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Electricity
Conservation of Charge
This attracting and repelling behavior is attributed
to a property called charge.
Electrons and protons have electric charge.
In a neutral atom, there are as many electrons as
protons, so there is no net charge.
The principle of conservation is that net electric
charge is neither created nor destroyed, but is
simply transferred from one material to another.
Charges cause Electrical Forces
Electrical Forces: A force that one charge exerts on
another.
What is the charge on the following?
Electrons: Negative
Protons: Positive
Neutrons: Neutral (no charge)
Charges cause Electrical Forces
• When the charges are the same sign, they repel.
• Like charges repel.
• When the charges are opposite signs, they attract.
• Opposites attract.
Electric Fields
 Just like the space around the earth is filled with a
gravitational pull, the space around every electric
charge is filled with an electric field.
Electric Field: An aura than extends through space.
 These vectors (magnitude and direction) are called
electric field lines.
 When the lines are farther apart, the field is weaker.
 When the lines are closer together, the field is stronger.
Static Electricity and Charging
Static electricity is the build up of charges on an
object.
 Charge can be transferred between objects.
 The total charge in an isolated system is
constant.
Ways Objects Acquire and
Transfer Charge
1. Friction: rubbing two objects together
 One object gains electrons and one object loses
electrons
Example:


Rubbing a balloon against your hair
Walking across the carpet
Ways Objects Acquire and
Transfer Charge
2. Conduction: electrons flow through one object to
another by contact
Example:

Electrical wires
Ways Objects Acquire and
Transfer Charge
3. Induction: electrons are rearranged in a neutral
object when a charged object comes near
Example:


Balloon stuck to a wall
Lightning
Ways Objects Acquire and
Transfer Charge
Static discharge is the loss of static electricity as
electric charges move off an object.
Lightning rods are used to protect
structures. The discharged electrons go
into the ground!
Conductors and Insulators
Whether a substance is classified as a conductor
or an insulator depends on how tightly the
atoms of the substance hold onto their
electrons.
Conductors: Outer electrons of the atom in a metal are not
anchored to the nuclei of particular atoms, they are free to
roam in the material.
 Their electrons are loose– “sea of electrons”.
Insulators: Electrons in other materials, rubber and glass for
example, are tightly bound and remain with particular
atoms.
 Electrons are not free to wander.
 These materials are poor conductors of heat and electricity.
Conductors and Insulators
Semiconductors: Materials can be made to behave as
an insulator and as a conductor.
NASA Solar Panels
Superconductor: At temperatures near absolute zero,
certain metals acquire infinite conductivity (zero
resistance to the flow of charge).
Electrical circuits have four basic
parts: source, path, load, and switch
Source: The power supply or battery
Path: The wire
Load: Lights, appliances, heaters
Switch: Opens/closes the circuit
Simple Circuit
Load
Battery
Path
Switch
An electric bell is a simple circuit…
• When you flip the
switch, the current
flows through the wire
to the electromagnet.
• From the
electromagnet, it goes
to the contact screw
to the hammer.
Open Circuit
Load
Battery
Path
Path is disconnected.
Closed Circuit
Load
Battery
Path
Path is connected, switch is on.
Short Circuit
Load
Battery
Path
Path goes around the load.
Preventing Overloaded Circuits
• To prevent overloading in
circuits, fuses are connected
in series along the supply line.
• The entire line of current must
pass through the fuse.
• The safety fuse is constructed
with a wire ribbon that will
heat up and melt at a certain
current, this stops the flow
when melted.
Fuses and Circuit Breakers
 Fuses open the circuit and stop the
flow of electricity.
 Fuses cannot be reused once
blown.
o
o
A circuit breaker uses bimetallic
strips or magnets to open the
switch.
Circuit breakers are used now
because they can be reused and
not thrown out.
Types of Circuits
Most circuits have more than one device that
receives electrical energy. These devices are
commonly connected in a circuit in one of two
ways, series or parallel.
When connected in series, they form a single
pathway for electron flow between the terminals of
the battery, generator, or wall socket.
When they are connected in parallel, they form
branches, each of which is a separate path for the
flow of electrons.
Schematic Diagrams
• Electric circuits are
frequently described by
simple diagrams called
schematic diagrams.
• The positive terminal of
the battery is represented
with a long line and the
negative terminal with a
short line.
Drawing Circuits with Schematic Diagrams
Series Circuit
Parallel Circuit
Series Circuits
o
o
An electrical circuit in
which devices are
arranged so that charge
flows through each in
turn.
If one part of the circuit
should stop the current, it
will stop throughout the
circuit.
• The more lights you have
in a series circuit, the
dimmer they are.
• A series circuit is like a
single lane road with no
alternate routes or paths.
Series Circuit
Load
Battery
Path
Loads are on same path.
Parallel Circuits
o
o
An electrical circuit in
which devices are connected
to the same two points of the
circuit, so that any single
device completes the circuit
independently of the others.
If one part of the circuit
should stop the current, the
current will continue to the
other parts of the circuit.
• All of the lights in a parallel
circuit will have the same
intensity.
• A parallel circuit is like a road
with alternative routes. When
one is blocked, you have other
choices.
Parallel Circuit
Load
Battery
Path
Loads are on different paths.
Parallel Circuits and Overloading
 Electricity is usually fed into a
house by way of load wires
called lines. These lines are
very low in resistance and are
connected to wall outlets in
each room.
 About 110 to 120 volts are
impressed on these lines by
generators at the power
utility.
 This voltage is applied to
appliances and other devices
that are connected in parallel
by plugs to these lines.
Electric Current:
Electric Current
The flow of electric charge
Conduction Electrons:
Electrons that are free to move throughout the atomic
network.
When electrons flow in a wire, the number entering
one end is the same as the number leaving the other
end.
What is Direct Current?
An electric current of constant direction. The
flow of electrons goes only in one direction.
It is abbreviated DC.
A battery produces direct current
What is Alternating Current?
Current that regularly reverses direction,
flowing first in one direction and then in the
opposite direction.
Power companies generate AC to make it
easier to transmit electricity over long
distances.
Voltage
 When you rub a balloon on your hair it (the balloon)
becomes negatively charged ~ several thousand volts!
Ampere (Amp)
The SI unit for electric current (I)
 a unit of electric current, or amount of electric charge
per unit time, in coulombs per second.
 The ampere is an SI base unit, and is named after
André-Marie Ampère.
 In practical household wiring terms, it is the
measurement of the number of electrons passing
through a given point in one second intervals

Voltage Sources
A sustained current requires a suitable “electric
pump” to provide a sustained potential difference.
Something that provides a potential difference is
known as a voltage source.
Remember: Voltage doesn’t go anywhere, it is the
charge that moves.
Sources of voltage
Dry cells, wet cells, and generators supply energy
that allows charges to move. In dry and wet cells,
energy released in a chemical reaction occurring
inside the cell is converted to electric energy.
Generators, such as alternators in
automobiles, convert mechanical energy into
electric energy.
Electric Resistance
The amount of current that flows in a circuit
depends on the voltage provided by the voltage
source.
The current flow also depends on the resistance
that the conductor offers to the flow of charge, which
is called the electric resistance.
Thick wires have less resistance than thin wires.
Longer wires have more resistance than short wires.
Electric resistance also depends on the conductivity
of the material and the temperature.
Ohm:

Ohm’s Law:
The relationship between voltage, current, and resistance.
This relationship states that you’ll get twice the current for twice the
voltage. The greater the voltage, the greater the current. The greater
the resistance, the less the current.
Current
V
I
R
Voltage
Resistance
What is the current produced
with a 9 volt battery through
a resistance of 100 ohms?
V 9V
R  100 
I?
Solution:
I=V/R
I=(9 V)/(100 ohms)
I=.09 Amps
What happens when there is less
resistance to the current?
Low resistance permits a large current, which
produces considerable heat.
If an appliance or circuit were to “overheat”, that
means there isn’t enough resistance, and the
electrons move too fast. It can catch on fire due to
the intense heat.
Inside electronic devices such as radio and
television receivers, the current is regulated by
circuit elements called resistors.