Download Chapter 16 Electricity

Section 1: Electric Charge and Force
Section 2: Current
Section 3: Circuits
Key Terms
Electric Charge
Electrical Conductor
Electrical Insulator
Electric Force
Electric Field
Electric Charge
An electric charge is an electrical property of
matter that creates electric and magnetic
forces and interactions
Like charges repel, and opposite charges attract
The two charges are positive and negative
Positive + Negative = Attraction
Positive + Positive = Repulsion
An object’s electric charge depends on the
imbalance of its protons and electrons
All matter is made up of protons, neutrons, and
electrons. When there is an imbalance in the
number of protons and electrons the object
becomes charged.
The SI unit of electric charge is coulomb
Proton = + 1.6x10-19 C
Electron = - 1.6x10-19 C
Conductors allow charges to flow; insulators do
not
Conductor a material in which charges can
move freely and that can carry an electric
current (Copper)
Insulator a material that does not transfer
current easily (Glass)
Objects can be charged by the transfer of
electrons
Protons and neutrons are relatively fixed in the
nucleus of an atom. The outermost electrons
can be easily transferred from on atom to
another.
The direction of transfer depends on the
material
One material gains electrons becoming
negatively charged and the other material
loses electrons become positively charged
The transfer of electrons by rubbing two
objects together is called charging by friction
Objects can also be charged without friction.
This is called charging by contact.
Charges move within uncharged objects
A negatively charged rod brought near a metal
doorknob induces a positive charge on the
side of the doorknob closest to the rod and a
negative charge on the side farthest from the
rod.
A charged object can attract and insulator by
inducing a slight change in position of the
electrons. This is called polarization
Electric Force
An electric force is the force of attraction or
repulsion between objects due to charge
Electric forces are responsible for events we
cannot see. They hold atoms together as well
as proteins which our bodies need
Electric force depends on charge and distance
The electric force between two objects is
proportional to the product of the charges on
the objects.
The electric force is also inversely proportional
to the square of the distance between two
objects
Electric force acts through a field
Electric field is a region in space around a
charged object that causes a stationary
charged object to experience an electric
field.
Electric field lines can be used to show an
electric field.
Key Terms
Electrical Potential Energy
Potential Difference
Cell
Current
Resistance
Voltage and Current
Electrical potential energy is the ability to
move an electric charge from one point to
another
Electrical potential energy between two
negative charges decrease as the distance
between them increases
Potential difference is measured in volts
Potential difference between any two points, the
work that must be done against electric forces
to move a unit charge from one point to the
other
The SI unit for potential difference is Volts (V)
1 Volt = 1Joule per Coulomb (1 J/C)
Potential difference is some times called voltage
There is a voltage across the terminals of a
battery
The voltage across two terminals of a battery
can range from 1.5 V (small battery) to 12 V
(car battery)
Cell is a device that is a source of electric
current because of a potential difference, or
voltage, between the terminals
Table 1 on pg 539 gives examples of different
types of cells.
Electrochemical cells contain an electrolyte, a
solution that conducts electricity, and two
electrodes, each a different conducting
material. Can be either a wet cell or dry cell
Wet cell – liquid electrolyte
Dry Cell – Paste-like electrolyte
A voltage sets charges in motion
When charges are accelerated by an electric
field to move to a position of lower potential
energy, an electric current is produced.
A current is the rate that electric charges
move through a conductor.
SI unit for current is Ampere (amp)
Ampere = 1 C of charge moving past a point in
1 second
Batteries are direct current sources because
the charge always moves from one terminal
to the other in the same direction.
Conventional current is defined as movement
of positive charge
A negative charge moving in one direction has
the same effect as a positive charge moving
in the opposite direction.
Electrical Resistance
Resistance is opposition posed by a material or
a device to the flow of current
Resistance can be calculated from current and
voltage
A conductor’s resistance indicates how much
the motion of charges within it is resisted
because of collisions
It is found by dividing the voltage across the
conductors by the current.
The SI unit is ohm Ω, which is equal to volts
per ampere.
Resistance Equation
resistance = voltage/current or R =V/I
Resistor is a special type of conductor used to
control current.
Equations
R = V/I = Ω
V = IR = volts
I = V/R = amp
Conductors have low resistance
A good conductor allows electrons to flow
easily under the influence of an electric
field. Metals (copper)
Some materials become superconductors below
a certain temperature
Certain metals and compounds have zero
resistance when their temperature falls
below a certain temperature called the
critical temperature.
Superconductors have been used in electrical
devices such as filters, powerful magnets,
and Maglev high-speed express trains.
Semiconductors are intermediate to
conductors and insulators
Materials that semiconductors that are in their
pure state, are insulators. As impurities are
added the semiconductor is loses some of its
insulating ability. Silicon and germanium are
examples of semiconductors .
Insulators have high resistance
Insulating materials are used to prevent
electric current from leaking.
Example: Plastic coating on wires
Sometimes it is important to provide a
pathway for current to leave a charged
object. This is done by using a ground.
Key Terms
Electric Circuit
Schematic Diagram
Series
Parallel
Electrical Energy
Fuse
Circuit Breaker
What are Circuits?
An electric circuit is a path through which
charges can be conducted
Closed Circuit - electrons are able to flow a
complete path
Open Circuit – Electrons are unable to flow a
incomplete path
Switches interrupt the flow of charges in a
circuit
Schematic diagrams are used to represent
circuits
Schematic diagram is a graphical
representation of a circuit that uses lines to
represent wires and different symbols to
represent components
Table 2 pg 548 has more examples of
Schematic Diagram Symbols
Series and Parallel Circuits
Series circuits have a single path for current
Figure 17A pg 549
If one element along the path in a series
circuit is removed, the circuit will not work
The voltage across each device in a series
circuit can be different
Parallel circuits have multiple paths for
current
The voltage across each device is the same in a
parallel circuit
When devices are connected in parallel,
charges have more than one path to follow.
The circuit can be complete even if one light
bulb burns out.
The current will take the path with least
resistance.
Electric Power and Electrical Energy
Electrical energy is the energy that is
associated with charged particles because of
their positions.
Electric power is the rate at which electrical
energy is used in a circuit
Electric power is the product of the total
current (I) in the voltage (V) across a circuit
Electric Power Equation
power = current x voltage
P = IV
Power is measured in watts
If you combine the electric power equation
with the equation V = IR, the power lost, or
dissipated, by a resistor can be calculated
P = I2 R = V2/R
Electric companies measure energy consumed
in kilowatt-hours
1kW x h = 3.6 x 106 J
Fuses and Circuit Breakers
If to much objects are plugged into an outlet,
the overall resistance will be lower. This
means that the electrical wires carry more
than a safe level of current
When this happens the circuit is said to be
overloaded which can lead to a fire.
When two wires touch, it creates an
alternative pathway for the electrical
current. This is a short circuit.
Grounding appliances can reduce the risk
electrical shocks from short circuits
Fuses melt to prevent circuit overloads
A fuse is an electrical device that contains a
metal strip that melts when current in the
circuit becomes too great.
A circuit overload will cause a fuse to blow out
Circuit breakers open circuits with high current
A circuit breaker is a switch that opens a
circuit automatically when the circuit
exceeds a certain value.
Circuit breakers uses a magnet or bimetallic
strip that respond to current overload by
opening the circuit
Unlike fuses circuit breakers can be reset by
turning the switch back on.