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Exam Review
Electricity
Electric Charges
• An electric charge is a negative or a
positive amount of electricity that builds
up in an object.
• Electrons have a negative charge and
protons have a positive charge.
• Like charges attract, opposite charges
repel.
Charged Objects
• Objects become negatively charged
when they have an excess of
electrons.
• Objects become positively charged
when they have lost electrons.
• Protons are never lost or gained.
Neutral Objects
• Neutral objects have the same
number of positive and negative
charges.
• The positive charges in one neutral
object will attract the negative
charges in another object.
Static Electricity
• Static electricity is a charge on a
substance that stays in the same
place.
• Electrostatics is the study of static
electric charge.
Conductors and Insulators
• Conductors allow electrons to move
freely throughout them. They carry,
conduct or transmit electricity.
– Ex: Silver, gold, copper wires
• Insulators hold their electrons very
tightly. They do not conduct electricity.
– Ex: Rubber, fur, glass
How Lightning is Produced
• Strong winds and the collisions of water
droplets and ice particles in the clouds
strip electrons from some particles and
deposit them on others.
• Negative charges collect at the bottom
of the cloud.
• The higher parts of the cloud are colder
and positively charged.
• The negatively charges on the bottom of
the cloud repel electrons on the surface
of the Earth, leaving the ground
positively charged just below the cloud.
• The strong attraction between the
negative cloud and the positive ground
pull electrons off atoms and molecules in
the air.
• Once a chain of ions forms, a giant
discharge occurs between the
cloud and the ground.
• Electrons crash through the air so
fast, colliding with other molecules,
heating and lighting up the air.
Current Electricity
• Current electricity is a continuous
flow of electric charge.
• For example: When we hook
something up to a battery, the
electrical current flows through a
path called a circuit.
Circuit Measurements
• Electric Current - Flow of electrons
through a material.
– Measured as the amount of current that
passes a conducting wire every second.
• Electrical Potential –
– Stored electric energy
Moving Charges
• The symbol for current is I and the
symbol for charge is Q, t is used to
represent time.
• Current = charge moving past a point
time
• I=Q
t
• Charge (Q) is measured in
Coulombs (C)
• Current (I) is measured in Amperes
(A)
• Time (t) is measured in seconds (s)
Voltage
• Voltage – causes current to flow
through an electrical circuit
• Volt – unit of measure to measure
the potential difference
• A Voltage Source (battery or
generator) is required to maintain
the electrical potential in a circuit.
Electrical Current
• Electric current is a measure of the
rate at which the electric charges
move past a given point in a circuit.
• Electrical Current is measured in
Amperes (A)
Measuring Electricity
• Electric Potential – the electrical
energy that an electron possesses.
• Current is measured by an Ammeter
• Voltage is measured by a Voltmeter
Resistance
• Amount of Electrical Current (
amps) depends on more than just
Voltage, it depends on the
Resistance found in the circuit.
Resistance
• Resistance – is the ability to hold
back the flow of electrons in a
conductor.
• The molecules found in every type
of conductor resists the flow of
electrons to some extent.
• When electrons flow through a
conductor the electrical resistance
causes a loss of electric potential
(voltage).
• There is a “difference” in the
amount of electric potential after
the electrons have flowed through
the conductor.
• This difference is referred to as the
potential difference.
Ohm’s Law
• The potential difference between
two points on a conductor is
proportional (directly related) to
the electric current flowing
through the conductor.
Ohm’s Law
Potential
Difference
(Voltage Drop)
= Electric Current x Electrical Resistance
V=IxR
Potential difference (V) is measured in volts (V)
Electric Current (I) is measured in ampere (A)
Resistance (R) is measured in ohms ()
Circuits
• Electric Circuit – a controlled path
in which electric current passes
through. Electric circuits are used
to convert electrical energy into
other forms of energy such as heat
or light.
• We can use a circuit diagram to
show the path of the electric
current.
Parts of a Circuit
1. Source of electrical Energy – the
place or thing that generates the
electrical energy.
2. Electrical Load – anything that
converts electrical energy into the
form of energy needed.
3. Electric Circuit Control Device – a
switch that controls the electricity
(turn on and off).
4. Connectors – the conducting wires
that provide the controlled path for
electric current to flow to each
part of the circuit.
Movement in a Circuit
• The movement in a circuit is continuous and
controlled.
• A battery serves as a source of energy for a
circuit.
• The electrical energy stored in the battery is
called electrical potential energy. The electrons
have the potential for doing work but are
unable to do it until the battery is connected to
the load and the circuit is closed.
Closed Circuit – a circuit that is
“on” or allowing electricity to flow
through.
Open Circuit – a circuit that is
“off” or not allowing electricity to
flow through.
Types of Circuits
• There are 2 types of circuits:
– Series Circuit: A circuit with only one
path for current to flow
– Parallel Circuit: A circuit with many
pathways for current to flow.
Series Circuits
• In a series circuit electrons travel around
one closed loop.
• The current at any one point in a series is
exactly the same as the current at any
other point.
• The charges pass from one load to the
next before returning to the power
source to be energized again.
Series Circuit
Parallel Circuits
• In a parallel circuit charges flow around
two or more different loops.
• After leaving the power source they
eventually reach a “fork in the road”.
Some charges take one path while other
charges take another path.
• The current in a parallel circuit is not
the same at every point in the circuit.
Parallel Circuits