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.