Chapter 20Chapter 20

Electric Current

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Solids: Conductors, Insulators and Semiconductors

Conductors: mostly metalsConductors: mostly metals

Insulators: mostly nonmetal Insulators: mostly nonmetal materialsmaterials

Semiconductors: metalloidsSemiconductors: metalloids

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Bonding in Metals The The electron-sea modelelectron-sea model is a is a

simple depiction of a metal as an simple depiction of a metal as an array of positive ions surrounded by array of positive ions surrounded by delocalized valence electrons.delocalized valence electrons.

– Metals are good conductors of electricity because of the mobility of these delocalized valence electrons.

– A metal also conducts heat well because the mobile electrons can carry additional kinetic energy.

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Bonding in Metals

Semiconductors Metalloids: semiconducting Metalloids: semiconducting

elementselements Low electrical conductivity at room Low electrical conductivity at room

temperaturetemperature Electrical conductivity increases Electrical conductivity increases

with temp.with temp.

Semiconductors Semiconducting elementsSemiconducting elements form form

the basis of solid state electronic the basis of solid state electronic devices.devices.

MetalloidsMetalloids (such as silicon or germanium) (such as silicon or germanium) are semiconducting elements whose are semiconducting elements whose electrical conductivity increases as electrical conductivity increases as temperature increases.temperature increases.

Shocking!

If you reach for a If you reach for a metal doorknob after metal doorknob after walking across a walking across a carpet, you might see carpet, you might see a spark.a spark.

The spark is caused The spark is caused by electrons moving by electrons moving from your hand to the from your hand to the doorknob.doorknob.

Conductor! A material in which electrons are A material in which electrons are

able to move easily is a able to move easily is a conductorconductor..

The best electrical conductors are The best electrical conductors are metals.metals.

Not a Conductor! A material in which electrons are A material in which electrons are

not able to move easily is an not able to move easily is an insulatorinsulator. .

Most plastics are insulatorsMost plastics are insulators

Charging Objects Rubbing two materials together can Rubbing two materials together can

result in a transfer of electrons.result in a transfer of electrons. Then one material is left with a positive Then one material is left with a positive

charge and the other with an equal charge and the other with an equal amount of negative charge.amount of negative charge.

The process of transferring charge by The process of transferring charge by touching or rubbing is called touching or rubbing is called charging charging by contactby contact..

INDUCTION! Because electrical forces act at a Because electrical forces act at a

distance, charged objects brought near distance, charged objects brought near a neutral object will cause electrons to a neutral object will cause electrons to rearrange their positions on the neutral rearrange their positions on the neutral object.object.

The rearrangement of electrons on a The rearrangement of electrons on a neutral object caused by a nearby neutral object caused by a nearby charged object is called charged object is called charging by charging by inductioninduction..

Induction

The balloon on the The balloon on the left is neutral. The left is neutral. The balloon on the right is balloon on the right is negatively charged. It negatively charged. It produces a positively produces a positively charged area on the charged area on the sleeve by repelling sleeve by repelling electrons.electrons.

Current and Potential The net movement of electric The net movement of electric

charges in a single direction is an charges in a single direction is an electric currentelectric current..

In a metal wire, or any material, In a metal wire, or any material, electrons are in constant motion in electrons are in constant motion in all directions. As a result, there is all directions. As a result, there is no net movement of electrons in no net movement of electrons in one direction.one direction.

Current and Voltage When an electric current flows in When an electric current flows in

the wire, electrons continue their the wire, electrons continue their random movement, but they also random movement, but they also drift in the direction that the current drift in the direction that the current flows. flows.

Electric current is measured in Electric current is measured in amperes.amperes.

Voltage difference

In a similar way, electric charge flows from higher In a similar way, electric charge flows from higher voltage to lower voltage.voltage to lower voltage.

A A voltage differencevoltage difference is related to the force that causes is related to the force that causes electric charges to flow. Voltage difference is measured electric charges to flow. Voltage difference is measured in volts.in volts.

Electric Circuits

This figure shows an This figure shows an electric current doing electric current doing work by lighting a work by lighting a lightbulb.lightbulb.

A closed path that electric A closed path that electric current follows is a current follows is a circuitcircuit..

If the circuit is broken by If the circuit is broken by removing the battery, or removing the battery, or the lightbulb, or one of the lightbulb, or one of the wires, current will not the wires, current will not flow.flow.

Resistance As the electrons flow through As the electrons flow through

the filament in a lightbulb, they the filament in a lightbulb, they bump into the metal atoms that bump into the metal atoms that make up the filament.make up the filament.

In these collisions, some of the In these collisions, some of the electrical energy of the electrical energy of the electrons is converted into electrons is converted into thermal energy.thermal energy.

Eventually, the metal filament Eventually, the metal filament becomes hot enough to glow, becomes hot enough to glow, producing radiant energy that producing radiant energy that can light up a dark room.can light up a dark room.

Electric Current Electrons in motion.Electrons in motion. Current:Current: The number of electrons The number of electrons

that pass a specific point in a circuit that pass a specific point in a circuit in one second in one second

I = I = Q/tQ/t Circuit:Circuit: electric current flows through electric current flows through

a closed, continuous path.a closed, continuous path.

HIGHLOW

Electric Current The reason electric charge flows

from one place to another is voltage. a. Voltage is the difference in

electrical potential between two places where e¯ are flowing.

b. Voltage is the “push” that makes electric charges move.

c. Measured in volts (V).

Batteries are e¯ pumps. a. They provide a voltage difference to a circuit. b. Types: wet-cells & dry-cells

Generating Electric Current Electrochemical cellElectrochemical cell: (battery) changes : (battery) changes

chemical energy into electric energy. chemical energy into electric energy. Two types wet cell and dry cell.Two types wet cell and dry cell.

Thermocouples:Thermocouples: a tool that uses a tool that uses differences in temperature to generate differences in temperature to generate electric currents.electric currents.

GeneratorGenerator- next chapter but make - next chapter but make alternating currentalternating current

Types of current Direct currentDirect current: electrons that flow in the : electrons that flow in the

same direction in a wire. (DC) same direction in a wire. (DC) From batteriesFrom batteries Alternating currentAlternating current: electrons that flow : electrons that flow

in different directions in a wire. (AC)in different directions in a wire. (AC) From GeneratorsFrom Generators Used in your homeUsed in your home Transformers change AC to DCTransformers change AC to DC

Measuring Electricity Current:Current: Measured in Amperes or amps Measured in Amperes or amps

(A) (A) Voltage:Voltage: Measured in volts (V) Measured in volts (V) Higher voltage, the more work the Higher voltage, the more work the

electrons can do.electrons can do.

Measuring Electricity Resistance:Resistance: the force opposing the flow the force opposing the flow

of electrons.of electrons. Measured in ohms Measured in ohms Symbol is Greek letter omega Symbol is Greek letter omega Thicker wire- less resistanceThicker wire- less resistance Longer wire- more resistanceLonger wire- more resistance Conductors- low resistanceConductors- low resistance Insulators- high resistanceInsulators- high resistance

Ohm’s Law The relationship among The relationship among

current, voltage, and current, voltage, and resistance.resistance.

Ohm’s law states that Ohm’s law states that the current in a circuit is the current in a circuit is equal to the voltage equal to the voltage divided by the resistancedivided by the resistance

I = I = VV R R

V

I R

Do the Math

A car has a 12 volt system. The A car has a 12 volt system. The headlights are on a 10 amp circuit. headlights are on a 10 amp circuit. How much resistance do they have?How much resistance do they have?

V

I R

Do the Math

A car has a 12 volt system. The A car has a 12 volt system. The headlights are on a 10 amp circuit. headlights are on a 10 amp circuit. How much resistance do they have?How much resistance do they have?

R =V / IR =V / I R = 12 V / 10 ampR = 12 V / 10 amp R =1.2 R =1.2

V

I R

Do the Math

Your house uses 120 volts. What Your house uses 120 volts. What amount of current would flow through amount of current would flow through a 20 ohm resistor?a 20 ohm resistor?

V

I R

Do the Math

Your house uses 120 volts. What Your house uses 120 volts. What amount of current would flow through amount of current would flow through a 20 ohm resistor?a 20 ohm resistor?

I = V / RI = V / R I = 120 V / 20 ohmI = 120 V / 20 ohm I = 6 amperes (A)I = 6 amperes (A)

V

I R

Electric power and energy

PowerPower: The rate at which electricity : The rate at which electricity does work or provides energy The does work or provides energy The amount of electric power a device uses amount of electric power a device uses to do work is determined by its to do work is determined by its resistance.resistance.

P = V x IP = V x I (P) power = (V) voltage x (I) current in (P) power = (V) voltage x (I) current in

the circuit.the circuit.

Energy Formula for energyFormula for energy E = P x tE = P x t (E) energy used = (P) power x (t) time(E) energy used = (P) power x (t) time The SI unit for energy is a joule.The SI unit for energy is a joule. Kilowatt-hour meters measure the Kilowatt-hour meters measure the

electricity used in your home.electricity used in your home. (kWh)(kWh)

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Write a Paragraph: (Ch 20) Investigate and explain the Investigate and explain the

relationships among:relationships among: currentcurrent VoltageVoltage ResistanceResistance powerpower

Grade the paragraph Current – Current –

number of electrons that pass a specific point in a circuit in one number of electrons that pass a specific point in a circuit in one second. second.

Reason it flows is due to voltage.Reason it flows is due to voltage. Voltage – Voltage –

is the push that makes the electric charge moveis the push that makes the electric charge move Resistance – Resistance –

force opposing the flow of electrons (or current)force opposing the flow of electrons (or current) Power – Power –

rate electricity does work. The amount of work is determined rate electricity does work. The amount of work is determined by its resistance.by its resistance.