Electric Electric Charges & Charges &

CurrentCurrent

Chapter Chapter 2020

Types of electric chargeTypes of electric charge ProtonsProtons w/ ‘w/ ‘++’ charge “’ charge “stuckstuck” in ” in

the nucleusthe nucleus ElectronsElectrons w/ ‘w/ ‘--’ charge ’ charge freely freely

moving moving around the nucleus in around the nucleus in orbitsorbits

ConductorsConductors Allow the Allow the easy flow easy flow of electricityof electricity loosely bound electrons loosely bound electrons that are free that are free

to move from atom to atomto move from atom to atom metalsmetals like aluminum, gold, copper like aluminum, gold, copper

and silver and silver

InsulatorsInsulators InsulatorsInsulators – – resists the flow resists the flow of electronsof electrons

hold more tightly to their valence hold more tightly to their valence electrons: electrons:

plastic, rubber, glassplastic, rubber, glass

Interactions between Interactions between chargescharges same as in magnetismsame as in magnetism

Unlike magnetism were on a magnet Unlike magnetism were on a magnet there is always a + on one end and a – there is always a + on one end and a – on the other end of the magneton the other end of the magnet

electrical charges can electrical charges can exist aloneexist alone Like charge repelLike charge repel Opposite charges attractOpposite charges attract

Electric FieldsElectric Fields Exert a force through the force field in all Exert a force through the force field in all

directions from the charged particledirections from the charged particle When a charged particle enters the force field of When a charged particle enters the force field of

another particle it is either another particle it is either attracted or repelledattracted or repelled The diagram represents The diagram represents stronger force stronger force as the lines as the lines

get get closer & closer togethercloser & closer together

Static ChargeStatic Charge Latin word “Latin word “StasisStasis” which means “” which means “StaysStays”” Objects are typically “Objects are typically “Neutral” w/ the same # of protons and Neutral” w/ the same # of protons and

electronselectrons They can become “They can become “chargedcharged” by ” by gaininggaining or or losinglosing electronselectrons – –

NOT PROTONSNOT PROTONS! – They stay in the nucleus!! – They stay in the nucleus! The The buildupbuildup of these charges is “ of these charges is “Static ElectricityStatic Electricity”” In In Static Electricity Static Electricity the charges the charges build up and build up and STAYSTAY; ;

they don’t flow as they do in they don’t flow as they do in electric currentselectric currents

TransferringTransferring Static Static ChargeCharge FrictionFriction – transferred from rubbing i.e. get shocked after – transferred from rubbing i.e. get shocked after

walking on the walking on the carpetcarpet ConductionConduction – transferred by direct contact w/ another – transferred by direct contact w/ another

object – hair object – hair standing on end w/ standing on end w/ Van de Graff machineVan de Graff machine InductionInduction – the force field of a highly negatively charged – the force field of a highly negatively charged

object pushes the object pushes the electrons away from nearby objects causing electrons away from nearby objects causing them to become + charged, they then are attracted to each other. them to become + charged, they then are attracted to each other. i.e. statically charged balloon attracts small pieces of torn up i.e. statically charged balloon attracts small pieces of torn up paperpaper

Static DischargeStatic Discharge Objects don’t hold a static charge forever – Objects don’t hold a static charge forever – objects tend toward objects tend toward

equilibriumequilibrium – – they “want” to be neutralthey “want” to be neutral When electrons move toward this equilibrium – static discharge occursWhen electrons move toward this equilibrium – static discharge occurs

HumidityHumidity – – water (a polar molecule) vapor in the air pulls electrons off negatively water (a polar molecule) vapor in the air pulls electrons off negatively charged objects, preventing static charges to build upcharged objects, preventing static charges to build up

Sparks & Lightning Sparks & Lightning - - objects reaching static equilibriumobjects reaching static equilibrium

Homework

Read pages: 682-689 Complete Guided Reading Handout

on Electric Charge and Static Electricity

Due Weds 6/4

Chp. 20 Electric Current & Circuits

Objectives: Define Voltage Define Electric Current Describe conditions that allow

current flow Explain how resistance affects

current flow

Circuit MeasurementsCircuit Measurements Electric Current Electric Current - - Flow of electrons through a Flow of electrons through a

materialmaterial Electrical Potential Electrical Potential – –

Similar to potential energy (lifting something higher Similar to potential energy (lifting something higher against the force of gravity gives it greater against the force of gravity gives it greater potential to potential to do workdo work, increasing its potential energy.), increasing its potential energy.)

When given the opportunity, objects will move from When given the opportunity, objects will move from higher potential energy to an area of lower potential higher potential energy to an area of lower potential energyenergy

Electrical potential is related to their electrical fields and Electrical potential is related to their electrical fields and not to height – not to height – as electrons build up on one side they as electrons build up on one side they “want” to flow to an area w/ less potential“want” to flow to an area w/ less potential

Voltage (Potential Difference)

Potential Difference (Voltage) Potential Difference is the work required to

move +1C of charge from one terminal through the device to the other terminals(Not through something attached to the device)

Unit is the volt, V

1 volt = 1 Joule/Coulomb

(Joule is unit for Energy & Work)

• Potential Difference is often called Voltage

VoltageVoltage VoltageVoltage – causes current to flow through an electrical circuit– causes current to flow through an electrical circuit VoltVolt – unit of measure to measure this potential– unit of measure to measure this potential A A Voltage Source Voltage Source ((batterybattery or or generatorgenerator) is required to maintain ) is required to maintain

the electrical potential in a circuit.the electrical potential in a circuit.

Potential Difference

Simple Battery Example showing electrons flowing in a simple circuit

http://phet.colorado.edu/en/simulation/battery-resistor-circuit

CURRENT Electricity is basically the movement of

electrons

Current Current – rate at which charges move in a

conductorCharges move to a position of lower potential

energy When this happens an electric current is

producedCurrent is abbreviated with an “I”

Unit of current is the “Ampere” or Amps, A 1 A = 1 Coulomb of Electrons / Second

Electrical CurrentElectrical Current Water flowing thru a pipe depends on more than the angle Water flowing thru a pipe depends on more than the angle

of the pipe. It also depends on the length of the pipe, of the pipe. It also depends on the length of the pipe, diameter of the pipe and if the pipe is clogged or open.diameter of the pipe and if the pipe is clogged or open.

Electrical Current Electrical Current is measured in is measured in AmperesAmperes Amount of Electrical Current ( amps) depends on more Amount of Electrical Current ( amps) depends on more

than just Voltage, it depends on the Resistance found in than just Voltage, it depends on the Resistance found in the circuit.the circuit.

Current

How do lights come on instantly when I flick the switch?

You don’t have to wait for 1 electron to make a complete circuit. There are electrons “waiting” all along the wire. Each electron pushes the one ahead of it, so as soon as the first electron starts to move, the last electron in the wire moves into the light bulb.

RESISTANCE Resistance works AGAINST current

Resistance – Ohms Resistance, R:

Caused by internal friction Slows the movement of charges

through a conducting materialUnit of measure is the ohm (Ω)

Recall… Friction results in Heat

Water Hose ExampleElectricity in a Wire is a lot like Water in a Hose

• Voltage is how much pressure is applied to push the water through the hose

• Current is how fast the water flows through the hose

• Resistance is the friction in the hose

Resistance Amount of Resistance can Vary:

In our water hose example, changing the following will change the amount of Resistance

– Diameter of hose– Length of hose– Material (changes the amount of

friction)

The Same would be true for a WIRE

• http://phet.colorado.edu/sims/resistance-in-a-wire/resistance-in-a-wire_en.html

What Influences Resistance in a wire? Material of wire – aluminum and copper

have low resistance Thickness – the thicker the wire the lower

the resistance Length – shorter wire has lower

resistance Temperature – lower temperature has

lower resistance

Resistance Resistance is a good thing because it

reduces the actual volts something gets

Just like a dam

• Resistor:

– Special type of conductor used to control current

Resistance Resistance can be a

bad thing… Energy can be lost as

electricity travels a long distance…

Electric Current

Homework: Read pages 692 to 699 Do Review & Reinforce/Guided

Reading Handout Due Monday 6/9

Chp. 20 Resistance and Ohms Law

Objectives: Define Resistance Describe Ohms Law Calculate current, voltage or

resistance in a circuit using ohms law

Current, Voltage & Resistance Pressure in the hose (V), rate the water

flows through the hose (I) and the type of hose (R) are all related. If you change one, it affects the others

The relationship is known asOhm’s Law…

Voltage = Current x Resistance

V = I x R

Ohms Law

Learning Target:

Use Ohm's Law to calculate current, voltage, and resistance using correct units.

Ohm’s Law

Practice:

There is a potential difference of 12 volts across a resistor with 0.50 amps of current in it. The resistance of the resistor is _________.A. 0.5 ohms

B. 6 ohms

C. 12 ohms

D. 24 ohms

Practice:

If there is a current of 6 amps flowing through a 3 ohm resistor, what is the potential difference in the circuit?A. 0.5 volts

B. 2 volts

C. 9 volts

D. 18 volts

Practice:

If a circuit has a potential difference of 120 volts and a resistance of 10 ohms, what is the current that flows through the circuit?A. 0.08 amps

B. 12 amps

C. 120 amps

D. 1200 amps

Ohms Law Homework:

Read pages 451 – 454 Do Review & Reinforce handout and

practice problems 5-9 Due Monday 6/9

Series CircuitsSeries Circuits Series Circuits Series Circuits – provides – provides only one path only one path for the for the

electrons to followelectrons to follow 1. A break in the circuit stops the flow of electricity to all other 1. A break in the circuit stops the flow of electricity to all other

parts of the circuitparts of the circuit 2. With multiple light bulbs (more resistance) the current 2. With multiple light bulbs (more resistance) the current

reduces & the dimmer the lights becomereduces & the dimmer the lights become 3. 3. Ammeters Ammeters should be wired in seriesshould be wired in series

Parallel CircuitsParallel Circuits Parallel circuits Parallel circuits – the different parts of the circuit are on – the different parts of the circuit are on

separate branchesseparate branches A break (burn out light bulb) in the circuit doesn’t stop the A break (burn out light bulb) in the circuit doesn’t stop the

flow to the remaining devicesflow to the remaining devices Multiple light bulbs will remain the same brightness since Multiple light bulbs will remain the same brightness since

the resistance is not decreasing as it does in a series the resistance is not decreasing as it does in a series circuit.circuit.

Each pathway can be separately switched off w/out Each pathway can be separately switched off w/out affecting the othersaffecting the others

Household circuits – Wired in parallelHousehold circuits – Wired in parallel, with a standard of , with a standard of 120 volts120 volts

Voltmeters are wired in parallelVoltmeters are wired in parallel

Parallel CircuitsParallel Circuits The The more paths the LESS the resistancemore paths the LESS the resistance

Water example again: added pipes coming from Water example again: added pipes coming from a large tank will allow more water to flow out a large tank will allow more water to flow out that a single pipe.that a single pipe.

Therefore Therefore as resistance degreases, current as resistance degreases, current increases; they are inversely proportionalincreases; they are inversely proportional

Schematic DiagramsSchematic Diagrams All circuits need at least the followingAll circuits need at least the following

Power supply, wire, resistors, other items include Power supply, wire, resistors, other items include switches, connectors, meters, etc. switches, connectors, meters, etc.

There is a set of standard symbols used to There is a set of standard symbols used to represent these items in a diagram of the circuitrepresent these items in a diagram of the circuit

That’s all That’s all for for

Chapter 20Chapter 20

Power –Watts (W)

Recall from last term… Power – rate that work is done at Units are Watts

The higher the Watts, the higher the energy output per unit of time.

Power - Electricity

Power = work/unit time P = v x I

=

• Unit of measure is the Watt (W)• 1 W = 1 J/sec

How is Electrical Power calculated?

Electrical Power is the product of the current (I) and the voltage (v)

The unit for electrical power is the same as that for mechanical power in the previous module – the watt (W)

Example Problem: How much power is used in a circuit which is 110 volts and has a current of 1.36 amps?

P = I V

Power = (1.36 amps) (110 V) = 150 W