Transcript Electricity - DarringtonScience

Electricity and
Magnetism
Chapter 2: Electricity
Section 1: Electric
Charge and Static
Electricity
Vocabulary
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Electric force
Electric field
Static electricity
Conservation of charge
Conduction
Induction
Static discharge
Electric Charge
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Remember:
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The charged parts of atoms are
electrons and protons
When two protons come near each
other, they repel
But if an electron and a proton
come near each other, they attract.
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Why do protons repel protons but
attract electrons?
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Because they have different types
of electric charge
Electric charge is a property of
electrons and protons
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Protons and electrons have opposite
electric charge
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Protons have + electric charge
Electrons have – electric charge
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The two types of electric charges
interact in specific ways
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This rule of repulsion/attraction is
the same as the rule for
magnetic poles
There is one important difference
between electric charges and
magnetic poles
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Magnetic poles cannot exist alone
We can have single electric charges
though.
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Meaning a negative charge can exist
without a positive charge.
Electric Force
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Remember, a force is any push
or pull.
In electricity, an electric force is
the attraction or repulsion
between electric charges.
Electric Field
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Just as magnetic poles exert
their forces over a distance, so
do electric charges.
Also, a magnetic field extends
around a magnet, so does
electric charges.
An electric field is an area of
electrical force around a charged
object.
When one charged object is
placed in the electrical field of
another charged object, it is
either pushed or pulled.
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Pushed if the objects have the
same charge, pulled if the objects
have opposite charges.
Electric Field Around a
Single Charge
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An electric field is invisible, just
like a magnetic field
Just like magnetic fields, we
represent electrical fields with
arrows to show the direction of
the electric force
Strength of the electric field is
related to the distance from the
charged object.
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The greater the distance, the
weaker the electric field is.
Like magnetic fields, the closer the
lines are to one another, the greater
strength of the field.
Electric Field Around
Multiple Charges
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When there are two or
more charges, the
shape of the electric
field of each charge is
altered.
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The electric field of
each charge will
combine by repelling or
attracting.
Static Electricity
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Most objects have no overall charge
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Some objects can become charged
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Meaning they are neutral
Meaning they have an equal number of
protons and electrons.
There would be no overall electric force on
the atom
But protons are bound tightly to the atom,
and can’t leave
But electrons can sometimes leave their
atom
With some materials, the electrons are
held loosely by their atoms
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So these electrons can move to other
atoms.
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Uncharged objects can become
charged by gaining or losing
electrons.
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If an object loses electrons, it is left with
more protons than electrons.
 This means it will have a positive charge
If an object gains electrons, it now has
more electrons than protons
 This means it will have a negative charge
The build-up of charges on an object
is called static electricity.
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In static electricity, charges build up on an
object, but do they do not flow
continuously.
Transferring Charge
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An object becomes charged only
when electrons are transferred
from one location to another.
Charges are never created or
destroyed.
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This is known as the conservation
of charge.
There are three methods by
which charges can be transferred
to build up static electricity
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Charging by friction
Charging by conduction
Charging by induction
Charging by Friction
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When two uncharged objects rub
together, some electrons from
one object can move onto the
other object.
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The object that gains electrons
becomes negative
Charging by friction is the
transfer of electrons by rubbing.
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Clothing sticking together after
coming out of the dryer
Rubbing a balloon against
something
Charging by
Conduction
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When a charged object touches
another object, electrons can be
transferred between the objects.
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Note: Objects must TOUCH!
Electrons will transfer from the
object that has a more negative
charge to the one with more
positive charge
So charging by conduction is
the transfer of electrons from a
charged object to another object
by direct contact.
Charging by Induction
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Objects to not have to be
touching for electrons to be
transferred
Charging by induction is the
movement of electrons from one
part of an object that is caused
by the electric field of another
object.
The electric field around a
charged object attracts or repels
the electric charges in a second
object.
Detecting Charge
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Electric charge is invisible, but
can be detected by an instrument
called an electroscope.
Static Discharge
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Charges that build up as static
electricity on an object don’t stay there
forever.
Electrons tend to move, returning the
object to neutral conditions.
Think of what happens when a
positively charged object touches a
negatively charged object
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Electrons transfer from the negatively
charged object to the positively charged
object until both have the same charge
The loss of static electricity as electric
charges transfer from one object to
another is called static discharge.
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Very often, static discharge produces a
spark.
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Lightning is a dramatic example of static
discharge
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Basically, just a large spark
Within clouds, water droplets can
become electrically charged
Lightning hits Earth when negative
charges at bottom of clouds causes
Earth to become positively charged
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As electrons move, they heat the air around
the path they travel until it glows
The glowing air is the spark we see
What type of charging would this be?
Induction!
Electrons will then jump between the
cloud and Earth’s surface, creating a
giant spark (lightning).
Van de Graff Generator
Electricity and
Magnetism
Chapter 2: Electricity
Section 2: Electric
Current
Vocabulary
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Electric current
Electric circuit
Conductor
Insulator
Voltage
Voltage source
Resistance
Flow of Electric
Charges
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Remember:
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Static electric charges don’t flow
continuously
However, when electric charges
are made to flow through a wire
(or other material) we can get an
electric current.
Electric current is the
continuous flow of electrical
charges through a material.
The amount of charge that flows
through the wire in a unit of time
is the rate of electric current.
Electric Current
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The unit for the rate of current is
the Ampere.
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Often shortened to Amp or just A.
This is like any other rate
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So this is how much charge moves
per second.
Current in a Circuit
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Electric current doesn’t
automatically exist in a material.
Current requires a specific path
to follow.
To produce electric current,
charges must flow
continuously from one place
to another.
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Current requires an electric circuit
An electric circuit is a complete,
unbroken path through which
electric charges can flow.
Conductors and
Insulators
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Charges low through a circuit
made of metal wires.
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But charge will not flow through a
circuit made of plastic.
Electric charges will not flow easily
through every material.
A conductor is a material that
transfers electrical charges easily
While an insulator is a material
that does not transfer electric
charges easily.
Conductors
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In a conductor, the atoms contain
electrons that are loosely bound.
These electrons are able to
move throughout the conductor.
As these electrons flow through
the conductor, they form an
electric current.
When you turn on a light switch,
are the charges coming from the
power station to the lightbulb?
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No
The charges are already in every
part of the circuit. The switch just
makes them start to flow!
Insulators
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Electrons in an insulator are
tightly bound to their atoms, so
they don’t move easily
Rubber, glass, sand, plastic, and
wood are all good insulators.
We generally use insulators to
stop the flow of charge
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This is why most wires come
wrapped in a rubber tube.
Charges Need to Flow
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Charges in an electrical circuit
move because of differences in
electrical potential energy
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Just like a roller coaster will fall
because of a difference in
gravitational potential energy at the
top of the track and the bottom of
the track.
A battery (or other power source)
in a circuit provides the potential
energy difference for the circuit.
This energy difference is related
to the charges inside the battery.
Voltage
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The difference in electrical
potential energy between two
places in a circuit is called
voltage.
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It is measured in the unit of the volt
(V).
Voltage is what causes current in an
electrical circuit.
To maintain voltage, a circuit
needs a source of energy.
A voltage source is a device that
creates a potential difference in
an electrical circuit
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Which provides the energy needed
to maintain voltage
Batteries and generators are
examples of voltage sources.
Resistance
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Current depends on resistance
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Resistance is the measure of how
difficult it is for charges to flow
through a material
In other words, conductors have low
resistance
Insulators have high resistance
The greater the resistance, the
less current there is for a given
charge.
The unit of resistance is the ohm
(Ω)
Factors effecting
resistance
There are four factors that
determine the resistance in an
object
The material from which the
wire is made
Length
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Long wires have more resistance
than short wires
Diameter of wire
3.
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Thin wires have more resistance
than thick wires
Temperature of the wire
4.
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Hotter wires have more resistance
than colder wires
Electricity and
Magnetism
Chapter 2: Electricity
Section 4: Electric
Circuits
Vocabulary
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Ohm’s law
Series circuit
Parallel circuit
Ohm’s Law
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To understand circuits, we need to
understand the relationship between
current, voltage and resistance.
In the 1800’s, Georg Ohm performed
experiments to show how these three
factors are related.
Ohm found that if he kept all of the
factors that affect resistance constant,
the resistance of most conductors
does not depend upon the voltage
across them
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So resistance does not depend upon
voltage
Voltage only affects the current
He hypothesized that conductors (and
other materials) have a constant
resistance regardless of applied voltage
Calculating with Ohm’s
Law
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The relationship between resistance,
voltage and current is summed up in
Ohm’s Law.
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States that the resistance is equal to the
voltage divided by the current
Voltage
Resistance 
Current
Voltage
Current 
Resistance
Voltage  Current  Resistance
What this tells us
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Direct relationship between
voltage and current
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So if we double voltage, we double
current
If we have 5x the voltage, we get 5x
the current
If I ½ the voltage, I get ½ the
current
Inverse relationship between
resistance and current
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If I double resistance, I ½x the
current
If I have 5x the resistance I get 1/5
the current
If I have 1/3 the resistance, I get 3x
the current
Features of a Circuit
All electrical circuits have the
same basic features
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Circuits have devices that are run
by electrical energy
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2.
Radio, computer, light bulb,
refrigerator are all devices that
transform electrical energy into some
other form of energy
Devices like light bulbs and fans act
like resistors in a circuit
A circuit has a source of electrical
energy
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In other words, a voltage source is
needed
3. Electric circuits are connected by
conducting wires
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The conducting wires complete the
path of the current.
Allows charges to flow throughout the
circuit
Circuit Diagrams
Series Circuits
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If all of the parts of an electric
circuit are connected one after
another, the circuit is called a
series circuit.
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Easy to build
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So in a series circuit, there is only
ONE path for current to take
Just hook up device after device
Disadvantages
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If one part of the circuit is broken,
the whole thing is
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Resistors
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As resistors are added to a series
circuit, the resistance increases
Parallel Circuits
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In a parallel circuit, the different parts
of the circuit are on separate
branches.
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Several Paths
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There are several paths for the charge to
take
If a light bulb burns out in a parallel
circuit…
 Only one path (branch) is broken, but
charge can still go through other branches
Resistors
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If you add resistors to a parallel circuit,
total resistance actually goes down
Too many appliances in an outlet
Electric Power
(Actually section 5)
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All electrical appliances
transform electrical energy into
some other form.
Remember, the rate at which
energy is transformed is known
as power.
Electrical power is dependent
upon voltage and current.
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Power = Voltage x Current