Post on 19-Jan-2016
Electricity & Magnetism
Electrostatics Review
Conductors allow charges to flow freely
Insulators hinder flow of chargesAre all conductors metal?Is earth a conductor or an insulator?
Electrostatics Review
Law of Electric Charges:Opposite charges attractSimilar charges repelCharged objects attract some neutral objects
Three ways to charge an object:FrictionContactInduction
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
Q
E
Q
WV
QVE or
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)
t
QI
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: parallelAmmeter: seriesExample - 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:
neq RRRRR
1...
1111
321
Series and Parallel Circuits
Practice Problems:
Pg. 642 # 33, 34
Pg. 646 #36, 37
Kirchhoff’s Laws
Kirchhoff’s Voltage Law (KVL)http://www.wisc-online.com/objects/
ViewObject.aspx?ID=DCE3002Vseries = V1 + V2 + V3 +....
Vparallel = V1 = V2 = V3 = ...
Kirchhoff’s Laws
Kirchhoff’s Current Law (KCL)http://www.wisc-online.com/objects/ViewObject.aspx
?ID=DCE3102http://www.youtube.com/watch?v=MnJS9RWbZwIIseries = I1 = I2 = I3 = ...
Iparallel = I1 + I2 + I3 + ...
Applying the Laws
Vsource = 40VVlamp1=10VVlamp3=20VVlamp2=?Vlamp4=?
Isource=0.40AI3=0.10AI1=?I2=?
Homework
P. 522 Practice # 1 – 2P. 522 Questions # 1 – 2
Ohm’s Law
Potential difference between any two points varies directly as the current between the two points.
IRV 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.
P = power (W)ΔE = energy used (J)Δt = time (s)
t
EP
tVIE IRV Other useful power equations derived using: and
VIP R
VP
2
RIP 2
Sample Problems: pg. 655 #41, 42
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
Uses Cancer detection and staging Stroke and MS detection
Spine evaluation Surgical planning and follow-up Sports injuries
Heart disease detection
Major AdvantagesNo radiation … non-invasive
Soft tissue visualization Image organ structure and function Spectroscopy, MRI
Image at any angle (3D)
Magnetic Resonance
Magnetic Resonance Imaging
Magnetic Force & Fields
Magnetic Force & Fields
Law of Magnetic Forces:
NN SS NNSS
FORCE
NN SS NN SS
FORCE
NN SSNN SS NNSS NNSS
FORCEFORCE
NN SSNN SS NN SSNN SS
FORCEFORCE
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 physicistDiscovered
electromagnetism in 1819 Was demonstrating the
heating effects of an electric current in a wire
Observed that a current-carrying 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 flowThumb 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
InductionAn 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.