MAGNETIC AND ELECTRIC FIELDS
ELECTRIC & MAGNETIC FIELDS
Spoiler: They are the same force viewed from differing frames of reference.
ELECTRIC FIELDS REVIEW (GRADE 9)
History of the Study of Charge
Ancient Greeks: Observed that rubbing amber with fur attracted light objects (water, fluff).
Amber in Greek means Elektron.
1700s:
Ben Franklin: Proposed that charge behaves like a fluid; objects can be neutral, positively, or negatively charged.
Charles François de Cisternay du Fay: Identified two types of charges: which can be created by rubbing fur on rubbery materials (resinous) or silk on glass (vitreous).
Early 1800s Developments
Atomic theory: Cathode rays studies showed that charge could deflect them.
1897: JJ Thomson formalized the existence of the electron with his plum pudding model.
1909: Robert Millikan conducted the oil drop experiment, determining the exact mass and charge of an electron (Nobel Prize in 1923).
Measuring Charge (q)
Fundamental Charge: Defined as electron charge = 1.602 x 10⁻¹⁹ C (commonly approximated as 1.6 x 10⁻¹⁹ C).
Coulombs: 1 Coulomb has approximately 6.25 x 10¹⁸ electrons.
Being "charged" indicates gain/loss of electrons:
Positive charge: lost electrons.
Negative charge: gained electrons.
Example Calculations
Uranium Atom Charge: Determine how many electrons are gained/lost for a charge of +9.6 x 10⁻¹⁹ C.
Neutral Object Charge: Find electrons needed to charge a neutral object to -30 µC.
Mr. Yost's Charge: Calculate total charge after gaining 78 quadrillion electrons.
iPhone Battery Charge: Determine total charge stored in an iPhone 16 battery with 3561 mAh.
ELECTRIC FIELDS
Definition: A region of space where force acts on an object due to its charge.
Gravitational Force Fields: Attract masses.
Electric Fields: Attract/repel charges.
Magnetic Fields: Attract/repel moving charges/magnetic objects.
Direction of Electric Fields
Defined by the direction of the force experienced by a positive charge.
Represented by Field Lines.
Field Lines
Conceptualized by Michael Faraday for visualizing force fields.
Density of Field Lines: Indicates relative strength of forces.
Direction Arrows: Show direction of force on a positive charge.
Field Lines Around Multiple Charges
Behavior changes when multiple charges coexist.
Dipole Field Lines: Characterize fields around positive and negative charges.
Identical Charges: Field lines around two similar charges.
Charged Plate: Field representation of a charged surface.
ELECTRIC FIELD STRENGTH (E)
Defined as the force experienced by a charge of +1C at a point in space.
Calculating Electric Field Strength
For a +2 C charge in an electric field causing a force of 350 N, find:
Electric field strength.
Force experienced by an electron at the same point.
Unknown charge based on the provided force.
A -40 µC charge under an electric force of 0.35 N, determine strength.
Combining Multiple Electric Fields
Net electric field strength calculated by considering contributions of multiple charges.
Analyze scenarios of different charge types and positions.
MILLIKAN OIL DROP EXPERIMENT
Conducted in 1909 at the University of Chicago to prove the existence of a fundamental charge.
Robert Millikan awarded the 1923 Nobel Prize for confirming elementary charge properties.
Found that charge is quantized; charged particles have whole numbers of elementary charges.
Oil Drop Experiment Process
Charged oil droplets using X-rays; selected droplets for study.
Applied electric field to balance forces on droplets and measure terminal velocity.
Examples of charge calculation from experiments:
Charge needed for a basketball to float in an electric field of 1.35 x 10⁵ N/C.
Excess electrons on an oil droplet based on its mass in an electric field of 1.15 x 10⁴ N/C.
MAGNETIC FIELDS
History: The term "magnet" derives from the Greek magnítis líthos meaning magnesian stone.
Natural Magnetite: Used in ancient compasses, with properties thought to be magical.
William Gilbert (1540-1603)
Known as the father of electricity and magnetism, first systematic study of magnetism.
Concluded that magnets have two poles and can attract or repel.
Magnetic Facts
Each magnet has polarity (north and south poles); cannot isolate them.
Like poles repel; opposite poles attract.
A compass is a suspended magnet that detects magnetic fields.
Permanent magnets made from iron, cobalt, nickel alloys.
Disruption from jarring, heating can demagnetize permanent magnets.
Magnetic Domains
Ferromagnetic materials consist of tiny magnetic regions called domains.
Aligned domains increase the strength of the magnet.
Alignment occurs under an external magnetic field and when electrically charged.
Field Lines in Magnetism
Proposed theory of magnetic lines of force by Michael Faraday.
Show direction of magnetic field at any point.
Magnetic Field Strength (B)
A magnetic field exists around magnets, moving charges, or wires carrying current.
Measured in Tesla (T) and indicated by vector quantity.
EARTH'S MAGNETIC FIELD (GEOMAGNETISM)
North Pole: Named for where a compass points towards the geographic North Pole.
Different types of North poles exist: Geographic, Geomagnetic, Magnetic.
Earth's Magnetic Field Cause
Described by Dynamo Theory: process by which fluid convection maintains Earth’s magnetic field.
Polarity Flips: Earth experiences random magnetic pole reversals approximately every 300,000 years, with the current flip delayed for 700,000 years.
Effects of Geomagnetism
Blocks radiation (Van Allen Belt) protecting Earth.
Impact on biological behaviors (e.g., migratory birds, compasses) and natural wonders (Northern Lights).
ELECTROMAGNETISM
Hans Christian Ørsted: Found relation between electric current and magnetic fields during lectures.
Concluded that moving charges create magnetic fields.
Magnetic Field Direction Around Wires
Use Right Hand Rule to find direction of magnetic field formed around current-carrying wires.
APPLICATIONS OF ELECTROMAGNETISM
Solenoids: Use coils of wire to produce strong magnetic fields, opening up many applications including loudspeakers.
Maglev Trains: Example of utilizing magnetic forces for fast transportation, reducing friction.