Electricity
Electric Charge and Force
Charge: property of matter that creates
electric and magnetic forces and
interactions
Observe property only through its
effects
Two types of charge: positive (+) and
negative (-)
Like charges repel, unlike charges
attract
--, ++: repel; +-, -+: attract
Charge depends on imbalance of
protons and electrons
Excess of electrons: (-)
Excess of protons: (+)
SI Unit: Coulomb (C)
Electron and proton have same
magnitude of charge, but opposite
signs
Electron: -1.6 x 10-19 C
Proton: 1.6 x 10-19 C
Conductors: Easily transfer charge
Charges move freely through material
Metals, some metalloids
Insulators: Transfer charge poorly
Charges not able to move freely
through material
Nonmetals and some metalloids
Methods of Charging
Charging normally performed through
movement of electrons
Protons and neutrons relatively fixed
in materials
Electrons have greater freedom of
movement
Charging by friction
When materials are rubbed together,
electrons can be transferred between
them
One material gains electrons:
becomes negatively charged
One material loses electrons:
becomes positively charged
Conduction: charging through contact
without rubbing
Touch neutral object with charged
object
Electrons flow towards more positive
object, creating charge on neutral
object
Induction: Charges on neutral object
redistributed to create polarity
Bring charged object in proximity to,
but not touching, neutral object
If charged object is (+): electrons
attracted to side of neutral object
closest to charged object
If charged object is (-): electrons
repelled to far side of neutral object
Total charge on neutral object still 0
Electric Force
Induction occurs through action of
electric force
Forces can create motion, push or pull
Like charges have repelling electric
force
Unlike charges have attracting
electric force
Size of force depends on magnitude of
charge and distance between objects
Coulomb’s Law
Example of an inverse-square law
Used to calculate magnitude of electric
force between two charged objects
Formula:
F = (kq1q2)/r2
F: electric force
k: constant = 9 x 109 Nm2/C2
q1: charge on first object (C)
q2: charge on second object (C)
r: distance between objects
2
If value of electric force is negative,
force is attractive
+,- or -,+
If value of electric force is positive,
force is repelling
+,+ or -,Inverse-square nature of formula
indicates that force diminishes quickly
with distance
Double the distance, force diminishes
to one-fourth
Force exerted through electric fields
Example of a field force, like gravity
Charged particle exerts electric field in
space around it
Another charged particle entering
field experiences an electric force
Electric fields can be represented by
drawing electric field lines around
charged particles
Lines point away from + charges
Lines point towards – charges
Field lines never cross one another
Fields strongest where lines are
densest and greater charges are
drawn with more field lines than
weaker charges
Electric Current
Charges that accumulate on an object –
static electricity
Charges in motion – electric current
Electric charges have potential energy
based on position in electric field
Like charges: highest potential energy
when charges are close
Unlike charges: highest potential
energy when charges are at distance
Often, potential difference is more useful to
study than potential energy
Potential difference occurs when a charge
changes position in electric field
Work that must be done against electric
forces to move charge between points
Potential difference = EPE/q
SI unit: Volts
1 volt = 1 joule/coulomb
Often called voltage
Batteries
Batteries function due to potential
difference between terminals
Convert chemical energy to electric
energy
One terminal (+), other is (-)
Two components of battery
Electrolyte: solution that conducts
electricity
Electrodes: two materials with
different conducting ability
Batteries can be dry cells or wet cells
Dry cell: electrolyte is paste-like
material
Wet cell: electrolyte is liquid
Voltage across terminals sets charges in
motion
Electrons move from negative
terminal to positive terminal
Connect terminals with wire and
charges flow through wire to get to
positive terminal
Charges move from higher potential
energy to lower potential energy
Produces electric current
Current: rate at which current moves
through a conductor
SI unit: ampere (A)
1 A = 1 coulomb/second
Batteries produce direct current (DC)
Charges always move in same
direction
Current can be made of positive or
negative charges or a combination of
the two
Metals: negative charges
Gases and chemical solutions: mix of
positive and negative charges
Conventional current defined as
movement of positive charges
Direction of current in a wire is
opposite the direction of the
movement of electrons in the wire
Electric Resistance
Resistance: opposition by material or
device to flow of current
Different wattage light bulbs shine with
different brightness due to resistance
across filament
Voltage from outlet always 120 V
Resistance created by internal friction
Slows movement of charges in
material
Resistance difficult to measure directly
Assess relationship between voltage
across conductor and current through
material
Ohm’s Law: relates voltage, current and
resistance
R = V/I or V = IR
SI unit for resistance: ohm (Ω)
Resistor: component in circuit that
provides specific resistance to flow of
charges
Collisions between moving charges and
atoms of conductor increase kinetic
energy of conductor atoms
Material shows temperature increase
Resistance affected by identity of
material, temperature, length and
cross-sectional area
temperature, resistance
length, resistance
cross-section area, resistance
Conductors have low resistance
Some materials become
superconductors below certain critical
temperatures
Insulators have high resistance
Semiconductors are intermediate
between conductors and insulators
Circuits
Electric circuit: set of electrical
components connected such that they
provide one or more complete paths
for the movement of electrons
Path between terminals of power
source is complete
Charges lose energy as travel through
circuit and regain energy when travel
through power source
Circuits may be open or closed
Closed circuit: path is complete and
electrons flow
Open circuit: path is broken in some
area and electrons cease to flow
Switches: devices for manually
opening and closing a circuit
On: circuit closed
Off: circuit open
Circuits visualized by using schematic
diagrams
Standard set of symbols to represent
components of circuit
Map out components of circuit in way
that it is understood worldwide
Types of Circuits
When different devices are connected
to power source, arrangement may
vary
Series circuit: Single pathway for
electrons to flow
Devices connected in linear fashion,
one after another
Electrons must flow through each
device to complete circuit
Current in each device is the same, but
resistances may vary
Different voltages across each device
See different potential change across
each device, if resistances vary
If one device burns out or is removed,
entire circuit ceases to function
Parallel circuits: multiple paths for
electrons to follow to complete circuit
Break in one path does not interfere
with other paths
Voltage is same across each device,
but current will vary according to
device’s resistance
Electric Power and Electric Energy
Electric energy – type of energy needed
for electric or electronic devices to
run
Electric power – rate at which electric
energy is used in a circuit
As charges travel circuit, they lose
energy that is converted into work
and is lost as heat
P = VI
SI unit: watt
Power lost or dissipated by a circuit can
be calculated:
P = I2R or P = V2/R
Electric energy used by home reported
on monthly electric bill
Electric meter tracks number of
kilowatt-hours of energy used
1 kilowatt-hour = 3.6 x 106 J
Set fixed cost per kilowatt hour
Fuses and Circuit Breakers
Mechanisms to protect circuits from
overload
Devices sensitive to electric energy –
too much and wiring and components
in device may be damaged
Fuses inserted into circuit possess
relatively weak filament that will burn
out before a power surge reaches
remainder of circuit
Must be replaced when burned out
Circuit breakers: respond to current
overloads by opening the circuit
Acts as a switch
Can be reset by turning on circuit
again
Circuit breaker boxes have switch for
every circuit in home
Ground Fault Interrupters (GFI’s)
monitor current in both wires
connected to socket
Change in current in a wire causes
GFI to open circuit
Touching bare wire would create
current change and open circuit