohm’s Law stated – The ratio of Ohm’s Law stated – The ratio of V/I is a constant, where V is the V/I is a constant, where V is the voltage applied across a piece of voltage applied across a piece of material (such as wire) and I is material (such as wire) and I is the current through the the current through the material. R is the resistance of material. R is the resistance of the piece of material.the piece of material.

– Units: Ohms (Units: Ohms () = Volt/amp) = Volt/ampCurrent

VoltageResistance

I

VR

Electric Power Electric Power

When there is current (I) in a When there is current (I) in a circuit as a result of a voltage (V), circuit as a result of a voltage (V), the electric power (P) delivered to the electric power (P) delivered to the circuit is:the circuit is:–– Units: watts = amp x volt = J/sUnits: watts = amp x volt = J/s

voltagecurrentPower VIP

Electric PowerElectric Power

Many electrical devices become hot when Many electrical devices become hot when provided with sufficient electric power. provided with sufficient electric power. Toasters, irons, space heaters, heating Toasters, irons, space heaters, heating elements in electric stoves, and incandescent elements in electric stoves, and incandescent light bulbs. In such cases it is possible to light bulbs. In such cases it is possible to obtain two equivalent power equationsobtain two equivalent power equations

When electric energy is transferred into When electric energy is transferred into thermal energy in a resistor, the increase in thermal energy in a resistor, the increase in thermal energy is:thermal energy is:

– units: Kilowatt hour or Joules units: Kilowatt hour or Joules

RIP 2 R

VP

2

RtIQ 2

Current in electric Current in electric CircuitsCircuits An electric Circuit is An electric Circuit is

a closed loop. a closed loop. Basic Circuits consist Basic Circuits consist

of three things:of three things:– Electron pump Electron pump

(Battery)(Battery)– Device that reduces Device that reduces

potential. (User)potential. (User)– Conducting Conducting

connectionsconnections (Wires) (Wires)

Direct Current Circuits:Direct Current Circuits:

Circuit – closed loop in Circuit – closed loop in which current can flowwhich current can flow

Basic simple circuitBasic simple circuit– Voltage sourceVoltage source– Voltage usesVoltage uses– conducting connections conducting connections

(wire)(wire) Diagrams for electric Diagrams for electric

circuitscircuits– Describe in wordsDescribe in words– use use

photographs/drawingsphotographs/drawings– Use symbols Use symbols

(schematics)(schematics)

Schematic SymbolsSchematic Symbols

Drawing Schematic Drawing Schematic CircuitsCircuits Draw symbol for battery and indicate “+” Draw symbol for battery and indicate “+”

and “-“ terminalsand “-“ terminals Draw wire from “+” to the voltage user and Draw wire from “+” to the voltage user and

draw its symbol.draw its symbol. If a point where there are two current paths, If a point where there are two current paths,

draw connection symbol and follow one path draw connection symbol and follow one path to the user. Return to the second path and to the user. Return to the second path and draw to its user and continue until paths join.draw to its user and continue until paths join.

Follow path to the “-“ terminal Follow path to the “-“ terminal Check workCheck work Connect ammeter in seriesConnect ammeter in series Connect voltmeter in parallelConnect voltmeter in parallel

ObjectivesObjectives

Describe the configuration of a Describe the configuration of a working circuitworking circuit

Interpret circuit diagramsInterpret circuit diagrams Describe the characteristics of series Describe the characteristics of series

connectionsconnections Determine equivalent resistance of Determine equivalent resistance of

circuits having two or more resistors.circuits having two or more resistors. Explain the cause of short circuitsExplain the cause of short circuits Homework:Homework:

– Read Pages 531-541Read Pages 531-541– Pg 534 #1-3, Pg 537 # 6-9Pg 534 #1-3, Pg 537 # 6-9

Series CircuitsSeries Circuits Only one path for current to followOnly one path for current to follow User followed by another user (ie. User followed by another user (ie.

Resistor followed by a light)Resistor followed by a light)

...321 VVVVequ

…

Series Circuit RulesSeries Circuit Rules

Current is the same for each user

The effective resistance is the sum of all resistors in the series

– The sum of the voltage drop is

equal to the total voltage drop –

...321 RRRRequ

...321 VVVVequ

Parallel CircuitsParallel Circuits

Two or more paths for Two or more paths for current to followcurrent to follow

Parallel Circuit RulesParallel Circuit Rules

Total current in the circuit is the sum Total current in the circuit is the sum of the current in all its paths of the current in all its paths (branches)(branches)

– The equivalent resistance decreases The equivalent resistance decreases

with more parallel resistorswith more parallel resistors–

Voltage is the same in each path.Voltage is the same in each path.

...321 IIIItot

...1111

321

RRRRequ

+ -

ObjectivesObjectives

Describe the configuration of a working circuitDescribe the configuration of a working circuit Distinguish between series and parallel circuitsDistinguish between series and parallel circuits Describe the characteristics of series Describe the characteristics of series

connections and of parallel connections.connections and of parallel connections. Interpret circuit diagramsInterpret circuit diagrams Determine equivalent resistance of circuits Determine equivalent resistance of circuits

having two or more resistors.having two or more resistors. Explain the cause and prevention of Explain the cause and prevention of

overloading household circuits.overloading household circuits. Homework:Homework:

– Read Pages 531-541Read Pages 531-541– Pg 534 #1-3, Pg 537 # 6-9Pg 534 #1-3, Pg 537 # 6-9

Part IPart I

Basic electric circuit and its Basic electric circuit and its diagram.diagram.

What causes the flow of electrons What causes the flow of electrons in a circuit.in a circuit.

Drift velocity.Drift velocity. Voltaic cell.Voltaic cell.

Electric CircuitElectric Circuit

Diagram of Electric Diagram of Electric CircuitCircuit

Remember: Electric Potential Remember: Electric Potential Energy- Energy- Two Unlike ChargesTwo Unlike Charges

Higher Potential Energy

Lower Potential Energy

+

-

•To cause movement of a charge, there must be a potential difference.

While the switch is While the switch is open:open: Free electrons (conducting electrons) Free electrons (conducting electrons)

are always moving in random motion.are always moving in random motion.

The random speeds are at an order of The random speeds are at an order of

101066 m/s. m/s. There is no There is no netnet movement of charge movement of charge

across a cross section of a wire.across a cross section of a wire.

http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/imgele/micohm.gif

What occurs in a wire when the circuit switch is closed?

What occurs in a wire What occurs in a wire when the circuit switch when the circuit switch is closed?is closed?

An electric field is established An electric field is established instantaneously (at almost the speed instantaneously (at almost the speed of light, 3x10of light, 3x1088 m/s). m/s).

Free electrons, while still randomly Free electrons, while still randomly moving, immediately begin drifting moving, immediately begin drifting due to the electric field, resulting in a due to the electric field, resulting in a net flow of charge.net flow of charge.

Average Average drift velocitydrift velocity is about is about 0.01cm/s. 0.01cm/s.

Closing the switch establishes a Closing the switch establishes a potential difference (voltage) and an potential difference (voltage) and an electric field in the circuit.electric field in the circuit.

Electrons Electrons flow in a flow in a net net direction direction away away from the from the (-) (-) terminal.terminal.

High PotentialLow

Potential

Question:Question:

If the drift velocity is about If the drift velocity is about 0.01cm/s, why do the lights turn 0.01cm/s, why do the lights turn on instantaneously when the on instantaneously when the circuit switch is closed?circuit switch is closed?

Conventional CurrentConventional Current By By

tradition, tradition, direction in direction in which which “positive “positive charges” charges” would flow.would flow.

Direction is Direction is opposite of opposite of electron electron flow.flow.

Question:Question:

What is required in order to have What is required in order to have an electric current flow in a an electric current flow in a circuit?circuit?

Answer:Answer:1.1. A voltage source.A voltage source.

2.2. The circuit must be closed. The circuit must be closed.

Battery (Chemical Battery (Chemical Cell):Cell): A device that converts chemical A device that converts chemical

energy to electricity.energy to electricity. A battery provides a potential A battery provides a potential

energy difference (voltage energy difference (voltage source).source).

Voltaic CellVoltaic Cell Alessandro Volta (1800’s)Alessandro Volta (1800’s) BatteryBattery

Cu and Zinc Cu and Zinc Electrodes. Why?Electrodes. Why?

Question: Why is the Question: Why is the bird on the wire safe?bird on the wire safe?

Question: Question: Why do electricians work Why do electricians work with one hand behind with one hand behind their back?their back?

Question:Question: Why is the ground prong Why is the ground prong longer than the other longer than the other two in a plug?two in a plug?

Example: Third rail of Example: Third rail of subwaysubway

http://static.howstuffworks.com/gif/subway-track.gif

Part IIPart II

Electric CurrentElectric Current AmmeterAmmeter ResistanceResistance ResistorResistor

Electric Electric Current:Current: The flow of The flow of

electric electric charges.charges.

Electric Electric Current, ICurrent, I

I = I = qq

tt

RateRate Unit:Unit: Coulomb / sec = Ampere Coulomb / sec = Ampere ( (AA)) Andre Ampere (1775-1836)Andre Ampere (1775-1836)

http://media-2.web.britannica.com/eb-media/36/236-004-D4AA985F.gif

Conventional current has Conventional current has the direction that the (+) the direction that the (+) charges would have in charges would have in the circuit.the circuit.

Direct Direct CurrentCurrent

DCDC Provided by Provided by

batteriesbatteries

Alternating Alternating Current Current

ACAC Provided by Provided by

power companiespower companies

AmmeterAmmeter Measures electric Measures electric

current.current.

Must be placed in Must be placed in series.series.

Example:Example:

What charge flows through a What charge flows through a cross sectional area of a wire in cross sectional area of a wire in 10min, if the ammeter measures 10min, if the ammeter measures a current of 5mA?a current of 5mA?

Answer: 3CAnswer: 3C

ResistanceResistance

Resistance of an object to the Resistance of an object to the flow of electrical current.flow of electrical current.

R= V / IR= V / I

Resistance equals the ratio of Resistance equals the ratio of voltage to current.voltage to current.

Unit: Ohm (Unit: Ohm (ΩΩ))

Ohm’s Law (Georg Ohm,Ohm’s Law (Georg Ohm, 1787-1854) 1787-1854)

V = IR V = IR

The voltage , V, across a resistor The voltage , V, across a resistor is proportional to the current, I, is proportional to the current, I, that flows through it. that flows through it.

In general, resistance does not In general, resistance does not depend on the voltage.depend on the voltage.

Ohmic ResistorOhmic Resistor

A device that obeys Ohm’s Law, A device that obeys Ohm’s Law, who’s resistance does not depend who’s resistance does not depend on the voltage.on the voltage.

ResistorResistor An object that An object that

has a given has a given resistance.resistance.

A Battery Provides EnergyA Battery Provides Energy

Electric CircuitElectric Circuit The battery The battery “pumps” positive “pumps” positive charges fromcharges from

low (-) tolow (-) to

high (+) high (+) potential.potential.

Resistors use up EnergyResistors use up Energy

Electric CircuitElectric Circuit A resistor uses A resistor uses up energy.up energy.

When the When the current goes current goes through the through the resistor it goes resistor it goes to a lower to a lower potential.potential.

Question:Question:

Electric CircuitElectric Circuit Which point has Which point has a lower a lower potential, A or B?potential, A or B?

Example:Example:

Calculate the current through Calculate the current through a 3 a 3 ΩΩ resistor when a voltage resistor when a voltage of 12V is applied across it.of 12V is applied across it.

Answer: 4 AAnswer: 4 A

Example:Example:

A 6 A 6 ΩΩ resistor has a power source resistor has a power source of 20V across it. What will happen of 20V across it. What will happen to the resistance if the voltage to the resistance if the voltage doubles? doubles?

Part IIIPart III

Factors that affect resistance.Factors that affect resistance. PotentiometerPotentiometer VoltmeterVoltmeter

ResistanceResistance Depends on type of material, Depends on type of material,

size and shape, temperature.size and shape, temperature.

R=R=ρρ LL

AA

L: length of the wireL: length of the wire

A: cross-sectional areaA: cross-sectional area

ρρ: resistivity (inherent to material): resistivity (inherent to material)

Example:Example:

What happens to the resistance What happens to the resistance when the length is doubled and when the length is doubled and the area is quadrupled?the area is quadrupled?

Answer: It changes by 1/2Answer: It changes by 1/2

Temperature Temperature Dependence of Dependence of ResistanceResistance

For metals: For metals: as temperature as temperature increases the resistance increases.increases the resistance increases.At very low temperatures resistance At very low temperatures resistance can become zero:can become zero: superconductivity superconductivity..

For semiconductors:For semiconductors: the opposite the opposite occurs.occurs.

PotentiometerPotentiometer A variable A variable

resistance.resistance.

Used for dimmers, Used for dimmers, fan speed controls, fan speed controls, etc.etc.

Potentiometer SymbolPotentiometer Symbol

VoltmeterVoltmeter

Measures the voltage Measures the voltage between two points in an between two points in an electric circuit.electric circuit.

Must be connected in Must be connected in parallel. parallel.

A voltmeter is A voltmeter is connected in parallel.connected in parallel.

AmmeterAmmeter Measures electric Measures electric

current.current.

Must be placed in Must be placed in series.series.