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Chapter 5
Electricity and Magnetism
What is electricity?
• Electricity describes all of the phenomena caused
by positive and negative charges
– Examples …
• Protons carry a positive charge (+) whereas
electrons carry a negative charge (-)
• If a body (atom/molecule or object) has MORE
PROTONS than electrons, it will be positively
charged.
• If a body has MORE ELECTRONS than protons, it
will be negatively charged.
Illustrating charges
• Draw two balloons, as shown below:
Positively charged balloon
Negatively charged balloon
Forces of Attraction / Repulsion
• Electrically charged objects, which have
gained or lost ELECTRONS, tend to exert a
force on other objects nearby.
• Like charges (+ + or - - ) will tend to REPEL
each other.
• Opposite charges (+ - ) will tend to ATTRACT
each other.
• Charges, like matter or energy, are conserved.
• http://www.youtube.com/watch?v=Fyko81W
AvvQ
Conductors and Insulators
Charging an Object
• Charging an object means creating an
imbalance between the number of protons
and electrons.
• When an object has the exact same number of
protons and electrons it is called NEUTRAL.
• Not all objects behave the same way when
they receive a charge.
Conducting Continuum
• Depending on what happens to an object
when it receives a charge, we can say that it is
a conductor or an insulator
• Conductors allow charges to flow freely and
spread evenly throughout the object
• Insulators are objects that impede the flow of
electrical charges and don’t allow them to
move easily through the material.
• Superconductors: a substance
capable of becoming
superconducting (lose all electrical
resistance) at sufficiently low
temperatures.
• Example: mercury, niobium-tin,
yttrium-barium-copper oxide.
• Semi-conductors: a substance that has a
conductivity between that of an insulator and
that of a conductor (metal).
• Examples: silicon, germanium
Conducting Continuum
• Every object fits somewhere along this
continuum
• Where do you think the human body would fit
in the above diagram?
Behaviour of charges
• When a charge is acquired by a conductor, the charges
will tend to distribute themselves across the surface.
When a charge is acquired by an insulator, the charges
will tend to remain at the initial location of charging.
Static electricity
• Static means “not moving”
• Static electricity describes all of the
phenomena related to charges that are at rest.
• Charges are at rest when they are INSULATED
• Static charge is often detected by a device
known as an electroscope.
• There are three methods for charging objects
Methods of charging an object
Method
Friction
Conduction
Induction
Before
During
After
Triboelectric Series
Dynamic Electricity
• Dynamic means “moving”
• Dynamic electricity describes all of the
phenomena related to electrical charges in
motion.
• Charges move when they are free to do so,
namely when they are in contact with
conductors
Electric Current
• Electric current: refers to the orderly flow of
electrons through a conductive medium
• Flow is from negative to positive
• Conventional current: is the direction in which a
positive particle would flow in an electrical circuit
[EVEN THOUGH THIS DOESN’T REALLY HAPPEN]
• Flow is from positive to negative!
Electric Circuits
Symbols
Explanation
• There are certain symbols
that will be important as we
study circuits. Here are
some of the most
important.
• Also learn the symbols for:
– Switch
– Light bulb
Electric Circuit
• There are three main categories of electric
circuits (but we’re only going to study two):
No choices…
I wish I had
choices!
Ah
choices…it’s
nice to have
choices!
Definitions in Electric Circuits
• There are three definitions that are very
important in the study of circuits.
• These are:
– Current Intensity
– Potential Difference
(Voltage)
– Resistance
Current Intensity
• Symbol: I
Unit: Amperes (Amps, A)
• is the number of charges that flow past a
given point in a circuit per second.
• Formula: I = q/t
• Measured with a device called an AMMETER,
which must be connected in series!
Potential Difference (Voltage)
• Symbol: V
Unit: Volts (V)
• the amount of energy transferred between
two points in an electrical circuit.
• Formula: V = E/q
• Measured with a device called a VOLTMETER,
which must be connected in parallel!
Resistance
• Symbol: R
Unit: Ohms (Ω)
• is the ability of a material within an electrical
circuit to hinder the flow of electric current.
• Formula: R = V/I
• Measured with a device called an
OHMMETER, which must be disconnected in
from the circuit to take a measurement!
• Resistance is dependent on many factors (see
notes)
Ohm’s Law
• Is the mathematical relationship between
current intensity, potential difference and
resistance.
• It states that for a given resistance, the
potential difference is directly proportional to
the current intensity.
• V=IxR
or
I=V/R
or R = V / I
Power
• Symbol: P
Unit: Watt (W)
• is the rate at which work is performed by an
electrical device in a circuit per second.
• Formulae: P = Work/time (W/t)
P = V•I (which can be measured directly from
the circuit by using an ammeter and
voltmeter)
Electrical Energy
• the amount of energy used by an
electrical device is often the same
as the work done by the device.
In order to calculate the amount
of money it costs to operate an
electrical device, it is important
to know the energy rating.
• You can convert between Energy
and Power by using E = P • t
• Pay attention to units!**
** Sometimes
Energy is
expressed in
kilowatt hours
(kWh) and
sometimes it’s
expressed in
Joules (J). **
Electricity costs $$$
Cost = cost of electricity • amount of electricity used
$ = $/kWh • kWh
Magnetism
• Magnetism describes all
of the phenomena caused
by magnets.
• Magnets are objects that
can attract other objects
containing primarily iron, cobalt or nickel.
Theory of Domains
Magnets have poles
• Just like in charges  opposites attract and likes repel
• The north pole of a magnet is the end that naturally seeks
the Earth’s magnetic pole near the geographic North Pole.
•The other side of the magnet is its south pole.
Compass
• A compass is a device that
has a freely turning
magnetized needle. This
needle will always try to
point north, and so is helpful
for navigational purposes.
Note: when a compass is
brought near a magnet, it will
align itself with the magnetic
field, and is no longer a
reliable navigation tool.
Magnetic Fields
• Magnetic fields are the areas in space in which
the force of a magnet can act on another magnet.
• These fields are represented by lines with arrows,
and the closer these lines are to one another, the
stronger the field.
• The direction of the arrows indicate the direction
of the magnetic field, which is always from
NORTH  SOUTH.
• Magnets come in different shapes and sizes.
Magnetic Fields in Straight Wires
• Current-carrying wires, like
magnets, will exert a
magnetic field.
• The direction of the magnetic
field is determined by the
RIGHT HAND RULE which
states: “when the thumb of
the right hand points from +
to -, the fingers of the right
hand point in the direction of
the magnetic field”.
Examples of Magnetic Fields