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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