Electricity
Chapter 7
Physical Science
Vocabulary for Electricity
Static electricity Current
Conductor Dry cell
Insulator Wet cell
Conduction (charging by contact) Resistance
Induction Ohm
Electroscope Series circuit
Voltage Parallel circuit
Circuit Circuit breaker
Electric Charge
All matter is made up of ATOMS
An atom is the smallest part of an element that has
all the properties of that element
Atoms are made of smaller particles called
“subatomic particles”
These particles include protons, neutrons and
electrons
Both protons and electrons have a basic property
called “electric charge”
Basic structure of an atom
Helium Atom
N
E- P+ P+
N E-
Nucleus of atom Energy level
Contains protons and neutrons Contains electrons
Is positively charged Has a negative charge
Charge and Force
Protons are positively charged P+
Electrons are negatively charged E-
Neutrons have no charge N
Opposite charges attract each other and can
cause a force of push or pull
An atom has no overall charge – why?
Equal number of protons and electrons causes
this to be true.
Protons cannot move, but electrons can move
freely
Electric Fields
An electric field is a An electric field is the
region surrounding a strongest near a charged
charged particle where particle
other charged particles The electric field is
are affected weaker when it is far
A force of attraction away
occurs if there is an So the strength of an
opposite charge. electric field depends
A force of repulsion upon the distance from
occurs if there is the the charged particle
same charge
Static Electricity
The word “static”, means not moving or
stationary
Static electricity is a build up of electric charges
on an object
The three ways electric charges can build are,
friction, conduction, and induction
Friction – Electrons are held loosely so rubbing a
balloon against a cloth will transfer the electrons
to the balloon.
Conduction – Direct contact is required for
electrons to move through a conductor
Induction – No contact is necessary.
Rearrangement occurs as repulsion and attraction
work together to cause a charge
Continued
A great example of a dramatic natural static electrical
discharge is lightning.
An electroscope is an instrument that detects a static
charge.
Static charges like to return to the ground and try to
balance themselves. (grounding)
We use grounding rods called lightning rods on our
homes to protect them from lightning. Ben Franklin
invented these devices for us.
There are six types of lightning. Heat, sheet, fork, bead,
ribbon, and ball.
Rapid expansion of the air from the heat of the spark is
what causes the thunder we hear.
Lightning can travel from ground to cloud, cloud to
cloud, or cloud to ground.
Lightning Facts
Conditions needed for lightning to flourish
are warmth and humidity.
Around the world there are 100 lightning
strikes every second.
Weather meteorologists use an instrument
called an interferometer to track electrical
fields.
An average of 3 people per year are struck
by lightning while talking on the telephone.
A lightning bolt is about an inch in
diameter.
4 out of 5 people struck by lightning will
survive.
Lightning will always take the path of least
resistance.
Voltage
The measure of energy that is used to move
electrons is called voltage.
Voltage is also known as “potential difference”.
Voltage is the push that makes electrons move.
The higher the voltage, the more energy the
electrons carry. More voltage – more energy.
The more energy each electron has, the more work
can be done.
The unit used for measuring voltage is the “volt”.
A voltmeter is used to measure voltage.
The letter “V”, is used to represent voltage.
The Flow of Electricity
The flow of electrons through a wire is called
electric current.
Current is measured by how many electrons pass a
given point in one second.
High current – more electrons.
The symbol for current is the letter “I”.
The unit used to measure current is the ampere
(A).
Ammeters and galvanometers are used to measure
current.
One amp is the amount of current that flows past a
given point in one second.
Resistance
Resistance - The opposition to the flow of
electricity.
Symbol for resistance is the letter “R”.
The unit for resistance is the Ohm.
Some materials conduct better than other materials.
Poor conductors will have a high resistance to the
flow of electricity.
Some materials are nonconductors and are used as
insulators.
Resistance is also affected by a wires thickness,
length and temperature. How?
Resistance continued….
Longer wire – more resistance
Thinner wire – more resistance
Can you think of a reason resistance might be
good to use?
While slowing down the flow of electrons, heat
and energy are released, sometimes used for
heating elements or filaments in light bulbs.
Super conductors often need extremely cold
temperatures to work efficiently.
Ohm’s Law
(law of electrical resistance)
Ohm’s law states that the current in a wire is
equal to the voltage divided by the resistance.
Current = voltage/resistance
I=V/R
What would happen if the resistance increases and
the voltage stays the same?
What would happen if the resistance decreases and
the voltage stays the same?
What must happen to the resistance if the voltage
increases and the current stays the same?
Producing a Current
We must have a source of electrons to produce a
current
Electrochemical cells provide a steady supply of
electric current from a chemical reaction occurring
in the cell.
Dry Cells – Are not really dry, but contain a paste
that reacts with the zinc container. Electrons then
travel to the carbon rod in the center for our use.
Electrons flow from the negative terminal to the
positive terminal of a battery.
Negative charges build up on the negative
terminal and then flow through the machine to
reach the positive terminal of the battery.
producing current continued…….
Wet Cells – (also called voltaic cell) Metal plates
called electrodes,(copper and zinc), are placed in
an electrolyte, usually a type of acid.
The chemical reaction between the electrodes and
the acid causes electrons to pile up on the zinc
electrode which is negatively charged.
Electron pressure pushes the electrons from the
zinc plate to the copper plate which is positively
charged.
The wire connecting the two electrodes gives us
the pathway for a steady flow of electric current.
Thermocouples
Thermocouple is a device that changes heat
energy into electrical energy.
If the ends of a piece of copper wire and iron wire
are joined and one of the connections is heated
while the other is cooled, we will be able to
produce a current.
The greater the difference in temperature, the
greater the current.
Thermocouples are used in cars, ovens, and gas
furnaces.
Current Direction
Electrons moving through a wire can move
in the same direction or they can travel back
and forth.
Direct Current – (DC) Electrons always
flow in one direction – negative to positive.
Alternating Current – (AC) Electrons
reverse their direction regularly.
The current in your home is AC and
changes direction about 60 times per second
or 60 cycles per second.
Electric Circuits
In order to flow, electrons need a closed path
through which to travel.
An electric circuit provides a complete, closed
path for an electric current.
Parts of a circuit include, load or resistance, wires
and a switch.
Load – The device that uses the electric energy.
Usually offers some resistance to the flow.
Switch – Opens and closes the circuit. Electricity
cannot flow through an open circuit.
Circuits
(Simple series) Switch
Battery
-
+
Load or resistance
Parallel Circuit
- +
The different parts of a parallel circuit are on separate branches.
Therefore, each can work independent of the other.
Electric Power
Power – The rate at which work is done or
energy is used.
Electric power – The measure of the rate at
which electricity does work or provides
energy.
Power = Voltage x Current or P=V x I
Also can be written;
Watts= Volts x Amperes
According to this formula, one watt of power
is delivered when a current of one ampere
flows through a circuit whose voltage is one.
Electric Energy
The formula for electric energy is:
energy = power x time, or, E = P x t
The unit for energy from electricity is
“kilowatt hours”.
Large quantities of power must be measure
in kilowatts or units of 1000 watts.
So, kilowatts times hours = kilowatt hours
Electrical Safety
Fuses are designed to protect your circuits
by burning out if too much current is
flowing at once.
Circuit Breaker – Takes the place of a fuse
by turning off the power if too much current
runs through them at once.
Fuses must be replaced, but circuit breakers
can be reset.
Respect electricity because it will always
take the easiest path to ground, which could
be through your body.
Circuits
Series Circuit
Parallel Circuit
Ohms (resistance)
Ohm’s circle
Will these Circuits Work?
1) Look at each circuit diagram below.
2) Do you think the bulb(s) will light? If you do, put a tick in BOX A under the diagram. If not, put a cross.
3) Now make the circuit using the equipment. Does the bulb light? If it does, put a tick in BOX B. If not, put a cross.
4) Repeat this for all of the circuit diagrams. Then compare your predictions (BOX A) to your results (BOX B).
Box A Box B Box A Box B Box A Box B
Box A Box B Box A Box B Box A Box B
Box A Box B Box A Box B Box A Box B