Download Electrostatics

SPH 3U
Electricity and Magnetism
Review of Matter
1. All matter is composed of atoms.
Our ideas about the nature of atoms have progressed over the last two centuries (and continue to
develop today).
 John Dalton introduced a new form of the ancient Greek idea of atoms at the beginning of
the nineteenth century.
 In 1897, J.J. Thomson discovered the electron and suggested the 'plum pudding' model of
the atom.
 In 1911, Rutherford suggested that electrons orbit the atomic nucleus like planets round
the Sun.
 In 1914, Bohr modified Rutherford's model by introducing the idea of energy levels.
2. Atoms contain protons, neutrons and electrons. Of these three subatomic particles, only the
proton (positively charged) and the electron (negatively charged) are considered charged
particles. A neutral object has an equal number of protons. A charged object has an unequal
number of electrons and protons.
(see hyperlink for more information)
http://www.broadeducation.com/htmlDemos/AbsorbChem/HistoryAtom/page.htm
Electrostatics
Static: still
Electrostatics: is the study of the phenomena arising from what seems to be stationary electric
charges.
Three themes – charge difference, medium, distance run through our study of electrostatics and
electricity
1. Charge Difference
 An object becomes charged if it gains or loses electrons. A neutral object has an equal
number of positive and negative charges! Charged objects obey the law of opposites as
follows
Fundamental Law of Electric Charges
A. Like charges repel.
B. Opposite charges attract.
C. Charged objects can attract some neutral objects, (depending on the level of
conductivity)

A historical look at charges
The choice of which type of electricity is called "positive" and which "negative" was
made around 1750 by Ben Franklin, early American scientist and man of many talents (the
stamp on the left commemorates his role as first US postmaster--and colonial postmaster
before that). Franklin studied static electricity, produced by rubbing glass, amber, sulfur etc.
with fur or dry cloth. Among his many discoveries was proof that lightning was a discharge
of electricity, by the foolhardy experiment (he claimed) of flying a kite in a thunderstorm.
The kite string produced large sparks but luckily no lightning, which could have killed
Franklin.
Franklin knew of two types of electric charge, depending on the material one rubbed.
He thought that one kind signified a little excess of the "electric fluid" over the usual amount,
SPH 3U
Electricity and Magnetism
and he called that "positive" electricity (marked by +), while the other kind was "negative"
(marked -), signifying a slight deficiency. It is not known whether he tossed a coin before
deciding to call the kind produced by rubbing glass "positive" and the other "resinous" type
"negative" (rather than the other way around), but he might just as well have.
Later, when electric batteries were discovered, scientists naturally assigned the direction of
the flow of current to be from (+) to (-). A century after that, electrons were discovered and it
was suddenly realized that in metal wires the electrons were the ones that carried the current,
moving in exactly the opposite direction. Also, it was an excess of electrons which produced
a negative electric charge. However, it was much too late to change Franklin's naming
convention
In 1896 Joseph John Thomson discovered the electron. However he didn't manage to define
its mass neither its charge. He only calculated the proportion of the charge to the mass of the
electron (q/m). He believed that the newly discovered particle was a part of an atom. But he
didn't give enough evidence for that hypothesis. Defining the charge and the mass could be
the final proof.
Many scientists tried to solve this problem. The first who managed to measure what was the
elementary charge was one of Thomson's students- J.S. Townsend. Unfortunately the final
result he got had a big measurement error. The first scientist who quite precisely defined that
charge was Robert Andrews Millikana. It must be said that he didn't measure the charge of
the electron. He only showed that there is some indivisible amount of charge in nature- the
elementary charge. All other charges are exact multiples of that elementary one. Later it
emerged that the elementary charge was the the charge of the electron.
Since protons are fixed in the nucleus of an atom, electrons are the charge which become
influenced and move. {electron flow} But, because charges obey the law of opposites, we can
analyze charges through the movement of either positive or negative charges, ignoring their
connection to the atom. {Current flow} The grade 11 curriculum encourages the use of the
current flow convention which assume that positive charges flow.

Charging an object
1. Friction
Whenever two different objects are rubbed together, electron transfer will occur due
to their electron affinity. Electron affinity refers to how much hold a substance has
on its electrons.
More information can be found at
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/estatics/u8l2a.html
including the triboelectric series
2. Arcing/Contact
If a charged object is brought near another charged object or a conductor, electrons
might jump through space due to their attraction. This is called arcing. Lightning is
an example of an arc – a large transfer of charge.
3. Induced Charge Separation / Grounding / Charging by grounding
As the diagram indicates the following explains another method of charging an object
 In the first picture, the charges are uniformly distributed.
 When a negative rod is brought near, the charges realign – called INDUCED
CHARGE SEPARATION
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Electricity and Magnetism
 If the neutrally charged ball (same number of positive and negatively charged
objects despite the new alignment) is then connected to ground, the charge will
flow to even out the local excess of charge.
 If the ground is removed, the once neutrally charged ball is charged.

Q=Ne
Robert Millikan hypothesized that all charges were based on some multiple of an elementary charge –
the electron. His claim was verified by his famous oil drop experiment in August 1913. For more
reading – see http://www.aps.org/publications/apsnews/200608/history.cfm
Q stands for the charge - units are Coulombs
N stands for a the number of electrons in excess or deficit
e stands for the charge of the elementary charge – the electron
e = 1.6021892  1019 Coulombs
2. Medium
Definition - the surrounding environment, substance. Examples include air, metals, etc..
Conductors vs. Insulators
More on conductivity....
The bonding nature (structure) of a material affects its ability to be influenced by a
charge.
A conductor allows a charge (electrons) to move within the material rather easily where
an insulator restricts any internal movement of charge (electrons). An insulator restricts
the movement of particles including electrons.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/conins.html
Humidity
Humidity is a measure of the amount of water vapour in the air. Water is a better conductor
then air and Lightning is more likely to happen when the humidity is high or when there is
rain.
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Electricity and Magnetism
3. Distance
Coulomb(1736-1806) mathematically analyzed the law of opposites.
More information - http://www.glenbrook.k12.il.us/gbssci/phys/Class/estatics/u8l3b.html
Coulomb’s law describes the force that one electric charge will exert on another:
 The magnitude of the electric force between any two charged objects is directly
proportional to the product of the charges on the objects and inversely proportional to the
square of the distance between their centers.
Fe 
kQ Q
1 2
d 2
k is the Coulomb's law constant, for air the value is approximately
9.0109







N  m2
C2
Summary
The smallest charge freely occurring in nature is the elementary charge.
The elementary charge is equal to about 1,602x10-19 Coulombs.
An electron is of the negative, elementary charge.
The first one to define the quantity of the elementary charge was Robert Andrews
Millikan.
positive charge comes from having more protons than electrons. i.e. +e
negative charge comes from having more electrons than protons. i.e. –e
charge is quantized, meaning that charge comes in integer multiples of the elementary
charge e
Q=N∙e
Q - the symbol used to represent charge
N - a positive or negative integer
e - the electronic charge, 1.60 x 10-19 Coulombs (C).
4. Movement
Stephen Gray , in 1707-8, determined that charge can flow through conductors and that
this flow could be controlled. Gray’s discovery marked the first step of the journey from
electrostatics to the control of electric current.
http://chem.ch.huji.ac.il/history/gray.html
The second more crucial step occurred in 1800, Alessandro Volta (1745-1827) invented the
electrochemical cell.
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Discovered that if you place salt water soaked paper between two different metals, such as silver
and zinc, an electric charge appeared on each of the metal disks and became known as a voltaic
cell.
When he made a pile of these cells the electric strength increased. This pile became known as a
battery or voltaic pile.
Question:
Suppose that two point charges, each with a charge of +1.00 Coulomb are separated by a distance
of 1.00m. Determine the magnitude of the electrical force of repulsion between them in (N), then
in units we can better make sense of.
Objects do not acquire charges on the order of 1.00 Coulomb.
Q values are on the order of 10-9 (nano, nC) or possibly 10-6 (micro, μC).
Question:
Two balloons are charged with an identical quantity and type of charge: -6.25 nC. They are held
apart at a separation distance of 61.7 cm. Determine magnitude of the electrical force of repulsion
between them. 9.2x10-7 N
Electric Current
The number of electrons flowing past a single point per second is referred to as electric current.
The SI unit for current is the ampere (A). We measure electric current with an ammeter.
quantity of charge
current 
change in time
Q
I
t
How fast do
electrons flow?
When a current of 1A flows through a conductor for 1s, 1C of charge passes any point in the
18
conductor. 1C of charge is equal to 6.2410 electrons.
What does this say
about electrons?
Question: A typical lightning flash lasts a quarter of a second and the peak current is about 15kA.
Determine the quantity of charge in coulombs transferred by this lightning strike. Approximately
how many electrons is this?
WHICH WAY DO ELECTRONS FLOW? DO WE KNOW?
2 THEORIES
Conventional Current
Today, the term current (I) means the flow of positive charge from the anode to cathode in a
circuit. (positive terminal to negative terminal) This was the convention chosen during the
discovery of electricity.
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Electricity and Magnetism
Electron Flow
The flow of negative charge from cathode to anode is called electron flow. (negative to positive
terminal)
Both Conventional Current and Electron Flow are used by industry. In fact, it makes no
difference which way current is flowing as long as it is used consistently. The direction of current
flow does not affect what the current does.
We will use the conventional current theory since a wealth of theory was based on this
convention.
Potential Difference
Imagine skiers on an electric hill, they represent the charges.
The chemical action inside a battery takes negative charges from the cathode (bottom of the
electric hill) to the anode (top of the electric hill), giving them electric potential energy.
The difference in electric potential energy (∆EQ) per unit of charge (Q) is defined as the potential
difference(V).
(V) is sometimes called the voltage of the cell, battery or power supply.
potential 
V
 potential energy
quantity of charge
E
Q
Q
The SI unit for potential difference is the volt. We measure potential difference with a voltmeter.
Question: A battery has a potential difference of 18.0V. How much work is done when a charge
of 64.0 C moves from the anode to the cathode?
Producing Electric Potential Energy
Electrochemical Cells
1. Different metals hold onto their electrons differently.
2. In a solution, the solute disassociates into ions. Thus possible charge carriers.
3. When these 2 conditions are present, electrons will move due to the potential difference
caused by the 2 metals. The current depends on the pull of the electrodes and the freedom
of electron movement.
Water Pump
Analogy
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Electricity and Magnetism
Voltaic Cell
 2 metals in an acidic solution.
Dry Cell
 2 metals in a paste
 safer because the acidic cannot leak
Piezoelectric Cell
 by squeezing a crystal, a current can be created.
Secondary Cell
 the electrons, through electrical energy, can be moved back to their origin causing the cell to
recharge.
Thermocouples
 2 metals coupled together can indicate temperature difference.
Photoelectric
 light comes down to free an electron.
The Simple Circuit
 Whatever current leaves the source must also return to the source. Because the charge carriers
must lose their energy during their trip & the resistance in the loop is predetermined, the rate
at which they travel through the loop adjust accordingly. This law is represented
mathematically as V = I R.
Kirchoff’s Voltage Law

The algebraic sum of the potential differences around any closed pathway or loop must
equal zero.
VT = V1 + V2 +…
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
Electricity and Magnetism
Via the conservation of energy, electrons gain as much energy at the source as they lose
at each load. (must spend all their energy). As a result the potential difference across the
components in a parallel circuit must be equal.
VT = V1 = V2 = V3
Kirchoff’s Current Law

At any junction in an electric circuit the total current flowing into the junction is equal to
the total current flowing out of the junction.

Electric charge is neither created nor destroyed in any electric circuit. (same current
returns) Since charge can only flow through one pathway in a series circuit, the electric
current at all points is the same.
IT = I1 = I2 = I3

At a parallel connection the total current flowing into the connection must be equal to the
sum of the currents flowing out.
IT = I1 + I2 + …
Ohm’s Law

The potential difference across a load is equal to the product of the current flowing
though the load and the loads resistance.

V=IR
Ohm’s Law is limited to metal conductors at stable temperatures. Therefore, does not
explain resistance in all circuits.

Some loads will not obey Ohm’s Law and are referred to as non-omhic. (i.e. light bulb)

As the current increases through the load the resistance increases.
Question:
Solve the following circuit. Find the unknown values.
V= 120V
I=
R= 90Ω
P=
V=
I=
R= 20Ω
P=
V=
I=
R=
P=
V=
I=
R= 30Ω
P=
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Electricity and Magnetism
Mixed Circuits
Your house is wired in parallel. One branch goes to your dryer, one to the kitchen, one to the
basement, etc.
Within each path we can further divide it and setup series sections.
V=
I=
R= 2Ω
P=
V=
I = 2A
R=
P=
V=
I=
R= 2Ω
P=
V=
I=
R= 2Ω
P=
V=
I=
R= 4Ω
P=
The net effect is you end up with a parallel circuit with a series section. To solve the circuit, you
must first simplify all subsections to 1 main type of circuit.
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Electricity and Magnetism
Resistance
The wire’s resistant nature is referred to as resistivity and depends upon:




the type of metal (gold is the best conductor)
the length of the loop
the cross sectional path (thickness)
the temperature (higher the temperature of the wire the higher the resistance)
If you combine the relationship of the resistance of a conductor to it’s length and cross-sectional
area, the result is
resistance  resistivit y 
R 
Length
Area
L
A
Temperature affects the resistivity values for different conducting materials.
Question:
Calculate the resistance of 15m length of copper wire at 20°C that has a diameter of 0.050cm.
(copper 1.7 x 10-8 Ωm)
Table of
resistivity values
Resistance in Circuits
 The total resistance in the circuit dictates the voltage and current based on the limits of the
power supply.
 The greater the resistance, the greater the need for energy to pull one electron around the
loop. This results in less electrons completing the loop (I ↓). (and vice versa)
 We look to the total Resistance in the circuit at the power supply because of this
RT 
V
I at the source.
adjustment of V and I,
 The variation in resistance calculation changes with the type of circuit (parallel, series), due
to the number of loads each electron must travel through.
In Series:
RT 
V
I at the power supply or
n
RT = R1+R2+R3+…(
In Parallel:
RT   Ri
i 1
)
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Electricity and Magnetism
RT 
V
I at the source or
n
1
1
1
1
1


 ... RT  
RT R1 R2
i 1 Ri )
(
Summary note on Current electricity.
Review of Equations
Circuits – a complete path for electrons to follow getting energy from a power source and
spending this energy at one or several loads. This energy is transferred to the load and is used to
do work on the load.
Laws
Series Circuit
Parallel Circuit
Mixed Circuit
- only one path for the
- multiple paths for the - a parallel and a
electron
electron
series combination
Ohm’s Law
R = V/I
R = V/I
R = V/I
(at each load)
Total Resistance
RT = R1+ R2+… Rn
the circuit must be
1
1
1


 ...
simplified to
RT R1 R2
properly solve – two
Kirchoff’s Voltage
VT = V1+ V2+… Vn
VT = V1= V2=… Vn
loads can be
Law
replaced with an
equivalent load
Kirchoff’s Current
IT = I1= I2=… In
IT = I1+ I2+… In
Law
I = current (Amps, A), R = resistance (Ohms, Ω ), V = voltage or potential difference (Volts, V)
Other equations
Power P = VI
Work = Energy change = VQ
(Watts, W = V∙A)
(Joules, J = Joules, J = V∙C)
Current I = Q/t
(Amperes, A = C/s)
Resistance
Resistivity
The wire’s resistant nature is referred to as resistivity and depends upon:




the type of metal (gold is the best conductor)
the length of the loop
the cross sectional path (thickness)
the temperature (higher the temperature of the wire the higher the resistance)
If you combine the relationship of the resistance of a conductor to it’s length and cross-sectional
area, the result is
resistance  resistivit y 
R 
L
A
Length
Area
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Electricity and Magnetism
The coefficient “ ρ” depends upon the material. Values can be found in Table 13.1 of your
PRACTICE PROBLEMS sheet.
Practice problems 16-20Resistance in Circuits
 The total resistance in the circuit dictates the voltage and current based on the limits of the
power supply.
 The greater the resistance, the greater the need for energy to pull one electron around the
loop. This results in less electrons completing the loop (I ↓). (and vice versa)
 We look to the total Resistance in the circuit at the power supply because of this
RT 
adjustment of V and I,

RT 
V
I at the source.
V
I is called Ohm’s Law. An ohmic resistor obeys this law and is one in which the
value of the resistor does not change over a fairly wide range of applied voltages and
currents. A non-ohmic resistor has a value that depends upon the applied voltage and
current and its resistance value will change. A non-ohmic resistor does not obey Ohm’s
Law.
Power, Energy and the Home
Power is defined as the rate at which work is done.
P = W/t (1 W = 1J/s) or P = VI (1 W = 1V∙A)
Homes use energy, not power. Power rates how fast energy is used. However, because the loads
are often listed by their power requirements – 60 W light bulbs – Electrical companies will
measure ones energy use through the KW∙h instead of J. Therefore , using an appliance for 1
hour might require 1 KW∙h or 1 KJ.
Practice problems – 51-53
Power
A power supply produces an electric potential (V) which when connected in a loop produces a
current.
P (Power) = I V (Watts, W)
P=IV
Power
A power supply produces an electric potential (V) which when connected in a loop produces a
current.
P (Power) = I V (Watts, W)
P=IV
Len’s Discovery (1843)
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Electricity and Magnetism
Len’s Law:
The magnetic field of an induced current always opposes the change in magnetic
field that is causing the induced current.
The electrons of an induced current flow in such a direction that the induced field
they create opposes the action of the inducing field.
Len’s Experiment
From Faraday: electron flow can be caused by changing the magnetic field either in magnitude or
position.
But which way would electrons flow? Only 2 possibilities.
#1.
Can’t be this! Law of Conservation of Energy. You must apply energy in some way. If this was
the case the magnet gets pulled in by itself.
Logic: Indirect Reasoning – if it’s not 1 than it has to be the other.
#2.
Must be this! You apply the energy by moving magnet. This KE is converted into electrical due
to the opposition of magnets motion. If you pull magnet away the induced current sets up a south
pole to attract the magnetic N, again opposing its motion.
PG 567 #8
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Electricity and Magnetism
Lesson 1: Title: Magnetism
Apparatus needed: big magnet, coins (iron filings, bar magnet, overhead)
Preliminaries: handout unit outline (on back of outline -- info on magnetic metals from Chem
Book)
Lesson: (be Socratic) Origin of magnet.
POLES:
All magnets have two opposite poles. DIPOLES. Electric quantities can have monopoles
(electron is just negative)
What are the two poles of a magnet called and why? North seeking, seeks the north direction.
Law of magnetic poles: Opposite magnetic poles attract. Similar magnetic poles repel.
(therefore the N pole is really a S pole!)
magnetic declination: difference between true north and magnetic north
MAGNETIC FIELD LINES
Forces which act over a distance (like gravity) with no physical contact. We think of a force field
extending from the magnet, that gets weaker with distance. (actually Fmag
We represent this field with lines:
(i) the lines form complete loops.
(ii) the field lines come out of the N pole and go into the S pole.
(iii) the closer the lines are, the stronger the field is.
(iv) field lines do not cross
Draw field lines around a bar magnet and a horseshoe magnet. Symbols for lines coming out of
DEMO: big magnet & coins.
How to map field lines: use iron filings, or small compasses.
Compasses point along the field lines -illustrate with world -- S is at top.
DOMAIN THEORY:
Electrons have a property called 'spin'. (not that they are actually spinning). Spin acts like a small
loop of electricity and makes a tiny magnetic field with a N and S pole. When there are a number
of unpaired electrons in an atom, the tiny magnetic fields can add up. Iron, nickel and cobalt are
the most common metals that have this effect. They are called "ferromagnetic materials". In
ferromagnetic materials the atomic dipoles of neighboring atoms align themselves. A region
where the atomic magnetic fields are aligned is called a domain. (Diagram of unmagnetized iron - random domains that cancel each other out) (Diagram of magnetized iron with N and S).
A region of a piece of iron where adjacent atoms are magnetically aligned is called a domain.
Unmagnetized iron consists of many tiny, disparate domains; a permanent magnet has more
aligned atoms and may have one domain the size of the magnet itself.
Rare earth magnets such as samarium, gadolinium, and neodymium owe their magnetism to the
orbital motion of electrons. When the magnetism related to this translational motion goes
uncanceled, the resulting field can be very strong. Rare earth alloy magnets are typically much
stronger than iron-based magnets.
SEATWORK: Read p 574-575. Summarize the effects a-g of the domain theory with one
sentence of explanation. (a. induction. b. demagnetization-dropping, Curie pt., c. reverse
magnetization. d. breaking. e. saturation. f. induced magnetism by earth. g. keepers.)
The Curie temperature of iron is 1043 K (m.p. = ), nickel 631 K (m.p. = )
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Homework: p579 #2,3,8,9 (or use for review)
Explain what causes the aurora borealis.
Draw magnetic field lines for two bar magnets: N---S N---S and N---S S---N
Magentism in depth
Types of Magentism:
(can leave this
for lesson 3)
Ordinary (weak) Magnetism
1. Repulsion from magnetic fields: diamagnetism.(e.g. Bi)
All the magnets in the atom (electron spin and orbital motion) exactly balance out: no average
magnetic moment. When you turn on an external field, little electric currents are generated
inside the atom by induction. According to Lenz’s Law these currents produce magnetic
2. Attraction to magnetic fields: paramagnetism (e.g. Al).
The atoms have a permanent magnetic moment (still have the diamagnetic effect - always)
When there is an external magnetic field, the moments (‘atoms’) try to line up with the
external field - enhancing it, and so the atoms are attracted/pulled to the stronger part of the
field. This effect is weak because the lining-up forces are small compared to the forces from
thermal motion which tend to randomize the order. Obviously paramagnetism is stronger at
lower temperatures.
1. ferroelectric materials: BaTiO3 permanent electrical polarization (one end positive, one
negative)
- predict that you could never get all of the magnetic moments lining up to make a permanent
magnet.
2. ferromagnetism (iron , cobalt, nickel and some alloys) 1000-1million times the magnetic
effects of other materials! Based on a quantum mechanical interaction between iron atoms.
Each small chunk of iron becomes spontaneously magnetized (below Curie Temp), but all of
the domains tend to cancel each other out.
Pauli Exclusion principle makes electrons that are close together want to have spins in
opposite directions (e.g. in chemical bonds can only have two electrons in each state - one
with spin up and one with spin down) -- very little magnetism. Now, there is something
similar that makes certain inner electrons want to align themselves from one atom to the next.
No one really knows why! It is a bulk property of iron.
3. ferrites: ferromagnetic insulators (Mg Al2O4) -- permanent magnets, but don’t conduct
electricity. Used a lot in high frequency systems (microwaves, AM antennas) since they don’t
have big eddy currents.
4. anti ferromagnets. (chromium & manganese) alternately aligned atoms -- no external field
The effect of a material on a magnetic field is described by a quantity called magnetic
permeability (
2 (just to confuse things, you
0
0 = 1 in gaussian units), describes how magnetic fields propagate in a vacuum.
Instead of absolute numbers, we commonly speak of relative permeability which is

r
=


0
.
>1
Magnetic Field: B =
H = ... (magnetizing field), B the result.
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Electricity and Magnetism
Magnetization: M = net magnetic moment per unit volume
= N atoms * average atomic magnetic moment (vector)
Electromagnetic Induction
Faraday Discovery (1821)
Principal of Electromagnetic Rotation – The Motor Principal:


A current carrying wire that cuts across a magnetic field experiences a force
perpendicular to both the direction of the current and the magnetic field.
The strength of this force depends on the strength of the field & the current.
Left Hand Rule: force comes out of palm
The Electric Motor involves changing the direction of the external magnetic field or the current
flowing through the coil every half rotation.
Otherwise the coil would only rotate until it is at right angles to the field and stop.
ex.
non coil
or loop
PG 554
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Electricity and Magnetism
(Oersted 1819: moving electrons create a magnetic field.)
Does the reverse hold true? Does a magnetic field cause electrons to move?
Faraday’s Discovery (1831)
Basic Principal of Electromagnetic Induction:

Whenever the conductor moves relative to a magnetic field or the strength of the field is
changing, a current will be induced to flow.

The strength of the current depends on:
1. # of turns in the coil
2. rate of change or motion
3. strength of the magnetic field
This is the basic principal for all generators and transformers.
Len’s Discovery (1843)
Len’s Law:
The magnetic field of an induced current always opposes the change in magnetic
field that is causing the induced current.
The electrons of an induced current flow in such a direction that the induced field
they create opposes the action of the inducing field.
Len’s Experiment
From Faraday: electron flow can be caused by changing the magnetic field either in magnitude or
position.
But which way would electrons flow? Only 2 possibilities.
#1.
SPH 3U
Electricity and Magnetism
Can’t be this! Law of Conservation of Energy. You must apply energy in some way. If this was
the case the magnet gets pulled in by itself.
Logic: Indirect Reasoning – if it’s not 1 than it has to be the other.
#2.
Must be this! You apply the energy by moving magnet. This KE is converted into electrical due
to the opposition of magnets motion. If you pull magnet away the induced current sets up a south
pole to attract the magnetic N, again opposing its motion.
The Northern Lights
The Northern Lights, or 'Aurora Borealis', are a natural light show that can be seen at the North
Pole. They also occur at the South Pole, where they are called the 'Southern Lights' or 'Aurora
Australis'. They are a spectacular display of different shades and colors of light rapidly moving in
the night sky.
When?
 Particles from the sun interact with the Earth's atmosphere.
 These particles are what's know as 'solar wind', and are quickly moving charged particles
that are emitted from the Sun.
 The solar wind is very closely linked to the 11 year solar sunspot cycle and to solar flares.
 Most of the world is protected from these particles by the magnetic field of the earth,
except the Poles. Here the solar wind can interact with the Earth's atmosphere.
How?
SPH 3U
Electricity and Magnetism
 charged particles collide with the Earth's air molecules, their energy is emitted as light.
 As the solar wind only interacts with the air at the Poles, this is where we see the lights, in
a ring around the Poles called the 'auroral ovals'.
 When there are more particles traveling from the sun, the ring expands and northern lights
have been reported in the UK.
 They are more common in the north of Scotland, as it is further north.
Where?
 very high above the Earth, usually over 40 miles high. This is even higher than jet planes
fly, but the lights have been seen as high as 600 miles above Earth, which is even higher
than space shuttles fly.
What Else?
 The solar wind can also have an effect on our telecommunications. Many radio signals are
'bounced' around the Earth using the top of the atmosphere. The charged particles of the
solar wind disturb parts of the atmosphere, which causes interference.
Transformers
A transformer is a basic component of the electric power system which convert power from
high voltages used in transmission to low voltage that are safe for use in our homes.
In all circuits there is internal resistance within the wires and connectors. In the short circuits
used in class the resistance is negligible. But in the transmission of power from generating
station to our homes there is energy that is lost in the form of heat.

The rate at which electrical energy is converted to heat can be found with this equation:
P = I2 R
This means there are only 2 ways to reduce power loss decrease the current (I) or decrease the
resistance (R).

Resistance has been minimized over the years by using power lines that are thick as
possible and made out of materials with low resistance.

Current can be reduced but the long distances over which the transmission occurs means
the resistance is still considerable. So reducing the current does not effectively reduce the
power loss due in the form of heat.
Recall the equation P = I V . With this equation we know that you can transfer the same
amount of power with high voltage and low current as you could with high current and low
voltage.
Situation 1:
Transmit Resistance Voltage -
1 000 000 W
10Ω
10 000 V
SPH 3U
Electricity and Magnetism
Current in the Wire
Power dissipated as Heat
P  I V
P  I2  R
P
V
1000000W
I 
10000V
I  100 A
P  100 2  10
I 
Situation 2:
Transmit Resistance Voltage -
Current in the Wire
P  100000W
Heat (W )
x100%
Power (W )
100000W

x100%
1000000W
 10%
% loss to heat 
1 000 000 W
10Ω
100 000 V
Power dissipated as Heat
P  I V
P  I2  R
P
V
1000000W
I 
100000V
I  10 A
P  10 2  10
I 
Percent loss due to Heat
P  1000W
Percent loss due to Heat
Heat (W )
x100%
Power (W )
1000W

x100%
1000000W
 0.1%
% loss to heat 
Therefore transmitting at a high voltage and low current reduces the power lost due to resistance
in the form of heat. Stepping up the voltage and stepping down the voltage occurs many times
before the power get to our appliances. Transmissions across the country can reach voltages of
500 000V in our home we use 120V.
Np < Ns
Vp < V s
Ip > Is
Np > Ns
Vp > V s
Ip < Is
SPH 3U

Electricity and Magnetism
Probably one of the main reasons for us using Tesla’s AC electrical system over Edison’s
DC system is the ease and efficiency with which transformers can change the voltage of
AC electricity.