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Electricity & Magnetism
Electrostatics Review
 Conductors allow charges to flow freely
 Insulators hinder flow of charges
 Are all conductors metal?
 Is earth a conductor or an insulator?
Electrostatics Review
Law of Electric Charges:
 Opposite charges attract
 Similar charges repel
 Charged objects attract some neutral objects
Three ways to charge an object:
 Friction
 Contact
 Induction
 The Bohr-Rutherford model of the atom is a planetary model in which
the electrons orbit around the atomic nucleus
 Electrons orbit the nucleus in orbits that have a set size and energy.
 The atoms of a solid are held tightly in place and the nuclei contain all
of the protons, the positive charges.
 The negative charges are free to move within a solid from atom to
atom.
Vocabulary
 Electric charge-a basic property of matter
described as negative or positive
 Static electricity-a build-up of stationery electric
charge on a substance
 Elementary charge (e)-electric charge of
magnitude equal to the charge on a proton and an
electron
Charging by Friction
Electrostatic Series:
A list that ranks the objects ability to take negative charges.
· Rubber (Items at top take negatives)
· Ebonite
· Polyethylene
· cotton
· silk
· wool
· glass
· acetate
· fur/hair (Items at bottom lose negatives)
Charging by Conduction
Charging by Induction
Electric Potential Difference
 Amount of work done per unit charge to
move a charge
V 
W E

Q
Q
or
E  QV
W = amount of work done to move a positive charge Q (J)
Q = amount of charge (C)
V = electric potential difference (voltage)
Where does C come from? e=1.60 x 10-19C
Electric Potential Difference
 Electric potential difference, or voltage, indicates the




difference in electric potential energy of the charges
(electrons) between two points in a circuit.
The amount of charge (Q), given by amount of electrons, is
measured in Coulombs
1 electron has a charge of e=1.60 x 10-19C, so Q=Ne,
where N is the # of electrons
Voltmeters measure electric potential difference and are
connected in parallel in a circuit. They have the symbol
Sources of electrical energy cause an increase in electric
potential (voltage gain), whereas loads cause a decrease in
the electric potential (voltage drop)
Electric Potential Difference
 Example 1: Calculate the electric potential
difference between the negative and positive
terminals of a battery if 1500 J of electric potential
energy is transformed to move 125 C of charge
between the terminals.
Practice Questions: p. 513 #1-5
Electric Current
 Electric charges moving from one place to
another
 Will only occur in a conductor (e.g. copper wire)
 It is measured in a unit called amperes (A)
Q
I
t
I = electric current (A)
Q = total charge (C)
Δt = total time (s)
Current + the Human Body
Electric Current
 Current flows from a region of _________ charge
to a region of __________ charge
 Here’s an example of a typical circuit diagram.
Please label with an arrow of the direction of
electric current flow.
 Just like a voltmeter measures voltage (electric
potential difference), an ammeter measures amps
(current)
 Voltmeter: parallel
 Ammeter: series
 Example - Calculate the amount of current through a
wire that has 0.85 C of electrons passing a point in
2.5 minutes
Current Practice
 Practice Questions: p. 518 # 1 - 9
Pictoral vs. Schematic
Electric Circuit
Symbols for elements of an electric circuit.
Series and Parallel Circuits
Equivalent Resistance
Series Circuit:
Req = R1 + R2 + R3 + … + Rn
Parallel Circuit:
1
1
1
1
1



 ... 
Req R1 R2 R3
Rn
Series and Parallel Circuits
Practice Problems:
Pg. 642 # 33, 34
Pg. 646 #36, 37
Kirchhoff’s Laws
Kirchhoff’s Voltage Law (KVL)
 http://www.wisconline.com/objects/ViewObject.aspx?ID=DCE3002
 Vseries = V1 + V2 + V3 +....
 Vparallel = V1 = V2 = V3 = ...
Kirchhoff’s Laws
Kirchhoff’s Current Law (KCL)
 http://www.wisconline.com/objects/ViewObject.aspx?ID=DCE3102
 http://www.youtube.com/watch?v=MnJS9RWbZwI
 Iseries = I1 = I2 = I3 = ...
 Iparallel = I1 + I2 + I3 + ...
Applying the Laws
Vsource = 40V
Vlamp1=10V
Vlamp3=20V
Vlamp2=?
Vlamp4=?
Isource=0.40A
I3=0.10A
I1=?
I2=?
Homework
 P. 522 Practice # 1 – 2
 P. 522 Questions # 1 – 2
Ohm’s Law
Potential difference between any two points varies
directly as the current between the two points.
V  IR
V = potential difference (V)
I = current (A)
R = resistance (Ω, ohm)
Sample problems:
Pg. 632 #24, 26
Power
Power is the rate at which energy is being used or
supplied. Same as previously defined in energy unit.
E
P
t
P = power (W)
ΔE = energy used (J)
Δt = time (s)
Other useful power equations derived using:
P  VI
V2
P
R
Sample Problems: pg. 655 #41, 42
E  VIt and V  IR
P  I 2R
Cost of Electricity
Electricity is charged by the amount of energy
used. The rate that the power companies use is
cost per kilowatt hour (kW·h)
kW·h = energy used in 1 hour by a load with a
power of 1 kW
For example, it costs $10.87 to operate a 40” LCD
television set for 30 days when used only 4.0h per
day.
Sample Problems: pg. 655 #41, 42
Electricity & Magnetism
Magnetic Resonance
Uses
Cancer detection and staging
Stroke and MS detection
Spine evaluation
Surgical planning and follow-up
Sports injuries
Heart disease detection
Major Advantages
No radiation … non-invasive
Soft tissue visualization
Image organ structure and function
Spectroscopy, MRI
Image at any angle (3D)
Magnetic Resonance Imaging
Magnetic Force & Fields
Magnetic Force & Fields
Law of Magnetic Forces:
N
S
S
N
FORCE
N
S
FORCE
N
S
Magnetic Force & Fields
Law of Magnetic Forces:
1. Opposite poles attract
Magnetic Force & Fields
Law of Magnetic Forces:
2. Similar poles repel
Magnetic Field Lines
Properties of magnetic field lines:
 Outside the magnet, begin on ________________
and end on ___________________.
 Inside the magnet, travel from _______________
to __________________.
 Never _______________.
 Spacing indicates the ___________________ of
the force (i.e. the ______________ the lines, the
greater the force.)
Magnetic Field Lines
Magnetic field around a
bar magnet
Magnetic field between
a pair of opposite poles
Magnetic Field Around the Earth
Oersted’s Discovery
 Danish physicist
 Discovered electromagnetism in
1819
 Was demonstrating the heating
effects of an electric current in
a wire
 Observed that a currentcarrying conductor caused the
needle of a compass to move
Electromagnetism
Principle of Electromagnetism:
Whenever an electric current moves
through a conductor, a magnetic field
is created in the region around the
conductor.
Right-Hand Rule #1
If a straight conductor with a current is held in the
right hand with the right thumb pointing in the
direction of the electric current, the curled
fingers will point in the direction of the magnetic
field lines.
Thumb points in direction of current
Magnetic lines
of force from
current
Fingers of right-hand curl around in
direction of field
Straight Conductor – Top View
The shaded inner circle represents the cross-section of a
straight conductor carrying a current.
Magnetic Field Around A Solenoid
Solenoid – large series of coils or loops (of wire).
Magnetic field created by a current flowing through a
solenoid is similar to the field of a bar magnet.
Direction of magnetic field depends on current direction.
Right-Hand Rule #2
 Fingers curl in direction of current flow
 Thumb points North
 The strength of the magnetic field of a coil depends on:



Current in the coil
Number of loops
Type of core material (e.g. air, iron…)
Motor Principle
A current-carrying conductor crossing an
external magnetic field, experiences a
force perpendicular to the magnetic field
and the direction of the current.
Parts of a Motor
-Commutator – split ring that rotates with the coil
-Brushes – connects commutator and cell
-Cell – provides current
-Field magnet – provides magnetic field
Motor
Motor
Right-Hand Rule #3
- Thumb in direction of current
- Fingers in direction of magnetic field
- Palm facing direction of force
Faraday’s Law of Induction
Law of Electromagnetic
Induction
An electric current is
induced in a conductor
whenever the magnetic
field in the region of the
conductor changes.
Faraday’s Law of Induction
Lenz’s Law
When a conductor interacts
with a magnetic field, there
must be an induced current
that opposes the interaction,
because of the law of
conservation of energy.