Chapter 20: Electricity
Section 20.1 Electric Charge and Static
Electricity
Section 20.2 Electric Current and Ohm’s Law
Section 20.3 Electric Currents
Section 20.1 Electric Charge
and Static Electricity
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Lightning and static cling result from the
movement of electric charges.
 Electric Charge
Def.-a property that causes subatomic
particles such as protons and electrons to
attract or repel each other
Two types of electric charge: positive and
negative
Protons (+ charge) and electrons (- charge)
Section 20.1 Electric Charge
and Static Electricity
Electric Charge
Examples of movement: lightening bolts
Examples of attraction: clothes taken out of
the dryer
Almost everything in our day to day lives is
affected some way by charges.
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Section 20.1 Electric Charge
and Static Electricity
Electric Charge
Cloud of (–) electrons surround (+) nucleus
Atom is neutral b/c #protons=#electrons
Atom gains 1 or more electrons becomes a
negatively charged ion
Atom loses electrons become a positively
charged ion
Key Concept: An excess or shortage or
electrons produce a net electric charge.
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Section 20.1 Electric Charge
and Static Electricity
Electric Charges
SI Unit of electric charge= C or coulomb
Takes 6.24 x 1018 electrons to produce a
single coulomb
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Section 20.1 Electric Charge
and Static Electricity
Electric Forces
Key Concept: Like charges repel, and
opposite charges attract.
Electric force-the force of attraction or
repulsion between electrically charged
objects
Charles-Augustin de Coulomb (1736-1806)discovered that electric forces obey laws
similar to the law of universal gravitation
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Section 20.1 Electric Charge
and Static Electricity
Electric Forces
Electric force b/t 2 objects directly proportional to
net charge on each object and inversely
proportional to the square of the distance between
them.
Doubling net charge on one object doubles the
electric force
Doubling distance between objects: electric force is
one fourth as strong
Electric forces inside atoms are much stronger than
gravitational forces
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Section 20.1 Electric Charge
and Static Electricity
Electric Fields
Def.-the effect an electric charge has on
other charges in the space around it
Key Concept: The strength of an electric
field depends on the amount of charge that
produces the field and on the distance from
the charge.
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Figure 4
Section 20.1 Electric Charge
and Static Electricity
Electric Fields
Exert forces on any charged object placed in
the field
Force depends on: net charge in the object,
strength and direction of the field at the
object’s position
The greater the net charge an object has,
the greater the force on it.
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Section 20.1 Electric Charge
and Static Electricity
Static Electricity and Charging
Static electricity-the study of the behavior of
electric charges, including how charge is
transferred between objects
Key Concept: Charge can be transferred by
friction, by contact, and by induction.
When charge transfer occurs, the total
charge is the same before and after the
transfer (law of conservation of charge)
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Section 20.1 Electric Charge
and Static Electricity
Charging by Friction
Ex. Balloon attracts hair
Electrons move from hair to balloon making
balloon negative and hair becomes positive
Ex. Walking across a carpet
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Section 20.1 Electric Charge
and Static Electricity
Charging by Contact
Hair standing on end
Person acquires a charge large enough
where hairs have like charges
Like charges repel
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Section 20.1 Electric Charge
and Static Electricity
Charging by Induction
Ex. Walk across carpet and reach for doorknob
You pick up electrons from the carpet: hand is
negatively charged
Net negative charge of hand repels the electrons in
the metal doorknob
Electrons move to base of doorknob=doorknob is
positively charged in part
Doorknob is still neutral but charge moved into it
Induction-a transfer of charge without contact
between materials
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Section 20.1 Electric Charge
and Static Electricity
Static Discharge
Shock from doorknobs or other objects=result of
static discharge
Key Concept: Static discharge occurs when a
pathway through which charges move forms
suddenly.
Air becomes charged suddenly when the gap b/t
your finger and the doorknob is small. Air provide
path for electrons to flow from your hand to the
doorknob. Spark can be seen in the dark.
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Section 20.1 Electric Charge
and Static Electricity
Static Discharge
Lightning-more dramatic discharge
Friction between moving air masses causes charge
to build up in storm clouds
Negative charge in lower part of cloud induces a
positive charge in the ground below it
Amt. of charge in cloud ↑, attraction b/t charges in
cloud and ground ↑
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Air becomes charged (forms pathway for electrons
to flow from cloud to the ground)
Section 20.2 Electric
Current and Ohm’s Law
Electric Current
Def.-the continuous flow of electric charge
SI Unit of electric current-ampere (A);
equals 1 coulomb/second
Key Concept: The two types of current are
direct current and alternating current.
Direct current (DC)-charge flows in one
direction
Flashlight; most battery-operated devices
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Section 20.2 Electric
Current and Ohm’s Law
Electric Current
Alternating current (AC)-a flow of electric
charge that regularly reverses its direction
Houses and schools
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A Flashlight
Figure 8
Section 20.2 Electric
Current and Ohm’s Law
Conductors and Insulators
Electrical conductor-a material through which
charge can flow easily
Electrical insulators-a material through which
charge cannot flow easily
Metals-have ions in a lattice that don’t move, but
have some electrons that a not bound to the lattice
=they can conduct charge
**Most materials don’t conduct charge b/c they
have no free electrons
Metal such as copper and silver are good electrical
conductors. Wood, plastics, rubber and air are
good electrical insulators.
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Section 20. 2 Electrical
current and Ohm’s Law
Resistance
Def.-opposition to the flow of charges in a
material (SI unit=ohm)
Key Concept: A material’s thickness, length,
and temperature affect its resistance.
Resistance > in long wire b/c charges have
to travel farther; Temp. >=more resistance
b/c electrons collide more often
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Section 20.2 Electric
Current and Ohm’s Law
Resistance
Can resistance be 0?
Superconductor-material that has almost
zero resistance when it is cooled to low
temperature.
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Section 20.2 Electric
Current and Ohm’s Law
Voltage
Key Concept: In order for charge to flow in a
conducting wire, the wire must be connected in a
complete loop that includes a source of electrical
energy.
 Potential Difference
Objects at greater height have more potential than
those at a lower height. So those objects fall from
higher to a lower potential energy. This is true of
charges. Charges flow spontaneously from a higher
to lower potential energy.
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Section 20.2 Electric
Current and Ohm’s Law
Voltage (Potential Difference)
Potential difference of a charge depends on its position in an
electric field.
Def.-the difference in electrical potential energy between two
places in an electric field
Measured in Joules/Coulombs (Volts) aka Voltage
 Voltage Sources
Batteries, solar cells, and generators.
Have terminals (positive and negative) that connect to wires
in a circuit
Battery-a device that converts chemical energy to electrical
energy.
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Section 20.2 Electric
Current and Ohm’s Law
Ohm’s Law
Ohm unit-German scientist-Georg Ohm (17891854)
1st determined how resistance and current affect
voltage.
Discovered voltage is not the same everywhere in a
circuit; Hypothesized that resistance reduces
voltage.
Found a relationship b/t voltage, current, and
resistance
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Section 20.2 Electric
Current and Ohm’s Law
Ohm’s Law
Voltage (V) in a circuit equals the product of the
current (I) and the resistance (R).
V=I x R or I=V/R
If current in amps and resistance in ohms, voltage
in volts
Key Concept: Increasing the voltage increases the
current. Keeping the same voltage and increasing
the resistance decreases the current.
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Section 20.3 Electric
Currents
Circuit Diagrams
Electric current-a complete path through which
charge can flow; wires in houses, etc. have
complex networks of circuits
Electricians use circuit diagrams to monitor how
elements in a circuit are connected.
Key Concept: Circuit diagrams use symbols to
represent parts of a circuit, including a source of
electrical energy and devices that run by the
electrical energy
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Section 20.3 Electric
Circuits
Circuit Diagrams
Show one or more complete paths where charge
can flow
Switches show where circuit can open; if open
circuit not in a complete loop so current
stops=open circuit
If switch closed, the circuit is complete and charge
can flow=closed circuit
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Section 20.3 Electric
Circuits
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Electrons in a wire flow in the opposite direction.
 Series Circuits
**Charge has only one path through which it can
flow
Key Concept: If one element stops functioning in a
series circuit, none of the elements can operate.
One light bulb blows it becomes an open circuit.
Bulbs are a source of resistance; the more present
the more resistance there is.
>resistance=<current;decrease brightness of bulbs
Section 20.3 Electric
Currents
Parallel Circuits
Circuits in homes are mostly parallel.
Def.-an electric circuit with two or more paths
through which charges can flow
Key Concept: If one element stops functioning in a
parallel circuit, the rest of the elements still can
operate.
Allows independent operation of devices.
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Figure 12
Series and Parallel Circuits
Section 20.3 Electric
Circuits
Power and Energy Calculations
Electric power-the rate at which electrical energy is
converted to another form of energy
Units of joules per second (watt, W). Power
measured in watts or kilowatts (kW)
Key Concept: Electric power can be calculated by
multiplying voltage by current.
P (watts)=I (amps) x V (volts)
**Appliances vary in the amount of power they
use.
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Section 20.3
Section 20.3 Electric
Currents
Electrical Safety
Inspectors check all new homes to make sure
electrical wiring is installed safely.
Key Concept: Correct wiring, fuses, circuit
breakers, insulation, and grounded plugs help make
electrical energy safe to use.
The amount of current in a circuit depends on how
many devices in the circuit. > # of devices turned
on the > the current
If current exceeds the circuits safety limit, wire may
overheat and start a fire.
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Section 20.3 Electric
Currents
Home Safety
Fuses prevent current overload in a circuit; wire in
the center of a fuse melts if there is too much
current passing through it “blowing a fuse”. Has to
be replaced with a new one to use circuit
Most homes use circuit breakers (a switch that
opens when current in a circuit is too high); has to
be reset for circuit to be used again
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Section 20.3 Electric
Currents
Personal Safety
Electrical wiring is insulated to protect from shock
Don’t touch electrical devices with wet hands
Insulation prevents short circuits; three pronged
plugs help prevent shocks from short circuits
Circular prong connects to ground; current takes
easier path to ground instead of entering your body
Grounding-the transfer of excess charge through a
conductor to Earth
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