Unit 10

Chapter 13

Consumer warning:You will have carpal tunnel syndrome by the

end of this unit if you don’t learn how to summarize the information contained in this powerpoint.

Think of all the ways electricity can be found in

your daily life.

Could you survive without electricity?

Electric charge and force

Electric charge is an electrical property of matter that creates a force between objects.

All matter is made of atomsAtoms are composed from 3 essential particles

Neutrons – in the nucleus with a neutral charge

Proton – in the nucleus with a (+) chargeElectron – outside the nucleus with a (-)

charge

Electric charge cont. SI unit of electric charge is the coulomb, CElectrons and protons have exactly the

same amount of charge… 1.6 x 10-19 CBecause they are oppositely charged.

Protons is a +1.6 x 10-19 CElectrons is a -1.6 x 10-19 C

If an object has excess electrons then it has an electric charge. The net electric charge is always a

multiple of 1.6 x 10-19 C.

Atoms become charged by gaining or losing electrons

Static electricity is the accumulation of excess electric charges on an object

What are the rules?

Law of Conservation of Charge

Charge may be transferred from object to object but it

cannot be created or destroyed

More rulesOpposites attract and like charges repel

Charges can act at great distances

Any charge placed in an electric field will be pushed or pulled by the field

Still more rulesElectrons move more easily through conductors like metals

Electrons do no move easily through insulators such as plastic, wood, rubber, and glass

Really dry air

With a big enough build up of charge, electricity WILL flow

Like say half a megavolt trying to pass through a wire

Conductors and InsulatorsA conductor is a material that transfers

charge easilyIt allows the charges to flowExample: metal

An insulator is a material that does not transfer charge easilyIt will not allow the charges to flowExamples: cardboard, glass, silk, and

plastic

Transferring a ChargeThree main methods

Charging by contactCharging by InductionStatic Discharge

Charging by contactDone by touching or rubbingExample: feet rubbing the carpet then touching a metal door knob

Charging by InductionElectrons move because of a nearby charged object

Example: negatively charged balloon near your sleeve causes your sleeve to be positively charged

Static DischargeA transfer of charge through the air between two objects because of the buildup of a static electricity – a spark!

Example: lightning

Last bitsGrounding – using a conductor to direct the charge into the groundIt an important part of electrical safety!

The presence of a charge can be determined by an electroscope

Electric forceElectric force is the force of attraction or repulsion between objects due to chargeIt is the reason why some clothes cling

together when you take them out of the dryer

It is part of what holds an atom togetherIt also plays a part in chemical bonding

Without electric force, life itself would be impossible

Electric force cont.The electric force between 2 charged

objects varies depending on the amount of charge on each object and the distance between them

Acts through a fieldElectric field is the region around a

charged object in which other charged objects experience an electric force

Electric force cont.Regardless of the charge, electric field

lines never cross one another

Current

Electrical potential energyElectrical potential energy is the potential energy of a charged object due to its position in an electric field

Potential difference is the change in the electrical potential energy per unit chargeSI unit for potential difference is the volt

(V) which is equivalent to 1 joule per coulomb

Often called voltage

Electric current The flow of charges through a wire or conductor is called electric current

Current is the rate that electric charges move through a conductor

SI unit of current is Amperes (A or amp)

VoltageCharges flow from high voltage to low voltage

A voltage difference is the push that causes charges to move

Voltage difference is measured in volts (V)

For charges to flow, the wire must always be connected in a closed path or circuit

Sources of electricityCell is a device that is a source of electric current because of a potential difference, or voltage between the terminals

Dry cell – standard battery – produces a voltage difference by a chemical reaction

Wet cell – car battery – produces a voltage difference by chemical reaction

Wall sockets – AC – produced at a generator

Types of electric cellsElectrochemical – Electrons transferred

between different metals immersed in an electrolyteUses – common batteries and automobile

batteries

Photoelectric and photovoltaic – Electrons are released from a metal when struck by light of sufficient energyUses –satellites, calculators, and

streetlights

Types of electric cellsThermoelectric – Two different metals are

joined together, and the junctions are held at different temperatures, causing electrons to flow.Uses – Thermostats for furnaces and ovens

Piezoelectric – Opposite surfaces of certain crystals become electrically charged when under pressure.Uses – crystal microphones and headsets,

computer keypads

Conventional current was the original idea……

Conventional current is the movement of positive charge

It is current made of positive charge that would have the same effect as the actual motion of charge in the material

Electrical ResistanceResistance is the ratio of the voltage across a conductor to the current it carries

The tendency for a material to oppose the flow of electrons, changing electrical energy into thermal energy and light

All materials have some resistance

Still resistingSI unit of resistance is ohms (Ω)It is equal to volts per ampere

Resistor is a special type of conductor used to control current

Making wires thinner, longer, or hotter increases resistance

Light bulbslight bulbs make use of resistance

Ohm’s Law

I = V R

I – current (A)V – voltage (V)R – resistance (Ω)

Practice problemWhat is the resistance of a wire that has

a current of 1.4 A in it when it is connected to a 6.0 V battery?

R = V I

Practice problemWhat is the resistance of a wire that has

a current of 1.4 A in it when it is connected to a 6.0 V battery?

R = V I = 6.0 V = 4.3 Ω 1.4 A

Practice problemAn electric space heater is plugged into a

120 V outlet. A current of 12.0 A is in the coils in the space heater. What is the resistance of the coils?

Practice problemAn electric space heater is plugged into a

120 V outlet. A current of 12.0 A is in the coils in the space heater. What is the resistance of the coils?

R = V I = 120 V = 10 Ω 12.0 A

Practice problemA steam ion has a current of 9.17 A when

plugged into a 120 V outlet. What is the resistance of the steam iron?

Practice problemA steam ion has a current of 9.17 A when

plugged into a 120 V outlet. What is the resistance of the steam iron?

R = V I = 120 V = 13 Ω 9.17 A

Practice problems……………………..

1

120.12V

2

4.8V

3

116.5V

4

119.56V

5

4.55V

1.5V per battery

6

2.9A

7

15A

8

4.79A

9

5.2A

10

230.77Ω

11

179.2 Ω

Anybody do #14?

5.74A

More on resistance….Conductors have low resistancesThey allow the electrons to flow easily and therefore have a low resistance

Still resistingSome materials become superconductors below a certain temperatureCertain metals and compounds have

zero resistance when their temperature falls below the critical temperature

The critical temperature varies between -272oC and -123oC

Examples – tin, mercury, and some metallic compounds containing barium, copper, and oxygen

Still resistingInsulators have high resistance to charge movementThey are used to prevent electric

current from leakingExamples - plastic coating around

copper wire of an electric cord keeps the current from escaping into the floor or your body

Circuits

What are circuits?Electric circuit is an electrical device connected so that it provides one or more complete paths for the movement of chargesClosed circuit is one in which there is

a closed-loop path for electrons to follow

Open circuit is one without a complete path, there is no charge flow and therefore no current

CircuitsSchematic diagram is a graphic representation of an electric circuit or apparatus, with standard symbols for the electrical devices

Electrical CircuitsCircuits rely on generators at power plants to produce a voltage difference across the outlet, causing the charge to move when the circuit is complete

Series and Parallel circuitsSeries describes a circuit or portion of a circuit that provides a single conducting path

Parallel describes components in a circuit that are connected across common points, providing two or more separate conducting paths

Series circuitThe current has only one loop to flow through

The parts of the series circuit are wired one after another so the amount of current is the same through every part

Series circuits cont. Open circuit – If any part of a series circuit is disconnected, no current flows through the circuitExample – Christmas lights

Parallel CircuitsContain two or more branches for current to move through

Individual parts can be turned off without affecting the entire circuitExample – circuits in a house

Electric Power and Electrical EnergyElectrical energy is the energy associated with electrical charges, whether moving or at restThe energy may come from a

battery or a power plantElectric Power

Power = current x voltageP = IV

SI unit for power is the watt (W)

Practice problemA flashlight has a potential difference of 3.0 V. The bulb has a current of 0.50 A. What is the electric power used by the flashlight?

P = IV = 3.0 V x 0.50 A = 1.5 W

Practice problemWhat is the current in a 60 W light bulb when it plugged into a 120 V outlet?

Practice problemWhat is the current in a 60 W light bulb when it plugged into a 120 V outlet?

I = P V = 60 W = 0.5 A 120 V

Household circuitsUse parallel circuits Each branch receives the standard voltage difference

Electricity enters your home at the circuit breaker or fuse box and branches out to the rest of your house

House guardsElectric fuse – contains a small piece of metal that melts if current becomes too high and breaks the circuit

Circuit breaker – uses a magnet or bimetallic strip (a strip with two different metals welded together) that responds to current overload by opening the circuit.It protects a circuit from current

overloadsUsed in many homes and businesses