Electricity and
Magnetism
What is magnetism?
Magnetism is the force of attraction
between magnets and magnetic
objects.
Any material that exerts a magnetic
force is considered a magnet.
Magnet vocabulary





Magnet: material that can both attract and
repel other magnets. Iron is most common.
Pole: Part of the magnet where the force is
the strongest.
S
N
Field: Region of magnetic force around a
magnet. Field lines run from north to south.
Bar magnet: bar-shaped magnet
Horseshoe magnet: U-shaped magnet.
Magnetic field of a bar magnet
Field lines curve around a magnet from N
pole to S pole and are strongest where
they are closer together.
How do magnetic poles interact?
• The north and south poles of two magnets
attract each other – the magnetic force pulls
them together.
• However, if you try to bring two of the same
poles (two norths or two souths) together they
will repel each other.
What happens if a magnet is broken?
When a magnet is broken, two
separate magnets are created,
each with their own N and S poles!
What causes magnetism?
Moving charges cause
magnetism, so…..
Electrons moving around an
atom produce a magnetic
field.
These spinning electrons
make each atom a tiny
magnet! Common magnetic
metals are Fe, Co and Ni.
Iron
What are magnetic domains?
•In most materials the
magnetic fields of atoms
point in random
directions.
•A cluster of billions of
atoms that all have
magnetic fields lined up
in the same way is known
as a magnetic domain.
How is magnetism related to domains?
In a magnetized material, all or most of the domains
are arranged in the same direction –the magnetic
properties of a substance depends on the electron
arrangement in the atoms.
Making Magnets
 In most materials, if you add energy to the electrons,
you can get them to move and align. By placing a
nonmagnetic object in a strong magnetic field, the
domains of the object can line up.
 In this way, a temporary magnet can be created.
Can a magnet be destroyed?
• Heating a magnet or striking a
magnet can cause the domains to
become misaligned and a magnet
can lose its magnetism.
• Above a certain temperature,
magnets will lose their magnetism.
The earth is one giant magnet…

Geographic North pole: the north end of the
axis around which the Earth rotates.
There is a difference
between the
geographic North
Pole and the
Magnetic North Pole


Magnetic North pole: the point on the Earth to
which a compass needle points.
The Earth creates magnets in some
materials, like iron.
The earth is surrounded by a
magnetic field
The earth’s magnetic field affects the movement of
electrically charged particles in space.
The Northern lights
The solar wind is a stream of electrically
charges particles from the sun.
What Is Electricity?
• Electricity is a form of energy made up
of moving charges that can produce
light, heat, or motion.
• The flow of electrons creates
electricity.
Kinds of Electricity
Static
• An electric charge that
collects or builds up on
the surface of an object,
but does not flow.
• Examples:
– Shocking
– Lightning
– Plastic wrap
Current
• The flow of electric
charges carried
through a material
such as wire or other
conductors.
• Examples:
– Power lines
– Computers
– Appliances
How does static electricity build up?
• A neutral object can become charged by gaining
or losing electrons.
• An object that loses electrons will become
positively charged, whereas an object that gains
electrons becomes negatively charged.
How are electrons transferred?
• There are three ways in which electrons can
be transferred:
friction –transfer by “stripping off”
electrons
conduction –transfer by direct contact
induction –transfer caused by the electric
field of another object
Static Discharge
• Negatively charged objects will attract positively
charged objects.
• When a negatively charged object and a positively
charged object are brought together, electrons
move until both objects have the same charge.
• The loss of static electricity as electric charges
move off an object is called static discharge.
• Sparks and lightning are examples of static
discharge.
Electrical Fields
Charges are similar to magnets -they
exert a force at a distance in the area
around the charged object creating an
electrical field.
Electrical fields pull or push on any
other charges in their vicinity.
Electrical Fields
Electrical fields are represented by arrows
pointing in the direction a positive charge
would be forced to move.
e-
p+
What are circuits?
• Circuits are a path along
which electricity moves.
• The path of electrons is from a
negative terminal, through the
various components, and on to a
positive terminal.
• Appliances such as toasters, radios
and hair dryers all have electric
circuits!
PROPERTIES OF ELECTRICITY
• Voltage is the electrical force, or "pressure", that causes
current to flow in a circuit. It is measured in VOLTS (V).
• Current is the movement of electrical charge - the flow
of electrons through the electronic circuit. Current is
measured in AMPERES (AMPS, A).
• Resistance is anything that causes an opposition to the
flow of electricity in a circuit. It is used to control the
amount of voltage and/or current in a circuit. Everything
in the circuit causes a resistance (even wire). It is
measured in OHMS (W).
The relationship between V, I and
R is similar to a waterfall where
V= height of waterfall, I=amount
of water flowing, and R=number
of rocks blocking the flow.
…or it can be thought of like a
football game… where volts are
the force that moves electrons
in a circuit, the resistance is the
force that tries to stop the
electrons, and the amperes are
the number of electrons that get
through.
Voltage
Resistance
V
I
R
What is the relationship
between these properties?
• The relationship between these properties is given by
“Ohm’s Law”: V=IR
where V=battery voltage, I=current and R=resistance
• Resistance is measured in Ohms (W)
Current is measured in Amperes (A)
• Example 1: What is I for a circuit with a 12 V battery and R
= 2 W?
I = V/R = 6 A
• Example 2: What is I for a circuit where resistance is 2 W
and the battery has 6 V?
Therefore, I = V/R = 3 A
Kinds of Circuits
Open/Closed
Open circuits do not
have a complete path to
follow. The power
cannot pass from the
source to the receiver.
Closed circuits allow
electricity to pass.
*Light bulb when it is
turned off (open).
*Light bulb when it is
turned on (closed).
Series
Parallel
These circuits
These
have a single circuits have
complete
more than
path for
one path for
electricity to electricity to
follow.
flow
through.
*Christmas
tree lights
*Your house
*Power lines
Circuit Materials
• Conductor: a conductor is a material (usually a
metal such as copper) that allows electrical
current to pass easily through. (ex: copper)
• Insulator: a material that is a poor conductor
and has electrons that are tightly bound to
individual atoms. (ex: neon)
• Semiconductor: a material having electrical
properties somewhere between those of good
electrical conductors and insulators. (ex: silicon)
Conductors and Insulators
Conductors
• Any material that
allows electric
charges to move
through it.
• Examples:
– Metals (esp.
copper)
– Your body
– Electric wires
Insulators
• Materials that do not
allow electric
charges to flow
freely through them.
• Examples:
– Wood
– Glass
– Rubber
• Simple Circuit
If we break a circuit down
to its basic parts we get:
1) A voltage source (ex: a
battery)
2) A flow path (a wire)
3) A load (ex: a lightbulb,
resistor)
4) A control switch
5) A meter (ex: voltage
indicator)
(RESISTOR)
Parts of a circuit
Symbols used in circuit diagrams
When drawing diagrams of circuits there are a
number of standard symbols.
A few of the more common ones are:
Battery or Voltage Source
Resistance or load
Switch
Meter
Series Circuit
• A series circuit is one
with all the loads in a
row, like links in a
chain.
• There is only ONE path
for the electricity to
flow. If this circuit was
a string of light bulbs,
and one burned out, the
remaining bulbs would
turn off.
Parallel Circuit
• A parallel circuit is one that has
two or more paths for the
electricity to flow. In other
words, the loads are parallel to
each other.
• If the loads in this circuit were
light bulbs and one burned out
there is still current flowing to
the others as they are still in a
direct path from the negative to
positive terminals of the
battery.
Combination Circuit
A combination
circuit is one that
has both series
and parallel paths
for the electricity
to flow. Its
properties are a
combination of
the two.
Check point…
What kind of circuit is shown in
each diagram?
Practice Problem
• The brightness of the bulbs in a circuit indicates the
amount of current flowing through the bulbs.
• How will the brightness of the bulbs change as we
add more in series?
+
+
+
• What does this imply about the current in the circuit?
Lights become dimmer as bulbs are added because there is more
total resistance. This means that the current is reduced.
More Practice…
• How will the brightness of the bulbs change as we
add more in parallel?
+
+
+
• What does this imply about the current in the
circuit?
Lights remain the same brightness as each bulb is added in parallel,
and the current is the same for all three.
Key Concepts
• Bulbs in series represent a source of resistance
to current flow, similar to boulders in a stream
which can block the flow of water. Adding bulbs
will SLOW the FLOW and thus reduce current.
• Bulbs in parallel offer additional pathways for
current flow, similar to branches of a stream
which can carry more water. Adding bulbs will
not decrease flow, so current is the same for all
paths.
More practice… A
• Is this circuit series or parallel?
B
• Combination circuit
• How do the bulbs compare in brightness?
+
• A is brighter than B and C
• How will the brightness of A and B change
if bulb C is removed?
• A will stay the same, but B will become
brighter.
• How will B and C change if A is removed?
• They will become brighter
C
Series Circuits:
The total resistance of a circuit is found by
adding all the resistance in the circuit.

RT


...
R1 R2 R3
Series Circuits:
The total voltage of the power source is equal
to the sum of voltage used (voltage drop)
across each resistor.
V



...
V
V
V
T
1
2
3
Series Circuits:
The total current through the circuit is equal to
the current across each resistor.
IT = I1 = I2 = I3 = …
V
I
R
How do you use Ohm’s Law
to describe circuits?
• Recall the relationship given by “Ohm’s Law”: V=IR
where V=battery voltage, I=current and R=resistance
• Example 1: What is I for a bulb in series with a 12 V battery
and R = 1 W?
I = V/R = 12 A
• Example 2: For two bulbs in series, R = 2 W and V= 12
Volts. Find I.
Therefore, I = V/R = 6 A (and each bulb is dimmer than in
the last example as it has less current through it)
V
I
R
Practice Problems:
• In a simple circuit, 0.5 A is flowing through a
bulb. The voltage across the bulb is 4.0 V. What is
the bulb’s resistance?
• ANSWER: R = V/I = 4.0V/0.5A = 8 W
• In order for a waffle iron to operate efficiently, a
current of 12 A must flow through it’s coils. If
the resistance is 10 W, what must the voltage be?
• ANSWER: V= IR = (12A)(10 W) = 120V
V
I
Practice Problem
R
• Find I for two bulbs connected in parallel to a
12V battery with R = 0.5 W.
ANSWER: I = V/R = 24 A
The total current is split into two pathways: half
the current goes down each pathway. So the
current in each pathway = (24/2) A = 12 A
• The bulbs in the parallel circuit are therefore
each as bright as the bulb in the single bulb
circuit.
Key Concepts
• A battery is a source of constant voltage, for
example 1.5 volts or 12 volts (labeled on the
battery).
• The current (I) flowing from the battery depends
on its voltage (V) and the total resistance (R) in
the circuit.
• More bulbs in series = more resistance
• More parallel pathways = less resistance
What is electric power?
• The rate at which energy is converted into another
form is known as power.
• Electrical power is calculated using the
relationship P= IV
where P = power (measured in Watts), I= current
(measured in amps) and V= voltage (Volts)
P
I
V
Power Practice Problem
• A household light bulb has approximately 0.5 A of
current flowing through it. Since the standard household
voltage is 120 volts, what is the power rating for this
bulb?
• ANSWER: P=IV = (0.5 A)(120 V) = 60 W
• A flashlight uses two 1.5 volt batteries in series to
create a current of 0.5 A. What is the power rating of the
bulb?
• ANSWER: VT= V1 + V2 = 1.5 V + 1.5 V = 3.0 V
PT = IVT = (3.0 V)(0.5 A) = 1.5 W
Summary of Terms and Units
(voltage)
Direct Current
• A current is considered to be one-directional if it
always maintains the same direction of flow; it is
two-directional if it changes direction.
• An example of a DC current source is a dry cell
battery.
• http://videos.howstuffworks.com/hsw/19268chemistry-connections-primary-and-secondarycells-video.htm
Alternating Current
• If the magnitude and direction of the current
varies with time, it is referred to as alternating
current (AC). When a conductor carries an
alternating current, all of the moving charges
continually change their directions of motion.
• An example of AC current are wall outlets in
your home. AC is preferred over DC in
transmitting electricity long distances, since
less energy is lost and it is easier to transform
into higher or lower voltages.
Magnets and electricity
• Magnets are used to generate, or produce,
electricity.
• Spinning a coil of wire inside a magnetic field
produces an electric force between the ends
of the coil. In other words, a moving magnet
can produce electricity.
• In a similar way, an electric current produces
a magnetic field around it. In other words, a
moving charge forms a magnet and creates a
magnetic field.
Electromagnets
• An electromagnet is a temporary magnet
made by passing electric current through a
wire coiled around an iron bar.
• EXAMPLE: A junkyard is one place to see an
electromagnet in action. A crane holding a
huge electromagnet can be used to pick up
scrap metal when current flows through it.
When the crane operator wants to drop the
scrap, he or she will simply shut off the
current to the electromagnet.
Strength of Electromagnets
Forming a wire into a loop reinforces the field in
the center of the loop, repeated loops increase
the strength of the magnetic field.
Adding an Iron core to the loops further
strengthens the magnetic field due to the
alignment of the domains in the iron.
A segment of wire that carries a current
creates a magnetic field around it.
By coiling the wire, the magnetic
field is strengthened around it.
Magnetic field lines around
an electromagnet
In an ac generator, a wire flips between
two magnets. The change in direction
produces the alternating current.
Voltage alternates between
positive and negative values.
THE END
ADDITIONAL
PRACTICE
QUESTIONS
NAME THE METAL THAT IS THE BEST CONDUCTOR OF
ELECTRICITY:
NAME THREE GOOD INSULATING MATERIALS:
WHAT ARE SOME GOOD INSULATORS?
NAME THREE ESSENTIAL PARTS OF A SIMPLE
ELECTRICAL CIRCUIT:
IN A SERIES CIRCUIT THERE ARE HOW MANY PATHS
FOR CURRENT FLOW?
IN A PARALLEL CIRCUIT THERE ARE HOW MANY PATHS
FOR CURRENT FLOW?
DEFINE A COMBINATION CIRCUIT:
VOLTAGE IS MEASURED IN WHAT UNIT?
CURRENT IS THE FLOW OF WHAT PART OF THE ATOM
THROUGH A CIRCUIT?
CURRENT IS MEASURED IN WHAT UNIT?
DEFINE RESISTANCE:
DEFINE POWER:
Parallel Circuits:
The total current (amperage) in the circuit is
the sum of the current in each branch of the
circuit.

IT


...
I1 I 2 I 3
Parallel Circuits:
The total resistance of the circuit is given
by the following formula:
1
R

T
1

1
R R
1

2
1
R
 ...
3
Parallel Circuits:
The formula for total resistance can
also be written:
R
T

1
1
R
1

1
R

2
1
R
 ...
3
Parallel Circuits:
The total voltage across the circuit is equal to
the voltage across each resistor.
VT = V1 = V2 = V3 = …
Two Birds on a Wire
When the switch is closed,
what happens to the birds?
a) Bird 1 gets a shock and
bird 2 does not
1
2
b) Bird 2 gets a shock and
bird 1 does not
c) They both get a shock
d) Neither gets a shock
12 V
bulb
switch
ANSWER: Bird #1 will get shocked because
there is a potential difference, whereas #2 will
not.